Peptidyl-Prolyl Isomerase Cpr7p of Yeast Prevents Protein Aggregation Upon Freezing

Abstract

Exposure to low temperatures may disturb proteome stasis due to cold denaturation and subsequent aggregation of proteins. The formation of reactive oxygen species, intracellular pH changes, and osmotic imbalance due to intracellular ice crystal formation may aggravate protein denaturation. In a previous study, deletion of some chaperone genes, particularly peptidyl-prolyl cis–trans isomerases, rendered yeast cells more vulnerable to freeze–thaw treatment. To elucidate their mode of action, an active site mutation was introduced into an identified peptidyl-prolyl isomerase, cpr7. Expression of mutant Cpr7p significantly recovered freeze survival in cpr7Δ yeast. Extensive protein aggregates were formed in cpr7Δ yeast cells upon freeze–thaw treatment, and introduction of either wild-type or mutant cpr7 significantly mitigated protein aggregation. Translation elongation factor 2 (EF-2) was predominantly found in the aggregated fraction in cpr7Δ yeast. Purified Cpr7p facilitated the refolding of unfolded Z-type antitrypsin proteins in vitro. Our results suggest that Cpr7p protects cells from freeze-induced protein aggregation and is potentially involved in the biosynthesis and/or folding of new proteins during recovery from freezing damage.