Mutational definition of functional domains within the Rev homolog encodedby human endogenous retrovirus K

Nuclear export of the incompletely spliced mRNAs encoded by several complex retroviruses, including human immunodeficiency virus type 1 (HIV-1), is de pendent on a virally encoded adapter protein, termed Rev in HIV-1, that dir ectly binds both to a cis-acting viral RNA target site and to the cellular Crm1 export factor. Human endogenous retrovirus K, a family of ancient endo genous retroviruses that is not related to the exogenous retrovirus HIV-1, was recently shown to also encode a Crm1-dependent nuclear RNA export facto r, termed K-Rev. Although HIV-1 Rev and K-Rev display little sequence ident ity, they share the ability not only to bind to Crm1 and to RNA but also to form homomultimers and shuttle between nucleus and cytoplasm, We have used mutational analysis to identify sequences in the 105-amino-acid K-Rev prot ein required for each of these distinct biological activities. While mutati ons in K-Rev that inactivate any one of these properties also blocked K-Rev -dependent nuclear RNA export, several K-Rev mutants were comparable to wil d type when assayed for any of these individual activities yet nevertheless defective for RNA export. Although several nonfunctional K-Rev mutants act ed as dominant negative inhibitors of K-Rev-, but not HIV-1 Rev-, dependent RNA export, these were not defined by their inability to bind to Crm1, as is seen with HIV-1 Rev. In total, this analysis suggests a functional archi tecture for K-Rev that is similar to, but distinct from, that described for HIV-1 Rev and raises the possibility that viral RNA export mediated by the similar to 25 million-year-old K-Rev protein may require an additional cel lular cofactor that is not required for HIV-1 Rev function.