THE PROTEASE OF HERPES-SIMPLEX VIRUS TYPE-1 IS ESSENTIAL FOR FUNCTIONAL CAPSID FORMATION AND VIRAL GROWTH

The herpes simplex virus type 1 protease and related proteins are invo lved in the assembly of viral capsids. The protease encoded by the UL2 6 gene can process itself and its substrate ICP35, encoded by the UL26 .5 gene. To better understand the functions of the protease in infecte d cells, we have isolated a complementing cell line (BMS-MG22) and con structed and characterized a null UL26 mutant virus, m100. The mutant virus failed to grow on Vero cells and required a complementing cell l ine for its propagation, confirming that the UL26 gene product is esse ntial for viral growth. Phenotypic analysis of m100 shows that (i) nor mal amounts of the c and d forms of ICP35 were produced, but they fail ed to be processed to the cleaved forms, e and f; (ii) viral DNA repli cation of the mutant proceeded at near wild-type levels, but DNA was n ot processed to unit length or encapsidated; (iii) capsid structures w ere observed in thin sections of m100-infected Vero cells by electron microscopy, but assembly of VP5 into hexons of the capsid structure wa s conformationally altered; and (iv) nuclear localizations of the prot ease and ICP35 are independent of each other, and the function(s) of N a, at least in part, is to direct the catalytic domain N-O to the nucl eus.