key: cord-0941743-lin7id2v authors: Hebditch, Max; Warwicker, Jim title: protein-sol pKa: prediction of electrostatic frustration, with application to coronaviruses date: 2020-07-19 journal: Bioinformatics DOI: 10.1093/bioinformatics/btaa646 sha: 2d1f602d8802ef9919204d377909e9e88dd340af doc_id: 941743 cord_uid: lin7id2v MOTIVATION: Evolution couples differences in ambient pH to biological function through protonatable groups, in particular those that switch from buried to exposed and alter protonation state in doing so. We present a tool focusing on structure-based discovery and display of these groups. RESULTS: Since prediction of buried group pKas is computationally intensive, solvent accessibility of ionizable groups is displayed, from which the user can iteratively select pKa calculation centers. Results are color-coded, with emphasis on buried groups. Utility is demonstrated with benchmarking against known pH sensing sites in influenza virus hemagglutinin and in variants of murine hepatitis virus, a coronavirus. A pair of histidine residues that are conserved in coronavirus spike proteins are predicted to be electrostatically frustrated at acidic pH in both pre- and post-fusion conformations. We suggest that an intermediate expanded conformation at endosomal pH could relax the frustration, allowing histidine protonation, and facilitating conformational conversion of coronavirus spike protein. AVAILABILITY: This tool is available at http://www.protein-sol.manchester.ac.uk/pka/ Since pKas underlie pH-dependent phenomena in biology, their prediction has received significant attention, largely through continuum electrostatics methods (Alexov et al., 2011) . We have contributed a server for predicting pH and ionic strength dependence with a Debye-Hückel (DH) model that accounts for solvent exposed groups, which are generally in the great majority (Hebditch and Warwicker, 2019) . However, conformational change often depends on the electrostatic frustration (destabilization) that develops when a buried group cannot ionize at a pH where it would in a more solvent accessible conformation (Narayan and Naganathan, 2018) . We reasoned that a web tool focusing on buried ionizable groups would be useful for studying pH-dependent conformational change, and have adapted our existing mixed finite difference Poisson-Boltzmann (FDPB) and DH model. The FDPB part of this implementation uses a previously defined separation of contributions from continuum electrostatics (Bashford and Karplus, 1990) and incorporates Monte Carlo sampling of protonation states (Beroza et al., 1991) . Our aim is to produce a tool that allows rapid screening of structures by non-expert users, which is therefore complementary to the more detailed and computationally intensive investigation, for example with constant pH molecular dynamics (Chen et al., 2014) , available to expert users. After benchmarking against measured data, the server is demonstrated with coronaviruses, some of which use the endocytotic pathway for membrane fusion, whereas others fuse at the plasma membrane. We focus on the pre-to post-fusion conformational changes in the S2 part of the spike protein, that mediates membrane fusion (Heald-Sargent and Gallagher, 2012). We have sought to limit FDPB/DH run time for pKa predictions (Warwicker, 2004) to about two minutes processing. Upon upload of a structure, the user is presented with a color-coded display (Rose et al., 2018) of solvent accessible surface area (ASA) values for ionizable groups. A user iteratively specifies centers, around which pKa calculations are made for spheres of radius 25 Å (about the size of lysozyme). Edge effects do not have a big effect on predicted pKas towards the sphere center. Results for Asp, Glu, Lys, Arg, His accumulate as more centers are © The Author(s) 2020. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. added, and are color-coded to show whether a group is stabilizing or destabilizing, assessed from the difference between calculated and intrinsic pKa (capped at -5 and 5). The pKas observed at a single site are representative of interactions within a charge network. In a simple case, for just two interacting groups, each of the two pKas measures the entire contribution of that pair interaction to stability. Users may either use ionizable group ASA/burial or literature knowledge of interesting sites, to select pKa calculation centers. It is envisaged that the server will allow a user to quickly survey a set of structures for potential pH-dependence hotspots, rather than provide a great depth of analysis for each structure. In order to assess the mapping of burial to electrostatic frustration, two quite different trimer examples are discussed (Fig. 1) . For one of a set of designed pH sensors, pRO-2.5 (6msr, Boyken et al., 2019) , the single (trimeric) signal for electrostatic frustration (H52) identifies with the only substantially buried region (dark green, Fig. 1a,c) . H52 is also the designed pH-sensing residue (Boyken et al. 2019) . Elsewhere on the surface, ionizable groups range from minimally to significantly stabilizing (more blue) pKas, part of the balance between folded state interactions and acid pH destabilization noted in the design study (Boyken et al., 2019) . A similar balance has been noted (Daniels et al., 1985) for the larger and more complex hemagglutinin trimer, HA1-HA2 cleaved H3 (2viu, Fleury et al., 1998) . Here, substantial burial does not necessarily lead to electrostatic frustration (Fig. 1b) , emphasising the role of pKa calculations in discriminating the balance of interactions, in particular destabilizing desolvation versus stabilizing charge networks. Most of the identified groups are known in the extensive literature for this H3 hemagglutinin. Of the adjacent H183, H184 grouping in HA1, H184 has been found to influence the fusion pH, tested in H5 (Mair et al., 2014) , whilst H183 is critical for receptor binding (Chen et al., 2018) . HA2 K51 is reported to abrogate sensitivity to a conformational change inhibitor, when mutated to arginine in the H1 sub-type (Yoshimoto et al., 2000) . HA2 D109 is buried and adjacent to the fusion peptide, upon cleavage of HA0 to HA1-HA2 (Trost et al., 2019) . Also close to the fusion peptide, mutation of HA1 H17 alters the fusion pH (Thoennes et al., 2008) . H142 of HA2 has been suggested to play a role in pH-dependent fusion (Kampmann et al., 2006) . Although HA2 H26 is the single residue from our calculation of electrostatic frustration that is not mentioned in the literature, it is highly conserved (Lee et al., 2018) . In summary, comparison with measurement, for one of the most studied pH-dependent systems, demonstrates that burial, although generally necessary, is insufficient to predict electrostatic frustration. Further, the electrostatically frustrated groups that are identified by our model compare well with known sites. Although there are about 40 structures of pre-fusion coronavirus spike proteins, there is just one post-fusion structure available (April 2020) that extends beyond the helical fusion core, for murine hepatitis virus (MHV) strain A59 (6b3o, Walls et al., 2017) . A pre-fusion structure for MHV A59 is also available (3jcl, Walls et al., 2016) . In a variant of mouse hepatitis virus type 4 (MHV4), spike protein Q1015H, Q1042H (MHV A59 numbering) and one further mutation (L to R) render the virus pHdependent, via the endocytotic pathway (Gallagher et al., 1991) . Modelling the mutations to histidine in the MHV A59 background, we find they are predicted to be buried in the pre-fusion form (destabilizing, not shown) and exposed in the post-fusion structure (not destabilizing, Fig. 2a ). The two panels in Fig. 2a demonstrate sequential use of the web tool, first (left) displaying burial (by color-coding given in Fig. 1c) , and second (right) showing pKa calculations around selected centers. Selected residue numbers in Fig. 2a are also color-coded by the presence (red) or absence (blue) of electrostatic frustration in the post-fusion spike protein. Our results are consistent with the relief of electrostatic frustration at endosomal pH biasing conformation away from the pre-fusion structure. The story is a little more complicated, since the MHV4 variant also loses fusion activity at neutral pH (Gallagher et al., 1991) , which could be due to additional stabilization of the pre-fusion form with pH-independent histidine interactions. Fig. 2b shows equivalent parts (972 -1118) of a monomer from pre-and post-fusion MHV A59, structurally aligned through a small common core around 1098/1114. Extending from the structurally aligned segments are helices that demonstrate the extensive spike protein structural changes that go along with cell fusion. Whereas the helical region wraps back around the aligned core in the pre-fusion structure, it extends upwards post-fusion, carrying the fusion peptide towards its target membrane, shown by the relative locations of 1015. Switching to conserved histidines and general upon exposure to acidic pH (Fig. 2a, red spacefill) . If these conserved histidines are electrostatically frustrated in both pre-and post-fusion conformations at endosomal pH, they would not bias towards either form. However, to allow the extensive changes exemplified in Fig. 2b , it is possible that the core region around H1098/H1114 loosens. If H1098 and/or H1114 were solvent exposed and protonated, then relief from frustration in a conformational intermediate could play a role in facilitating transfer between post-and pre-fusion structures in coronaviruses that use the endocytotic pathway, including SARS-CoV-2 (Ou et al., 2020) . In this proposal, H1098/H1114 assistance in crossing the pre-to post-fusion conformational barrier would be available to those viruses that are unable to fuse at the plasma membrane. Interestingly, both H1098A and H1114A mutations in MHV A59 prevented virus growth (Li et al., 2018) , perhaps indicative of (pH-independent) packing stabilizations in their buried environments, so that evolutionary retention for fusion could be afforded by a more direct structural imperative. This would be in line with the coupling of factors that determine infection pathways, including spike protein stability, receptor binding, proteolytic cleavage, as well as endosomal pH (Heald-Sargent and Gallagher, 2012; Millet and Whittaker, 2015) . If the proposed loosening around the conserved post-and pre-fusion core is borne out, then although it may not be universally necessary (in pH-dependent entry), it could be the basis for a novel, albeit transient, coronavirus drug target. Our web tool will allow users to look for ionizable groups that could mediate pH-dependence in coronaviruses and other systems. Progress in the prediction of pKa values in proteins pKa's of ionisable groups in proteins: atomic detail from a continuum electrostatic model Protonation of interacting residues in a protein by a Monte Carlo method: application to lysozyme and the photosynthetic reaction center of Rhodobacter sphaeroides De novo design of tunable, pH-driven conformational changes Recent developments and application of constant pH molecular dynamics Structural insight into a human neutralizing antibody against influenza virus H7N9 Fusion mutants of the influenza virus hemagglutinin glycoprotein Antigen distortion allows influenza virus to escape neutralization Alteration of the pH-dependence of coronavirusinduced cell fusion: effect of mutations in the spike glycoprotein Ready, set, fuse! The coronavirus spike protein and acquisition of fusion competence Web-based display of protein surface and pHdependent properties for assessing the developability of biotherapeutics The role of histidine residues in low-pH-mediated viral membrane fusion Deep mutational scanning of hemagglutinin helps predict evolutionary fates of human H3N2 influenza variants Identification of H209 as essential for pH8-triggered receptorindependent syncytium formation by S protein of mouse hepatitis virus A59 A histidine residue of the influenza virus hemagglutinin controls the pH-dependence of the conformational change mediating membrane fusion Host cell proteases: critical determinants of coronavirus tropism and pathogenesis Switching protein conformational states by protonation and mutation Characterization of spike glycoprotein of SARS-Cov-2 on virus entry and its immune cross-reactivity with SARS-CoV Analysis of residues near the fusion peptide in the influenza hemagglutinin structure for roles in triggering membrane fusion A conserved histidine in Group-1 influenza subtype hemagglutinin proteins is essential for membrane fusion activity Identification of amino acids of influenza virus HA responsible for resistance to a fusion inhibitor Cryo-electron microscopy structure of a coronavirus spike glycoprotein structure Tectonic conformational changes of a coronavirus spike glycoprotein promote membrane fusion Improved pKa calculations through flexibility based sampling of a water-dominated interaction scheme) We thank David Steinhauer for discussion of influenza virus hemagglutinin. This work has been supported by the UK EPSRC (grant EP/N024796/1).Conflict of Interest: none declared.