Posttranslational processing of infected cell protein 22 mediated by viralprotein kinases is sensitive to amino acid substitutions at distant sites and can be cell-type specific

Infected cell protein 22 (ICP22) is posttranslationally phosphorylated by t he viral kinases encoded by U(S)3 and U(L)13 and nucleotidylylated by casei n kinase II. In rabbit and rodent cells and in primary human fibroblasts in fected with mutants from which the alpha 22 gene encoding ICP22 had been de leted, a subset of late (gamma (2)) gene products exemplified by U(L)38 and U(S)11 proteins are expressed at a reduced level, as measured by the accum ulation of both mRNA and protein. The same phenotype was observed in cells infected with mutants lacking the U(L)13 gene. The focus of this report is on three serine- and threonine-rich domains of ICP22. Two of these domains are homologs located between residues 38 to 66 and 300 to 328. The third do main is near the carboxyl terminus and contains the sequence T374SS. The re sults were as follows. (i) Alanine substitutions in the amino-terminal homo log precluded the posttranslational processing of ICP22 in rabbit skin cell s and in Vero cells but had no effect on the accumulation of either U(S)11 or U(L)38 protein. (ii) Alanine substitutions in the carboxyl-terminal homo log had no effect on posttranslational processing of ICP22 accumulating in Vero cells but precluded full processing of ICP22 accumulating in rabbit sk in cells. The effect on accumulation of U(L)38 and U(S)11 proteins was insi gnificant in Vero cells and minimal in rabbit skin cells. (iii) Substitutio ns of alanine for the threonine and serines in the third domain precluded f ull processing of ICP22 and caused a reduction of accumulation of U(S)11 an d U(L)38 proteins. These results indicate the following. (i) The posttransl ational processing of ICP22 is sensitive to mutations within the domains of ICP22 tested and is cell-type dependent. (ii) Posttranslational processing of ICP22 is not required for accumulation of U(L)38 and U(S)11 proteins to the same level as that seen in cells infected with the wild-type virus. (i ii) The T374SS sequence shared by ICP22 and the U(S)1.5 proteins is essenti al for the accumulation of a subset of gamma (2) proteins exemplified by U( S)11 and U(L)38 and is the first step in mapping of the sequences necessary for optimal accumulation of U(S)11 and U(L)38 proteins.