Energy requirements for maltose transport in yeast

Maltose transport in yeast (Saccharomyces cerevisiae) is inhibited by uncouplers under conditions where the intracellular concentration of the sugar is lower than in the medium. The uncouplers did not deplete the ATP content of the yeast cells and a 50--100-fold reduction in ATP caused by antimycin and 2-deoxyglucose had no effect on maltose transport. In ATP-depleted cells, the maltose transported is partially hydrolyzed to glucose but not further metabolized and therefore a mechanism of transport involving phosphorylation can be discarded. One proton is cotransported with every maltose molecule. The fact that maltose transport is inhibited by KCl but not by NaCl, Tris-Cl or KSCN suggest that the electroneutrality during maltose and proton uptake can be maintained by the exit of K+ from the cells or by the entry of a permeable anion as SCN-. These results indicate that the translocation of maltose across the yeast plasma membrane is not dependent on ATP and is coupled to the electrochemical gradient of protons in this membrane. When this gradient is abolished by uncouplers, the transport system is not able to function even in favour of a concentration gradient of the sugar.