ALBERT

Notes: Abstract The yeast Saccharomyces cerevisiae consumes mono- and disaccharides preferentially to any other carbon source. Since sugars do not freely permeate biological membranes, cellular uptake of these compounds requires the action of ‘transporters’. The purpose of this review is to summarize the present knowledge on sugar transport in this organism. Yeast cells show two transporters for monosaccharides, the so-called glucose and galactose transporters that act by a facilitated diffusion mechanism. In the case of glucose transport, which also acts upon d-fructose and d-mannose, two components with high- and low-affinity constants have been identified kinetically. Activity of the high-affinity component is dependent on the presence of active kinases whereas activity of the low-affinity component is independent of the presence of these enzymes. Three genes, SNF3, HXT1 and HXT2, encode three different glucose transporters with a high affinity for the substrates and are repressed by high concentrations of glucose in the medium. Kinetic studies suggest that at least one additional gene exists that encodes a transporter with a low affinity and is expressed constituently. The present view is that there are several additional transporters for glucose that have not yet been identified. Galactose transport has only one natural substrate, d-galactose, and is encoded by the gene GAL2. Expression of this gene is induced by galactose and repressed by glucose. Two transporters for disaccharides have been identified in S. cerevisiae: maltose and α-methylglucoside transporters. These transporters are H+-symports that depend on the electrochemical proton gradient and are independent of the ATP level. The gene that encodes the maltose transporter is clustered with the other two genes required for maltose utilization in a locus that is found repeated at different chromosomal locations. Its expression is induced by maltose and repressed by glucose. The rate of sugar uptake in yeast cells is controlled by changes in affinity of the corresponding transporters as well as by an irreversible inactivation that affects their Vmax. The mechanisms involved in these regulatory processes are unknown at present.