Human cytomegalovirus UL36 protein is dispensable for viral replication incultured cells

Consistent with earlier analyses of human cytomegalovirus UL36 mRNA, we fin d that the UL36 protein is present throughout infection. In fact, it is del ivered to the infected cell as a constituent of the virion. Curiously, much less UL36 protein accumulated in cells infected with the AD169 strain of h uman cytomegalovirus than in cells infected with the Towne or Toledo strain , and localization of the protein in cells infected with AD169 is strikingl y different from that in cell infected with the Towne or Toledo strain. The variation in steady-state level of the proteins results from different sta bilities of the proteins. The UL36 proteins from the three viral strains di ffer by several amino acid substitutions, However, this variability is not responsible for the different half-lives because the AD169 and Towne protei ns, which exhibit very different half-lives within infected cells, exhibit the same half-life when introduced into uninfected cells by transfection wi th expression plasmids, We demonstrate that the UL36 protein is nonessentia l for growth in cultured cells, and we propose that the ability of the viru s to replicate in the absence of UL36 function likely explains the striking strain-specific variation in the half-life and intracellular localization of the protein.