Key Role for Intracellular K + and Protein Kinases Sat4/Hal4 and Hal5 in the Plasma Membrane Stabilization of Yeast Nutrient Transporters

Abstract

ABSTRACT K + transport in living cells must be tightly controlled because it affects basic physiological parameters such as turgor, membrane potential, ionic strength, and pH. In yeast, the major high-affinity K + transporter, Trk1, is inhibited by high intracellular K + levels and positively regulated by two redundant “ hal otolerance” protein kinases, Sat4/Hal4 and Hal5. Here we show that these kinases are not required for Trk1 activity; rather, they stabilize the transporter at the plasma membrane under low K + conditions, preventing its endocytosis and vacuolar degradation. High concentrations (0.2 M) of K + , but not Na + or sorbitol, transported by undefined low-affinity systems, maintain Trk1 at the plasma membrane in the hal4 hal5 mutant. Other nutrient transporters, such as Can1 (arginine permease), Fur4 (uracil permease), and Hxt1 (low-affinity glucose permease), are also destabilized in the hal4 hal5 mutant under low K + conditions and, in the case of Can1, are stabilized by high K + concentrations. Other plasma membrane proteins such as Pma1 (H + -pumping ATPase) and Sur7 (an eisosomal protein) are not regulated by halotolerance kinases or by high K + levels. This novel regulatory mechanism of nutrient transporters may participate in the quiescence/growth transition and could result from effects of intracellular K + and halotolerance kinases on membrane trafficking and/or on the transporters themselves.