title: Aquaporin expression correlates with freeze tolerance in baker's yeast, and overexpression improves freeze tolerance in industrial strains.
abstract: Little information is available about the precise mechanisms and determinants of freeze resistance in baker's yeast, Saccharomyces cerevisiae. Genomewide gene expression analysis and Northern analysis of different freeze-resistant and freeze-sensitive strains have now revealed a correlation between freeze resistance and the aquaporin genes AQY1 and AQY2. Deletion of these genes in a laboratory strain rendered yeast cells more sensitive to freezing, while overexpression of the respective genes, as well as heterologous expression of the human aquaporin gene hAQP1, improved freeze tolerance. These findings support a role for plasma membrane water transport activity in determination of freeze tolerance in yeast. This appears to be the first clear physiological function identified for microbial aquaporins. We suggest that a rapid, osmotically driven efflux of water during the freezing process reduces intracellular ice crystal formation and resulting cell damage. Aquaporin overexpression also improved maintenance of the viability of industrial yeast strains, both in cell suspensions and in small doughs stored frozen or submitted to freeze-thaw cycles. Furthermore, an aquaporin overexpression transformant could be selected based on its improved freeze-thaw resistance without the need for a selectable marker gene. Since aquaporin overexpression does not seem to affect the growth and fermentation characteristics of yeast, these results open new perspectives for the successful development of freeze-resistant baker's yeast strains for use in frozen dough applications.
authors.affiliation: Laboratorium voor Moleculaire Celbiologie, Institute of Botany and Microbiology, Katholieke Universiteit Leuven, B-3001 Leuven-Heverlee, Flanders, Belgium.
authors.forename: An
authors.initials: A
authors.lastname: Tanghe
authors.forename: Patrick
authors.initials: P
authors.lastname: Van Dijck
authors.forename: Françoise
authors.initials: F
authors.lastname: Dumortier
authors.forename: Aloys
authors.initials: A
authors.lastname: Teunissen
authors.forename: Stefan
authors.initials: S
authors.lastname: Hohmann
authors.forename: Johan M
authors.initials: JM
authors.lastname: Thevelein
mesh_terms: D051398:Aquaporin 1; D020346:Aquaporins / Q000235:genetics*; D001789:Blood Group Antigens; D005285:Fermentation; D005615:Freezing; D015966:Gene Expression Regulation, Fungal; D006801:Humans; D007218:Industrial Microbiology; D020411:Oligonucleotide Array Sequence Analysis; D012441:Saccharomyces cerevisiae / Q000502:physiology*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics
chemical_list: C496020:AQP1 protein, human; C115153:AQY1 protein, S cerevisiae; C422454:AQY2 protein, S cerevisiae; D020346:Aquaporins; D001789:Blood Group Antigens; D029701:Saccharomyces cerevisiae Proteins; D051398:Aquaporin 1