Function of exon 2 in optimal trans-activation by Tat of HIV type 2.

HIV-1 and HIV-2 are human retroviruses whose life cycles require viral regulatory proteins, one of which is the trans-activator, Tat. Tat of HIV-1 (Tat-1) displays modular function with independent activation function localized to the amino-terminal, cysteine-rich, and core regions and independent RNA-binding function localized to a basic region. These functional domains are contained in the first of two exons encoding Tat-1; deletion of exon 2 does not contribute to functional domains of Tat-1. Tat of HIV-2 (Tat-2) has structurally analogous regions, but the amino terminus, basic region, and carboxy terminus encoded by exon 2 display amino acid sequence and functional divergence compared to Tat-1. We have shown that, in contrast to Tat-1, exon 2 of Tat-2 (residues 100 to 130) is required for optimal trans-activation of HIV-1 and HIV-2 long terminal repeats (LTRs). Here we demonstrate that a series of basic residues in exon 2 are required for these effects. Exon 2 does not alter the level of protein expression of Tat-2. Further, in the context of heterologous DNA binding, exon 2 does not contribute to activation function. These data suggest that full-length Tat-2 results in optimal trans-activation through enhanced RNA-binding function of exon 1 by involvement of a basic region in exon 2. Differential expression of short and full-length Tats during different stages of the HIV-2 life cycle might regulate levels of viral expression, viral replication, and resultant cytopathology.