Proteolytic Cleavage of the Amino Terminus of the UL15 Gene Product of Herpes Simplex Virus Type 1 Is Coupled with Maturation of Viral DNA into Unit-Length Genomes

ABSTRACT The UL15 gene of herpes simplex virus type 1 (HSV-1), like UL6, UL17, UL28, UL32, and UL33, is required for cleavage of concatameric DNA into genomic lengths and for packaging of cleaved genomes into preformed capsids. A previous study indicated that the UL15 gene encodes minor capsid proteins. In the present study, we have shown that the amino-terminal 509 amino acids of the UL15-encoded protein are sufficient to confer capsid association inasmuch as a carboxyl-terminally truncated form of the UL15-encoded protein with an M r of approximately 55,000 readily associated with capsids. This and previous studies have shown that, whereas three UL15-encoded proteins with apparent M rs of 83,000, 80,000, and 79,000 associated with wild-type B capsids, only the full-length 83,000-M r protein associated with B capsids purified from cells infected with viruses lacking functional UL6, UL17, UL28, UL32, and UL33 genes (B. Salmon and J. D. Baines, J. Virol. 72:3045–3050, 1998). Thus, all viral mutants that fail to cleave viral DNA into genomic-length molecules also fail to produce capsid-associated UL15 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 cells infected with a UL25 null virus that cleaves, but does not package, DNA. The conclusion that the amino terminus of the 83,000-M r protein is truncated 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 first 35 amino acids of the UL15 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(UL15M36V), an engineered virus encoding valine rather than methionine at codon 36. Thus, initiation 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 capsid-associated UL15-encoded protein is proteolytically cleaved near the N terminus and indicate that this modification is tightly linked to maturation of genomic DNA.