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 β‐1,3‐glucan synthase. Analysis of fks1 Δ mutants showed a partial K1 killer toxin‐resistant phenotype and a 30% reduction in alkali‐soluble β‐1,3‐glucan that was accompanied by a modest reduction in β‐1,6‐glucan. The gas1 Δ mutant lacking a 1,3‐β‐glucanosyltransferase displayed a similar reduction in alkali‐soluble β‐1,3‐glucan but did not share the β‐1,6‐glucan defect, indicating that β‐1,6‐glucan reduction is not a general phenotype among β‐1,3‐glucan biosynthetic mutants. Overexpression of FKS2 suppressed the killer toxin phenotype of fks1 Δ mutants, implicating Fks2p in the biosynthesis of the residual β‐1,6‐glucan present in fks1 Δ cells. In addition, eight out of 12 fks1 ts fks2 Δ mutants had altered β‐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 β‐1,6‐glucan, while the in vitro hyperactive allele FKS1 T605I M761T increased both β‐glucan levels. These β‐1,6‐glucan phenotypes may be due to altered availability of, and structural changes in, the β‐1,3‐glucan polymer, which might serve as a β‐1,6‐glucan acceptor at the cell surface. Alternatively, Fks1p and Fks2p could actively participate in the biosynthesis of both polymers as β‐glucan transporters. We analysed Fks1p and Fks2p in β‐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 β‐1,6‐glucan defects. Copyright © 2002 John Wiley & Sons, Ltd.