CYTOMEGALOVIRUS PROTEIN SUBSTRATES ARE NOT CLEAVED BY THE HERPES-SIMPLEX VIRUS TYPE-1 PROTEINASE

The herpesvirus maturational proteinase, assemblin, is made as a precu rsor that undergoes at least two autoproteolytic cleavages-one in a se quence toward its carboxyl end, called the maturational (M) site, and one in a sequence toward its midpoint, called the release (R) site. Th e M- and R-site sequences are both well conserved among the herpesviru s proteinase homologs, suggesting that the proteinase of one herpesvir us might be able to cleave the substrates of another. To test this pos sibility, we cloned and expressed in human cells the long (i.e., full- length open reading frame of proteinase gene) and short (i.e., proteol ytic domain, assemblin) forms of the proteinase from human and simian cytomegalovirus (HCMV and SCMV, respectively) and from herpes simplex virus type 1 (HSV-1), as well as the genes for their respective assemb ly protein precursor substrates. Data from cotransfections of these pr oteinase genes with appropriate homologous and heterologous substrates showed that although the SCMV and HCMV enzymes cleaved the M-sites of the assembly protein substrates of all three viruses and an SCMV R-si te substrate, the HSV-1 proteinase cleaved only its own substrate. Thi s finding demonstrates that the substrate specificity properties of th e HSV-1 enzyme differ from those of the two CMV enzymes.