NA, AN AUTOPROTEOLYTIC PRODUCT OF THE HERPES-SIMPLEX VIRUS TYPE-1 PROTEASE, CAN FUNCTIONALLY SUBSTITUTE FOR THE ASSEMBLY PROTEIN ICP35

The herpes simplex virus type 1 (HSV-1) protease and its substrate, th e assembly protein ICP35, are involved in virion maturation. Both prot eins are encoded by a single open reading frame but are translated ind ependently from 3'-coterminal mRNAs of different sizes and are in fram e. The herpesvirus shell assembles around an internal scaffold which i s subsequently lost during packaging of the viral genome. The scaffold is composed of ICP35, which is the major component, and autoproteolyt ically processed forms of the viral protease containing sequences comm on to ICP35 (Nb). In the baculovirus system, HSV-1 intact capsids can be formed in the presence of the protease or ICP35, indicating that th e protease mag substitute for ICP35 (Thomsen et al., J. Virol. 68:2442 -2457, 1993). This is further supported by the fact that ICP35, in con trast to the protease, is not absolutely essential for viral growth. T he processed intermediate of the protease analogous to ICP35 is the 38 8-amino-acid (aa) protein, Na, which is an N-terminal 59-aa extension of the 329-aa ICP35. To directly examine whether Na can functionally s ubstitute for ICP35 during viral replication, we first constructed a m utant virus, Na Delta 35, in which 35 aa from the N terminus of Na wer e deleted. Phenotypic analysis of the mutant showed that this deletion had no effect on protease function. The function of Na was further ex amined by construction of a plasmid expressing Na alone and testing it s ability to complement the growth of the mutant Prb virus in the abse nce of ICP35. Our results demonstrate that Na can functionally substit ute for ICP35 during viral replication.