Selective translational repression of HIV-1 RNA by Sam68DeltaC occurs by altering PABP1 binding to unspliced viral RNA

HIV-1 structural proteins are translated from incompletely spliced 9 kb and 4 kb mRNAs, which are transported to the cytoplasm by Crm1. It has been assumed that once in the cytoplasm, translation of incompletely spliced HIV-1 mRNAs occurs in the same manner as host mRNAs. Previous analyses have demonstrated that Sam68 and a mutant thereof, Sam68 Delta C, have dramatic effects on HIV gene expression, strongly enhancing and inhibiting viral structural protein synthesis, respectively. While investigating the inhibition of incompletely spliced HIV-1 mRNAs by Sam68 Delta C, we determined that the effect was independent of the perinuclear bundling of the viral RNA. Inhibition was dependent upon the nuclear export pathway used, as translation of viral RNA exported via the Tap/CTE export pathway was not blocked by Sam68 Delta C. We demonstrate that inhibition of HIV expression by Sam68 Delta C is correlated with a loss of PABP1 binding with no attendant change in polyadenosine tail length of the affected RNAs. The capacity of Sam68 Delta C to selectively inhibit translation of HIV-1 RNAs exported by Crm1 suggests that it is able to recognize unique characteristics of these viral RNPs, a property that could lead to new therapeutic approaches to controlling HIV-1 replication.