Highly divergent lentiviral Tat proteins activate viral gene expression bya common mechanism

The human immunodeficiency virus type 1 (HIV-1) Tat protein (hTat) activate s transcription initiated at the viral long terminal repeat (LTR) promoter by a unique mechanism requiring recruitment of the human cyclin T1 (hCycT1) cofactor to the viral TAR RNA target element. While activation of equine i nfectious anemia virus (EIAV) gene expression by the EIAV Tat (eTat) protei n appears similar in that the target element is a promoter proximal RNA, eT at shows little sequence homology to hTat, does not activate the HIV-1 LTR, and is not active in human cells that effectively support hTat function. T o address whether eTat and hTat utilize similar or distinct mechanisms of a ction, we have cloned the equine homolog of hCycT1 (eCycT1) and examined wh ether it is required to mediate eTat function. Here, we report that express ion of eCycT1 in human cells fully rescues eTat function and that eCycT1 an d eTat form a protein complex that specifically binds to the EIAV, but not the HIV-1, TAR element. While hCycT1 is also shown to interact with eTat, t he lack of eTat function in human cells is explained by the failure of the resultant protein complex to bind to EIAV TAR. Critical sequences in eCycT1 required to support eTat function are located very close to the amino term inus, i.e., distal to the HIV-1 Tat-TAR interaction motif previously identi fied in the hCycT1 protein. Together, these data provide a molecular explan ation for the species tropism displayed by eTat and demonstrate that highly divergent lentiviral Tat proteins activate transcription from their cognat e LTR promoters by essentially identical mechanisms.