Cytomegaloviral protein kinase pUL97 interacts with the nuclear mRNA export factor pUL69 to modulate its intranuclear localization and activity

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Abstract
Human cytomegalovirus encodes a number of phosphorylation-regulated proteins, including the autophosphorylating protein kinase pUL97 and the nuclear mRNA export factor pUL69. Recently, it was reported that the kinase inhibitor roscovitine induces an intranuclear aggregation of pUL69 in infected fibroblasts. Here, we demonstrate that pUL97-specific kinase inhibitors induce a similar pUL69 aggregation. Furthermore, a direct pUL69–pUL97 interaction was demonstrated by coimmunoprecipitation analyses. Deletion mapping identified the domains required for interaction in both proteins (1–140/478–532 in pUL69 and 231–336 in pUL97). Further analysis of the immunoprecipitates by in vitro kinase assays demonstrated the phosphorylation of pUL69 by pUL97. However, catalytically inactive mutants of pUL97 and interaction-negative fragments of pUL69 were phosphorylation-negative. Moreover, an analysis of the pUL69-mediated nuclear RNA export indicated a correlation of the export efficiency with the presence of active pUL97 kinase. These data suggest a specific pUL69–pUL97 interaction and pUL97-mediated phosphorylation which influences the regulatory activities of pUL69.