Recruitment of the Crm1 nuclear export factor is sufficient to induce cytoplasmic expression of incompletely spliced human immunodeficiency virus mRNAs

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.