Current antiretroviral therapy is instituted upon immunosuppression when HIV has already entered the brain. Further, this mode of treatment has no effect on the integrated virus or on the production of early viral gene transcripts such as Tat, Nef and Rev. Of these gene products, only Tat is released extracellularly and has a variety of effects on neurons and glial cells. To date, most studies have been performed by only 1 or 2 Tat proteins derived from HIV clade B, but there is significant divergence in the amino acid sequences between clade B and C derived Tat proteins, even in the so called conserved regions of the gene. Further, HIV clade C virus infects the largest populations in the world, and interestingly little or no dementia has been reported from the clade C infected countries. Although there may be multiple reasons for the differences, very little is known about the functional properties of Tat derived from clade C. Further, the role of the second exon of both clade B and C is poorly understood. Tat also has residues that undergo post-translational modification the functional significance of needs to be characterized. Because of the critical role of Tat in HIV replication and its effects on brain cells, there is need to establish therapeutic approaches that would target this protein or host factors that are critical in mediating Tat effects. We thus propose to 1) determine HIV clade specific differences in Tat effects on brain cells. Tat will be sequenced from brain and spleen from well-characterized patients with HIV clade B and C infections. Recombinant Tat proteins and Tat expressing cell lines will be compared for their functional differences. 2) determine physical properties of Tat necessary for mediating Tat effects on brain cells. The amino acids involved in posttranslational modification will be mutated to determine their role in Tat function. 3) identify novel therapeutic approaches for targeting Tat mediated effects on brain cells. Drugs approved for human use targeted against host proteins that interact with Tat will be screened to determine their therapeutic potential against clade B and C derived Tat proteins.