Transmembrane ion transport by polyphosphate/poly-(R)-3-hydroxybutyrate complexes

Transmembrane ion transport, a critical process in providing energy for cell functions, is carried out by pore-forming macromolecules capable of discriminating among very similar ions and responding to changes in membrane potential. It is widely regarded that ion channels are exclusively proteins, relatively late arrivals in cell evolution. Here we discuss the formation of ion-selective, voltage-activated channels by complexes of two simple homopolymers, namely, inorganic polyphosphates (polyPs) and poly-(R)-3-hydroxybutyrates (PHBs), derived from phosphate and acetate, respectively. Each has unique molecular characteristics that facilitate ion selection, solvation, and transport. Complexes of the two polymers, isolated from bacterial plasma membranes or prepared from the synthetic polymers, form voltage-dependent, Ca2+-selective channels in planar lipid bilayers that are selective for divalent over monovalent cations, permeant to Ca2+, Sr2+, and Ba2+, and blocked by transition metal cations in a concentration-dependent manner. Recently, both polyP and PHB have been found to be components of ion-conducting proteins: namely, the human erythrocyte Ca2+-ATPase pump and the Streptomyces lividans potassium channel. The contribution of polyP and PHB to ion selection and/or transport in these proteins is yet unknown, but their presence gives rise to the hypothesis that these and other ion transporters are supramolecular structures in which proteins, polyP, and PHB cooperate in forming well-regulated and specific cation transfer systems.