THE ROLE OF POLYPHOSPHATES IN SUGAR TRANSPORT ACROSS THE PLASMA MEMBRANE OF YEAST

Abstract

For sugar transport in yeast, at least three different mechanisms are operative. Some sugars are taken up via facilitated diffusion in various yeast strains. Other carbohydrates are transported actively, via a sugar-proton support system. A third mechanism, namely, transport-associated phosphorylation, has been suggested for glucose and glucose derivatives in Saccharomyces cerevisiae and in Saccharomyces fragilis. This hypothesis of transport-associated phosphorylation was based on several lines of circumstantial evidence. Based on this hypothesis, the following predictions can be made: (1) the transported sugar should not enter the cytoplasm as the free sugar but as the corresponding sugar-phosphate; (2) polyphosphates should be present outside the plasma membrane; (3) if sugar phosphorylation is prevented, this should be reflected by changes of the transport parameters; and (4) a glucose kinase activity, utilizing polyphosphate as phosphate donor, should be associated with the plasma membrane. This chapter discusses experimental results obtained in laboratories in the framework of these predictions.