Nuclear pores mediate the transport of proteins and RNA between the nucleus and cytoplasm. Mature mRNA is exported through the nuclear pores bound to proteins as an RNP particle. It is thought one or more of the bound proteins act as a carrier to target the mRNA to the pore and mediate its interaction with various pore proteins. Once the mRNA reaches the cytoplasm, the carrier dissociates and reenters the nucleus, thereby shuttling between the nucleus and the cytoplasm. Genetic and biochemical studies in yeast, mammalian and viral systems have led to identification of several proteins that could be considered as potential carriers of mRNA. For a protein to be considered an export carrier of mRNA it must be functionally linked to mRNA export, bind mRNA, shuttle between the nucleus and cytoplasm and exit the nucleus in an NES dependent manner. We have used fission yeast Schizosaccharomyces pombe as a model system to study how eukaryotic cells export their mRNA out of the nucleus. Through genetic approaches we have identified genes whose products influence the export of mRNA from the nucleus into the cytoplasm. We identified mRNA export factor spmex67 through its genetic interaction with rae1. Both spMex67p and Rae1p genetically and functionally interact with nucleoporins, Npp106p and Nup184p to export mRNA out of the nucleus. Our results suggest a model in which both Rae1p and spMex67p can function in mRNA export by interacting with common mRNA export complexes. Furthermore, we demonstrate that spMex67p acts as an accessory factor in Rae1p-dependent mRNA export by participating at various common steps in mRNA export with Rae1p. In S. pombe we have identified an RNA binding protein, Crp79p that fullfills the requirements of an mRNA export carrier. It binds mRNA, complements the mRNA export defects associated with rae1-167 nup184-1 double mutant, shuttles between the nucleus and cytoplasm, and contains a nuclear export acitivity at the C-terminus that is essential for Crp79p to fullfill its role in mRNA export. The nuclear import of Crp79p requires the functioning of the Ran-GTPase switch system. We have found that in S. pombe, Crp79p is functionally redundant in mRNA export and that it functions in mRNA export idependent of the Rae1p mRNA export pathway.
NIH ExPorter
NIH ExPorter