Our study of HIV-1-host interaction has generated new and fundamental insight into the translational control of HIV-1 mRNAs and the HIV-1 infected cell. Using multiple approaches, we demonstrated that the host cell translation is suppressed during acute HIV-1 infection, an effect associated with reduced levels of phosphorylated eIF4E and 4E-BP1. The changes in protein modification and translation are in response to the cell cycle arrest induced by the viral accessory protein Vpr. Exogenous expression of Vpr is sufficient for global translation suppression and mutations that ablate Vpr-induced cell cycle arrest eliminate this effect. Despite the reduction in total protein synthesis, HIV-1 sustains translation of its structural and enzymatic proteins. We hypothesize that this effect is attributable to the unique composition of the viral RNP, in particular the retention of its interaction with the nuclear cap binding complex (CBC) throughout its lifetime in the cytoplasm. These findings outline a novel viral strategy to circumvent downregulation of eIF4E-dependent translation, a mechanism that might also be exploited by selected cellular mRNAs during conditions of cellular stress. The retention of CBC on the mRNA allows for continued viral mRNA translation when global translation is reduced by limited eIF4E function, such as during cellular stress response, induction of apoptosis, and progression from the G2/M phase of the cell cycle 
. We hypothesize that CBC-bound HIV-1 mRNAs and selected host mRNAs represent a novel class of RNA regulon 
whose protein products need to be synthesized during conditions of limited eIF4E activity.
Previous studies have determined that CBC is required for the nuclear export of U snRNA to the cytoplasm 
and U snRNP assembly 
. The U snRNA is tethered to the CRM1 nuclear export receptor by the phosphorylated adapter for RNA export (PHAX) protein 
. In the case of incompletely spliced HIV-1 RNAs, the tethering function is provided by the virus-encoded adapter protein Rev. Rev interacts with the cis-acting Rev responsive element (RRE) in the env open reading frame and mediates the export of the unspliced viral RNAs to the cytoplasm, in a CRM1-RanGTP-dependent manner 
. In contrast, completely spliced viral mRNAs utilize NXF1/NXT1, the same adaptor proteins used by the spliced cellular mRNAs. Once in the cytoplasm, the RNP undergoes rearrangement, part of which involves the exchange of the CBC for cytoplasmic eIF4E. The CBC is then reimported into the nucleus by importin-α 
Biophysically, CBC and eIF4E employ two aromatic side chains to interact with the 7-methylguanine of the bound mRNA in an induced-fit structure 
. However, the CBC has significantly greater affinity for the 7-methylguanosine cap structure than eIF4E 
. Interaction with the translation initiation cofactor eIF4G is necessary to provide eIF4E with a competitive advantage for binding 7-methylguanosine cap structure 
. However, recent work by Yedavalli and Jeang determined that cytoplasmic unspliced HIV-1 RNA has a tri-methlyguanosine cap, making these RNAs structurally distinct from cellular mRNAs, which have a mono-methylguanosine cap 
. The difference in RNA cap structure is a function of the export pathway used since the same RNA sequence transported via NXF1/NXT1 lacked the tri-methlyguanosine modification. We hypothesize that the distinct modifications of the 5′-cap generates a unique molecular signature that dictates the fate of this mRNA template during G2/M phase, cellular stress that limits eIF4E activity, and/or stimulation of the innate immune response 
. Biophysical measurements have determined that the CBC has a ~100 fold higher affinity for tri-methylguanosine relative to mono-methylguanosine 
. Such differences in affinity may provide a mechanism for preferential retention of the CBC by HIV-1 gag mRNA allowing it to circumvent reduced eIF4E-dependent translation, sustain viral protein synthesis and remain bound during virion assembly. Selective translation of HIV-1 structural protein mRNAs may also be facilitated by a distinct set of interactions that mediate translation initiation of the CBC-bound mRNAs. A recent study determined that CBC-bound mRNAs had a selective advantage in engaging the translational apparatus 
perhaps as a result of using different factors (CTIF versus eIF4G) to support translation 
. The possibility that CBC retention is a result of the presence of an IRES within HIV-1 unspliced RNA was also considered. To address this issue, we evaluated HIV-1 Rev-dependent env RNA that lacks the previously mapped IRES 
. As shown in the supplementary material (Figures S2
), the env transcript that lacks the IRES maintained the preferential retention of CBC and the spliced product was associated with eIF4E.
The differences by which HIV-1 mRNAs engage the translational machinery may be important to allow HIV-1 to subvert NMD 
. Both unspliced and singly spliced HIV-1 RNAs have multiple termination codons throughout the sequence that would normally signal degradation by NMD. Other retroviruses, such as the Rous sarcoma virus, use an RNA stability element to antagonize recognition of premature termination codons 
. In a similar fashion, the distinct composition of the HIV-1 RNPs may serve to antagonize the induction of the NMD, by preventing recognition of termination codons present in the incompletely spliced and unspliced HIV-1 mRNAs.
Taken together, the data presented in this report indicate that the unspliced and singly spliced HIV-1 transcripts have a distinct pattern of regulation from the bulk of host mRNA in the cytoplasm, conferred, in part, by differences in the composition of the RNPs. Such differences permit HIV-1 to maximize viral protein synthesis and virion production under conditions that reduce cell survival. Our discovery also opens new avenues to investigate the mechanistic control of CBC/eIF4E cap exchange that is fundamental to translational regulation in all metazoan cells.