| Aquaporin water channel proteins have been characterized in animals, plants, and bacteria. The completion of the Saccharomyces cerevisiae genome enabled characterization of aquaporins in yeast. The genome database contains two ORFs with homology to aquaporins, AQY1 and AQY2. Using Xenopus oocytes to determine water channel activity, Aqy1p is a functional aquaporin in some strains such as Sigma1278b, but is nonfunctional in most lab strains due to point mutations. Aqy2p is disrupted by a stop codon in most strains; however, Sigma1278b has an intact ORF. Because Sigma1278b Aqy2p has an intracellular localization in Xenopus oocytes and in yeast, other strains of yeast were examined. Aqy2p from Saccharomyces chevalieri has a single amino acid that differs from Sigma1278b Aqy2p. S. chevalieri Aqy2p is a functional water channel in oocytes and traffics to the plasma membrane of yeast. The Sigma1278b parental strain, aqyl null yeast, aqy2 null yeast, aqy1-aqy2 double-null yeast, and null yeast expressing S. chevalieri Aqy2p were examined under various conditions. When survival of the parental strain and the aquaporin null strains were compared during a series of osmotic shocks, the null strains showed significantly increased viability. Therefore, the extreme osmolarities that lab strains are subjected to may explain the negative selection of aquaporins in yeast. Comparison of the strains also revealed that the aquaporin null cells were more aggregated and their surface was more hydrophobic. Aquaporins may provide an advantage by keeping cells disperse during starvation or during sexual reproduction. The aquaporin homolog from Candida albicans was studied to look at the affect on cell surface hydrophobicity, a factor that is important in virulence. Aqy1p from Candida albicans functions as a water channel in oocytes. However, there was no difference in cell aggregation or hydrophobicity between wild-type and aquaporin null Candida cells. Mice injected with wild-type and null yeast survived equivalently. Although there is no effect of aquaporins on the virulence of Candida albicans, water channels may play a role in the pathogenesis of other microorganisms. In addition, there may be other subtle roles of aquaporins in additional processes that may be difficult to examine in the laboratory. |
