HIGH-AFFINITY GLUCOSE-UPTAKE IN SACCHAROMYCES-CEREVISIAE IS NOT DEPENDENT ON THE PRESENCE OF GLUCOSE-PHOSPHORYLATING ENZYMES

Glucose uptake in Saccharomyces cerevisiae is believed to consist of t wo kinetically distinguishable components, the affinity of which is mo dulated during growth on glucose. It has been reported that triple hex ose-kinase deletion mutants do not exhibit high-affinity glucose uptak e. This raises the question of whether and how high-affinity glucose u ptake is related to the presence of glucose-phosphorylating enzymes. I n this study the kinetics of glucose uptake in both wild-type cells an d cells of hexose-kinase deletion mutants, grown on either glycerol or galactose, were determined using a rapid-uptake method. In wild-type cells glucose uptake measured over either 5 s or 200 ms exhibited high affinity. In contrast, in cells of hexose-kinase deletion mutants the apparent affinity of glucose uptake was dependent on the time scale d uring which uptake was measured. Measurements on the 5-s scale showed apparent low-affinity uptake whereas measurements on the 200-ms scale showed high-affinity uptake. The affinity and maximal rate of the latt er were comparable to those in wild-type cells. Using a simple model f or a symmetrical facilitator, it was possible to simulate the experime ntally determined relation between apparent affinity and the time scal e used. The results suggest that high-affinity glucose transport is no t necessarily dependent on the presence of glucose-phosphorylating enz ymes. Apparent low-affinity uptake kinetics can arise as a consequence of an insufficient rate of removal of intracellular free glucose by p hosphorylation. This study underlines the need to differentiate betwee n influences of the translocator and of metabolism on the apparent kin etics of sugar uptake in yeast.