Aquaporin expression and freeze tolerance in yeast.
An Tanghe, Johan Thevelein, Patrick Van Dijck
Labo Moleculaire Celbiologie, Instit. Plantk. en Microbiol., K. park Arenberg 31, Heverlee, 3001, Belgium
Microbial MIP channel proteins have been correlated with various phenotypes, but their physiological function remains controversial. In Saccharomyces cerevisiae, aquaporin expression has previously been shown to clearly correlate with freeze tolerance. This finding has now been extended to two other yeast species. Deletion of both alleles of the Candida albicans water channel protein encoding gene AQY1 significantly reduces freeze tolerance, whereas re-introduction of AQY1 in the homozygous deletion strain restores freeze tolerance. In addition, the low intrinsic freeze tolerance of Schizosaccharomyces pombe which has no bona fide aquaporins could be significantly improved by heterologous overexpression of the S. cerevisiae aquaporin encoding gene AQY2-1. These results support a role for plasma membrane water transport activity in determination of freeze tolerance in yeast. They raise the question whether aquaporin-mediated water transport might play a role in freeze stress survival in the natural environment of microorganisms. Moreover, they indicate that aquaporin overexpression might be a tool to improve cryopreservation of many other cell types as well. We are now studying the effect of three other proteins for their anticipated synergistical or complementary effect to the aquaporin-mediated improvement of yeast (fast and slow) freeze tolerance, namely a fish antifreeze protein (rQAE m1.1), a plant hydrophylin (Le25) and a bacterial ice-nucleating protein (InaZ).