The Essential Protein Encoded by the UL31 Gene of Herpes Simplex Virus 1 Depends for Its Stability on the Presence of UL34 Protein

To pursue an earlier observation that the protein encoded by the U(L)34 gene binds to intermediate chain of dynein, we constructed a series of mutants from which sequences encoding the entire protein (DeltaU(L)34) or amino-terminal [U(L)34Delta(3-119)] or carboxyl-terminal [U(L)34Delta(245-275)] domains were deleted. The mutant lacking the sequence encoding the carboxyl-terminal domain grew in all cell lines tested. The two other mutants replicated only in cell type-dependent manner and poorly. Rescue of DeltaU(L)34 mutant with a fragment that does not encompass the U(L)31 ORF restored wild-type phenotype. U(L)34 protein interacts physically with U(L)31, and the U(L)31 deletion mutant appears to have a phenotype similar to that of U(L)34 deletion mutant. Experiments designed to determine whether the phenotypes of the deletion mutants have a common base revealed that cells infected with the DeltaU(L)34 mutant accumulate U(L)31 RNA but not the corresponding protein. The U(L)31 protein accumulated, however, to near wild-type virus-infected cell levels in cells infected with DeltaU(L)34 mutant and treated with the MG132 proteosomal inhibitor at 6 h after infection. This is evidence that the stability of an essential viral protein requires the presence of another protein. The observation raises the bar for identification of gene function on the basis of analyses of the phenotype of mutants in which the gene has been deleted or rendered inoperative.