Isolation and molecular characterization of mRNA transport mutants in Schizosaccharomyces pombe

Nucleocytoplasmic transport of mRNA is essential for eukaryotic gene expression. However, how mRNA is exported from the nucleus is mostly unknown. To elucidate the mechanisms of mRNA transport, we took a genetic approach to identify genes, the products of which play a role in that process. From about 1000 temperature-sensitive (ts(-) or cs(-)) mutants, we identified five ts(-) mutants that are defective in poly(A)(+) RNA transport by using in situ hybridization with an oligo(dTT)(50) as a probe. These mutants accumulate poly(A)(+) RNA in the nuclei when shifted to a nonpermissive temperature. All five mutations are tightly linked to the ts growth defects, are recessive, and fall into four different groups designated as ptr 1-4 (poly(A)(+) RNA transport). Interestingly, each group of mutants has a differential localization pattern of poly(A)(+) RNA in the nuclei at the nonpermissive temperature, suggesting that they have defects at different steps of the mRNA transport pathway. Localization of a nucleoplasmin-green fluorescent protein fusion suggests that ptr2 and ptr3 have defects also in nuclear protein import. Among the isolated mutants, only ptr2 showed a defect in pre-mRNA splicing. We cloned the ptr2(+) and ptr3(+) genes and found that they encode Schizosaccharomyces pombe homologues of the mammalian RCC1, a guanine nucleotide exchange factor for Ran/TC4, and the ubiquitin-activating enzyme E1 involved in ubiquitin conjugation, respectively. The ptr3(+) gene is essential for cell viability, and Ptr3p tagged with green fluorescent protein was localized in both the nucleus and the cytoplasm. This is the first report suggesting that the ubiquitin system plays a role in mRNA export.