A new family of outwardly rectifying potassium channel proteins with two pore domains in tandem

POTASSIUM channels catalyse the permeation of K + ions across cellular membranes and are identified by a common structural motif, a highly conserved signature sequence of eight amino acids in the P domain of each channel's pore-forming α-subunit 1,2 . Here we describe a novel K + channel (TOK1) from Saccharomyces cerevisiae that contains two P domains within one continuous poly-peptide. Xenopus laevis oocytes expressing the channel exhibit a unique, outwardly rectifying, K + -selective current. The channel is permeable to outward flow of ions at membrane potentials above the K + equilibrium potential; its conduction–voltage relationship is thus sensitive to extracellular K + ion concentration. In excised membrane patches, external divalent cations block the channel in a voltage-dependent manner, and their removal in this configuration allows inward channel current. These attributes are similar to those described for inwardly rectifying K + channels 3,4 , but in the opposite direction, a previously unrecognized channel behaviour. Our results identify a new class of K + channel which is distinctive in both its primary structure and functional properties. Structural homo-logues of the channel are present in the genome of Caenorhabditis elegans .