Fps1p channel is the mediator of the major part of glycerol passive diffusion in Saccharomyces cerevisiae: artefacts and re-definitions

Abstract

Glycerol has been shown to cross the plasma membrane of Saccharomyces cerevisiae through (1) a H(+)/symport detected in cells grown on non-fermentable carbon sources, (2) the constitutively expressed Fps1p channel and (3) by passive diffusion. The Fps1p channel has been named a facilitator for mediating glycerol low affinity transport of the facilitated diffusion type. We present experimental evidence that this kinetic is an artefact created by glycerol kinase activity. Instead, the channel is shown to mediate the major part of glycerol's passive diffusion. This is not incompatible with Fps1p's major role in vivo, which has been previously shown to be the control of glycerol export under osmotic stress or in reaction to turgor changes. We also verified that FPS1 overexpression caused an increase in H(+)/symport V(max). Furthermore, yfl054c and fps1 mutants were equally affected by exogenously added ethanol, being the correspondent passive diffusion stimulated. For the first time, to our knowledge, a phenotype attributed to the functioning of YFL054c gene is presented. Glycerol passive diffusion is thus apparently channel-mediated. This is discussed according to glycerol's chemical properties, which contradict the widely spread concept of glycerol's liposoluble nature. The discussion considers the multiple roles that the intracellular levels of glycerol and its pathway regulation might play as a central key to metabolism control.

New insights on glycerol transport in Saccharomyces cerevisiae

“…Either strain cells, cultivated on YPD and collected during exponential growth phase, presented only a first order kinetics, indicative of channel-mediated entry [4,6]. This is in accordance with previously published results that show the active transport system to be under glucose repression [5,6].…”

“…Consistently, no glycerol active transport could be measured on a gup1gup2gut1 mutant grown on YPE [6]. On the other hand, as expected [4,6], Fps1p-mediated glycerol entry by first order kinetics was always present.…”

“…Glycerol, the main compatible solute used by yeasts under low a w in high osmolarity environments [1], can permeate Saccharomyces cerevisiae plasma membrane through a constitutively expressed channel, Fps1p [2][3][4], and a H þ /symport, active in cells derepressed by growth on non-fermentable carbon sources [5]. This transport system was associated with the expression of the genes GUP1 and GUP2 [6].…”

Aspergillus glaucus Aquaglyceroporin Gene glpF Confers High Osmosis Tolerance in Heterologous Organisms

“…Instead, those of the yeast Saccharomyces cerevisiae have been the most thoroughly studied. The two genes encoding AQGPs in the yeast genome, Fps1 (15,16) and Yfl054 (16,17), are functional glycerol facilitators. Fps1 plays a key role in yeast osmoregulation by regulating intracellular glycerol levels during changes in external osmolarity (18)(19)(20), whereas the cellular function of Yfl054 remains uncertain (16).…”

Conditional Osmotic Stress in Yeast

“…Rather, intracellular glycerol levels are controlled by passive glycerol export, which is mediated by Fps1 (2,(11)(12)(13). Upon a hyperosmotic shock the transport capacity of Fps1 is rapidly diminished to ensure that glycerol is maintained inside the cell (2,11).…”