Yeast Cyclophilins Prevent Cold Denaturation of Proteins

Abstract

Freezing environments are one of the major challenges faced by many organisms, including Saccharomyces cerevisiae. Exposure to low temperatures reduces protein folding rates and induces the cold denaturation of proteins, necessitating aid from chaperones. In a previous study, we identified 19 chaperone genes, the deletion of which makes the host more vulnerable to freeze‐and‐thaw treatments. Among those, peptidyl–prolyl cis–trans isomerases (PPIases) were the most frequently identified. At low temperatures, peptidyl–prolyl isomerization is a rate‐limiting step in protein folding, and folding intermediates, which are prone to protein aggregation, tend to accumulate. To characterize their mode of function, the identified PPIases were overexpressed in Escherichia coli. Not only did they increase the survival of E. coli during freeze‐and‐thaw treatment at –20 °C, but they also protected β‐galactosidase against freeze‐induced protein denaturation. Purified Cpr1p facilitated the refolding of a slow‐folding substrate protein in vitro. These results suggest that the identified PPIases enhance cold survival of cells by preventing cold‐induced protein denaturation and promoting protein folding.