Tanghe An, Van Dijck Patrick, Dumortier Françoise, Teunissen Aloys, Hohmann Stefan, Thevelein Johan M
Laboratorium voor Moleculaire Celbiologie, Institute of Botany and Microbiology, Katholieke Universiteit Leuven, B-3001 Leuven-Heverlee, Flanders, Belgium.
Appl Environ Microbiol. 2002 Dec;68(12):5981-9. doi: 10.1128/AEM.68.12.5981-5989.2002.
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.
关于面包酵母酿酒酵母抗冻的确切机制和决定因素，目前所知甚少。对不同抗冻和冻敏菌株进行的全基因组基因表达分析和Northern分析现已揭示，抗冻性与水通道蛋白基因AQY1和AQY2之间存在相关性。在实验室菌株中缺失这些基因会使酵母细胞对冷冻更敏感，而各自基因的过表达以及人类水通道蛋白基因hAQP1的异源表达则提高了抗冻性。这些发现支持质膜水运输活性在酵母抗冻性测定中发挥作用。这似乎是首次为微生物水通道蛋白确定的明确生理功能。我们认为，在冷冻过程中，水通过渗透作用快速外流可减少细胞内冰晶形成及由此导致的细胞损伤。水通道蛋白的过表达还提高了工业酵母菌株在细胞悬液以及冷冻保存或经历冻融循环的小面团中的活力维持能力。此外，基于其提高的冻融抗性，可以选择水通道蛋白过表达转化体，而无需选择标记基因。由于水通道蛋白的过表达似乎不会影响酵母的生长和发酵特性，这些结果为成功开发用于冷冻面团应用的抗冻面包酵母菌株开辟了新的前景。
