The essential protein encoded by the U(L)31 gene of herpes simplex virus 1depends for its stability on the presence of U(L)34 protein

To pursue an earlier observation that the protein encoded by the U(L)34 gen e binds to intermediate chain of dynein, we constructed a series of mutants from which sequences encoding the entire protein (Delta U(L)34) or amino-t erminal [U(L)34 Delta(3-119)] or carboxyl-terminal [U(L)34 Delta(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 repli cated only in cell type-dependent manner and poorly. Rescue of Delta U(L)34 mutant with a fragment that does not encompass the U(L)31 ORF restored wil d-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)3 4 deletion mutant. Experiments designed to determine whether the phenotypes of the deletion mutants have a common base revealed that cells infected wi th the Delta U(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-i nfected cell levels in cells infected with Delta U(L)34 mutant and treated with the MG132 proteosomal inhibitor at 6 h after infection. This is eviden ce that the stability of an essential viral protein requires the presence o f another protein. The observation raises the bar for identification of gen e function on the basis of analyses of the phenotype of mutants in which th e gene has been deleted or rendered inoperative.