Finding and using local symmetry in identifying lower domain movements in hexon subunits of the herpes simplex virus type 1 B capsid.

A characteristic of virus assembly is the use of symmetry to construct a complex capsid from a limited number of different proteins. Many spherical viruses display not only icosahedral symmetry, but also local symmetries, which further increase the redundancy of their structural proteins. We have developed a computational procedure for evaluating the quality of these local symmetries that allows us to probe the extent of local structural variations among subunits. This type of analysis can also provide orientation parameters for carrying out non-icosahedral averaging of quasi-equivalent subunits during three-dimensional structural determination. We have used this procedure to analyze the three types of hexon (P, E and C) in the 8.5 A resolution map of the herpes simplex virus type 1 (HSV-1) B capsid, determined by electron cryomicroscopy. The comparison of the three hexons showed that they have good overall 6-fold symmetry and are almost identical throughout most of their lengths. The largest difference among the three lies near the inner surface in a region of about 34 A in thickness. In this region, the P hexon displays slightly lower 6-fold symmetry than the C and E hexons. More detailed analysis showed that parts of two of the P hexon subunits are displaced counterclockwise with respect to their expected 6-fold positions. The most highly displaced subunit interacts with a subunit from an adjacent P hexon (P'). Using the local 6-fold symmetry axis of the P hexon as a rotation axis, we examined the geometrical relationships among the local symmetry axes of the surrounding capsomeres. Deviations from exact symmetry are also found among these local symmetry axes. The relevance of these findings to the process of capsid assembly is considered.