Proteolytic cleavage of the amino terminus of the U(L)15 gene product of herpes simplex virus type 1 is coupled with maturation of viral DNA into unit-length genomes

The U(L)15 gene of herpes simplex virus type 1 (HSV-l), like U(L)6, U(L)17, U(L)28, U(L)32, and U(L)33, is required for cleavage of concatameric DNA i nto genomic lengths and for packaging of cleaved genomes into preformed cap sids. A previous study indicated that the U(L)15 gene encodes minor capsid proteins. In the present study, we have shown that the amino-terminal 509 a mino acids of the U(L)15-encoded protein are sufficient to confer capsid as sociation inasmuch as a carboxyl-terminally truncated form of the U(L)15-en coded protein with an M-r of approximately 55,000 readily associated with c apsids. This and previous studies have shown that, whereas three U(L)15-enc oded proteins with apparent M(r)s of 83,000, 80,000, and 79,000 associated with wild-type B capsids, only the full-length 83,000-M-r protein associate d with B capsids purified from cells infected with viruses lacking function al U(L)6, U(L)17, U(L)28, U(L)32, and U(L)33 genes (B. Salmon and J. D, Bai nes, J. Virol. 72:3045-3050, 1998). Thus, all viral mutants that fail to cl eave viral DNA into genomic-length molecules also ;fail to produce capsid-a ssociated U(L)15 80,000- and 79,000-M-r proteins, In contrast, the 80,000- and 79,000-M-r proteins were readily detected in capsids purified from cell s infected with a U(L)25 null virus that cleaves, but does not package, DNA . The conclusion that the amino terminus of the 83,000-M-r protein is trunc ated to produce the 80,000- and/or 79,000-M-r protein was supported by the following observations, (i) Whereas the C termini of the 83,000-, 80,000-, and 79,000-M-r proteins are identical, immunoreactivity dependent on the fi rst 35 amino acids of the U(L)15 83,000-M-r protein was absent from the 80, 000- and 79,000-M-r proteins. (ii) The 79,000- and 80,000-M-r proteins were detected in capsids from cells infected with HSV-1(U(L)15M36V), an enginee red virus encoding valine rather than methionine at codon 36, Thus, initiat ion at codon 36 is unlikely to account for production of the 80,000- and/or 79,000-M-r protein. Taken together, these data strongly suggest that capsi d-associated U(L)15-encoded protein is proteolytically cleaved near the N t erminus and indicate that this modification is tightly linked to maturation of genomic DNA.