SSP2 and OSW1, Two Sporulation-Specific Genes Involved in Spore Morphogenesis in Saccharomyces cerevisiae

Abstract

Spore formation in Saccharomyces cerevisiae requires the synthesis of prospore membranes (PSMs) followed by the assembly of spore walls (SWs). We have characterized extensively the phenotypes of mutants in the sporulation-specific genes, SSP2 and OSW1, which are required for spore formation. A striking feature of the osw1 phenotype is asynchrony of spore development, with some spores displaying defects in PSM formation and others spores in the same ascus blocked at various stages in SW development. The Osw1 protein localizes to spindle pole bodies (SPBs) during meiotic nuclear division and subsequently to PSMs/SWs. We propose that Osw1 performs a regulatory function required to coordinate the different stages of spore morphogenesis. In the ssp2 mutant, nuclei are surrounded by PSMs and SWs; however, PSMs and SWs often also encapsulate anucleate bodies both inside and outside of spores. In addition, the SW is not as thick as in wild type. The ssp2 mutant defect is partially suppressed by overproduction of either Spo14 or Sso1, both of which promote the fusion of vesicles at the outer plaque of the SPB early in PSM formation. We propose that Ssp2 plays a role in vesicle fusion during PSM formation.S PORULATION in the budding yeast, Saccharomyces cerevisiae, involves two overlapping processesmeiosis and spore morphogenesis. During meiosis, a diploid cell undergoes a single round of DNA replication, followed by two rounds of chromosome segregation, to generate four haploid nuclei. During spore morphogenesis, the four daughter nuclei are packaged into spores, which are contained within an ascus.Spore morphogenesis (reviewed by Neiman 2005) initiates with enlargement of the outer surface of the spindle pole body (SPB) to form the meiotic outer plaque. As cells undergo meiosis II, vesicles coalesce on the meiotic plaque to form a novel intracellular membrane called the prospore membrane (PSM). The doublelayered PSM extends outward from each SPB to surround the adjacent nucleus. When the leading edges of the PSM meet and become fused, the haploid nucleus is fully encapsulated to form a prospore. PSM closure triggers the final step of spore morphogenesis, spore wall (SW) formation. The SW is assembled between the two membranes of the PSM and consists of four layers. The two inner layers are composed of glucan and mannan, which are components of vegetative cell walls.

Sporulation in the Budding Yeast Saccharomyces cerevisiae

“…In addition to the cda1D cda2D mutant mentioned above, mutation of the transcription factor GIS1 or of the OSW1 or MUM3 genes produces a similar phenotype (Coluccio et al 2004a). While the effect of the gis1D mutation is likely indirect, the Osw1 protein localizes to the spore wall and so may be directly involved in assembly of the chitosan layer (Coluccio et al 2004a;Li et al 2007). The Mum3 protein has not been localized but has homology to acyltransferases (Neuwald 1997), suggesting that it has an enzymatic activity that could play a role in assembly of this spore wall layer as well.…”

“…In addition to the assembly enzymes, additional sporulation-specific factors influencing the assembly of the b-glucan layer have been identified including Spo73, Spo77, and Ssp2 (Sarkar et al 2002;Coluccio et al 2004a;Li et al 2007). Though no molecular function has been ascribed to any of these proteins, each localizes to the cytoplasmic side of the prospore membrane, suggesting that they affect assembly indirectly (e.g., as regulators of the synthase).…”

“…For example, the Osw1 protein is localized to the spore wall but lacks an obvious signal sequence for secretion to the prospore membrane lumen. Moreover, it is localized in the cytoplasm earlier in sporulation (Coluccio et al 2004a;Li et al 2007). Thus, Osw1 might remain in the ascal cytoplasm and only enter the spore wall after outer membrane dissolution.…”

Heterozygote Advantage Is a Common Outcome of Adaptation inSaccharomyces cerevisiae

“…The regions haploidized and duplicated are approximately the same across strains although the boundaries are not identical across chemostats (Table 1 and Figure S7). In all cases the haploidized region affects 393 genes, among which are 69 essential genes (Deutschbauer et al 2005), and OSW1 and SSP2, which are necessary for sporulation (Sarkar et al 2002;Li et al 2007). Also found among the haploidized genes is STD1, an MTH1 paralog (Kellis et al 2004) involved in glucose sensing (Gancedo 2008).…”

The Ras/cAMP Pathway and the CDK-Like Kinase Ime2 Regulate the MAPK Smk1 and Spore Morphogenesis in Saccharomyces cerevisiae

“…It has been proposed that the APC Ama1 plays a role in coupling completion of the meiotic divisions to gamete formation (Oelschlaegel et al 2005;Penkner et al 2005). Ssp2, a middle meiosis-specific protein that regulates spore formation (Sarkar et al 2002;Coluccio et al 2004;Li et al 2007) is also required for efficient phosphorylation of Smk1's activation loop (McDonald et al 2005). Thus, while Smk1 is a MAPK family member on the basis of its sequence and its activation by T-loop phosphorylation, it is unlike other MAPKs in several key respects; it is tightly controlled at the transcriptional level, it is activated by a CDK activating kinase, it is regulated by the APC, and its activation state is largely controlled by internally generated signals rather than extracellular ligands.…”