A Dissection of the Protein–Protein Interfaces in Icosahedral Virus Capsids

We selected 49 icosahedral virus capsids whose crystal structures are reported in the Protein Data Bank. They belong to the T = 1, T = 3, pseudo T = 3 and other lattice types. We identified in them 779 unique interfaces between pairs of subunits, all repeated by icosahedral symmetry. We analyzed the geometric and physical chemical properties of these interfaces and compared with interfaces in protein–protein complexes and homodimeric proteins, and with crystal packing contacts. The capsids contain one to 16 subunits implicated in three to 66 unique interfaces. Each subunit loses 40–60% of its accessible surface in contacts with an average of 8.5 neighbors. Many of the interfaces are very large with a buried surface area (BSA) that can exceed 10,000 Å2, yet 39% are small with a BSA < 800 Å2 comparable to crystal packing contacts. Pairwise capsid interfaces overlap, so that one-third of the residues are part of more than one interface. Those with a BSA > 800 Å2 resemble homodimer interfaces in their chemical composition. Relative to the protein surface, they are non-polar, enriched in aliphatic residues and depleted of charged residues, but not of neutral polar residues. They contain one H-bond per about 200 Å2 BSA. Small capsid interfaces (BSA < 800 Å2) are only slightly more polar. They have a similar amino acid composition, but they bury fewer atoms and contain fewer H-bonds for their size. Geometric parameters that estimate the quality of the atomic packing suggest that the small capsid interfaces are loosely packed like crystal packing contacts, whereas the larger interfaces are close-packed as in protein–protein complexes and homodimers. We discuss implications of these findings on the mechanism of capsid assembly, assuming that the larger interfaces form first to yield stable oligomeric species (capsomeres), and that medium-size interfaces allow the stepwise addition of capsomeres to build larger intermediates.