THE MINK POTASSIUM CHANNEL EXISTS IN FUNCTIONAL AND NONFUNCTIONAL FORMS WHEN EXPRESSED IN THE PLASMA-MEMBRANE OF XENOPUS-OOCYTES

The minK protein induces a slowly activating voltage-dependent potassi um current when expressed in Xenopus oocytes. In order to measure the levels of minK protein in the plasma membrane, we have modified the mi nK gene by inserting a 9 amino acid epitope into the N-terminal domain of the protein sequence. When intact live oocytes are injected with t he modified minK RNA and subsequently incubated with an antibody to th is epitope, specific binding is detected, indicating that the N-termin al domain is extracellular. We found that when oocytes are injected wi th amounts of minK mRNA up to 50 ng, the levels of protein at the surf ace are proportional to the amount of injected mRNA. In contrast, the amplitude of the minK current recorded in the oocytes saturates at 1 n g of injected mRNA. Although the amplitude of the currents is not alte red by increasing mRNA levels above 1 ng, the kinetics of activation o f the current differ in oocytes with high or low levels of minK RNA. I n particular, activation is slower with higher levels of minK protein in the plasma membrane. Finally, we find that increasing intracellular cAMP levels, which increases the amplitude of minK currents, does not alter surface expression of the minK protein but produces a small inc rease in the rate of activation of the current. Our results support a model in which minK protein forms functional potassium channels by ass ociation with a factor endogenous to the oocyte.