The Rex protein of human T-cell leukemia virus type 1, like the functionally equivalent Rev protein of human immunodeficiency virus type 1, contains a leucine-rich activation domain that specifically interacts with the human nucleoporin-like Rab/hRIP cofactor. Here, this Rex sequence is shown to function also as a protein nuclear export signal (NES). Rex sequence libraries containing randomized forms of the activation domain/ NES were screened for retention of the ability to bind Rab/hRIP by using the yeast two-hybrid assay. While the selected sequences differed widely in primary sequence, all were functional as Rex activation domains. In contrast, randomized sequences that failed to bind Rab/hRIP lacked Rex activity. The selected sequences included one with homology to the Rev activation domain/NES and a second that was similar to the NES found in the cellular protein kinase inhibitor ␣. A highly variant, yet fully active, activation domain sequence selected on the basis of Rab/hRIP binding retained full NES function even though this sequence preserved only a single leucine residue. In contrast, nonfunctional activation domain mutants that were unable to bind Rab/hRIP had also lost NES function. These data demonstrate that NES activity is a defining characteristic of the activation domains found in the Rev/Rex class of retroviral regulatory proteins and strongly support the hypothesis that the Rab/hRIP cofactor plays a critical role in mediating the biological activity of these NESs. In addition, these data suggest a consensus sequence for NESs of the Rev/Rex class.The pathogenic complex retroviruses human T-cell leukemia virus type 1 (HTLV-1) and human immunodeficiency virus type 1 (HIV-1) belong to distinct retroviral families and display little sequence homology. The approaches used by these viruses to regulate proviral gene expression are nevertheless remarkably similar (reviewed in reference 5). Thus, each virus encodes a transcriptional regulatory protein that acts on the respective viral long terminal repeat promoter element to dramatically enhance viral gene expression. In addition, each virus also encodes an essential posttranscriptional regulatory protein, termed Rex in HTLV-1 and Rev in HIV-1, that induces the sequence-specific nuclear export, and hence translation, of the incompletely spliced viral mRNA species that encode the various viral structural proteins (8,11,16,17,20,23,27).Initial evidence favoring the hypothesis that Rex and Rev, despite lacking any significant sequence identity, might nevertheless mediate the nuclear export of target RNAs via the same mechanism came from the finding that Rex could partly rescue the replication of a Rev-deficient HIV-1 provirus (36). Subsequently, both Rev and Rex were shown to bind directly to structured RNA response elements present in their target RNAs (3,6,14,19,29,38,41) and to contain multimerization domains that are essential for the recruitment of additional Rev or Rex molecules (Fig. 1) (2, 26, 34). In addition, both Rev and Rex contain short, leu...

SummaryThe nefgene product encoded by the mac239 proviral done of simian immunodeficiency virus (SIV) markedly enhances viral replication and pathogenesis in vivo. We have used this biologicaUy active nefisolate to examine the phenotype of Nef in retrovirally transduced human T cells in culture. SIV Nefis shown to dramatically inhibit cell-surface expression of the CD4 glycoprorein without significantly affecting the total steady-state level of cellular CD4. This downregulation of the cell-surface CD4 receptor for human immunodeficiency virus type 1 (HIV-1) infection correlated with the acquisition of resistance to superinfection by HIV-1. However, SIV Nef did not affect the level of gene expression directed by the HIV-1 long terminal repeat. It is hypothesized that downregulation of cell-surface CD4 by Nef facilitates the efficient release of infectious progeny virions and, hence, viral spread in vivo.