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POTASSIUM channels catalyse the permeation of Potassium sup + 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 alpha-subunit [1,2]. Here we describe a novel Potassium sup + channel (TOK1) from Saccharomyces cerevisiae that contains two P domains within one continuous polypeptide. Xenopus laevis oocytes expressing the channel exhibit a unique, outwardly rectifying, Potassium sup + -selective current. The channel is permeable to outward flow of ions at membrane potentials above the Potassium sup + equilibrium potential; its conduction-voltage relationship is thus sensitive to extracellular Potassium sup + 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 Potassium sup + channels [3,4], but in the opposite direction, a previously unrecognized channel behaviour. Our results identify a new class of Potassium sup + channel which is distinctive in both its primary structure and functional properties. Structural homologues of the channel are present in the genome of Caenorhabditis elegans.