Here we present functional evidence for involvement of poly-(R)-3-hydroxybutyrate (PHB) and inorganic polyphosphate (polyP) in ion conduction and selection at the intracellular side of the Streptomyces lividans potassium channel, KcsA. At less than or equal to25 degreesC, KcsA forms channels in planar bilayers that display signal characteristics of PHB/polyP channels at the intracellular side; i.e., a preference for divalent Mg2+ cations at pH 7.2, and a preference for monovalent K+ cations at pH 6.8. Between 25 and 26 degreesC, KcsA undergoes a transition to a new conformation in which the channel exhibits high selectivity for K+, regardless of solution pH. This suggests that basic residues of the C-terminal polypeptides have moved closer to the polyP end unit, reducing its negative charge. The data support a supramolecular structure for KcsA in which influx of ions is prevented by the selectivity pore, whereas efflux of K+ is governed by a conductive core of PHB/polyP in partnership with the C-terminal polypeptide strands. (C) 2004 Elsevier Inc. All rights reserved.