UC Irvine

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-subunit1,2. 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+ channels3,4, 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.