Mutations in Fks1p affect the cell wall content of β-1,3-and β-1,6-glucan in Saccharomyces cerevisiae

Fks1p and Fks2p are related proteins thought to be catalytic subunits of the beta-1,3-glucan synthase. Analysis of fks1Delta mutants showed a partial K1 killer toxin-resistant phenotype and a 30% reduction in alkali-soluble beta-1,3-glucan that was accompanied by a modest reduction in beta-1,6-glucan. The gas1Delta mutant lacking a 1,3-beta-glucanosyltransferase displayed a similar reduction in alkali-soluble beta-1,3-glucan but did not share the beta-1, 6-glucan defect, indicating that beta-1,6-glucan reduction is not a general phenotype among beta-1,3-glucan biosynthetic mutants. Overexpression of FKS2 suppressed the killer toxin phenotype of fks1Delta mutants, implicating Fks2p in the biosynthesis of the residual beta-1, 6-glucan present in fks1Delta cells. In addition, eight out of 12 fks1(ts) fks2Delta mutants had altered beta-glucan levels at the permissive temperature: the partial killer resistant FKS1(F1258Y) (N1520D) allele was severely affected in both polymers and displayed a 55% reduction in beta-1,6-glucan, while the in vitro hyperactive allele FKS1(T6051 M761T) increased both beta-glucan levels. These beta-1,6-glucan phenotypes may be due to altered availability of, and structural changes in, the beta-1,3-glucan polymer, which might serve as a beta-1,6-glucan acceptor at the cell surface. Alternatively, Fks1p and Fks2p could actively participate in the biosynthesis of both polymers as beta-glucan transporters. We analysed Fks1p and Fks2p in beta-1,6-glucan deficient mutants and found that they were mislocalized and that the mutants had reduced in vitro glucan synthase activity, possibly contributing to the observed beta-1,6-glucan defects. Copyright (C) 2002 John Wiley Sons, Ltd.