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 ...

The yeast orthologue of mammalian TCTP is here proposed to be named Mmi1p (microtubule and mitochondria interacting protein). This protein displays about 50% amino acid sequence identity with its most distantly related orthologs in higher organisms and therefore probably belongs to a small class of yeast proteins which have housekeeping but so far incompletely known functions needed for every eukaryotic cell. Previous investigations of the protein in both higher cells and yeast revealed that it is highly expressed during active growth, but transcriptionally down-regulated in several kinds of stress situations including starvation stress. In human cells, TCTP presumably has anti-apoptotic functions as it binds to Bcl-XL in vivo. TCTP of higher cells was also shown to interact with the translational machinery. It has acquired an additional function in the mammalian immune system, as it is identical with the histamine releasing factor. Here, we show that in S. cerevisiae induction of apoptosis by mild oxidative stress, replicative ageing or mutation of cdc48 leads to translocation of Mmi1p from the cytoplasm to the mitochondria. Mmi1p is stably but reversibly attached to the outer surface of the mitochondria and can be removed by digestion with proteinase K. Glutathionylation of Mmi1p, which is also induced by oxidants, is not a prerequisite or signal for translocation as shown by replacing the only cysteine of Mmi1p by serine. Mmi1p probably interacts with yeast microtubules as deletion of the gene confers sensitivity to benomyl. Conversely, the deletion mutant displays resistance to hydrogen peroxide stress and shows a small but significant elongation of the mother cell-specific lifespan. Our results so far indicate that Mmi1p is one of the few proteins establishing a functional link between microtubules and mitochondria which may be needed for correct localization of mitochondria during cell division.

The de novo formation of multilayered spore walls inside a diploid mother cell is a major landmark of sporulation in the yeast Saccharomyces cerevisiae. Synthesis of the dityrosine-rich outer spore wall takes place toward the end of this process. Bisformyl dityrosine, the major building block of the spore surface, is synthesized in a multistep process in the cytoplasm of the prospores, transported to the maturing wall, and polymerized into a highly cross-linked macromolecule on the spore surface. Here we present evidence that the sporulation-specific protein Dtr1p (encoded by YBR180w) plays an important role in spore wall synthesis by facilitating the translocation of bisformyl dityrosine through the prospore membrane. DTR1 was identified in a genome-wide screen for spore wall mutants. The null mutant accumulates unusually large amounts of bisformyl dityrosine in the cytoplasm and fails to efficiently incorporate this precursor into the spore surface. As a result, many mutant spores have aberrant surface structures. Dtr1p, a member of the poorly characterized DHA12 (drug:H ؉ antiporter with 12 predicted membrane spans) family, is localized in the prospore membrane throughout spore maturation. Transport by Dtr1p may not be restricted to its natural substrate, bisformyl dityrosine. When expressed in vegetative cells, Dtr1p renders these cells slightly more resistant against unrelated toxic compounds, such as antimalarial drugs and food-grade organic acid preservatives. Dtr1p is the first multidrug resistance protein of the major facilitator superfamily with an assigned physiological role in the yeast cell.Diploid a/␣ cells of the budding yeast Saccharomyces cerevisiae undergo a specialized developmental program termed sporulation when transferred to a nitrogen-free medium containing potassium acetate as nonfermentable carbon source. The final product of sporulation is an ascus that consists of four haploid spores surrounded by the ascus wall, the former vegetative cell wall (for a review, see reference 27). Spores are protected from adverse environmental conditions by the spore wall. Especially the surface layers contribute both to the spores' mechanical rigidity and their resistance against chemical and enzymatic attack (3). Spore wall synthesis begins with the formation of the prospore membrane, a bilayered electrondense structure that starts to form during the second meiotic division on the cytoplasmic side of each of the four spindle pole bodies by fusion of secretory vesicles (9, 13, 23, 31-33). As meiosis progresses, the prospore membrane extends along the outer surface of the nuclear envelope. Septins, among them the sporulation-specific proteins Spr3p and Spr28p, localize at the leading tip of the prospore membrane under control of the Gip1p-Glc7p phosphatase complex and might be involved in its extension and directed growth (14,16,44). Other proteins localized to the growing prospore membrane are SspIp, Ady3p, and Don1p, which form the leading edge protein coat (23,33) and presumably Sps2p (11,38), but th...

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A clone was isolated from a cDNA library from early embryos of Xenopus laevis that codes for a highly charged protein containing 339 amino acids. Two putative nuclear localization signals could be identified in its sequence, but no other known motifs or domains. Closely related ORFs are present in the genomes of man, C. elegans, yeast and Arabidopsis. A fusion protein with GFP expressed in HeLa cells or Xenopus oocytes was found to be localized in the nucleolus and coiled (Cajal) bodies. Moreover, immunoprecipitation experiments demonstrated that the new Xenopus protein interacts with 5S, 5.8S and 28S RNAs of large ribosomal subunits. The name Brix (biogenesis of ribosomes in Xenopus) is proposed for this protein and the corresponding gene. In Saccharomyces cerevisiae, the essential gene YOL077c, now named BRX1, codes for the Brix homolog, which is also localized in the nucleolus. Depletion of Brx1 p in a conditional yeast mutant leads to defects in rRNA processing, and a block in the assembly of large ribosomal subunits.