Transport in Saccharomyces cerevisiae

Abstract

INTRODUCTION
One of the most challenging problems for a cell is to regulate and integrate its metabolism. In the case of eukaryotic microorganisms, this must often be done in the face of changing internal needs and an uncertain external environment. Past studies addressing such control have focused on gene expression. Transport, on the other hand, has been cast as the means by which cells collect needed metabolites and precursors from their environment.1 To be sure, metabolite accumulation is a central function of transport systems, but such a parochial view of transport in Saccharomyces cerevisiae is becoming increasingly unsatisfactory. It ignores the complex internal structure of a yeast cell and the complexity of the membranes forming these structures.

The following example illustrates the need for adopting a more dynamic view of transport processes and their role in metabolic control. For the past 20 years, induction and repression of enzyme synthesis have been thought to be the principal means of shifting from one metabolic mode to another; the molecular events associated with operation of the lac operon form the paradigm. The model involves switching on synthesis of an enzyme when it is needed and switching it off again when the need is past. This is exactly what is observed for the allantoin-degradative enzymes in S. cerevisiae (Cooper and Lawther 1973; Lawther and Cooper 1973; Whitney et al. 1973). If allantoin or one of its degradation products is added to a culture of yeast, synthesis of the allantoin-degradative enzymes is induced. If a...