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Schizosaccharomyces pombe Sst4p, a Conserved Vps27/Hrs Homolog, Functions Downstream of Phosphatidylinositol 3-Kinase Pik3p To Mediate Proper Spore Formation ^,

Masayuki Onishi,^1, Michihiro Iida,¹ Takako Koga,¹ Sadayuki Yamada,¹ Aiko Hirata,² Tomoko Iwaki,³ Kaoru Takegawa,³ Yasuhisa Fukui,^1* and Hiroyuki Tachikawa¹

Laboratory of Biological Chemistry, Graduate School of Agricultural and Life Science, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan,¹ Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8562, Japan,² Department of Life Sciences, Faculty of Agriculture, Kagawa University, Miki-cho, Kagawa 761-0795, Japan³

Received 18 June 2007/ Accepted 28 September 2007

Sporulation of the fission yeast Schizosaccharomyces pombe is a developmental process that generates gametes and that includes the formation of spore envelope precursors called the forespore membranes. Assembly and development of forespore membranes require vesicular trafficking from other intracellular membrane compartments. We have shown that phosphatidylinositol 3-kinase (PtdIns 3-kinase) is required for efficient and proper development of forespore membranes. The role of a FYVE domain protein, Sst4p, a homolog of Vps27p/Hrs, as a downstream factor for PtdIns 3-kinase in sporulation was investigated. sst4 asci formed spores with oval-shaped morphology and with reduced viability compared to that of the wild-type spores. The extension of forespore membranes was inefficient, and bubble-like structures emerged from the leading edges of the forespore membranes. Sst4p localization was examined using fluorescent protein fusions and was found to be adjacent to the forespore membranes during sporulation. The localization and function of Sst4p were dependent on its FYVE domain and on PtdIns 3-kinase. Sst4p colocalized and interacted with Hse1p, a homolog of Saccharomyces cerevisiae Hse1p and of mammalian STAM. Mutations in all three UIM domains of the Sst4p/Hse1p complex resulted in formation of spores with abnormal morphology. These results suggest that Sst4p is a downstream factor of PtdIns 3-kinase and functions in forespore membrane formation.

Published ahead of print on 19 October 2007.

Present address: Department of Genetics, Stanford University School of Medicine, Stanford, CA 94305-5120.