TRK2 IS NOT A LOW-AFFINITY POTASSIUM TRANSPORTER IN SACCHAROMYCES-CEREVISIAE

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 trkl1 Delta 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 trk1 Delta TRK2(D) cells increases the V-max of the moderate-affinity component, and this component disappears in trk1 Delta trk2 Delta cells. In contrast, the low-affinity component of Rb+ influx in trk1 Delta TRK2 cells is not affected by mutations in TRK2. Consistent with the different levels of activity of the moderate-affinity Rb+ influx, trk1 Delta TRK2 cells grow slowly in micromolar K+, trk1 Delta TRK2(D) cells grow rapidly, and trk1 Delta trk2 Delta cells fail to grow. The existence of a unique K+ uptake system composed of several proteins is also discussed.