MIN-K CHANNELS FORM BY ASSEMBLY OF AT LEAST 14 SUBUNITS

Injection of min K mRNA into Xenopus oocytes results in expression of slowly activating voltage-dependent potassium channels, distinct from those induced by expression of other cloned potassium channels, The mi n K protein also differs in structure, containing only a single predic ted transmembrane domain. While it has been demonstrated that all othe r cloned potassium channels form by association of four independent su bunits, the number of min K monomers which constitute a functional cha nnel is unknown. In rat min K, replacement of Ser-69 by Ala (S69A) cau ses a shift in the current-voltage (I-V) relationship to more depolari zed potentials; currents are not observed at potentials negative to 0 mV. To determine the subunit stoichiometry of min K channels, wild-typ e and S69A subunits were coexpressed, Injections of a constant amount of wild-type mRNA with increasing amounts of S69A mRNA led to potassiu m currents of decreasing amplitude upon voltage commands to -20 mV. Ap plying a binomial distribution to the reduction of current amplitudes as a function of the different coinjection mixtures yielded a subunit stoichiometry of at least 14 monomers for each functional min K channe l, A model is presented for how min K subunits may form a channel.