Nuclear pores channel the transport of proteins and mRNA in a bi-directional fashion between the nucleus and cytoplasm. The fission yeast Schizosaccharomyces pombe has been utilized as a model system to study nucleocytoplasmic trafficking. A temperature (ts) sensitive mutant rae1-1 (ribonucleic acid export) is defective in the nuclear export of polyA+ RNA when grown at restrictive temperature and cells arrest at the G2/M boundary. In evolution, the rae1 gene is highly conserved both in structure and function. The human rae1 gene functionally complements the S. pombe rae1-1 mutation for growth and mRNA export. We have shown that rae1 function is required for a process which is necessary for mitosis other than mRNA export. When rae1 function is inactivated, either through depletion or through inactivation of rae1-1 mutant protein, cells arrest after the activation of cdc2 kinase without the formation of mitotic spindle and without separation of the spindle pole bodies. The isolation of an extragenic suppressor of rae1-1 mutation in rad24 checkpoint further supports the hypothesis for cell cycle role for rae1. Several possibilities exist as to how rae1 protein may be required for cell cycle progression: 1) mislocalization of the mitotic activators, e.g., cdc2-cyclinB complex or the substrates for cdc2 kinase 2) mislocalization of proteins required for spindle pole body separation. Mutations in several nucleoporins in budding yeast, S. cerevisiae, have revealed their role in protein import, mRNA export and for maintaining the integrity of the nuclear envelope. The localization of the rae1 protein to the nuclear periphery is consistent with its role in transport. We have demonstrated that it functionally interacts with a nuclear pore protein, Nic96. Similar to known nuclear pore and the rae1 proteins, a GFP-Nic96 fusion protein revealed its localization to the nuclear periphery as punctate structures. The N-terminal coil-coil domain of Nic96 is sufficient for its localization to the nuclear pores and for its function. These and other observations have led us to suggest that rae1 protein functions in the nucleocytoplasmic transport of proteins required for cell cycle progression and for mRNA export.
| 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 |
