key: cord-298503-l60cdllh
authors: Saraste, J.; Marie, M.
title: Intermediate Compartment: A Sorting Station between the Endoplasmic Reticulum and the Golgi Apparatus
date: 2015-08-20
journal: Encyclopedia of Cell Biology
DOI: 10.1016/b978-0-12-394447-4.20013-8
sha: 
doc_id: 298503
cord_uid: l60cdllh

The intermediate compartment (IC) is a pleiomorphic membrane system that mediates two-way trafficing in the early biosynthetic-secretory pathway of mammalian cells. The IC associates with the cytoskeleton, binds COPI (coat protein I) coats and generates vesicular, tubular and saccular transport carriers. It recieves newly made proteins and lipids from the endoplasmic reticulum (ER) and sorts them for transport to the Golgi apparatus or recycling back to the ER. Although the IC appears to be functionally complex and resides at the crossroads of multiple transport routes, it is still disputed whether it represents a transient or stable structure.

Newly synthesized proteins and lipids leave the endoplasmic reticulum (ER) at specialized transitional regions called ER exit sites (ERES) (Jamieson and Palade, 1967; Sesso et al., 1994; Bannykh et al., 1996; Hammond and Glick, 2000; Tang et al., 2005) and enter the intermediate compartment (IC) that has been shown to operate as an obligatory a post-ER sorting station in the early biosynthetic-secretory trafficking of mammalian cells. From the IC they are typically transported to the cisternal stacks of the Golgi apparatus, prior to their delivery to the different organelles of the endomembrane system or secretion to the extracellular space. Bidirectional ER-Golgi trafficking involves the sequential operation of membrane-bound coat protein II (COPII) and COPI coats (Aridor and Balch, 1996; Scales et al., 1997; Stephens et al., 2000) . ER-derived COPII vesicles mediate forward (anterograde) transport, while IC-and Golgi-derived COPI vesicles are thought to act in the opposite (retrograde) direction (Lee et al., 2004; Rabouille and Klumperman, 2005) . Retrograde transport also involves COPIindependent routes (Kano et al., 2009) .

Despite the conservation of the transport machineries (such as the COP coats), the organization of the ER-Golgi interface varies in different eukaryotic cells. For example, in plants, certain yeasts, and the fruit fly Drosophila melanogaster, the individual Golgi stacks lie next to the widespread ERES, establishing units for short range ER-Golgi communication. By contrast, in animal cells the Golgi stacks are linked together into a continuous ribbon around the microtubule-organizing center (MTOC)/centrosome, whereas the ER extends throughout the cytoplasm. Hence, a large proportion of the ERES reside at the cell periphery and ER-Golgi trafficking depends on the long distance movements of the IC elements along MT tracks (Saraste and Svensson, 1991; Presley et al., 1997; Scales et al., 1997; Brandizzi and Barlowe, 2013; Day et al., 2013) . Thus, it has been proposed that the IC represents a late evolutionary invention, which developed to meet the special sorting, transport and recycling requirements of the large-sized animal cells, but lacks lower eukaryotes. However, results showing that the IC constitutes an extensive membrane system that can be compared with the endosomal network in complexity, are questioning this view.

Electron microscopic (EM) studies using a temperature-sensitive mutant of Semliki Forest virus (SFV ts-1) to synchronize the transport of viral membrane glycoproteins from ER to the plasma membrane (PM) showed that when the cells are shifted from 39 1C to 15 1C the proteins exit the ER, but accumulate in vacuoles/saccules (up to 0.5 mm in diameter), tubules, and vesicles in the cis-Golgi region and more peripheral locations (Saraste and Kuismanen, 1984) . By light microscopy (LM) the proteins were localized at 15 1C to scattered globular structures in the cytoplasm, a pattern distinct from that of ER or Golgi (Kuismanen and Saraste, 1989) . The transport block was readily reversible, and the proteins entered the Golgi stacks and reached the PM following the transfer of cells from 15 1C to 28 1C, showing that these structures are normal transport intermediates.

Studies employing a similar mutant of vesicular stomatitis virus (VSV tsO45) showed that the '15 1C compartment' also acts as a way station during the transport of the VSV G glycoprotein (Balch et al., 1986; Bonatti et al., 1989; Schweizer et al., 1990; Duden et al., 1991; Lotti et al., 1992) . Like the SFV proteins, the G-protein was found to maintain its ER-type high-mannose glycans at 15 1C, indicating lack of processing by Golgi enzymes. Furthermore, cell fractionation experiments showed that newly synthesized secretory proteins are arrested in a post-ER location when pancreatic exocrine cells are kept at 16 1C (Saraste et al., 1986) . Live cell imaging of green fluorescent protein (GFP) equipped with an ER targeting signal (ssGFP) and EM studies of procollagen and growth hormone constructs verified that the transport of membrane and secretory proteins is similarly affected at 15-16 1C (Blum et al., 2000; Volchuk et al., 2000; Trucco et al., 2004) . In addition, the transport of virus glycoproteins and cholesterol appears to be blocked at the same site at this temperature (Urbani and Simoni, 1990; Heino et al. 2000) .

EM of mouse hepatitis virus (MHV)-infected cells showed that the budding of progeny virus initially takes place at tubulovesicular membranes located in the transitional region between the ER and the Golgi apparatus (Tooze et al., 1984) . The first step of O-glycosylation, the attachment of Nacetyl-galactosamine (GalNAc) to the viral membrane (M) glycoprotein, was suggested to occur in this compartment, causing its reactivity with the lectin Helix pomatia, which specifically binds GalNAc (Tooze et al., 1988; Krijnse-Locker et al., 1994) . Subsequent studies showed that the intracellular maturation of various coronaviruses occurs at the same budding site, which corresponds to the IC (Klumperman et al., 1994) .

The discovery of the lys-asp-glu-leu tetra-peptide (KDEL)motif in abundant, lumenal ER proteins lead to the proposal that the 15 1C/budding compartment is the post-ER site from which these proteins are retrieved to the ER (Munro and Pelham, 1987; Warren, 1987; Pelham, 1989 ). The first mammalian KDEL-receptor, a multispanning membrane protein, was identified and shown to predominantly localize to the IC and cis-Golgi (Lewis and Pelham, 1990; Tang et al., 1993; Griffiths et al., 1994; Orci et al., 1997) . Also, KDEL proteins, such as the immunoglobulin binding protein (BiP/GRP78), glucoseregulated protein of 94 kDa (GRP94), protein disulphide isomerase (PDI) and calreticulin (CR), are present in the IC Connolly et al., 1994; Zuber et al., 2001; Ying et al., 2002; Breuza et al., 2004) . Following binding to their receptor, the proteins are retrieved to the ER in COPI vesicles (Orci et al., 1997; Martínez-Menárguez et al., 1999; Majoul et al., 2001) . Attachment of the KDEL-motif to lysosomal enzymes and the use of the 15 1C block suggested that the enzyme that initiates the formation of their lysosomal targeting signal (mannose-6-phosphate) resides in the IC (Pelham, 1988; Lazzarino and Gabel, 1988; Dittmer and von Figura, 1999) . The cytoplasmic tails of certain type I integral membrane proteins were shown to contain dilysine (KKXX)motifs, which by directly interacting with COPI coats result in their retrieval from the IC/cis-Golgi to the ER (Nilsson et al., 1989; Jackson et al., 1990 Jackson et al., , 1993 Cosson and Letourneur, 1997) .

The first endogenous IC markers rat p58 and human ER-Golgi intermediate compartment (ERGIC)-53 (89% homology) were identified by the generation of antibodies against the 16 1C post-ER fraction isolated from pancreatic acinar cells (Saraste et al., 1987) and a Golgi fraction derived from epithelial Caco-2 cells (Schweizer et al., 1988) , respectively. The cytoplasmic C-terminal tails of these non-glycosylated, hexameric, type-1 integral membrane proteins (Schindler et al., 1993; Lahtinen et al., 1992 Lahtinen et al., , 1996 Neve et al., 2005) contain a KKFF-motif, which interacts with COPII and COPI coats and gives rise to their continuous cycling between ER, IC, and cis-Golgi (Kappeler et al., 1997; Tisdale et al., 1997) . At 15 1C the recycling of p58/ERGIC-53 to the ER is inhibited and the proteins pile up in the same pre-Golgi structures that contain the SFV or VSV membrane proteins Saraste and Svensson, 1991; Plutner et al., 1992) , verifying that the p58/ERGIC-53 compartment (Figures 1 and 4) is equivalent to the site where cargo is arrested at low temperature.

ERGIC-53 and the related VIP36 were shown to share homology with leguminous plant lectins (Fiedler and Simons, 1994) and to be identical with a mannose-binding protein MR60 of human myelomonocytic (HL60) cells (Arar et al., 1995) . The N-terminal domain of p58/ERGIC-53 binds mannose in a calcium-dependent manner (Itin et al., 1996; Velloso et al., 2003; ; hence the name lectin mannose-binding protein 1 (LMAN1). It is the best characterized mammalian cargo receptor, facilitating the COPII vesicle-mediated export of a subset of soluble glycoproteins from the ER (Nichols et al., 1998; Appenzeller et al., 1999; Hauri et al., 2000) .

Most well-characterized IC proteins are components of the transport machinery. Many of them cycle at the ER-Golgi boundary and are also found in cis-Golgi, supporting the view of the IC as a transient structure (see below). In fact, EM studies showing the specific metal (osmium) staining of the IC and cis-Golgi provided the first indication of their compositional similarity (Friend and Murray, 1965; Rambourg et al., 1974) . However, the fungal compound brefeldin A (BFA) helps to discriminate between IC and Golgi components. It releases COPI coats, disassembles the Golgi stacks and redistributes Golgi components to the ER, whereas cycling proteins (such as p58/ERGIC-53/LMAN1 and the KDELreceptor) are arrested in the drug-resistant IC elements (Lippincott-Schwartz et al., 1990; Saraste and Svensson, 1991; Tang et al., 1995b; Füllekrug et al., 1997; Ward et al., 2001; Marie et al., 2009) . The BFA resistance of the IC has been utilized for its proteomics analysis (Breuza et al., 2004) , and indicates its stability.

Like p58/ERGIC-53, the p24 family proteins contain motifs for COPII and COPI binding, resulting in their cycling between the ER, IC, and cis-Golgi (Rojo et al., 1997; Dominguez et al., 1998; Blum et al., 1999; Gommel et al., 1999; Strating and Martens, 2009 ). As abundant type-1 transmembrane proteins they participate in the biogenesis of COPI vesicles (Stamnes et al., 1995; Majoul et al., 2001; Beck et al., 2009) , but have also been implicated in tubulation and the formation of membrane domains (Lavoie et al., 1999; Emery et al., 2003) . Studies in yeast first suggested their function as receptors for the exit of glycosylphosphatidylinositol (GPI)-anchored proteins from the ER (Schimmöller et al., 1995) .

The COPI coats are mainly recruited to the cytoplasmic surface of IC and cis-Golgi membranes, but also associate with the lateral rims of the Golgi stacks (Duden et al., 1991; Oprins et al., 1993; Griffiths et al., 1995; Orci et al., 1997; Klumperman et al., 1998) . COPI vesicle budding depends on the activation of small GTPases of the ADP-ribosylation factor (Arf) family (Popoff et al., 2011) . The guanine nucleotide exchange factor (GEF) of these Arfs, GBF1, is the master regulator of COPI recruitment and the target of BFA (Niu et al., 2005) . It localizes to the IC and cis-Golgi and plays a key role in ER-to-Golgi trafficking (Kawamoto et al., 2002; Garcia-Mata et al., 2003; Zhao et al., 2006; Szul et al., 2007) . GBF1 knock down or specific inhibition by Golgicide A releases COPI coats and arrests the VSV G-protein in the IC, indicating its involvement in anterograde IC-to-Golgi transport (Manolea et al., 2008; Sáenz et al., 2009) . However, the prevailing view is that the main function of COPI vesicles is to recycle membrane and selected proteins to the ER. In pancreatic exocrine cells approximately 70% of the coats associate with the VTCs, suggesting that the IC represents the main point for recycling (Martínez-Menárguez et al., 1999) .

The roles of two GTPases of the large Rab family, Rab1 and Rab2, in ER-Golgi trafficking are relatively well understood (Plutner et al., 1991; Tisdale et al., 1992) . Both interact with multiple effectors suggesting that they coordinate successive transport steps (Barnekow et al., 2009) . The association of Rab1 with the cytoplasmic surface of IC and cis-Golgi membranes, and its absence from the ER, has been demonstrated by EM Saraste et al., 1995; Satoh et al., 2003; Marie et al., 2012 ; Figure 2 ). The two Rab1 isoforms, Rab1A and RAB1B (93% homology), are recruited to IC membranes at ERES, show similar localizations by LM (Mochizuki et al., 2013 ; Figure 3 (a)), but seem to play distinct roles in tubular and vesicular (long and short range) trafficking within the IC. Accordingly, live cell imaging and cell fractionation have shown that Rab1A mainly associates with IC tubules (Sannerud et al., 2006; Marie et al., 2009) , while Rab1B interacts with GBF1 and evidently modulates COPI recruitment to globular IC domains Monetta et al., 2007; see below) . Also, Rab1A is specifically

Nu * * * * Figure 1 (a) Immunofluorescence LM localization of p58/ERGIC-53 in baby hamster kidney, (b) mouse myeloma, (c) rat neuroendocrine PC12, and (d) human HeLa cells. The protein marks the IC elements accumulating in the perinuclear Golgi region (asterisks), scattered throughout the cytoplasm, and located close to the PM (arrows). The reticular staining indicates the ER pool of p58/ERGIC-53, which varies in the different cell types. Nu, nucleus. Bar: 10 mm. phosphorylated at mitosis (Bailly et al., 1991) , correlating with the cessation of tubular IC dynamics, whereas COPI-mediated vesicular transport continues (Marie et al., 2012) . Rab1A can functionally replace Ypt1, which coordinates two-way ER-Golgi trafficking in the yeast Saccharomyces cerevisiae (Haubruck et al. 1989; Segev, 2001; Kamena et al., 2008) .

Rab2 has been localized to IC and cis-Golgi membranes (Chavrier et al., 1990; Lotti et al., 1992) and proposed to regulate the formation of retrograde COPI vesicles that contain p58/ERGIC-53 (Tisdale, 1999) . It has also been implicated in the recruitment of the motor protein dynein to IC membranes and the association of IC elements with MTs (Tisdale et al., 2009; see below) .

Most of the known Rab1 and Rab2 effectors are cytoplasmically oriented, long coiled-coil proteins, which function in the tethering of vesicle and organelle membranes preceding their soluble NSF attachment protein receptor (SNARE)-mediated fusion (Barnekow et al., 2009; Sztul and Lupashin, 2009 ). Besides the IC/cis-Golgi tethers GM130 and GMAP-210 (Rios et al., 2004) , Rab2 has been shown to interact with medialand trans-Golgi tethers, suggesting a more widespread function (Short et al., 2001; Sinka et al., 2008) . The Rab1 effector p115 is functional already at the peripheral ERES (Alvarez et al., 1999) , while GM130 (bound to its membrane anchor GRASP65) cycles between central IC and cis-Golgi (Marra et al., 2001) and regulates membrane tethering at a later transport step (Alvarez et al., 2001; Marra et al., 2007) . Rab1 also interacts with the transmembrane tethers golgin-84 and giantin, which appear to act in COPI vesicle trafficking at the lateral rims of the Golgi stacks (Diao et al., 2003; Malsam et al., 2005; Barnekow et al., 2009; Munro, 2011) .

The membrane fusion machinery (SNARE proteins) operating in ER-Golgi trafficking in the yeast Saccharomyces cerevisiae has been well characterized (Cai et al., 2007; Barlowe and Miller, 2013) , whereas the fusion events that take place in mammalian cells remain enigmatic. The determination of the exact fusion steps is complicated by the continuous cycling of the SNAREs in COPI vesicles (Hay et al., 1998; Martinez-Menárguez et al., 2001) . In analogy to yeast, Rab1 (Ypt1) has been suggested to recruit p115 (Uso1p) to COPII vesicles at ERES (Allan et al., 2000) , followed by the formation of a SNARE complex (Sec22B, membrin, Bet1 and syntaxin 5), which mediates either homotypic fusion of COPII vesicles or their heterotypic fusion with the stationary IC (Zhang et al., 1997; Xu et al., 2000; see below) . Another SNARE complex (syntaxin 5, GS28, Bet1 and Ykt6) is involved in a later cis-Golgi transport step (Zhang and Hong, 2001) .

By EM the IC elements can be readily distinquished from the ER and Golgi, but share structural similarity with endosomes. They reside close to peripheral and central ERES as clusters of vesicles and tubules (VTCs; Figure 4 (f)) that frequently contain COPI coats (Balch et al., 1994; Bannykh et al., 1996; Martínez-Menárguez et al., 1999) . However, they display considerable size heterogeneity (Ying et al., 2000) and also include large saccules that are found within the membrane clusters, free in the cytoplasm, or at the cis-face of the Golgi stacks (Saraste and Svensson, 1991; Lahtinen et al., 1992 , Connolly et al., 1994 Stinchcombe et al., 1995; Ladinsky et al., 1999; Fan et al., 2003 ; Figures 4(a)-4(e)). These pleiomorphic saccules can accommodate large-sized cargo, such as procollagen or lumenal protein aggregates (Volchuk et al., 2000; Trucco et al., 2004; Zuber et al., 2004) , and based on correlative microscopy (LM/EM) correspond to many of the mobile carriers that are visible in living cells (Mironov et al., 2003) . Like endosomes, they extend narrow tubules and also bind COPI coats, indicating that they represent sites for vesicle budding (Saraste and Kuismanen, 1984; Volchuk et al., 2000; Horstman et al., 2002; Figures 4(c) and 4(d)). The hypertrophy of the saccular domains most likely gives rise to the pre-Golgi vacuoles seen in cells kept at 15 1C (Saraste and Kuismanen, 1992; Trucco et al., 2004) .

LM shows the division of the IC into globular and tubular domains (Presley et al., 1998) . The former contain anterograde cargo, cargo receptors and COPI coats, most likely corresponding to individual VTCs or free saccules, while the latter are highlighted by Rab1A (Sannerud et al., 2006) . The tubules extend from the globular domains ( Figure 5) . Some of them contain recycling proteins, but lack anterograde cargo, indicating that they function in retrograde transport (Palokangas et al., 1998; Simpson et al., 2006) . However, under synchronized conditions cargo is also detected in the tubular domain, due to overload or the existence of different types of IC tubules ( Figure 6 ). For example, incubation of cells at 15-16 1C inhibits the formation of IC tubules, but causes the expansion of the globular domain, while the shift of cells to 37 1C generates tubular networks containing both antero-and retrograde markers (Blum et al., 2000; Ben-Takaya et al., 2005; Simpson et al., 2006) . Proliferation of the tubules is induced when COPI function is compromised (Szul et al., 2007; Marie et al., 2009; Ben-Takaya et al., 2010; Tomás et al., 2010; Hamlin et al., 2014) , but also occurs under physiological conditions. In differentiating neuroendocrine PC12 cells the Rab1A-positive tubular IC domain expands and the tubules move from the cell body to the forming neurites accumulating in their growth cones (Sannerud et al., 2006 ; Figure 5 ). An analogous pathway connects the IC with the leading edge of fibroblasts ( Figure 6) .

Live imaging of various fluorescent IC markers indicates that the tubules are highly dynamic, while the globular domains are typically more stationary. Due to their differential localization within these domains, the constructs highlight different aspects of IC dynamics (see below). Long distance ER-to-Golgi transport involves the movement of IC elements from the cell periphery to the central cis-Golgi region, resulting in the division of the IC into spatially distinct early (ERESadjacent) and late (cis-Golgi-adjacent) domains (Saraste and Svensson, 1991; Presley et al., 1997; Scales et al., 1997; Marra et al., 2001) . Two types of anterogradely moving IC elements can be resolved by LM, narrow tubules and large pleiomorphic structures. Some of the latter appear to represent saccular elements that transform into elongated structures ('thick tubules') (Presley et al., 2002; Marie et al., 2009 ). In addition, narrow tubules establish dynamic connections between the globular IC domains (Ben-Tekaya et al., 2005; Sannerud et al., 2006) .

The long distance movements of the IC elements depend on MTs (Murshid and Presley, 2004; Palmer et al., 2005 (b) and 6). The plus-and minus-end directed motor proteins kinesin and dynein associate with the IC elements (Lippincott-Schwartz et al., 1995; Roghi and Allan, 1999; Stauber et al., 2006) , explaining their bidirectional movements even along the same MT tracks (Sannerud et al., 2006) . When the MTs in mammalian cells are depolymerized by nocodazole the IC elements become immobile and accumulate close to ERES (Saraste and Svensson, 1991; Hammond and Glick, 2000; Ben-Tekaya et al., 2005; Sannerud et al., 2006) . Although the central Golgi ribbon breaks down, the formation of Golgi ministacks in the vicinity of ERES re-establishes ER-Golgi communication as a short range process (as in plants), explaining the ongoing Golgi modification and secretion of proteins (Saraste and Svensson, 1991; Cole et al., 1996; Storrie et al., 1998) . The Rho family GTPase cdc42, which regulates actin dynamics, interacts with COPI coats and affects dynein function, suggesting functional coupling between the actin filament system and MT-dependent motility of IC/cis-Golgi carriers (Luna et al., 2002; Chen et al., 2005) . Similarly, WHAMM, which promotes actin nucleation and interacts with MTs, has been localized to the IC and implicated in the formation and transport of pre-Golgi tubules (Campellone et al., 2008) .

Based on imaging of fluorescent cargo, such as the VSV G-protein and procollagen, in living cells (Presley et al., 1997; Scales et al., 1997; Stephens and Pepperkok, 2002) , it has been proposed that the IC represents a collection of large, pleiomorphic transport complexes (TCs) (Bannykh and Balch, 1997; Stephens and Pepperkok, 2001) , which form via homotypic fusion of COPII vesicles, or protrude directly from the ER (Mironov et al., 2003; Xu and Hay, 2004; Yu et al., 2006) . Thereafter, they move in a MT-and dynein-dependent (Burkhardt et al., 1997) 'stop-and-go' fashion to the Golgi region, where they either fuse with or transform into cis-Golgi cisternae (Figure 7(a) ). In other words, the IC elements are transient structures which are first formed de novo at ERES and then consumed at cis-Golgi. These mobile structures (speed of about 1 mm/sec) corresponding to saccules or more complex, pleiomorphic elements (Mironov et al., 2003) appear to consist of subdomains enriched in antero-or retrograde cargo, or COPI (Shima et al., 1999; Stephens et al., 2000) . They have also been shown to contain other machinery proteins, such as p23/24, p58, VIP36, membrin and Rab1A (Blum et al., 1999;  Golgi Nu Figure 5 Immunofluorescence LM localization of Rab1 in differentiating PC12 cells, illustrating the interconnected globular (arrowheads) and tubular (arrows) domains of the IC. Two confocal sections of the same cell are shown. The IC has expanded in response to a 24 h treatment with the nerve growth factor (NGF) and the tubules are found both in the cell body and the neurite-like extensions (right panel). Nu, nucleus; Golgi, perinuclear Golgi region. Bar: 5 mm.

Time projection Figure 6 Bidirectional movements of GFP-Rab1A-positive tubules in a living NRK cell. Selected confocal images from a movie obtained by timelapse microscopy are shown. Tubules extending from the same relatively stationary globular IC element (black arrowhead) pinch off and move either in the direction of the cell's leading edge (upper panels; yellow arrowheads), or toward the Golgi indicated by an asterisk (lower panels; red arrowheads). The paths taken by the tubules are highlighted in the time projection on the right. Adapted from Sannerud, R., Marie, M., Nizak, C., et al., 2006 Chao et al., 1999; Dahm et al., 2001; Sannerud et al., 2006; Monetta et al., 2007; Tomás et al., 2010) .

Another model of the IC (Figure 7(b) ) is largely based on the imaging of GFP-ERGIC-53 dynamics in living cells (Ben-Tekaya et al., 2005) . It proposes that the IC consists of stationary, long-lived membrane clusters, located close to the ERES, which communicate with the ER and Golgi via distinct transport carriers (Appenzeller-Herzog and Hauri, 2006) . Thus, in a two-step transport process cargo is first transfered from ER to the IC via heterotypic fusion of COPII vesicles. The IC then generates novel (possibly COPI-coated) anterograde carriers (ACs) that move along MTs to the cis-Golgi. The recycling of GFP-ERGIC-53 to the ER evidently occurs mainly from the ERES-associated IC, since it was not detected in the ACs containing soluble, Golgi-destined cargo. The identification of a motif in the neuronal GABA transporter, that seems to be required for its exit from the IC, supports the stable nature of the IC (Farhan et al., 2008) .

Visualization of IC dynamics using GFP-Rab1A showed that the pleiomorphic transport carriers arriving along MTs from the cell periphery do not move directly to cis-Golgi, but are targeted to the MTOC/centrosome that is normally positioned next to the Golgi ribbon. In cells displaying centrosome motility, for example, cells that are migrating or entering mitosis, the centrosome-targeted membranes can be resolved as a separate compartment, called the pericentrosomal IC (pcIC), which is distinct from the cis-Golgi-adjacent IC domain (Marie et al., , 2012 Mochizuki et al., 2013 ; Figure 3 (a)). Live imaging further showed that the separated pcIC and the Golgi communicate via tubular and globular carriers. The pcIC contains its own pool of COPI coats, and mediates the BFAinduced, COPI-independent backflow of Golgi enzymes to the ER, suggesting that forward pcIC-to-Golgi transport depends on COPI coats, and thus is blocked by BFA .

Both the peripheral IC elements and the pcIC persist upon Golgi disassembly by BFA and maintain their communication with via dynamic tubules . Accordingly, the IC has been proposed to constitute a dynamic, but stable membrane network due to its anchoring next to the ERES and the centrosome Figure 7(c) ). This model is supported by studies of mitotic cells, showing that the IC persists despite Golgi breakdown and the rearrangement of the MTs, and maintains its spatial organization due to its ongoing association with the spindle MTs and the centrosomes at the spindle poles (Marie et al., 2012 ; Figure 3(a) ).

In the endocytic pathway, soluble proteins and particles bound to lysosomes accumulate in the lumen of vacuolar endosomes, while many membrane proteins are sorted into narrow tubules for recycling to the PM. The observed major The IC consists of stationary membrane clusters located close to ERES. In this case ER-to-Golgi trafficking involves two distinct transport steps, since the IC recieves cargo from the ER via heterotypic fusion of COPII vesicles and forms special anterograde carriers (AC) that move along MTs to cis-Golgi. (c) The IC represents a stable interconnected network that is anchored both next to ERES and the centrosome. Bidirectional transport between these sites and between the central IC elements and the Golgi stacks involves vesicular, tubular or saccular carriers. ERES-IC communication via COPII vesicles occurs as in model b. COPI, COPII and clathrin coats are shown in gray, orange, and blue, respectively, and the centrosome in red.

concentration of soluble secretory proteins within the IC (Oprins et al., 2001) could be explained by similar geometrybased sorting, namely, their exclusion from vesicles and tubules, which recycle lipids and membrane-bound proteins back to the ER (Martínez-Menárguez et al., 1999) . Membrane recycling is a major function of both the IC and endosomes in mammalian cells. Due to the similarity of the tubular domain of the IC with the endocytic recycling compartment (ERC), its 'mirror compartment' next to the centrosome, it has been designated as the biosynthetic recycling compartment (BRC; Saraste and Goud, 2007; Marie et al., 2009) . Lumenal conditions: Receptor-mediated retrieval of KDEL proteins from the IC requires that it is discontinuous with the ER and maintains special lumenal conditions (Pelham, 1989) . The finding of low pH-dependent binding of KDEL-ligands to their receptor in vitro suggested the existence of a pH gradient between the ER and cis-Golgi (Wilson et al., 1993) . The pHsensitive interactions of the low density lipoprotein (LDL)receptor-related protein (LRP) and procollagen with the chaperones RAP and Hsp47, respectively (Bu et al., 1995; Satoh et al., 1996) , and partial co-localization of DAMP (a marker for acidic compartments) and p58/ERGIC-53 in central IC elements (Palokangas et al., 1998) , are in accordance with this idea. The pH of the ER and cis-Golgi has been estimated to be about 7.2 and 6.5-6.7, respectively (Paroutis et al., 2004; Vavassori et al., 2013) .

Although the effect pH on the binding of mannose-containing cargo to p58/ERGIC-53/LMAN1 remains unclear, it appears that cargo release is caused by a drop in free calcium concentration between the ER and IC lumen (Appenzeller-Herzog et al., 2004; Bentley et al., 2010; . On the other hand, the depletion of lumenal calcium stores affects the morphology of the IC and the recycling of cargo receptors at the ER-Golgi boundary, and the IC has been reported to contain the calcium-ATPase SERCA, as well as the calciumbinding proteins BiP, GRP94, CR, and CALNUC (Ying et al., 2002; Breuza et al., 2004; Howe et al., 2009; Bentley et al., 2010) , suggesting its role in intracellular calcium storage.

Role of COPI coats: There are at least three subtypes of COPI coats (Beck et al., 2009) , and four Arf GTPases that regulate their membrane binding (Popoff et al., 2011) . Three Arfs appear to act at the ER-Golgi boundary and two of these associate with membranes in a BFA-resistant manner (Volpicelli-Daley et al., 2005; Chun et al., 2008; Duijsings et al., 2009; Ben-Tekaya et al., 2010; Hamlin et al., 2014) , suggesting that different types of COPI vesicles mediate two-way trafficking at the level of the IC. The role of COPI in anterograde transport has been considered for some time (Hosobuchi et al., 1992; Pepperkok et al., 1993; Peter et al., 1993; Orci et al., 1997; Malsam et al., 2005) . In addition, although both p58/ERGIC-53 and KDEL-receptor employ COPI vesicles in their recycling, their transport itineraries differ (Tang et al., 1995a; Marie et al., 2009 Marie et al., , 2012 . In addition to acting in vesicle budding the COPI coats may form membrane domains (Presley et al., 2002) .

Golgi bypass: Besides mediating bidirectional ER-Golgi trafficking, the IC has been suggested to be involved in Golgiindependent pathway(s) (Marie et al., 2008; Saraste et al., 2009) . The route between the IC and the leading edge or growth cone of motile fibroblasts or neurons, respectively ( Figures 5 and 6) , could participate in cholesterol and integrin trafficking (Urbani and Simoni, 1990; Sannerud et al., 2006; Wang et al., 2010) or correspond to the BFA-resistant ER-IC-PM route that supports phagocytosis (Becker et al., 2005; Saraste and Goud, 2007; see below) . Direct pericentrosomal communication between the IC and the endosomal system seems to be used by the cystic fibrosis transmembrane conductance regulator (CFTR) during its Golgi-independent transport to the cell surface (Yoo et al., 2002; Marie et al., 2009; Gee et al., 2011) . Further, the presence of the IC in neuronal dendrites may provide a Golgi-independent satellite pathway for local dendritic trafficking (Krijnse-Locker et al., 1995; Sannerud et al., 2006; Ehlers, 2013) .

Questioning the studies on coronavirus maturation, which suggested that O-glycosylation is initiated in the IC (see above), subsequent EM studies showed that the GalNActransferases are predominantly found in the Golgi. However, the activation of cells (by epidermal growth factor) causes their incorporation into COPI vesicles and relocation to the IC and ER, which consequently become positive for the lectin Helix pomatia (Gill et al., 2010) . The cycling of cis-Golgi proteins to the IC could be a constitutive event (Lin et al., 1999; Marra et al., 2001; Jarvela and Linstedt, 2012) . Moreover, the construction of the sugar chains on proteoglycans has been suggested to begin in the IC (Jönsson et al., 2003) .

The KDEL-containing molecular chaperones present in the IC (BiP, PDI, and GRP94; see above) could be cycling while still bound to their unfolded client proteins, opening for a post-ER level of protein folding and quality control. The presence of quality control machinery in the IC and the finding that the ER-associated degradation of certain proteins requires their ER export is in accordance with this idea (Zuber et al., 2001; Anelli and Sitia, 2008) . The PDI family member ERp44 and p58/ ERGIC-53 cooperate in the IC/cis-Golgi in sequential assembly of IgM polymers (Anelli and Sitia, 2008) . Unassembled IgM or T-cell antigen receptor subunits bound to ERp44 or BiP, respectively, can be retrieved to the ER by the KDEL-receptor in a pH-dependent manner (Yamamoto et al., 2001; Vavassori et al., 2013) , similarly as the overexpressed, misfolded VSV G-protein (Hammond and Helenius, 1994) and mutant V2 vasopressin receptors (Hermosilla et al., 2004) . Additional proof for a post-ER checkpoint is provided by studies showing the accumulation of the deletion mutant DF508 of CFTR (Gilbert et al., 1998) , misfolded MHC class I proteins (Hsu et al., 1991; Raposo et al., 1995) , and proinsulin (Zuber et al., 2004) in expanded IC elements.

The level of p58/ERGIC-53 mRNA is up-regulated by the unfolded protein response (UPR), which also requires yeast Ypt1 function, supporting a link between this signaling pathway and ER-Golgi trafficking (Nyfeler et al., 2003; Tsvetanova et al., 2012) . Protein kinases Src, aPKC, and Scyl1 have been implicated in COPI function at the level of the IC (Tisdale and Artalejo, 2006; Hamlin et al., 2014) and PKC and its downstream effectors appear to control IC morphology (Ben-Tekaya et al., 2010; Sugawara et al., 2012) . The activation of neuronal Trk receptor tyrosine kinases in the IC initiates signaling via the MEK pathway leading to Golgi fragmentation (Schecterson et al., 2010) .

Membranes enriched in IC markers (p58/ERGIC-53/LMAN1, KDEL-receptor, and Sec22B) play a role in the biogenesis of autophagosomes by representing the primary membrane source for the lipidation of LC3, which triggers this process (Ge et al., 2013) . Moreover, Ypt1/Rab1 is a key regulator of autophagy in yeast and mammals (Lynch- Day et al. 2010; Winslow et al., 2010; Zoppino et al., 2010; Huang et al., 2011; Lipatova et al., 2012) . A phosphatidylinositol 3-phosphatase (MTMR6) acting in vesicle transport and autophagy is regulated by Rab1B and localizes predominantly to the pcIC (Mochizuki et al., 2013) , further supporting the role of the IC in autophagy.

Supporting the existence of an unconventional pathway that connects the ER with the PM-derived phagosome (Gagnon et al., 2002) , the IC-enriched SNARE Sec22B/ERS-24 (Zhang et al., 1999) was shown to influence phagocytosis independently of its role in ER-Golgi trafficking (Becker et al., 2005; Hatsuzawa et al., 2009) . In dendritic cells the delivery of certain proteinssuch as the peptide transporter (TAP), CR and tapasinfrom ER to the phagosome is required for antigen cross-presentation. This pathway depends on the interaction between Sec22B and the PM SNARE syntaxin 4 and involves efficient recruitment of the integral IC components sec22B and p58/ERGIC-53 to the phagosomes (Cebrian et al., 2011) , in accordance with the idea that the IC provides an important membrane source for their formation. The presence of CR, tapasin, and functional TAP in the IC/cis-Golgi support this possibility (Kleijmeer et al., 1992; Howe et al., 2009; Ghanem et al., 2010) .

The IC plays a role in the Golgi-independent trafficking of CFTR (see above). Mutations in p58/ERGIC-53/LMAN1, or its partner MCFD2 (multiple coagulation factor deficiency protein 2) result in an autosomal recessive bleeding disorder called combined deficiency of coagulation factors V and VIII. They disrupt the lectin mannose-binding protein 1 (LMAN1)-MCFD2 receptor complex, thereby inhibiting the secretion of these factors and reducing their serum levels (Nichols et al., 1998; Zheng and Zhang, 2013) . Parkinson's disease-related cytotoxic protein, α-synuclein, has been suggested to interfere with ER-Golgi trafficking and to arrest autophagy by inhibiting Rab1 function (Cooper et al., 2006; Winslow et al., 2010) . Many phagocytosed bacterial pathogens, such as Legionella, hijack the ER-to-IC-to-phagosome pathway during their intracellular replication (Isberg et al., 2009; Arasaki et al., 2012; see above) . In addition to coronaviruses, the IC has been implicated in the replication of bunya-, entero-, flavi-, picorna, and vacciniaviruses (Jäntti et al., 1997; Risco et al., 2002; Miller and Krijnse-Locker, 2008; Hsu et al., 2010) . Surprisingly, p58/ ERGIC-53 interacts in a lectin-independent manner with fusion proteins of a number of membrane viruses and participates in different stages of their life cycle (Klaus et al., 2013) . 

Rab1 recruitment of p115 into a cis-SNARE complex: Programming budding COPII vesicles for fusion

ER to Golgi transport: Requirement for p115 at a pre-Golgi VTC stage

COPI recruitment is modulated by a Rab1b-dependent mechanism

The p115-interactive proteins GM130 and giantin participate in endoplasmic reticulum-Golgi traffic

Protein quality control in the early secretory pathway

The lectin ERGIC-53 is a cargo transport receptor for glycoproteins

The ER-Golgi intermediate compartment (ERGIC): In search of its identity and function

pH-induced conversion of the transport lectin ERGIC-53 triggers glycoprotein release

ERGIC-53, a membrane protein of the endoplasmic reticulum-Golgi intermediate compartment, is identical to MR60, an intracellular mannose-specific lectin of myelomonocytic cells

The Legionella pneumophila effector DrrA is sufficient to stimulate SNARE-dependent membrane fusion

Principles of selective transport: Coat complexes hold the key

Phosphorylation of two small GTP-binding proteins of the Rab family by p34 cdc2

ATP-coupled transport of vesicular stomatitis virus G protein between the endoplasmic reticulum and Golgi

Vesicular stomatitis virus glycoprotein is sorted and concentrated during export from the endoplasmic reticulum

Membrane dynamics at the endoplasmic reticulum-Golgi interface

The organization of endoplasmic reticulum export complexes

Secretory protein biogenesis and traffic in the early secretory pathway

Rab proteins and their interaction partners

The COPI system: Molecular mechanisms and function

Differential use of endoplasmic reticulum membrane for phagocytosis in J774 macrophages

ADP ribosylation factors 1 and 4 and group VIA phospholipase A 2 regulate morphology and intraorganellar traffic in the endoplasmic reticulum-Golgi intermediate compartment

Live imaging of bidirectional traffic from the ERGIC

Vesicular calcium regulates coat retention, fusogenicity, and size of pre-Golgi intermediates

Intracellular localization and in vivo trafficking of p24A and p23

Lumenal targeted GFP, used as a marker of soluble cargo, visualises rapid ERGIC to Golgi traffic by a tubulo-vesicular network

Palmitylation of viral membrane glycoproteins takes place after exit from the endoplasmic reticulum

Organization of the ER-Golgi interface for membrane traffic control

Proteomics of endoplasmic reticulum-Golgi intermediate compartment (ERGIC) membranes from brefeldin A-treated HepG2 cells identifies ERGIC-32, a new cycling protein that interacts with human Erv46

Binding and endocytosis of 39 kDa protein by MDBK cells

Overexpression of the dynamitin (p50) subunit of the dynactin complex disrupts dynein-dependent maintenance of membrane organelle distribution

Coats, tethers, Rabs, and SNAREs work together to mediate the intracellular destination of a transport vesicle

WHAMM is an Arp2/3 complex activator that binds microtubules and functions in ER to Golgi transport

Sec22b regulates phagosomal maturation and antigen crosspresentation by dendritic cells

SNARE membrane trafficking dynamics in vivo

Localization of low molecular weight GTP binding proteins to exocytic and endocytic compartments

Coatomer-bound Cdc42 regulates dynein recruitment to COPI vesicles

Characterization of class I and class II ADP-ribosylation factors in live cells: GDP-bound class II Arfs associate with the ER-Golgi intermediate compartment independently of GBF1

Golgi dispersal during microtubule disruption: Regeneration of Golgi stacks at peripheral endoplasmic reticulum exit sites

Transport into and out of the Golgi complex studied by transfecting cells with cDNAs encoding horseradish peroxidase

Alpha-synuclein blocks ER-Golgi traffic and Rab1 rescues neuron loss in Parkinson's models

Coatomer (COPI)-coated vesicles: Role in intracellular transport and protein sorting

Quantitative ER 2 Golgi transport kinetics and protein separation upon Golgi exit revealed by vesicular integral membrane protein 36 dynamics in live cells

A three-stage model of Golgi structure and function

The coiled-coil membrane protein golgin-84 is a novel Rab effector required for Golgi ribbon formation

Phosphorylation of arylsulphatase A occurs through multiple interactions with the UDP-N-acetylglucosamine-1-phosphotransferase proximal and distal to its retrieval site by the KDEL receptor

gp25L/emp24/p24 protein family members of the cis-Golgi network bind both COP I and II coatomer

Beta-COP, a 110 kd protein associated with non-clathrin-coated vesicles and the Golgi complex, shows homology to beta-adaptin

Differential membrane association properties and regulation of class I and class II Arfs

Dendritic trafficking for neuronal growth and plasticity

The transmembrane protein p25 forms highly specialized domains that regulate membrane composition and dynamics

Ultrastructural analysis of transitional endoplasmic reticulum and pre-Golgi intermediates: A highway for cars and trucks

Signal-dependent export of GABA transporter 1 from the ER-Golgi intermediate compartment is specified by a C-terminal motif

A putative novel class of animal lectins in the secretory pathway homologous to leguminous lectins

Osmium impregnation of the Golgi apparatus

Characterization of brefeldin A-induced vesicular structures containing cycling proteins of the intermediate compartment/cis-Golgi network

Endoplasmic reticulummediated phagocytosis is a mechanism of entry into macrophages

ADP-ribosylation factor/COPIdependent events at the endoplasmic reticulum-Golgi interface are regulated by the guanine nucleotide exchange factor GBF1

The ER-Golgi intermediate compartment is a key membrane source for the LC3 lipidation step of autophagosome biogenesis

Rescue of ΔF508-CFTR trafficking via a GRASP-dependent unconventional secretion pathway

The transporter associated with antigen processing (TAP) is active in a post-ER compartment

Delta F508 CFTR localizes in the endoplasmic reticulum-Golgi intermediate compartment in cystic fibrosis cells

Regulation of O-glycosylation through Golgi-to-ER relocation of initiation enzymes

p24 and p23, the major transmembrane proteins of COPI-coated transport vesicles, form heterooligomeric complexes and cycle between the organelles of the early secretory pathway

Localization of the Lys, Asp, Glu, Leu tetrapeptide receptor to the Golgi complex and the intermediate compartment in mammalian cells

Immunocytochemical localization of beta-COP to the ER-Golgi boundary and the TGN

Scyl1 scaffolds class II Arfs to specific subcomplexes of coatomer through the γ-COP appendage domain

Dynamics of transitional endoplasmic reticulum sites in vertebrate cells

Quality control in the secretory pathway: Retention of a misfolded viral membrane glycoprotein involves cycling between the ER, intermediate compartment, and Golgi apparatus

Sec22b is a negative regulator of phagocytosis in macrophages

The ras-related mouse ypt1 protein can functionally replace the YPT1 gene product in yeast

ERGIC-53 and traffic in the secretory pathway

Localization, dynamics, and protein interactions reveal distinct roles for ER and Golgi SNAREs

Dissecting the role of the Golgi complex and lipid rafts in biosynthetic transport of cholesterol to the cell surface

Disease-causing V(2) vasopressin receptors are retained in different compartments of the early secretory pathway

Ultrastructural characterization of endoplasmic reticulum-Golgi transport containers (EGTC)

SEC21 is a gene required for ER to Golgi protein transport that encodes a subunit of a yeast coatomer

Calreticulin-dependent recycling in the early secretory pathway mediates optimal peptide loading of MHC class I molecules

Viral reorganization of the secretory pathway generates distinct organelles for RNA replication

A recycling pathway between the endoplasmic reticulum and the Golgi apparatus for retention of unassembled MHC class I molecules

Antibacterial autophagy occurs at PI(3)P-enriched domains of the endoplasmic reticulum and requires Rab1 GTPase

The Legionella pneumophila replication vacuole: Making a cosy niche inside host cells

ERGIC-53 is a functional mannose-selective and calcium-dependent human homologue of leguminous lectins

Identification of a consensus motif for retention of transmembrane proteins in the endoplasmic reticulum

Retrieval of transmembrane proteins to the endoplasmic reticulum

Intracellular transport of secretory proteins in the pancreatic exocrine cell. I. Role of the peripheral elements of the Golgi complex

Immunocytochemical analysis of Uukuniemi virus budding compartments: Role of the intermediate compartment and the Golgi stack in virus maturation

Irradiation-induced protein inactivation reveals Golgi enzyme cycling to cell periphery

Initiation of the decorin glycosaminoglycan chain in the endoplasmic reticulum-Golgi intermediate compartment

Ypt1p is essential for retrograde Golgi-ER transport and for Golgi maintenance in S. cerevisiae

Yip1A regulates the COPIindependent retrograde transport from the Golgi complex to the ER

The recycling of ERGIC-53 in the early secretory pathway. ERGIC-53 carries a cytosolic endoplasmic reticulum-exit determinant interacting with COPII

GBF1, a guanine nucleotide exchange factor for ADP-ribosylation factors, is localized to the cis-Golgi and involved in membrane association of the COPI coat

The intracellular cargo receptor ERGIC-53 is required for the production of infectious arenavirus, coronavirus, and filovirus particles

Location of MHC-encoded transporters in the endoplasmic reticulum and cis-Golgi

Coronavirus M proteins accumulate in the Golgi complex beyond the site of virion budding

The recycling pathway of protein ERGIC-53 and dynamics of the ER-Golgi intermediate compartment

Characterization of the budding compartment of mouse hepatitis virus: Evidence that transport from the RER to the Golgi complex requires only one vesicular transport step

The organization of the endoplasmic reticulum and the intermediate compartment in cultured rat hippocampal neurons

Low temperature-induced transport blocks as tools to manipulate membrane traffic

Golgi structure in three dimensions: Functional insights from the normal rat kidney cell

Characterization of a 58 kDa cis-Golgi protein in pancreatic exocrine cells

Molecular cloning and expression of a 58-kDa cis-Golgi and intermediate compartment protein

Roles for alpha(2)p24 and COPI in endoplasmic reticulum cargo exit site formation

Biosynthesis of the mannose-6-phosphate recognition marker in transport-impaired mouse lymphoma cells. Demonstration of a two-step phosphorylation

Bi-directional protein transport between the ER and Golgi

A human homologue of the yeast HDEL receptor

ER/Golgi intermediates acquire Golgi enzymes by brefeldin A-sensitive retrograde transport in vitro

Regulation of selective autophagy onset by a Ypt/Rab GTPase module

Kinesin is the motor for microtubule-mediated Golgi-to-ER membrane traffic

Microtubuledependent retrograde transport of proteins into the ER in the presence of brefeldin A suggests an ER recycling pathway

Immunocytochemical analysis of the transfer of vesicular stomatitis virus G glycoprotein from the intermediate compartment to the Golgi complex

Regulation of protein transport from the Golgi complex to the endoplasmic reticulum by CDC42 and N-WASP

Trs85 directs a Ypt1 GEF, TRAPPIII, to the phagophore to promote autophagy

KDEL-cargo regulates interactions between proteins involved in COPI vesicle traffic: Measurements in living cells using FRET

Golgin tethers define subpopulations of COPI vesicles

Distinct functions for Arf nucleotide exchange factors at the Golgi complex: GBF1 and BIGs are required for assembly and maintenance of the Golgi stack and TGN, respectively

Division of the intermediate compartment at the onset of mitosis provides a mechanism for Golgi inheritance

The function of the intermediate compartment in pre-Golgi trafficking involves its stable connection with the centrosome

Take the 'A' train: On fast tracks to the cell surface

The GM130 and GRASP65 Golgi proteins cycle through and define a subdomain of the intermediate compartment

The biogenesis of the Golgi ribbon: The roles of membrane input from the ER and of GM130

Vesicular tubular clusters between the ER and Golgi mediate concentration of soluble secretory proteins by exclusion from COPI-coated vesicles

Peri-Golgi vesicles contain retrograde but not anterograde proteins consistent with the cisternal progression model of intra-Golgi transport

Modification of intracellular membrane structures for virus replication

ER-to-Golgi carriers arise through direct en bloc protrusion and multistage maturation of specialized ER exit domains

Phosphatidylinositol 3-phosphatase myotubularin-related protein 6 (MTMR6) is regulated by small GTPase Rab1B in the early secretory and autophagic pathways

Rab1B interacts with GBF1 and modulates both ARF1 dynamics and COPI association

The golgin coiled-coil proteins of the Golgi apparatus

A C-terminal signal prevents secretion of luminal ER proteins

ER-to-Golgi transport and cytoskeletal interactions in animal cells

Oligomerization and interacellular localization of the glycoprotein receptor ERGIC-53 is independent of disulfide bonds

Mutations in the ER-Golgi intermediate compartment protein ERGIC-53 cause combined deficiency of coagulation factors V and VIII

Short cytoplasmic sequences serve as retention signals for transmembrane proteins in the endoplasmic reticulum

Dynamics of GBF1, a brefeldin A-sensitive Arf1 exchange factor at the Golgi

The cargo receptor ERGIC-53 is a target of the unfolded protein response

Beta-COP localizes mainly to the cis-Golgi side in exocrine pancreas

The ER-to-Golgi interface is the major concentration site of secretory proteins in the exocrine pancreatic cell

Bidirectional transport by distinct populations of COPI-coated vesicles

The role of microtubules in transport between the endoplasmic reticulum and Golgi apparatus in mammalian cells

Retrograde transport from the pre-Golgi intermediate compartment and the Golgi complex is affected by the vacuolar H þ -ATPase inhibitor bafilomycin A1

The pH of the secretory pathway: Measurement, determinants, and regulation

Evidence that luminal ER proteins are sorted from secreted proteins in a post-ER compartment

Control of protein exit from the endoplasmic reticulum

Beta-COP is essential for biosynthetic membrane transport from the endoplasmic reticulum to the Golgi complex in vivo

β-COP is essential for transport of protein from the endoplasmic reticulum to the Golgi in vitro

Rab1b regulates vesicular transport between the endoplasmic reticulum and successive Golgi compartments

Morphological analysis of protein transport from the ER to Golgi membranes in digitonin-permeabilized cells: Role of the p58-containing compartment

Several ADP-ribosylation factor (Arf) isoforms support COPI vesicle formation

Dissection of COPI and Arf1 dynamics in vivo and role in Golgi membrane transport

ER-to-Golgi transport visualized in living cells

Golgi membrane dynamics

The maturing role of COPI vesicles in intra-Golgi transport

Three-dimensional structure of the osmium-impregnated Golgi apparatus as seen in the high voltage electron microscope

Misfolded major histocompatibility complex class I molecules accumulate in an expanded ER-Golgi intermediate compartment

GMAP-210 recruits γ-tubulin complexes to cis-Golgi membranes and is required for Golgi ribbon formation

Endoplasmic reticulum-Golgi intermediate compartment membranes and vimentin filaments participate in vaccinia virus assembly

Dynamic association of cytoplasmic dynein heavy chain 1a with the Golgi apparatus and intermediate compartment

Involvement of the transmembrane protein p23 in biosynthetic protein transport

Golgicide A reveals essential roles for GBF1 in Golgi assembly and function

Rab1 defines a novel pathway connecting the pre-Golgi intermediate compartment with the cell periphery

Emerging new roles of the pre-Golgi intermediate compartment in biosynthetic-secretory trafficking

Functional symmetry of endomembranes

Pre-and post-Golgi vacuoles operate in the transport of Semliki Forest virus membrane glycoproteins to the cell surface

Pathways of protein sorting and membrane traffic between the rough endoplasmic reticulum and the Golgi complex

Localization of the small GTP-binding protein Rab1 to early compartments of the secretory pathway

Temperature-sensitive steps in the transport of secretory proteins through the Golgi complex in exocrine pancreatic cells

Antibodies to rat pancreas Golgi subfractions: Identification of a 58 kDa cis-Golgi protein

Distribution of the intermediate elements operating in ER to Golgi transport

Golgin-84 is a Rab1 binding partner involved in Golgi structure

Intracellular interaction of collagen-specific stress protein HSP47 with newly synthesized procollagen

Visualization of ER-to-Golgi transport in living cells reveals a sequential mode of action for COPII and COPI

Trk activation in the secretory pathway promotes Golgi fragmentation

The absence of Emp24p, a component of ER-derived COPII-coated vesicles, causes a defect in transport of selected proteins to the Golgi

ERGIC-53, a membrane protein of the ER-Golgi intermediate compartment, carries an ER retention motif

Identification, by a monoclonal antibody, of a 53-kD protein associated with a tubulo-vesicular compartment at the cis-side of the Golgi apparatus

Identification of an intermediate compartment involved in protein transport from endoplasmic reticulum to Golgi apparatus

Ypt and Rab GTPases: Insight into functions through novel interactions

A three-dimensional reconstruction study of the rough ER-Golgi interface in serial thin sections of the pancreatic acinar cell of the rat

Segregation of COPI-rich and anterograde-cargo-rich domains in endoplasmic-reticulum-to-Golgi transport complexes

A GRASP55-rab2 effector complex linking Golgi structure to membrane traffic

Biogenesis of tubular ER-to-Golgi transport intermediates

Golgi coiled-coil proteins contain multiple binding sites for Rab family G proteins

An integral membrane component of coatomer-coated transport vesicles defines a family of proteins involved in budding

A role for kinesin-2 in COPI-dependent recycling between the ER and the Golgi complex

COPI-coated ER-to-Golgi transport complexes segregate from COPII in close proximity to ER exit sites

Illuminating the secretory pathway: When do we need vesicles

Imaging of procollagen transport reveals COPIdependent cargo sorting during ER-to-Golgi transport in mammalian cells

Anterograde and retrograde traffic between the rough endoplasmic reticulum and the Golgi complex

Recycling of Golgi-resident glycosyltransferases through the ER reveals a novel pathway and provides an explanation for nocodazole-induced Golgi scattering

The p24 family and selective transport processes at the ER-Golgi interface

PKCδ and ε regulate the morphological integrity of the ER-Golgi intermediate compartment (ERGIC) but not the anterograde and retrograde transports via the Golgi apparatus

Role of vesicle tethering factors in ER-Golgi membrane traffic

Dissecting the role of the ARF guanine nucleotide exchange factor GBF1 in Golgi biogenesis and protein trafficking

Segregation of ERGIC-53 and the mammalian KDEL-receptor upon exit from the 15 1C compartment

Differential response of resident proteins and cycling proteins of the Golgi to brefeldin A

COPII and exit from the endoplasmic reticulum

Molecular cloning, characterization, subcellular localization and dynamics of p23, the mammalian KDEL receptor

A Rab2 mutant with impaired GTPase activity stimulates vesicle formation from pre-Golgi intermediates

Src-dependent atypical protein kinase C iota/lambda (aPKCiota/lambda) tyrosine phosphorylation is required for aPKCiota/lambda association with Rab2 and glyceraldehyde-3-phosphate dehydrogenase on pre-Golgi intermediates

Rab2 utilizes glyceraldehyde-3-phosphate dehydrogenase and protein kinase C{iota} to associate with microtubules and to recruit dynein

GTPbinding mutants of Rab1 and Rab2 are potent inhibitors of vesicular transport from the endoplasmic reticulum to the Golgi complex

p53/58 binds COPI and is required for selective transport through the early secretory pathway

Regulation of ER-Golgi intermediate compartment tubulation and mobility by COPI coats, motor proteins and microtubules

Replication of coronavirus MHV-A59 in saccells: Determination of the first site of budding of progeny virions

Site of addition of N-acetyl-galactosamine to the E1 glycoprotein of mouse hepatitis virus-A59

Secretory traffic triggers the formation of tubular continuities across Golgi subcompartments

The yeast Rab GTPase Ypt1 modulates unfolded protein response dynamics by regulating the stability of HAC1 RNA

Cholesterol and vesicular stomatitis virus G protein take separate routes from the endoplasmic reticulum to the plasma membrane

A pH-regulated quality control cycle for surveillance of secretory protein assembly

The crystal structure of the carbohydrate-recognition domain of the glycoprotein sorting receptor p58/ERGIC-53 reveals an unpredicted metal-binding site and conformational changes associated with calcium ion binding

Megavesicles implicated in the rapid transport of intracisternal aggregates across the Golgi stack

Isoform-selective effects of the depletion of ADP-ribosylation factors 1-5 on membrane traffic

Regulation of Integrin β1 recycling to lipid rafts by Rab1a to promote cell migration

Maintenance of Golgi structure and function depends on the integrity of ER export

Signals and salvage sequences

pH-dependent binding of KDEL to its receptor in vitro

α-Synuclein impairs macroautophagy: Implications for Parkinson's disease

Reconstitution of COPII vesicle fusion to generate a pre-Golgi intermediate compartment

Subunit structure of a mammalian ER/Golgi SNARE complex

The KDEL receptor mediates a retrieval mechanism that contributes to quality control at the endoplasmic reticulum

The p58-positive pre-Golgi intermediates consist of distinct subpopulations of particles that show differential binding of COPI and COPII coats and contain vacuolar H þ -ATPase

Colocalization of Ca 2 þ -ATPase and GRP94 with p58 and the effects of thapsigargin on protein recycling suggest the participation of the pre-Golgi intermediate compartment in intracellular Ca 2 þ -storage

Non-conventional trafficking of the cystic fibrosis transmembrane conductance regulator through the early secretory pathway

mBet3p is required for homotypic COPII vesicle tethering in mammalian cells

Ykt6 forms a SNARE complex with syntaxin 5, GS28, and Bet1 and participates in a late stage in endoplasmic reticulum-Golgi transport

The mammalian protein (rbet1) homologous to yeast Bet1p is primarily associated with the pre-Golgi intermediate compartment and is involved in vesicular transport from the endoplasmic reticulum to the Golgi apparatus

Morphological and functional association of Sec22b/ERS-24 with the pre-Golgi intermediate compartment

GBF1, a cis-Golgi and VTCs-localized ARF-GEF, is implicated in ER-to-Golgi protein traffic

Structural characterization of carbohydrate binding by LMAN1 protein provides new insight into the endoplasmic reticulum export of factors V (FV) and VIII (FVIII)

Combined deficiency of coagulation factors V and VIII: An update

Autophagosome formation depends on the small GTPase Rab1 and functional ER exit sites

Immunolocalization of UDP-glucose: glycoprotein glucosyltransferase indicates involvement of pre-Golgi intermediates in protein quality control

Misfolded proinsulin accumulates in expanded pre-Golgi intermediates and endoplasmic reticulum subdomains in pancreatic beta cells of Akita mice

Further Reading

Getting into the Golgi

Molecular motors and the Golgi complex: Staying put and moving through

The KDEL-receptor: New functions for an old protein

Protein sorting receptors in the early secretory pathway

The Golgi apparatus: 100 years of progress and controversy

Architecture of the mammalian Golgi

Secretory protein trafficking and organelle dynamics in living cells

COPII and the regulation of protein sorting in mammals