Involvement of kinases in glucose and fructose uptake by Saccharomyces cerevisiae.

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Abstract
Uptake of glucose, fructose and the nonmetabolizable analog 6-deoxyglucose was measured in wild-type S. cerevisiae and 2 mutant strains, 1 (hxk1 hxk2) lacking both hexokinase A (P-I) and B (P-II) but containing glucokinase (and hence able to grow on glucose but not fructose) and the other (hxk1 Hxk2 glk) also lacking glucokinase (and not able to grow on glucose either). Uptake of the nonmetabolized substances (i.e., 6-deoxyglucose in all three strains, fructose in the 2 mutants and glucose in the triple mutant) reached a plateau at or below the external concentration. The kinetic characteristics of uptake were determined from 5-s incubations by plotting velocity (V) vs. velocity/substrate concentration (V/S) curves. According to such plots, in the wild-type strain uptake had two components, high affinity uptake with Km values of .times. 1 mM for glucose and 6 mM for fructose and low affinity uptake with Km values of .apprx. 20 and 50 mM, respectively. The double kinase mutant showed both components for glucose but only the high Km component for fructose, while the triple kinase mutant showed only high Km uptake for both glucose and fructose. Genetic analysis showed that only in strains lacking both hexokinases (hxk1 hxk2) was the low Km system for fructose absent. Low Km uptake was restored to the triple mutant by introduction of the cloned wild-type genes: HXK1 or HXK2, for fructose uptake, and HXK1, HXK2 or GLK1, for glucose uptake. A phosphoglucose isomerase mutant had both low and high Km uptake for glucose. These results indicate the presence of 2 types of uptake mechanism for glucose and fructose in yeast, the functioning of one of which, the low Km system, is influenced by the cognate kinases.