Multicopy suppression screening of Saccharomyces cerevisiae Identifies the ubiquitination machinery as a main target for improving growth at low temperatures.

A decrease in ambient temperature alters membrane functionality and impairs the proper interaction between the cell and its external milieu. Understanding how cells adapt membrane properties and modulate the activity of membrane-associated proteins is therefore of major interest from both the basic and the applied points of view. Here, we have isolated multicopy suppressors of the cold sensitivity phenotype of a trp1 strain of Saccharomyces cerevisiae. Three poorly characterized genes, namely, ALY2 encoding the endocytic adaptor, CAJ1 encoding the J protein, and UBP13 encoding the ubiquitin C-terminal hydrolase, were identified as mediating increased growth at 12°C of both Trp⁻ and Trp+ yeast strains. This effect was likely due to the downregulation of cold-instigated degradation of nutrient permeases, since it was missing from cells of the rsp5Δ mutant strain, which contains a point mutation in the gene encoding ubiquitin ligase. Indeed, we found that 12°C treatments reduced the level of several membrane transporters, including Tat1p and Tat2p, two yeast tryptophan transporters, and Gap1, the general amino acid permease. We also found that the lack of Rsp5p increased the steady state level of Tat1p and Tat2p and that ALY2-engineered cells grown at 12°C had higher Tat2p and Gap1p abundance. Nevertheless, the high copy number of ALY2 or UBP13 improved cold growth even in the absence of Tat2p. Consistent with this, ALY2- and UBP13-engineered cells of the industrial QA23 strain grew faster and produced more CO₂ at 12°C than did the parental when maltose was used as the sole carbon source. Hence, the multicopy suppressors isolated in this work appear to contribute to the correct control of the cell surface protein repertoire and their engineering might have potential biotechnological applications.