Morphogenetic Pathway of Spore Wall Assembly in Saccharomyces cerevisiae

Abstract

The Saccharomyces cerevisiae spore is protected from environmental damage by a multilaminar extracellular matrix, the spore wall, which is assembled de novo during spore formation. A set of mutants defective in spore wall assembly were identified in a screen for mutations causing sensitivity of spores to ether vapor. The spore wall defects in 10 of these mutants have been characterized in a variety of cytological and biochemical assays. Many of the individual mutants are defective in the assembly of specific layers within the spore wall, leading to arrests at discrete stages of assembly. The localization of several of these gene products has been determined and distinguishes between proteins that likely are involved directly in spore wall assembly and probable regulatory proteins. The results demonstrate that spore wall construction involves a series of dependent steps and provide the outline of a morphogenetic pathway for assembly of a complex extracellular structure.As a response to nitrogen starvation in the presence of a poor carbon source, MATa/MAT␣ diploid cells of the baker's yeast Saccharomyces cerevisiae exit the cell cycle, undergo meiosis, and form haploid spores (18). These spores are a quiescent, stress-resistant cell type that can survive until nutrients are reintroduced. Much of the spores' resistance to environmental damage is provided by a specialized extracellular coat, the spore wall (41).The spore wall is a stratified extracellular matrix that is more complex than the normal vegetative cell wall (15,31,40). The vegetative wall consists primarily of an inner layer (closest to the plasma membrane) of ␤-glucans interspersed with a small amount of chitin and an outer layer of heavily mannosylated proteins (mannans) (15,31). By contrast, the spore wall consists of four distinct layers. The first two strata, an innermost layer composed primarily of mannan and a second layer of ␤1-3-linked glucans, are similar in composition to the vegetative wall but are reversed in position with respect to the spore plasma membrane (16). The outer portion of the spore wall is comprised of two polymers that are unique to the spore and confer much of the spore's resistance to environmental damage (4, 32). Immediately outside of the ␤-glucan is a layer composed primarily of chitosan, a glucosamine polymer synthesized by the deacetylation of chitin (7,25,32). Outside of the chitosan is a layer that consists largely of cross-linked tyrosine molecules (4-6).In addition to being more complex than the cell wall, the spore wall is also unique in that it is constructed without a preexisting matrix to act as a template. Spore morphogenesis begins with the formation of prospore membranes within the cytoplasm of the cell that envelop each of the haploid nuclei produced by the meiotic divisions (26). Closure of the prospore membrane results in each nucleus being surrounded by a double membrane (26). The unit membrane closest to the nucleus serves as the plasma membrane of the spore. The spore wall is constructed in the luminal ...

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

“…Concurrent with our studies, SSP2 and OSW1 were also characterized by two other groups (Sarkar et al 2002;Coluccio et al 2004). Our results indicate that the ssp2 and osw1 null mutants are proficient in meiotic recombination and chromosome segregation, but defective in the subsequent processes of PSM and SW formation.…”

“…OSWs are rarely seen in osw1; the aberrant shapes of some prospores might result from lack of the OSW for stabilization. Coluccio et al (2004) also observed an absence of OSWs in osw1; they did not report any other defects.…”

“…The ssp2 mutant phenotype has been characterized previously by two groups (Sarkar et al 2002;Coluccio et al 2004). Both groups reported that the PSM develops properly.…”

“…Several other mutants have been described that display heterogeneous defects in the development of PSMs and SWs (Friesen et al 1994;Krisak et al 1994;Wagner et al 1997;Straight et al 2000;Coluccio et al 2004). These include mutations in four genes encoding protein kinases, SMK1, SPS1, CAK1 and MPS1, as well as mutations in the SPO75 gene, which encodes an integral membrane protein.…”

“…Both of these ORFs were identified in previous screens for sporulationdefective mutants (Rabitsch et al 2001;Sarkar et al 2002;Enyenihi and Saunders 2003). YOR242c has been termed SSP2 by Sarkar et al (2002), and YOR255w has been named OSW1 by Coluccio et al (2004). SSP2 encodes a protein of 371 amino acids, which shares significant homology with IPF16670, a predicted gene product of Candida albicans.…”