CATABOLITE INACTIVATION OF WILD-TYPE AND MUTANT MALTOSE TRANSPORT PROTEINS IN SACCHAROMYCES-CEREVISIAE

The maltose transporter of Saccharomyces cerevisiae is subject to rapi d, irreversible inactivation in the presence of glucose. Loss of trans port function was paralleled by a decrease in amount of transporter pr otein and most likely involves endocytosis and degradation of the prot ein in the vacuole, This (catabolite) inactivation of Mal61p was trigg ered not only by glucose but also by 2-deoxy-D-glucose, which cannot b e metabolized beyond 2-deoxy-D-glucose phosphate. The signal that targ ets membrane proteins specifically for catabolite inactivation is unkn own. To investigate whether or not specific modification of Mal61p tri ggers the inactivation putative protein kinase A and C phosphorylation sites were removed, and the transport activities and levels of the mu tant proteins upon addition of glucose were followed in time. Three Ma l61p mutants, i.e. S295A, T363A, and S487A, exhibited significantly re duced rates of inactivation in the presence of glucose. Likewise, in w ild-type Mal61p the rate of inactivation and degradation of the protei n paralleled each other in the case of T363A. On the contrary, for the S295A and S487A mutants the rates of protein degradation were slowed down more profoundly than was the loss of transport activity. These ob servations indicate that (i) some form of modification (e.,g. phosphor ylation) of the protein precedes breakdown, (ii) the modification inac tivates MaI61p, and (iii) the inactivation of Mal61p is not necessaril y followed by proteolytic degradation.