Nuclear export of mRNA is a fundamentally important process that is conserved in evolution. Mature mRNA is exported through the nuclear pores bound to proteins as an RNP-particle. In the cytoplasm, the RNP-complex is disassembled, the mRNA is released for translation and the proteins involved in the export of mRNA are returned to the nucleus utilizing import pathways so that they can participate in another round of mRNA export. We have used fission yeast Schizosaccharomyces pombe (S. pombe) as a model system to study the mechanism of how eukaryotic cells export their mRNA out of the nucleus. A variety of experiments suggest that both yeast Mex67/metazoan Tap and Rae1 are involved in the export of mRNA at steps after the maturation of mRNA, including those that link the mRNA-protein complex to the nuclear pore. The identity of many of the soluble factors, including some protein-protein interactions of the Mex67 mediated mRNA export pathway are known. Our studies are designed to identify protein-protein interactions of the Rae1p mediated mRNA export pathway. We have found that the S. pombe homologs of S. cerevisiae Yra1p, Sub2p, Sac3p that function with Mex67/Mtr2 proteins in S. cerevisiae are components of the Rae1p-mediated mRNA export pathway in S. pombe. In S. pombe they are known as Mlo3p, Uap56p and Sac3p respectively. Additionally, Dss1p, the S. pombe homolog of human breast cancer tumor suppressor (BRAC2) cofactor DSS1, is a novel mRNA export factor that functions with Rae1p. By using purified proteins we found that Dss1p, Mlo3p, Uap56p and Rae1p interact in vitro. Importantly, Dss1p and Uap56p appear to act as linkers between Mlo3p and Rae1p. Tap-tagged Dss1 could immunoprecipitate the interacting proteins in vivo confirming the in vitro data. Based on these experiments we have developed a mechanistic model for the essential Rae1p-mediated mRNA export route in S. pombe. In essence, in this model Dss1p and Uap56p could link mRNA to the nuclear pores by bridging Mlo3p to Rae1p.
| Thakurta, Anjan G; Gopal, Ganesh; Yoon, Jin Ho et al. (2004) Conserved nuclear export sequences in Schizosaccharomyces pombe Mex67 and human TAP function in mRNA export by direct nuclear pore interactions. J Biol Chem 279:17434-42 |
