Homologous Subunits of 1,3-Beta-Glucan Synthase Are Important for Spore Wall Assembly in Saccharomyces cerevisiae

Duringsporulation inSaccharomyces cerevisiae , the fourhaploid nuclei are encapsulated within multilayered spore walls.Glucan, the major constituent of the spore wall, is synthesized by1,3-β-glucan synthase, which is composed of a putativecatalytic subunit encoded byFKS1 andFKS2 . Althoughanother homolog, encoded byFKS3 , was identified by homologysearching, its function is unknown. In this report, we show thatFKS2 andFKS3 are required for spore wall assembly.The ascospores offks2 andfks3 mutants wereenveloped by an abnormal spore wall with reduced resistance to diethylether, elevated temperatures, and ethanol. However, deletion of theFKS1 gene did not result in a defective spore wall. Theconstruction of fusion genes that expressed Fks1p and Fks2p under thecontrol of theFKS2 promoter revealed that asci transformedwithFKS2p -driven Fks1p and Fks2p were resistant to elevatedtemperatures, which suggests that the expression ofFKS2 playsan important role in spore wall assembly. The expression ofFKS1p -driven Fks3p during vegetative growth did not affect1,3-β-glucan synthase activity in vitro but effectivelysuppressed the growth defect of the temperature-sensitivefks1 mutant by stabilizing Rho1p, which is a regulatory subunit of glucansynthase. Based on these results, we propose thatFKS2 encodesthe primary 1,3-β-glucan synthase in sporulation and thatFKS3 is required for normal spore wall formation because itaffects the upstream regulation of 1,3-β-glucansynthase.