Most enzymes in Succlzuromyces cerevisiae are fairly
stable in maintaining their catalytic activities for long
periods under different metabolic conditions [l-4].
Sugar transport systems behave differently in this respect
since a rapid inactivation is observed upon inhibition
of protein synthesis [4]. This inactivation, that follows
first-order kinetics indicating half-lives in the
range of 1-5 h for these carriers [5 -81, is an energydependent
process stimulated by fermentable substrates.
The characteristics of this inactivation suggest that
it is due to proteolysis of these proteins [5-81. To see
whether a low stability is a peculiarity of the sugar
transports or also affects other carriers of the yeast
plasma membrane we have investigated the stability of
the K’ transport system.
K’ transport has been extensively studied in S. cerevisiae
[9-l 11. This transport is dependent on plasma membrane
potential [12] and exists in two interconvertible
forms with different kinetic constants depending on the
K’ content of the cells [13,14]. To investigate the stability
of this carrier we have followed its activity upon
inhibition of protein synthesis by addition of cycloheximide.