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Sla1, a Schizosaccharomyces pombe Homolog of the Human La Protein, Induces Ectopic Meiosis when Its C Terminus Is Truncated

Department of Life Science and Biotechnology, Faculty of Life and Environmental Science, Shimane University, Matsue, Shimane 690-8504, Japan

Received 25 June 2003/ Accepted 18 August 2003

Sla1 is a Schizosaccharomyces pombe homolog of the human La protein. La proteins are known to be RNA-binding proteins that bear conserved RNA recognition motifs (La and RRMs), but their biological functions still have not been fully resolved. In this study, we show that the S. pombe La homolog (Sla1) is involved in regulating sexual development. Sla1 truncated in the C terminus (Sla1C) induced ectopic sporulation in the ras1 strain and several other sporulation-deficient mutants. The C terminus contains a nuclear localization signal. While full-length Sla1 localizes in the nucleus, Sla1C is found throughout the cell, suggesting the cytoplasmic localization of Sla1C is involved in its sporulation-inducing activity. Further deletion analysis of Sla1 indicated that a small region (35 amino acids) that includes a portion of RRM2 is sufficient to induce sporulation. The La motif (RRM1) is not involved in this activity. Strikingly, Sla1C induced haploid meiosis in a heterothallic strain, similar to the pat1-114 or mei2-SATA mutation. Sla1C induced sporulation in a mei3 disruptant but not in a mei2 disruptant, indicating that Sla1C requires Mei2 to induce haploid meiosis. Deletion of the chromosomal sla1 gene lowered the temperature sensitivity of the pat1-114 mutant. Two-hybrid analysis indicated that Pat1 interacts with Sla1C but not full-length Sla1. Thus, Sla1C may block Pat1 activity. This block would remove the inhibition on Mei2, which would then drive the cell into haploid meiosis. Finally, Sla1 was degraded prior to the start of meiosis when we monitored Sla1 in cells in which meiosis was synchronously induced. The ability of truncated Sla1 to induce ectopic meiosis represents a very novel function that has hitherto not been suspected for the La family of proteins.

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