Objectives:To investigate some of the cellular consequences of HIV-1 Tat expression in human astrocytic cells. This study is based on evidence that cellular factors play a critical role in facilitating transcriptional activation by Tat through its interaction with thetrans-acting-response (TAR) RNA element and upstream HIV-1 long terminal repeat (LTR) promoter binding site.Study Design-Methods:Using the previously established astrocytic cell line of human origin stably transfected with Tat cDNA, we analyzed the formation of a nucleoprotein complex consisting of three cellular proteins associated with TAR RNA using ultraviolet (UV) crosslinking and glutathione-S-transferase (GST) pull-down assays.Results:UV crosslinking experiments reveal that the molecular masses of the proteins range from 50 to 62 kd. Transient transfection studies demonstrate that the presence of these proteins correlates with the ability of Tat to transactivate the HIV-1 LTR in the absence of the trinucleotide bulge, a region within TAR that has been shown to be important for Tat-TAR interaction. A combination of GST pull-down assays and RNA binding studies demonstrates that the 50-kd protein interacts with both Tat and TAR and is likely to be NF-&kgr;B p50.Conclusions:Taken together, these data suggest that in the absence of a functional Tat binding site such as TAR (which tethers the viral protein to the RNA), cellular protein NF-&kgr;B p50 may be able to bring Tat into the RNA binding complex. Tat has been shown to activate expression of a variety of cellular genes that may not contain a binding site for Tat but do contain binding sites for NF-&kgr;B family members. The results presented in this study may be relevant for Tat-mediated transactivation of cellular as well as viral genes, both of which might contribute to the central nervous system damage associated with HIV-1 infection.Journal of Human Virology 1999;2:72-80 © Lippincott Williams & Wilkins, Inc.