Sodium and acetate inhibit cell growth and ethanol fermentation by different mechanisms in Saccharomyces cerevisiae. We identified the substitution of a conserved Thr255 to Ala (T255A) in the essential Nedd4-family ubiquitin ligase Rsp5, which enhances cellular sodium acetate tolerance. The T255A mutation selectively increased the resistance of cells against sodium acetate, suggesting that S. cerevisiae cells possess an Rsp5-mediated mechanism to cope with the composite stress of sodium and acetate. The sodium acetate tolerance was dependent on the extrusion of intracellular sodium ions by the plasma membrane-localized sodium pumps Ena1, Ena2, and Ena5 (Ena1/2/5) and two known upstream regulators: the Rim101 pH signaling pathway and the Hog1 mitogen-activated protein kinase. However, the T255A mutation affected neither the ubiquitination level of the Rsp5 adaptor protein Rim8 nor the phosphorylation level of Hog1. These data raised the possibility that Rsp5 enhances the function of Ena1/2/5 specifically in response to sodium acetate through an unknown mechanism other than ubiquitination of Rim8 and activation of Hog1-mediated signaling. Also, an industrial yeast strain that expresses the T255A variant exhibited increased initial fermentation rates in the presence of sodium acetate. Hence, this mutation has potential for the improvement of bioethanol production from lignocellulosic biomass.

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Watcharawipas A, Watanabe D, Takagi H

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