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 both itself and the UL26.5 gene product. To study the structure and function 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 expressed in wild-type baculovirus. The UL26 polypeptide behaved in a similar manner to the protein made in HSV-1-infected cells, cleaving itself 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 second 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 numbers 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 occasionally in UL26.5-recombinant baculovirus-infected cells. The results suggest that the proteinase, in the absence of other HSV capsid proteins, stimulates the formation of large numbers of scaffold-like particles present either as semi-crystalline arrays or as dispersed structures.