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Eukaryotic Cell doi:10.1128/EC.00200-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Homologous subunits of yeast 1,3-beta-glucan synthase are important for spore wall assembly in Saccharomyces cerevisiae

Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Bldg. FSB-101, 5-1-5 Kashiwanoha, Kashiwa, Chiba Prefecture 277-8562, Japan; Unite des Aspergillus, Institut Pasteur, 25 Rue du Docteur Roux, 75015 Paris, France

^* To whom correspondence should be addressed. Email: ohya{at}k.u-tokyo.ac.jp.

	Abstract

During sporulation in Saccharomyces cerevisiae, the four haploid nuclei are encapsulated within multilayered spore walls. Glucan, the major constituent of the spore wall, is synthesized by 1,3--glucan synthase, which is composed of a putative catalytic subunit encoded by FKS1 and FKS2. Although another homolog, encoded by FKS3, was identified by homology searching, its function is unknown. In this report, we show that FKS2 and FKS3 are required for spore wall assembly. The ascospores of fks2 and fks3 mutants were enveloped by an abnormal spore wall with reduced resistance to diethyl ether, elevated temperature, and ethanol. However, deletion of the FKS1 gene did not result in a defective spore wall. The construction of fusion genes that expressed Fks1p and Fks2p under the control of the FKS2 promoter revealed that asci transformed with FKS2p-driven Fks1p and Fks2p were resistant to elevated temperature, which suggests that the expression of FKS2 plays an important role in spore wall assembly. The expression of FKS1p-driven Fks3p during vegetative growth did not affect 1,3--glucan synthase activity in vitro but effectively suppressed the growth defect of the fks1^ts mutant by stabilizing Rho1p, which is a regulatory subunit of glucan synthase. Based on these results, we propose that FKS2 encodes the primary 1,3--glucan synthase in sporulation, and that FKS3 is required for normal spore wall formation by affecting the upstream regulation of 1,3--glucan synthase.