Posttranscriptional control of gene expression

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Our research focuses on regulation of gene expression, in particular the mechanisms controlling cellular and viral mRNA expression. The use of retroviral model systems, pioneered by research on HIV-1, have led to major discoveries in the field of mRNA metabolism. These studies have resulted in the elucidation of the major transport pathways from the nucleus utilized by both mRNAs and proteins. Our work led to the discovery of mechanisms governing posttranscriptional regulation of two key retroviral systems: human immunodeficiency virus type 1 (HIV-1) and simian type D retrovirus (SRV). Whereas the posttranscriptional expression of HIV-1 is mediated through the essential viral Rev protein interacting with the cis-acting Rev-responsive element (RRE), the expression of SRV is mediated through a cellular factor interacting with the cis-acting constitutive transport element (CTE). The mechanisms promoting expression of viral mRNAs also involve cellular factors; therefore, their identification and characterization is important for our understanding of cellular gene expression. My lab investigates the molecular steps mediating cellular mRNA transport and expression of HIV-1 and SRV using a combination of biochemistry and tools of functional genomics and proteomics. The dissection of the mechanisms of posttranscriptional control and nucleocytoplasmic trafficking of macromolecules are relevant to understand the processes involved in carcinogenesis. In previous studies, we have characterized the CTE RNA transportelements of the several simian type D retroviruses and some intracisternal A particle (IAP) retroelements. These studies identified that the overall structure and the sequence of the two internal loops are essential for function. By in vitro RNA selection, we have recently identified a genomic element with the properties of a primordial CTE, which is able to mediate RNA export from Xenopus oocyte nucleus (Zolotukhin, A. S., et al. J Virol. 75:5567-5575, 2001). These findings constitute direct evidence of an evolutionary link between these elements and CTE. We had identified NXF1 (formerly named TAP) as essential export receptor for the CTE RNA and as key transporter of cellular mRNA. NXF1 is the first identified export receptor for cellular mRNAs, thereby providing a direct link between mRNA and the nuclear pore complex. We further found that the NXF1 ortholog in C. elegans is an essential mRNA export factor, demonstrating conservation of NXF1 function in metazoa (Tan, W. et al. RNA. 6:1762-1772, 2000). Recent data from my lab revealed that NXF1 binds directly to the splicing factor U2AF (Zolotukhin et al, submitted). This interaction provides a novel link between splicing and export of mRNA. In collaboration with George Pavlakis, we identified a novel potent RNA transport element RTE (Nappi, F., et al.. J Virol. 75:45584569, 2001). We found that RTE function is conserved in vertebrates. The use of RTE provides us with a powerful tool for further dissection of mRNA transport mechanisms.