Herpes Simplex Virus Type 1 Cleavage and Packaging Proteins UL15 and UL28 Are Associated with B but Not C Capsids during Packaging

At least seven viral genes encode proteins (UL6, UL15, UL17, UL25, UL28, UL32, and UL33) that are required for DNA cleavage and packaging of herpes simplex virus type 1 (HSV-1) DNA. Sequence analysis reveals that UL15 shares homology with gp17, the large catalytic subunit of the bacteriophage T4 terminase. Thus, UL15 may play a direct role in the cleavage of viral DNA replication intermediates into monomers. In this study, we asked whether UL15 and other cleavage and packaging proteins could be detected in capsids isolated from infected cells. Consistent with previous studies showing that UL6 and UL25 are minor protein constituents of the capsids, we detected these proteins in both B and C capsids. In contrast, the previously identified full-length version (81 kDa) of UL15 was found predominantly in B capsids and in much smaller amounts in C capsids. In addition, the UL28 protein was found predominantly in B but not C capsids in a distribution similar to that of the 81-kDa version of UL15. These results suggest that UL28 and the 81-kDa form of UL15 are transiently associated with capsid intermediates during the packaging process. Surprisingly, however, a previously unidentified 87-kDa form of UL15 was found in the B and C capsids and in virions. Analysis of cells infected with mutants individually lacking UL6, UL15, UL25, UL28, or UL32 demonstrates that the lack of one cleavage and packaging protein does not affect the expression of the others. Furthermore, this analysis, together with guanidine HCl extraction analysis of purified capsids, indicates that UL6, UL25, and UL28 are able to associate with B capsids in the absence of other DNA cleavage and packaging proteins. On the other hand, the two UL15-related proteins (81 and 87 kDa) do not associate efficiently with B capsids in cells infected with UL6 and UL28 mutants. These results suggest that the ability of the UL15-related proteins to bind to B capsids may be mediated through interactions with UL6 and UL28.