Fps1, a yeast member of the MIP family of channel proteins, is a facilitator for glycerol uptake and efflux and is inactive under osmotic stress

The Saccharomyces cerevisiae FPS1 gene, which encodes a channel protein belonging to the MIP family, has been isolated previously as a multicopy suppressor of the growth defect of the fdp1 mutant (allelic to GGS1/TPS1) on fermentable sugars. Here we show that overexpression of FPS1 enhances glycerol production. Enhanced glycerol production caused by overexpression of GPD1 encoding glycerol-3-phosphate dehydrogenase also suppressed the growth defect of ggs1/tps1Δ mutants, suggesting a novel role for glycerol production in the control of glycolysis. The suppression of ggs1/tps1Δ mutants by GPDI depends on the presence of Fps1. Mutants lacking Fps1 accumulate a greater part of the glycerol intracellularly, indicating that Fps1 is involved in glycerol efflux. Glycerol-uptake experiments showed that the permeability of the yeast plasma membrane for glycerol consists of an Fps1-independent component probably due to simple diffusion and of an Fps1-dependent component representing facilitated diffusion. The Escherchia coli glycerol facilitator expressed in a yeast fps1Δ mutant can restore the characteristics of glycerol uptake, production and distribution fully, but restores only partially growth of a ggs1/tps1Δ/fps1Δ double mutant on glucose, Fps1 appears to be closed under hyperosmotic stress when survival depends on intracellular accumulation of glycerol and apparently opens rapidly when osmostress is lifted. The osmostress-induced High Osmolarity Glycerol (HOG) response pathway is not required for inactivation of Fps1. We conclude that Fps1 is a regulated yeast glycerol facilitator controlling glycerol production and cytosolic concentration, and might have additional functions.