THE HERPES-SIMPLEX VIRUS GENE UL26 PROTEINASE IN THE PRESENCE OF THE UL26.5 GENE-PRODUCT PROMOTES THE FORMATION OF SCAFFOLD-LIKE STRUCTURES

The herpes simplex virus type 1 (HSV-1) polypeptides encoded by genes UL26 and UL26.5 are thought to form a scaffold around which the capsid shell assembles. The UL26 gene specifies a proteinase that cleaves bo th itself and the UL26.5 gene product. To study the structure and func tion of the UL26 and UL26.5 gene products, the proteins were expressed in cells infected with recombinant baculoviruses containing the genes under the control of the polyhedrin promoter. Both polypeptides were made in large amounts, approaching the levels of polyhedrin protein ex pressed in wild-type baculovirus. The UL26 polypeptide behaved in a si milar manner to the protein made in HSV-1-infected cells, cleaving its elf rapidly into the capsid proteins VP21 and VP24 and converting the UL26.5 product more slowly into the capsid protein VP22a. The results of immunoblot analysis using antisera specific for the amino-terminal region of the UL26 polypeptide suggested that both the first and secon d ATGs in the UL26 open reading frame were recognized as translational start signals but the first ATG was the preferred initiation codon as is the case in HSV-1-infected cells. Electron microscopic examination of thin section preparations of cells infected with both the UL26.5- and UL26-recombinant baculoviruses revealed the presence of large numb ers of small spherical particles, often arranged in a semi-crystalline array. These clusters of scaffold-like particles were not present in cells infected with UL26-recombinant baculovirus but were observed occ asionally in UL26.5-recombinant baculovirus-infected cells. The result s suggest that the proteinase, in the absence of other HSV capsid prot eins, stimulates the formation of large numbers of scaffold-like parti cles present either as semi-crystalline arrays or as dispersed structu res.