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 min K protein also differs in structure, containing only a single predicted transmembrane domain. While it has been demonstrated that all other cloned potassium channels form by association of four independent subunits, the number of min K monomers which constitute a functional channel is unknown. In rat min K, replacement of Ser-69 by Ala (S69A) causes a shift in the current-voltage (I-V) relationship to more depolarized potentials; currents are not observed at potentials negative to 0 mV. To determine the subunit stoichiometry of min K channels, wild-type and S69A subunits were coexpressed, Injections of a constant amount of wild-type mRNA with increasing amounts of S69A mRNA led to potassium currents of decreasing amplitude upon voltage commands to -20 mV. Applying 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 channel, A model is presented for how min K subunits may form a channel.