Lytic replication of Kaposi's sarcoma-associated herpesvirus results in the formation of multiple capsid species: Isolation and molecular characterization of A, B, and C capsids from a gammaherpesvirus

Despite the discovery of Epstein-Barr virus more than 35 years ago, a thoro ugh understanding of gamma-herpesvirus capsid composition and structure has remained elusive. We approached this problem by purifying capsids from Kap osi's sarcoma-associated herpesvirus (KSHV), the only other known human gam ma-herpesvirus. The results from our biochemical and imaging analyses demon strate that KSHV capsids possess a typical herpesvirus icosahedral capsid s hell composed of four structural proteins. The hexameric and pentameric cap somers are composed of the major capsid protein (MCP) encoded by open readi ng frame 25. The heterotrimeric complexes, forming the capsid floor between the herons and pentons, are each composed of one molecule of ORF62 and two molecules of ORF26. Each of these proteins has significant amino acid sequ ence homology to capsid proteins in alpha- and betaherpesviruses. In contra st, the fourth protein, ORF65, lacks significant sequence homology to its s tructural counterparts from the other subfamilies, Nevertheless, this small , basic, and highly antigenic protein decorates the surface of the capsids, as does, for example, the even smaller basic capsid protein VP26 of herpes simplex virus type 1. We have also found that, as with the alpha-and betah erpesviruses, lytic replication of KSHV leads to the formation of at least three capsid species, A, B, and C, with masses of approximately 200, 230, a nd 300 MDa, respectively. A capsids are empty, B capsids contain an inner a rray of a fifth structural protein, ORF17.5, and C capsids contain the vira l genome.