CA150, A NUCLEAR-PROTEIN ASSOCIATED WITH THE RNA-POLYMERASE-II HOLOENZYME, IS INVOLVED IN TAT-ACTIVATED HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1TRANSCRIPTION

Maximal human immunodeficiency virus type 1 (HIV-1) gene expression re quires specific cellular factors in addition to the virus-encoded tran s-activator protein Tat and the RNA element TAR. We developed a functi onal assay, based on transcriptional activation in vitro, to identify these cellular factors. Here, we describe the purification and molecul ar cloning of CA150, a nuclear protein that is associated with the hum an RNA polymerase II holoenzyme and is involved in Tat-dependent HIV-1 transcriptional activation. The sequence of CA150 contains an extensi ve glutamine-and alanine-rich repeat that is found in transcriptional modulators such as GAL11 and SSN6 in Saccharomyces cerevisiae and Zest e in Drosophila melanogaster. Immunodepletion of CA150 abolished Tat t rans activation in vitro. Moreover, overexpression of a mutant CA150 p rotein specifically and dramatically decreased Tat-mediated activation of the HIV-1 promoter in vivo, strongly suggesting a role for CA150 i n HIV-1 gene regulation. Immunoprecipitation experiments demonstrated that both CA150 and Tat associate with the RNA polymerase II holoenzym e. Furthermore, we found that functional Tat associates with the holoe nzyme whereas activation-deficient Tat mutants do not. Thus, we propos e that Tat action is transduced,ia an RNA polymerase II holoenzyme tha t contains CA150.