The minK potassium channel exists in functional and nonfunctional forms when expressed in the plasma membrane of Xenopus oocytes

Abstract

The minK protein induces a slowly activating voltage-dependent potassium current when expressed in Xenopus oocytes. In order to measure the levels of minK protein in the plasma membrane, we have modified the minK gene by inserting a 9 amino acid epitope into the N- terminal domain of the protein sequence. When intact live oocytes are injected with the modified minK RNA and subsequently incubated with an antibody to this epitope, specific binding is detected, indicating that the N-terminal domain is extracellular. We found that when oocytes are injected with amounts of minK mRNA up to 50 ng, the levels of protein at the surface are proportional to the amount of injected mRNA. In contrast, the amplitude of the minK current recorded in the oocytes saturates at 1 ng of injected mRNA. Although the amplitude of the currents is not altered by increasing mRNA levels above 1 ng, the kinetics of activation of the current differ in oocytes with high or low levels of minK RNA. In 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 increase in the rate of activation of the current. Our results support a model in which minK protein forms functional potassium channels by association with a factor endogenous to the oocyte.

Molecular Mechanism and Functional Significance of the MinK Control of the KvLQT1 Channel Activity

“…This model explains (i) the complex kinetic behavior of the slow KvLQT1/MinK channel after a long-lasting depolarization (compare Fig. 4, G and H) and (ii) previous results demonstrating that the kinetics of activation of the slow K ϩ channel formed in the Xenopus oocyte depends on the levels of the MinK protein present in the membrane (22,23). Thus, MinK appears to be a regulatory protein that finely tunes the KvLQT1 channel activity in a concentration-dependent manner.…”

Cytoplasmic and Extracellular IsK Peptides Activate Endogenous K and Cl Channels in Xenopus Oocytes

“…It has been suggested recently that IsK might become functional by interacting with either a rare lipid, a cytoskeletal protein, or a channel protein subunit (19). Regarding our data, we favor the latter proposal for two main reasons.…”

MinK-KvLQT1 Fusion Proteins, Evidence for Multiple Stoichiometries of the Assembled I Channel

“…Key experiments in Xenopus oocytes suggested that the MinK protein was a regulator of expressed channel activity and not sufficient by itself to form functional I sK channels (11,12). This work has been confirmed based on genetic linkage analysis of an inherited cardiac arrhythmia, the Long QT (LQT1) 1 syndrome and positional cloning strategies (13,14).…”