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Eukaryotic Cell doi:10.1128/EC.00321-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

GAS2 and GAS4 : a pair of developmentally regulated genes required for spore wall assembly in Saccharomyces cerevisiae

Dipartimento di Scienze Biomolecolari e Biotecnologie, Università degli Studi di Milano, 20133 Milano, Italy, Department of Biochemistry and Cell biology, SUNY Stony Brook, Stony Brook, NY 11794-5215, USA and Departamento de Microbiología II, Universidad Complutense de Madrid, CP 28040 Madrid, Spain

^* To whom correspondence should be addressed. Email: Laura.Popolo{at}unimi.it.

	Abstract

The GAS multi-gene family of Saccharomyces cerevisiae is composed of five paralogs (GAS1-5). GAS1 is the only one of these genes that has been characterized to date. It encodes a glycosylphosphatidylinositol-anchored protein functioning as a (1,3)-glucan elongase and required for proper cell wall assembly during vegetative growth. In this study we characterize the role of the GAS2 and GAS4 genes. These genes are expressed exclusively during sporulation. Their mRNA levels showed a peak at 7 h from induction of sporulation and then decreased. Gas2 and Gas4 proteins were detected and reached a maximum level between 8 and 10 h of sporulation, a time roughly coincident with spore wall assembly. The double null gas2 gas4 diploid mutant showed a severe reduction in the efficiency of sporulation, an increased permeability of the spores to exogenous substances and produced inviable spores, whereas the single gas2 and gas4 null diploids were similar to the parental strain. An analysis of spore ultrastructure indicated that the loss of Gas2 and Gas4 proteins affected the proper attachment of the glucan to the chitosan layer probably as a consequence of the lack of coherence of the glucan layer. The ectopic expression of GAS2 and GAS4 genes in a gas1 null mutant revealed that these proteins are redundant versions of Gas1p specialized to function in a compartment at a pH value close to neutrality.

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