The export of mRNA from the nucleus to the cytoplasm is a multistep process that is mediated by mRNA-protein complexes (mRNP). The formation of the mRNP begins co-transcriptionally involving RNA and export factors. One such step is the recruitment of export factors on to elongating transcripts. A second major step involves targeting of the mRNP to the nuclear pore complex (NPC). Following targeting to the NPC the translocation of the mRNP occurs through the nuclear pore channel. This is thought to involve interactions between the mRNP and the hydrophobic phenylalanine gycine(-FG) patches lining the wall of the pore channel. Our studies where designed to define these different steps of mRNA export, and identify protein-protein interactions that are required for progression of the mRNP-complexes through various stages of the export process.The mammalian Uap56 (S. cerevisiae Sub2) and its functional homologs belong to the conserved DECD-class of ATP-dependent RNA helicases that function in splicing of mRNA and its export. We found that Uap56 is not essential for splicing but is essential for mRNA export in S. pombe. Moreover, the presumptive helicase function was not required for mRNA export. In contrast, mutations in its conserved RNA binding domain inhibited mRNA export. In addition, we found that Uap56 has both import (NLS) and export (NEA) activities thus, suggesting Uap56 shuttles between the nucleus and the cytoplasm. In addition, we found Uap56 interacts with Rae1p via its NEA and its interaction is essential for the nuclear export of Uap56. Finally we showed that Uap56 not Rae1 is recruited early during transcriptional elongation. Taken together our results suggest that: (1) Uap56 function both early as well in the late stages of mRNA export. (2) mRNP targeting complex formation involving Uap56-Rae1 takes place at a latter step ion mRNA export. (3) The need for RNA binding and functional NEA for Uap56 suggests that it likely functions as a carrier of mRNA from the nucle
| 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 |
