Cytoplasmic expression of the incompletely spliced RNA transcripts that encode the late, structural proteins of human immunodeficiency virus type 1 (HIV-1) is dependent on the viral Rev regulatory protein. General agreement exists that Rev acts, at least in part, by recruiting the cellular Crm1 nuclear export factor to HIV-1 transcripts bearing the Rev response element RNA target, and thereby inducing their nuclear egress. However, several groups have argued that Crm1 recruitment may not be sufficient for Rev function. Thus, several additional candidate cofactors for Rev have been proposed, and Rev has also been suggested to also inhibit the nuclear splicing of HIV-1 transcripts and/or to directly enhance their cytoplasmic translation. To examine whether Crm1 recruitment is, instead, sufficient to activate the nuclear export of viral mRNAs, we targeted a leucine-rich Crm1 binding domain, derived from a heterologous protein that normally plays no role in RNA metabolism, to HIV-1 RNAs and showed that this tethered Crm1 binding domain is sufficient to induce the nuclear export and cytoplasmic translation of late HIV-1 mRNA species. More importantly, we show that direct tethering of the Crm1 nuclear export factor to target mRNAs, by fusion to a heterologous RNA binding domain, is in and of itself sufficient to induce the nuclear export and cytoplasmic expression of the unspliced HIV-1 mRNAs that encode the viral Gag proteins.Retroviral replication requires the cytoplasmic expression of both fully spliced and incompletely spliced forms of the initial, genome-length viral RNA transcript (reviewed in references 7 and 37). However, cells have evolved mechanisms to prevent the nuclear export of incompletely spliced cellular mRNAs, i.e., pre-mRNAs (4, 23). Retroviruses have therefore had to develop mechanisms that allow intron-containing viral transcripts to exit the nucleus in the face of this cellular proofreading mechanism. In several simple retroviruses, nuclear export of the genome-length transcript is mediated by cellular factors that are recruited to a cis-acting RNA target termed a constitutive transport element (CTE) (3,36,49). The cellular protein specific for the CTE found in simian type D viruses has been identified as Tap, a nuclear export factor that also plays a key role in mediating cellular mRNA export (7,15,19,43).In contrast to simple retroviruses, complex retroviruses encode a regulatory protein that is critical for the cytoplasmic expression of their incompletely spliced transcripts. In human immunodeficiency virus type 1 (HIV-1), this protein is termed Rev. Rev interacts with a cis-acting viral RNA target site, the Rev response element (RRE), and with Crm1, a host cell protein that is a member of the karyopherin or importin/exportin family of nucleocytoplasmic transport factors (12,14,29,31,35,42,50). Crm1 binds specifically to a short leucinerich motif found in the Rev protein that also functions as a nuclear export signal (NES) (2,11,30,48). NES binding by Crm1 requires a cellular cofactor, the GTP...