HIV-I TAT inhibits PKR activity by both RNA-dependent and RNA-independent mechanisms

Replication of the human immunodeficiency virus type 1 (HIV-1) is inhibited by interferons (IFNs), in part through activity of the IFN-inducible prote in kinase PKR, To escape this antiviral effect, HIV-1 has developed strateg ies for blocking PKR function. We have previously shown that the HIV-1 Tat protein can associate with PKR in vitro and in vivo and inhibit PKR activit y. Here we present evidence that Tat can inhibit PKR activity by both RNA-d ependent and RNA-independent mechanisms. Tat inhibited PKR activation by th e non-RNA activator heparin, and also suppressed PKR basal level autophosph orylation in the absence of RNA. However, when Tat and dsRNA were preincuba ted, the amount of Tat required to inhibit PKR activation by dsRNA depended on the dsRNA concentration. In addition to its function in vitro, Tat can also reverse translation inhibition mediated by PKR in COS cells. The Tat a mino acid sequence required for interaction with PKR was mapped to residues 40-58, overlapping the hydrophobic core and basic region of HIV-1 Tat. Ali gnment of amino acid sequences of Tat and eIF-2 alpha indicates similarity between the Tat-PKR binding region and the residues around the eIF-2 alpha phosphorylation site, suggesting that Tat and eIF-2 alpha may bind to the s ame site on PKR. (C) 2000 Academic Press.