The residual enzymatic phosphorylation activity of hexokinase II mutants is correlated with glucose repression in Saccharomyces cerevisiae

Saccharomyces cerevisiae mutants containing different point mutations in the HXK2 gene were used to study the relationship between phosphorylation by hexokinase II and glucose repression in yeast cells. Mutants showing different levels of hexokinase activity were examined for the degree of glucose repression as indicated by the levels of invertase activity. The levels of hexokinase activity and invertase activity showed a strong inverse correlation, with a few exceptions attributable to very unstable hexokinase II proteins. The in vivo hexokinase II activity was determined by measuring growth rates, using fructose as a carbon source. This in vivo hexokinase II activity was similarly inversely correlated with invertase activity. Several hxk2 alleles were transfered to multicopy plasmids to study the effects of increasing the amounts of mutant proteins. The cells that contained the multicopy plasmids exhibited less invertase and more hexokinase activity, further strengthening the correlation. These results strongly support the hypothesis that the phosphorylation activity of hexokinase II is correlated with glucose repression.