Transport receptors mediating the nuclear export of intronless messages.

Résumé

The hepatitis B virus and the woodchuck hepatitis virus posttranscriptional regulatory elements (HPRE and WPRE, respectively) are cis-acting RNA sequences that allow the efficient expression of intronless viral messages. The exact mechanism by which PREs facilitate the cytoplasmic localization of unspliced RNAs is unknown. Recent data indicated that HPRE could affect many steps in mRNA processing, including inhibition of splicing and the enhancement of both polyadenylation and mRNA export. In addition, both can increase the amount of intronless cytoplasmic RNA of a normally intron-dependent &beta-globin cDNA, consistent with the idea that they functionally replace an intron during RNA processing. Although the function of the PREs is unclear, their ability to work in a Rev-dependent assay and their requirement for RNA cytoplasmic accumulation has been suggestive of a role in export. Mapping studies have demonstrated that HPRE and WPRE contain two homologous cis-acting sub-elements, designated PRE&alpha and PRE&beta. However, there are significant functional differences between HPRE and WPRE. WPRE is significantly more active than HPRE and also has the unique ability to stimulate the expression of heterologous cDNAs. The increased activity correlates with the presence of an additional sub-element, PRE&gamma, which is not found in HPRE. Thus, WPRE, compared to HPRE, has an additional posttranscriptional activity. The activity of the PREs is independent of any viral-encoded proteins, suggesting that they require trans-acting factors for function. Using a combination of CAT and HBV surface antigen assays, as well as Northern blot analysis, we demonstrate that the cellular export factor Tap mediates HPRE function. We also demonstrate that WPRE posttranscriptional activity has aspects that are both Tap-dependent and CRM1 dependent. Our results demonstrate that PREs function, at least in part, to mediate nuclear export of intronless mRNAs. Moreover, the results identify WPRE as the first example of an RNA element whose activity is mediated by several alternative pathways that are not mutually exclusive, but instead cooperative.