presence of fermentable substrates. Rogers et al. [25] have observed that some proteins with short intracellular half-lives contain one or more regions rich in Pro (P), Glu (E), Ser (S), and Thr (T) that are flanked by positively charged amino acids. In addition, proteins containing Arg-Arg pairs are also rapidly degraded 1251. Taking these facts into account, it has
been recently predicted that K+ transport should be a target for rapid degradation because it contains three PEST sequences and six Arg-Arg pairs [26]. Our results confirm this prediction. Acknowledgements: We are very grateful to Dr. A. Rodriguez-Navarro for help in the Rb’ uptake measurements and for the stimulating discussions, to Dr. R. Serrano for the gift of the ATPase mutants and to Dr. C. Gancedo for critical reading of this manuscript. This work
was supported by Direction General de Investigation Cientifica y Tecnica (PB87-0294), by Commision of the European Communities (BAP 0389E), and by Fondo de Investigaciones Sanitarias (88/1448).