TheSchizosaccharomyces pombe spo20⁺Gene Encoding a Homologue ofSaccharomyces cerevisiaeSec14 Plays an Important Role in Forespore Membrane Formation

Abstract

The Schizosaccharomyces pombe spo20-KC104 mutation was originally isolated in a screen for sporulation-deficient mutants, and the spo20-KC104 mutant exhibits temperature-sensitive growth. Herein, we report that S. pombe, spo20(+) is essential for fission yeast cell viability and is constitutively expressed throughout the life cycle. We also demonstrate that the spo20(+) gene product is structurally homologous to Saccharomyces cerevisiae Sec14, the major phosphatidylinositol transfer protein of budding yeast. This structural homology translates to a significant degree of functional relatedness because reciprocal complementation experiments demonstrate that each protein is able to fulfill the essential function of the other. Moreover, biochemical experiments show that, like Sec14, Spo20 is a phosphatidylinositol/phosphatidylcholine-transfer protein. That Spo20 is required for Golgi secretory function in vegetative cells is indicated by our demonstration that the spo20-KC104 mutant accumulates aberrant Golgi cisternae at restrictive temperatures. However, a second phenotype observed in Spo20-deficient fission yeast is arrest of cell division before completion of cell separation. Consistent with a direct role for Spo20 in controlling cell septation in vegetatively growing cells, localization experiments reveal that Spo20 preferentially localizes to the cell poles and to sites of septation of fission yeast cells. We also report that, when fission yeasts are challenged with nitrogen starvation, Spo20 translocates to the nucleus. This nuclear localization persists during conjugation and meiosis. On completion of meiosis, Spo20 translocates to forespore membranes, and it is the assembly of forespore membranes that is abnormal in spo20-KC104 cells. In such mutants, a considerable fraction of forming prespores fail to encapsulate the haploid nucleus. Our results indicate that Spo20 regulates the formation of specialized membrane structures in addition to its recognized role in regulating Golgi secretory function.

Role of the α‐glucanase Agn2p in ascus‐wall endolysis following sporulation in fission yeast

“…pombe, during ascospore-wall formation, vacuoles become extremely enlarged (Kashiwazaki et al, 2005). Consistent with this hypothesis, the membrane-associated proteins Spo14p and Spo20p and t-SNARE protein Psy1p are transported away from the cell periphery during the sporulation process Nakase et al, 2001;Nakamura-Kubo et al, 2003). Furthermore, the vacuolar serine protease Ips6p, which is shown to be involved in large-scale degradation of proteins, is highly upregulated during sporulation.…”

Roles of Phosphoinositides and of Spo14p (phospholipase D)-generated Phosphatidic Acid during Yeast Sporulation

“…Although our study was in progress, it was found that the spo20 ϩ gene of the fission yeast Schizosaccharomyces pombe, which was identified originally on the basis of its requirement for prospore membrane formation (Hirata and Shimoda, 1992), encodes a homolog of S. cerevisiae Sec14p (Nakase et al, 2001). Unlike sec14-1 cells, which we found were unable to carry out meiosis, the spo20 mutant S. pombe cells progressed through meiosis.…”

“…This difference is probably attributable to a partial retention of function by the spo20 allele used, based on its other phenotypes. In addition, the PtdInsbinding-deficient Sec14(K66A K239A)p, when expressed from a strong inducible promoter on a high-copy plasmid, fully compensated for the loss of Spo20p function during S. pombe sporulation (Nakase et al, 2001). However, these investigators did not examine lower levels of expression.…”

Live Observation of Forespore Membrane Formation in Fission Yeast

“…To address the molecular mechanism of sporulation, we have identified and analyzed a number of sporulation-specific genes (Bresch et al, 1968;Ikemoto et al, 2000;Asakawa et al, 2001;Nakamura et al, 2001Nakamura et al, , 2002Nakamura et al, , 2005Nakase et al, 2001Nakase et al, , 2004Nakase et al, , 2008Nakamura-Kubo et al, 2003;Yoshida et al, 2005;Ye et al, 2007). Spo2, Spo13, and Spo15 proteins localize to the SPB.…”

“…Sec12 is responsible for vesicle transport from the endoplasmic reticulum (ER) to the Golgi apparatus in S. cerevisiae (Nakano et al, 1988). Spo20 is structurally and functionally related to the major S. cerevisiae phosphatidylinositol/phosphatidylcholine-transfer protein Sec14, which is required for vesicle formation from the Golgi apparatus (Bankaitis et al, 1990;Nakase et al, 2001). Spo20 regulates formation of the FSM, in addition to its known roles in post-Golgi vesicle trafficking (Nakase et al, 2001).…”