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. Our studies suggest that several redundant pathways can export mRNA utilizing different complexes composed of both common and unique factors. Rae1p and Mex67p are two critical export factors that share several common interactions with other export factors but also share distinct interactions. Our studies have also identified a novel mRNA export carrier, Crp79p in S. pombe. Crp79p contains three RNA binding domains and two nuclear export activities: a leucine rich HIV-Rev-like NES which is not essential for its function, and a novel nuclear export activity at the C-terminus which is essential for its mRNA export function. The C-terminal export activity can also function in directing nuclear export of a heterologous protein in HeLa cells, suggesting a similar export pathway may exist in human cells. Additionally, we have identified and characterized Elf1p, a novel mRNA export factor in S. pombe. It is a member of ABC class of ATPases. elf1 mutations are genetically linked to rae1 and mex67 mutations in mRNA export. Elf1p expressed and purified from Escherichia coli binds and hydrolyzes ATP. A mutation of a conserved glycine-to-aspartic acid within the second Walker A domain does not affect the protein's ability to bind ATP, but the mutated protein is unable to hydrolze ATP. This mutant protein is unable to participate in exporting mRNA. From these results we conclude that ATPase activity of Elf1p is essential for its mRNA export function in vivo. These results provide a connection between enzymatic reaction that consumes energy and the process ofmRNA 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 |
