ASSEMBLY OF HERPES-SIMPLEX VIRUS CAPSIDS USING THE HUMAN CYTOMEGALOVIRUS SCAFFOLD PROTEIN - CRITICAL ROLE OF THE C-TERMINUS

An essential step in assembly of herpes simplex virus (HSV) type I cap sids involves interaction of the major capsid protein (VP5) with the C terminus of the scaffolding protein (encoded by the UL26.5 gene). The final 12 residues of the HSV scaffolding protein contains an A-X-X-F- V/A-X-Q-M-M-X-X-R motif which is conserved between scaffolding protein s found in other alphaherpesviruses but not in members of the beta- or gamma-herpesviruses. Previous studies have shown that the bovine herp esvirus 1 (alphaherpesvirus) UL26.5 homolog will functionally substitu te for the HSV UL26.5 gene (E. J. Haanes et al., J. Virol. 69:7375-737 9, 1995). The homolog of the UL26.5 gene in the human cytomegalovirus (HCMV) genome is the UL80.5 gene. In these studies, we tested whether the HCMV UL80.5 gene would substitute for the HSV UL26.5 gene in a bac ulovirus capsid assembly system that we have previously described (D. R. Thomsen et al., J. Virol. 68:2442-2457, 1994). The results demonstr ate that (i) no intact capsids were assembled when the full-length or a truncated (missing the C-terminal 65 amino acids) UL80.5 protein was tested; (ii) when the C-terminal 65 amino acids of the UL80.5 protein were replaced with the C-terminal 25 amino acids of the UL26.5 protei n, intact capsids mere made and direct interaction of the UL80.5 prote in with VP5 was detected; (iii) assembly of intact capsids was demonst rated when the sequence of the last 12 amino acids of the UL80.5 prote in was changed from RRIFVA A<(LN)under bar>KLE to RRIFVAA<(MM)under ba r>KLE; (iv) self-interaction of the scaffold proteins is mediated by s equences N terminal to the maturation cleavage site; and (v) the UL26. 5 and UL80.5 proteins will not coassemble into scaffold structures. Th e results suggest that the UL26.5 and UL80.5 proteins form a scaffold by self-interaction via sequences in the N termini of the proteins and emphasize the importance of the C terminus for interaction of scaffol d with the proteins that form the capsid shell.