Membrane transport in an osmotically fragile mutant of Saccharomyces cerevisiae

Isolation and characterization of Schizosaccharomyces pombe mutants defective in cell wall (1-3)beta-D-glucan

Schizosaccharomyces pombe thermosensitive mutants requiring the presence of an osmotic stabilizer to survive and grow at a nonpermissive temperature were isolated. The mutants were genetically and biochemically characterized. In all of them, the phenotype segregated in Mendelian fashion as a single gene which coded for a recessive character. Fourteen loci were defined by complementation analysis. Studies of cell wall composition showed a reduction in the amount of cell wall beta-glucan in three strains (JCR1, JCR5, and JCR10) when growing at 37 degrees C. Galactomannan was diminished in two others. Strains JCR1 and JCR5, with mutant alleles cwg1-1 and cwg2-1, respectively, were further studied. The cwg1 locus was mapped on the right arm of chromosome III, 18.06 centimorgans (cM) to the left of the ade5 marker; cwg2 was located on the left arm of chromosome I, 34.6 cM away from the aro5 marker. (1-3)beta-D-Glucan synthase activities from cwg1-1 and cwg2-1 mutant strains grown at 37 degrees C were diminished, as measured in vitro, compared with the wild-type strain; however, Km values and activation by GTP were similar to the wild-type values. Mutant synthases behaved like the wild-type enzyme in terms of thermostability. Analyses of round shape, lytic behavior, and low (1-3)beta-D-glucan synthase activity in cultures derived from ascospores of the same tetrad showed cosegregation of all these characters. Detergent dissociation of (1-3)beta-D-glucan synthase into soluble and particulate fractions and subsequent reconstitution demonstrated that the cwg1-1 mutant was affected in the particulate fraction of the enzymatic activity while cwg2-1 was affected in the soluble component. The antifungal agents Papulacandin B and Aculeacin A had similar effects on the enzymatic activities of the wild type and the cwg2-1 mutant strain, whereas the cwg1-1 mutant, when growing at 37 degrees C, had a more inhibitor-resistant (1,3)beta-D-glucan synthase. It is concluded that the cwg1+ and cwg2+ genes are related to (1,3)beta-D-glucan biosynthesis.

Cell fusion of Saccharomyces cerevisiae fragile mutants

Fragile mutants of Saccharomyces cerevisiae are defective in the structure of the cell wall and plasma membrane. The mutant cells lyse in hypotonic solutions but grow exponentially when osmotic stabilizer is included in the medium. These mutants display a general increase in the permeability of the plasma membrane. We show here that fragile yeast cells of the same mating type can fuse without protoplast formation. The frequency of cell x cell fusion is lower than that observed for protoplast x protoplast fusion and can be significantly increased if the cells of one partner are converted to protoplasts. Microscopic observations and genetic analysis demonstrate that the hybrids obtained are fusion products. The fusion between fragile cells is explained in terms of the existence of local defects on their surface where the cell wall is thinner (or even missing), thus allowing a direct contact of cells by means of their plasma membranes.