LOCALIZATION OF THE NOVEL RNA BINDING PROTEIN SLA1 AND ITS ROLE IN CELL DIFFERENTIATION IN FISSION YEAST

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Sla1, a homolog of human La protein in fission yeast, has been known to be an RNA binding protein that has an RRM (RNA-Recognition Motif) and the La motif which are highly conserved among La protein families. In this study, we present the evidence, for the first time among the La homologs, that a truncated form of Sla1 is important for regulation of sexual development in Schizosaccharomyces pombe. We found that Sla1 truncated in the COOH-terminus (Sla1DC) induced ectopic sporulation in ras1 strain and other several sporulation deficient strains such as disruptants of the components of the MAPK-cascade, the SAPK-cascade and the cAMP-cascade. The full-length of Sla1 located in nucleus, but sporulation inducible Sla1DC located in cytoplasm. Consistent with this result, Sla1 has a basic NLS (Nuclear Localization Signal)-like motif in its COOH terminus. Further deletion analysis of Sla1 suggested that for sporulation inducible effect a small region that includes only a portion of RRM, but neither a whole RRM nor the La motif was required. In addition, Sla1DC induced ectopic haploid meiosis, like pat1-114 or mei2-SATA mutant do. Sla1DC induced sporulation in a mei3 disruptant but not in a mei2 disruptant. These results strongly suggested that Sla1DC acts at the upstream of Mei2 and at the downstream of Mei3.