Identification of a novel cellular cofactor for the Rev/Rex class of retroviral regulatory proteins

HIV-1 Rev is the prototype of a class of retroviral regulatory proteins that induce the sequence-specific nuclear export of target RNAs. This function requires the Rev activation domain, which is believed to bind an essential cellular cofactor. We report the identification of a novel human gene product that binds to not only the HIV-1 Rev activation domain in vitro and in vivo but also to functionally equivalent domains in other Rev and Rex proteins. The Rev/Rex activation domain-binding (Rab) protein occupies a binding site on HIV-1 Rev that precisely matches that predicted by genetic analysis. Rab binds the Rev activation domain when Rev is assembled onto its RNA target and can significantly enhance Rev activity when overexpressed. We conclude that Rab is the predicted activation domain-specific cofactor for the Rev/Rex class of RNA export factors.