Ion channels form transmembrane water-filled pores that allow ions to cross membranes in a rapid and selective fashion, The amino acid residues that line these pores have been sought to reveal the mechanisms of ion conduction and selectivity(1-7). The pore (P) loop(8) is a stretch of residues that influences single-channel-current amplitude, selectivity among ions and open-channel blockade(2,3,5) and is conserved in potassium-channel subunits previously recognized to contribute to pore formation(5,9). To date, potassium-channel pores have been shown to form by symmetrical alignment of four P loops around a central conduction pathway(10-12). Here we show that the selectivity-determining pore region of the voltage-gated potassium channel of human heart through which the I-Ks current passes includes the transmembrane segment of the non-P-loop protein minK. Two adjacent residues in this segment of minK are exposed in the pore on either side of a short barrier that restricts the movement of sodium, cadmium and zinc ions across the membrane, Thus, potassium-selective pores are not restricted to P loops or a strict P-loop geometry.