Interactions between HIV Rev and nuclear import and export factors: The Rev nuclear localisation signal mediates specific binding to human importin-β

The human immunodeficiency virus type 1 (HIV-1) Rev protein binds to unspliced HIV-1 pre-mRNA and exports it from the nucleus. Rev itself can 'shuttle' between the nucleus and cytoplasm. This bi-directional transport is mediated by two specific Rev sequences: a nuclear localisation signal (NLS), which overlaps the RNA-binding domain, and a distinct nuclear export signal (NES). In this study we characterised new monoclonal antibodies that bind different epitopes of Rev, including the import and export sequences. In RNA bandshift assays, we observed that formation of a multimeric complex between Rev and its target RNA completely masks the Rev NLS, whereas the NES remains readily accessible. We then tested for signal-mediated interactions between Rev and different nuclear transport receptors, using mutations in the Rev NES or NLS to control for specificity. Extensive biochemical analyses did not reveal any direct NES-dependent interaction between Rev (free or RNA-bound) and the previously proposed export co-factors, human RIP/Rab and eIF-5A. By contrast, similar tests showed that Rev binds directly via its arginine-rich NLS to the human nuclear import receptor, importin-β. This interaction was highly specific and was abolished by mutation in the Rev NLS. Importin-β did not bind to the RNA-bound form of Rev, providing a mechanism to ensure that Rev is imported only following release of its RNA cargo. Unlike many NLS-containing proteins that bind stably to an importin-α/β heterodimer, the binding of Rev to importin-β was actually blocked by importin-α receptor. Our findings suggest that Rev and importin-α bind (via an arginine-rich sequence) to a similar region on importin-β. In addition, we show that the complex between Rev and importin-β can be dissociated by the nuclear Ran GTPase, but only when Ran is in the GTP-bound form. The series of interactions we describe provide a novel pathway for the import of Rev across the nuclear pore complex, and a mechanism for its release into the nucleoplasm.