CELL BIOLOGY AND METABOLISM
Catabolite Inactivation of Wild-type and Mutant Maltose Transport Proteins in Saccharomyces cerevisiae *

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The maltose transporter of Saccharomyces cerevisiae is subject to rapid, irreversible inactivation in the presence of glucose. Loss of transport function was paralleled by a decrease in amount of transporter protein and most likely involves endocytosis and degradation of the protein in the vacuole. This (catabolite) inactivation of Mal61p was triggered not only by glucose but also by 2-deoxy-d-glucose, which cannot be metabolized beyond 2-deoxy-d-glucose phosphate. The signal that targets membrane proteins specifically for catabolite inactivation is unknown. To investigate whether or not specific modification of Mal61p triggers the inactivation, putative protein kinase A and C phosphorylation sites were removed, and the transport activities and levels of the mutant proteins upon addition of glucose were followed in time. Three Mal61p mutants, i.e. S295A, T363A, and S487A, exhibited significantly reduced rates of inactivation in the presence of glucose. Likewise, in wild-type Mal61p the rate of inactivation and degradation of the protein 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 observations indicate that (i) some form of modification (e.g. phosphorylation) of the protein precedes breakdown, (ii) the modification inactivates Mal61p, and (iii) the inactivation of Mal61p is not necessarily followed by proteolytic degradation.

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This work was supported by the Ministry of Economic Affairs, the Ministry of Education, Culture and Science, the Ministry of Agriculture, Nature Management and Fishery in the framework of an industrial relevant research programme of the Netherlands Association of Biotechnology Centres in the Netherlands (ABON), by a grant from the Royal Gist-Brocades NV, Delft, The Netherlands, and by the Foundation for Technical Sciences (Stichting Technische Wetenschappen) with financial aid from the Netherlands Organization for Scientific Research (Nederlandse Organisatie voor Wetenschappelijk Onderzoek). The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.