Viral and host factors involved in HIV-1 post-transcriptional control

The full-length 9-kb unspliced gRNA is used as the mRNA template to synthesize the major structural viral protein Gag and Pol, which contains the viral enzymes. This viral mRNA avoids splicing through a not well understood mechanism that is probably influenced by weak splice sites and assisted by the viral protein Rev.
gRNA expression
Despite the gRNA is expected to recruit co-transcriptionally the nuclear cap-binding complex (CBC, CBP20/CBP80) and other mRNP components, it may lack most of the classical cellular mRNP components such as the EJC and TAP/NXF1, which are recruited during the processing steps. As a consequence, the HIV-1 gRNA do not exits the nucleus by the canonical mRNA export pathway neither can benefit from the stimulating effects on nuclear export and translation provided by splicing through the recruitment of the EJC. Instead, the viral protein Rev binds to the RRE (Rev Responsive Element, an RNA motif present at the 3´ end of the gRNA and all partially spliced transcripts) and to CRM1 a karyopherin that shuttles between the nucleus and the cytoplasm and thus, exports the Rev/RRE complex as a cargo in a Ran-GTP-dependent manner.
Several cellular proteins including hRIP, eIF5A, Sam68 and the DEAD-box RNA helicases DDX1 and DDX3 have been shown to assist the Rev-mediated nuclear export yet, the mechanisms at play are poorly understood. One of the identified Rev partners is the methyltransferase, PIMT, which trimethylates the cap structure of the gRNA in a Rev-dependent manner. Interestingly, while the affinity of eIF4E and the CBC for a trimethylated cap is largely reduced resulting in the inhibition of translation, cap trimethylation was shown to increase Gag expression from the genomic RNA. Another Rev associated factor involved in gRNA translation is DDX3, which plays critical roles during the very early steps leading to ribosome recruitment and polysome association (see above).

As has been shown for cellular mRNAs, nuclear events are pivotal for the cytoplasmic destiny of the gRNA. Indeed, the viral protein Rev has been involved in cytoplasmic processes of the gRNA such as translation and packaging. Thus, we are investigating how nuclear host factors together with viral protein Rev are coupling nuclear export with translation initiation in order to gain insights into the molecular and cellular processes that regulate expression of this viral transcript, which is associated with non-canonical mRNP components.