Existence of a tightly regulated water channel in Saccharomyces cerevisiae

The Saccharomyces cerevisiae strain Sigma 1278b possesses two putative aqua porins, Aqy1-1p and Aqy2-1p. Previous work demonstrated that Aqy1-1p functi ons as a water channel in Xenopus oocyte. However, no function could be att ributed to Aqy2-1p in this system. Specific antibodies were used to follow the expression of Aqy1-1p and Aqy2-1p in the yeast. Aqy1-1p was never detec ted whatever the growth phase and culture conditions tested. In contrast, A qy2-1p was detected only during the exponential growth phase in rich medium containing glucose. Aqy2-1p expression was repressed by hyper-osmotic cult ure conditions. Both immunocytochemistry and biochemical subcellular fracti onation demonstrated that Aqy2-1p is located on the endoplasmic reticulum ( ER) as well as on the plasma membrane. In microsomal vesicles enriched in E R, a water channel activity due to Aqy2-1p was detected by stopped-flow ana lysis. Our results show that the expression of aquaporins is tightly contro lled. The physiological relevance of aquaporin-mediated water transport in yeast is discussed.