Expression of minK protein in Xenopus oocytes induces a slowly activating, voltage-dependent, potassium-selective current. Point mutations in minK that alter current gating kinetics, ion selectivity, pharmacology, and response to protein kinase C all support the notion that minK is a structural protein for a channel-type transporter. Yet, minK has just 130 amino acids and a single transmembrane domain. Though larger cloned potassium channels form functional channels through tetrameric subunit association, the subunit composition of minK is unknown. Subunit stoichiometry was determined by coexpression of wild-type minK and a dominant lethal point mutant of minK, which reaches the plasma membrane but passes no current. The results support a model for complete minK potassium channels in which just two minK monomers are present, with other, as yet unidentified, non-minK subunits.

MinK has neither the P region nor signature sequence that characterizes pore-forming subunits of all known K+ channels. A specific minK region has now been identified that affects external blockade by 2 common probes of K+ channel pores. When mutated to cysteine, residues in this region render minK susceptible to covalent blockade by methanethiosulfonate ethylsulfonate and alter reversible inhibition by tetraethylammonium. The 2 blockers are found to share overlapping binding site determinants and to interact. Since inhibition by external tetraethylammonium is sensitive to voltage and to the internal concentration of permeant ions, we argue that tetraethylammonium blocks by occluding the external end of a water-filled transmembrane pore. These findings support the view that minK is directly involved in forming a K+-selective ion conduction pathway.

MinK is a transmembrane protein of 130 amino acids found in the kidney, heart, and vestibular system of mammals. Its expression in Xenopus laevis oocytes induces a voltage-dependent potassium current similar to that seen in vivo. Indirect evidence has fueled speculation that function requires association of MinK and another protein endogenous to oocytes and native tissues. In this report, we show that direct covalent modification of an oocyte membrane protein alters properties of the MinK ion conduction pore; modified channels exhibit decreased potassium conduction and increased permeability to sodium and cesium. The modifying reagents, two membrane-impermeant, sulfhydryl-specific methanethiosulfonate derivatives, react only from the extracellular solution at rates that are determined by the conformational state of the channel. These findings indicate that MinK is intimately associated with an oocyte protein whose exposure to the external solution changes during channel gating and which acts with MinK to establish ion conduction pore function.

MinK is a widely expressed protein of relative molecular mass approximately 15K that forms potassium channels by aggregation with other membrane proteins. MinK governs ion channel activation, regulation by second messengers, and the function and structure of the ion conduction pathway. Association of minK with a channel protein known as KvLQT1 produces a voltage-gated outward K+ current (I[sK]) resembling the slow cardiac repolarization current (I[Ks]). HERG, a human homologue of the ether-a-go-go gene of the fruitfly Drosophila melanogaster, encodes a protein that produces the rapidly activating cardiac delayed rectifier (I[Kr]). These two potassium currents, I(Ks) and I(Kr), provide the principal repolarizing currents in cardiac myocytes for the termination of action potentials. Although heterologously expressed HERG channels are largely indistinguishable from native cardiac I(Kr), a role for minK in this current is suggested by the diminished I(Kr) in an atrial tumour line subjected to minK antisense suppression. Here we show that HERG and minK form a stable complex, and that this heteromultimerization regulates I(Kr) activity. MinK, through the formation of heteromeric channel complexes, is thus central to the control of the heart rate and rhythm.