TRK2 is not a low-affinity potassium transporter in Saccharomyces cerevisiae

Abstract

TRK1 and TRK2 encode proteins involved in K+ uptake in Saccharomyces cerevisiae. A kinetic study of Rb+ influx in trk1 TRK2, trk1 TRK2(D), and trk1 trk2 mutants reveals that TRK2 shows moderate affinity for Rb+. K+- starved trk1Δ TRK2 cells show a low-affinity component accounting for almost the total V(max) of the influx and a moderate-affinity component exhibiting a very low V(max). Overexpression of TRK2 in trk1Δ TRK2(D) cells increases the V(max) of the moderate-affinity component, and this component disappears in trk1Δ trk2Δ cells. In contrast, the low-affinity component of Rb+ influx in trk1Δ TRK2 cells is not affected by mutations in TRK2. Consistent with the different levels of activity of the moderate-affinity Rb+ influx, trk1Δ TRK2 cells grow slowly in micromolar K+, trk1Δ TRK2(D) cells grow rapidly, and trk1Δ trk2Δ cells fail to grow. The existence of a unique K+ uptake system composed of several proteins is also discussed.