key: cord-287729-pl88otue authors: Gray, Stewart M. title: Plant virus proteins involved in natural vector transmission date: 1996-07-31 journal: Trends in Microbiology DOI: 10.1016/0966-842x(96)10040-8 sha: doc_id: 287729 cord_uid: pl88otue Abstract Plant viruses transmitted by invertebrate vectors either reversibly bind to vector mouthparts or are internalized by the vector and later secreted. Viral proteins mediate the binding of plant viruses to vector mouthparts and the transport of virus across vector-cell membranes. Both mechanisms probably involve conformational changes of virus proteins during their association with the vector. The terminology used to describe plant virus transmission is biased towards aphid vectors, and does not accurately reflect the transmission processes of other vector taxa". In this review, I divide plant viruses into two broad cazgories with different transmission processes: circulative and noncirculative. Circulative viruses are usually defined as moving from the abmentary canal of an insect vector into its hemocoel (its open circulatory system) and back out +hrougb the salivary secretory system. However, in this review, I expand the definition of circulative transmissioal to include any plant virus that must be actively transported across vector membranes and survive inside the vector to be transmitted. Noncirculative viruses associate with the cuticular lining of the insect mouthpam or foregut and are released as the insect expels digestive secretions into the plant when it begins to feed. These viruses are not actively transported across vector-cell membranes, nor are they carried internally. The cxrernal cuticular lining of insects (and nematodes) exrecds well ir?to the mouthparts and foregut, but is shed when the animal molts. comoviruses, bromoviruses and sobemoviruses could also be included. as these viruses are tramported across gut membranes into the hemolymph of the insect"; however, this mipht not be an e-xntial requirement for transmis\Inn". crrcf4larif 'C rransrrrcssion in i#se?cr rvcrors Luteoviruses and PEMV are the best studied of the circulative nonpropagative viruses, ard thcv share many fundamental katures and mechanisms related to their transmisGon by aphids'l'. Afiet uptake into the alimentary canal, virus particles attach to and are transported across the hindgut, and occasionally the midgut, epitbelium into the hcmc~ ccl via a receptor-mediated & cytotic pathway's2'. The hindgut can act as a barrier to ttrteovirus movement into the hemocoel, but it does nor appear to inhibit the uptake of most luteovirus isolates=*. Virus parricles are carried by the hemolymph From the abdomen to the head, where they can associatewiththea ccessory salivary glared (MC). Here, virus is actively transported across two distinct barriers to transmi~ion, the ASC, basal lamina and the -4% plasmalemma, and then released into the salivary canal. Virus is then injected into a plant as the aphid feeds" (Figs l-3) . The .4SG b-1 lamina is a fibrous network consisting primarily of collagen, which provides support snd may also act as a filtee. Lutcovirus isolates differentially bind the ASG basal lamina and selectively but move across the basal lamina matrix. Luteoviruses do not interact with rhe basal lamina of other rissues, including the principal salivary gland""'-. The transport of luteoviruses across the ASG plasma!emma, which is also selective for specific isolates, appears to occur by receptor-mediated endocytosis". The hindgut, ASG basal lamina and ASG plasmalemma have various effects on the transmission efficiency of different cornhinations of aphid species and virus isolate'-.".'-, suggesting that the moIecular mechanisms involved in virus movement across these three harriers are likely to involve different viral proteins or protein domains. The capsid of luteoviruses and PEMV contains two proteins, a predominant coat prc,tc#u and R srcondary protein that is present in small amounts and isrranslated via .cadrhrough of the coat protein stop codon-."x-r'. Heterologous encapsidation IS common between barley yellow dwarf luteoviruses when multiple isolates infect the same plant"-". If complete or partial exchange of capsid proteins occurs, the vector-specific transmission phenotype of one or both isolates is altered".". The readthrough protein, although not required ior particle assembly or plant infection' '-.4. is required for aphid transmission-*-". Although luteovirus particles lacking the readthrough protein are acquired by aphids and can cross the hindgut harrier to accumulate in he hemocoel, no transmission to plants occurs"*". These observations suggest that rhe readthrough protein is required for viruses to move across transmiscicm 5arriers in the ASG., and that the coat protein is probably therefore responsible for virus movement across the hindgut. The circulative transmrssion of geminiviruses through their whitefly or leafhopper vectors has not been studied in detail, but is thought to be similar to that of luteoviruses in aphids. Unlike the Iuteovituses, only the coat protein is required for transmission, and the coar prorein appears to regulate the specificity of transmission by both whirefiies and leafhoppers'"*'-. The whiteflytransmitted gemirliviruses have a highly conserved amino acid sequence in their coat proteins, and are all transmitted efficiently by one whitefly species, Bemisia tahci. in the amino terminus UC the coat protein is essential for aphid transmission. Proteolytic treatment of virions, which removes the amino terminus of the coar-protein subunits, prevents the particles from being transmitted by aphids, although they remain infectious when mechanically inoculated into plantsas. The HC is presumably acquired along with virus particles as the aphid feeds on plant sap, hut the role of this protein in transmission is unknown. One hypothesis is that it mediates the binding of virus to sites within the aphid food canal, either directly, by iinking the virus to the aphid (Fig. 4A ), or indirectly, by modifying viral attachment compounds in the aphid to allow virus binding (Fig. 48) . Virus-like particles have been observed embedded in a matrix material assocaated with the stylers and foregut of aphids fed on a mlxrurc of Gus and HC. No bound particles were oherved when aphids were fed on virus alone*. In \ it-u+ mutants ccmraining deletions or substitutions making either the coat protein or the HC incompetent for aphid transmission, theassocianon of virus-like particles with the cuticle did not occur (T.P. Pironc. pers. commun.). Transmission -ompete; 7 HL is required for virus association and retention in the aphid mouthpatts and must be present before the virus or simultaneously with it'; this suggests that HC is involved in me&ating binding between the aphid cuticle and the virus. An alternative hypothesis is that I-K acts Indirectly to modify the coat protein and allows a direct interaction between the virus partick md theaphid (Fig. 4C) . When a recombinant protein containing the aminoterminal region: of; ;atyvirus coat protein was fed to aphids before feeding them virus lnd HC, transmission was abolished'-. Salomon and Bemardi interpret these data tosuggest that the recombinant protein saturated vircs-binding sites in the aphid and prevented subsequenr virus binding. They hypothesize that the aminoterminal region of the coat protein, rather than the HC. attacha to sites on the aphid. They further suggest that the am&-terminal region of the coat-protein monomers assembled into virus particles is not normally available for interaction with the aphids, but that HC mediates a conformational change in the amino termiof thecoat protein that allows binding of the virus ;o the aphid (Fig. 4C) .mission is regulated mainly by the codt protein'". Site-specific mutagenesis of cucumber mosaic virus (CuMV) has identified two regions of the coat protein thar are involved in efficient transmission, and has pinpointed the amino acids needed<'. The same regions of the coat protein, but not the same amino B acids, have been implicated in a second poorly transmissible CuMV strain. The rm Isu&don effiiency of CuMV has been attributed to properties of the coat protein, but not to identifiable linear amino acid sequences. These observations biggest that one or a few amino acid changes in the coat protein could alter the vectortransmission phenotype by pre\ enring direct interaction between the virus and the vector. Alternatively, rhe amino C acid changes couid influence the threedimensional structure of the coat protein or capsid. and indirectly affect the ability of the virus to interact with its vector. irr LEO, but cannot mediate transmission. If supplied along with functional HC and virus, the GST-HC fusion protein inhibits transmission'". Schmidt eta/. suggest that the GST-HC protein outcompetes the native HC and saturates virus-binding sites. As the amino-terminal portion of the HC in the fusion is attached zo GST, it cannot interact with binding sites in the aphid, and so transmission is prevented. Noncrrculuti~~e rims fransmissiorr by nentamdes The noncirculative nematode-transmitted nepoviruses and tobraviruses also associate with the cuticular lining of the vector food canal. Acquired virus particles bind to specific regions of the stylet sheath, pharynx or esophagus, and a carbohydrate-containing material of unknown origin is associated with bound virus particles. The involvement of nonstructural virus proteins has not been established for the nepoviruses, but recent evidence suggests that a nonstructural protein might be required for transmission of to'uraviruses in addition to the coat proteitP. Virus release is thought to be mediated by a pH change resulting from salivary secretions flowing through the foocl canal when the nematode begins to feed on a plantA(l. Like the amino-terminal domain of rhe potyvirus capsid protein, the carboxy-terminal domain of the tobravirus coat protein can be cleaved from the virus particle by some proteases without adversely affecting the virus". It is not known whether or not the lnge&ion the role of HC in mediating binding between the aphid cuticle and the virus. Biologically active HC, isolated from infected plants or a baculovirus expression system, binds coat protein in &ro48*49. Two of the carboxy-terminal 31 amino acids of the HC are required for HC to bind coat protein, and loss of binding abolishes aphid transmission. HC expressed in E.scbtichia ccli as an amino-terminat glutathione-S-transferase-HC (GST-HC) fusion protein can bind coat protein REVIEWS ------modified particles can hc transmitted by nematodes. Interestirgty, the aggregation state of coat-protcln monomers does rcrpond to chnngrs in pH. and the carboxyl terminus of the tobacco rattle tobravirus coat protein contains a segment that does not appear to he part of the struct!tral framework of the virus particlecl. Could this be a conformationally active rcg)l,n of the coat protein that is exposed only in the nematode foregut and that acts as a cleavage site for virui release? In noncirculative transmission, the virus must associate with the cuticular lining of the food canal of the vector. binding require5 \ iral protein sequences and perhaps specific vector substances. The binding rnl!rt be reversible. and release can involve specific proieolyric cleavage events or can be passive. The ends f cleavage sites on the coat proteins. Exposure of these sites to digestive uxre;ions during feeding would create a mechanism by which bou.~d \irt:s could bc released from rhr \'cilor and inirs---cl into a plant host (Fig. 4) . Recently. rapid progress has been m; de in dis=+ng the relatively simple genomes of p!ant vuuses and identitying the proteins that are involved in their rransmission by k-actors. Changes in the linear sequence of transmission-associated proteins can alter the tratzsmission phenotype of a virus. but the three-dimensional structure of protein subunits or viruscapsid is also likely to be involved in virus-vector interactions. Conformational changes in 3 virus protein could alter in response to environmental changes, and this could prevetrt virus transmission. However, most plant-Infecting viruses do not replicate in their vectors and do not appear it) undergo major morphological changes within the \~'ctar. The environment within titc dumentary system or hemolymph of a vector 1s probably significantly different from that within a plant cell or in Gtro. Possibly. viruses undergo more-subtle conformational changes within the veczor that are not readily detectable. It is well known that changes in pH or ionic strength can alter the conformarion of plant viruses".". Hence, the biochemical environment within a vector might alter the virion struc~~~re such that different protein domains are accessible for interaction with a vector. 1 discuss this hypothesis for noncirculative viruses, but it is also likely to apply to circulative viruses. The readthrough protein of luteoviruses is required for efficient transmission of vtrus through the aphid ASG. The readthrough protein is thought to protrude from the particle surface, or at least to be part of the surface topography znJv.U. However, antibodies that specifically label the proteins in vitro do not iabel whole ----------. cd.). pp. l-14 Ph~opofl?o&~ X4, I 155-1 I $6 IYYS) V~I/~~~~ 112. i83-!LJl 5) \'rrobqy 2 I ;. (r Blanc. 5. el d ( i 9931 Vrro/r,i Vtro/q,, l95,692-6Y9 .