LENTIVIRUS TAT PROTEINS SPECIFICALLY ASSOCIATE WITH A CELLULAR PROTEIN-KINASE, TAK, THAT HYPERPHOSPHORYLATES THE CARBOXYL-TERMINAL DOMAIN OF THE LARGE SUBUNIT OF RNA-POLYMERASE .2. CANDIDATE FOR A TAT COFACTOR

Efficient replication of human immunodeficiency virus types 1 and 2 (H IV-1 and HIV-2) requires the virus transactivator proteins known as Ta t. In order to understand the molecular mechanisms involved in Tat tra nsactivation, it is essential to identify the cellular target(s) of th e Tat activation domain. Using an in vitro kinase assay, we previously identified a cellular protein kinase activity, Tat-associated kinase (TAK), that specifically binds to the activation domains of Tat protei ns. Here it is demonstrated that TAK fulfills the genetic criteria est ablished for a Tat cofactor. TAK binds in vitro to the activation doma ins of the Tat proteins of HIV-1 and HIV-2 and the distantly related l entivirus equine infectious anemia virus but not to mutant Tat protein s that contain nonfunctional activation domains. In addition, it is sh own that TAK is sensitive to dichloro-1-beta-D-ribofursnosylbenzimidaz ol a nucleoside analog that inhibits a limited number of kinases and i s known to inhibit Tat transactivation in vivo and in vitro. We have f urther identified an in vitro substrate of TAK, the carboxyl-terminal domain of the large subunit of RNA polymerase II. Phosphorylation of t he carboxyl-terminal domain has been proposed to trigger the transitio n from initiation to active elongation and also to influence later sta ges during elongation. Taken together, these results imply that TAK is a very promising candidate for a cellular factor that mediates Tat tr ansactivation.