Ion channels are macromolecular pores in membranes that permit transfer of selected ions through the lipid bilayer. Most of these channels are composed of proteins or peptides. However, a non-proteinaceous calcium-selective channel, composed of two simple linear homopolymers (poly(3-hydroxybutyrate) and inorganic polyphosphate), has been discovered in Gram-negative and Gram-positive bacteria. Both polymers are ubiquitous components of prokaryotic and eukaryotic cells, and each has unique physical-chemical properties relevant to ion conduction. The two polymers associate to form channels in lipid bilayers in vitro that exhibit voltage-activated conductance, are selective for divalent over monovalent cations, are permeant to Ca, Sr, and Ba, and are blocked by transition metal cations. These studies will define the structural determinants and basic molecular mechanisms that give rise to these properties. The polymer components, either isolated from bacteria or prepared synthetically, will be incorporated into lipid bilayers, and single-channel currents will be examined by voltage-clamp techniques under different defined conditions. Studies are designed to determine how factors that influence bilayer fluidity affect conductance, permeance, selectivity, and voltage-gating. Additional studies will determine the influence of pH on cation selectivity, the way Mg and other cations block Ca conductance, and the possible effects of agents known to be agonists and antagonists of protein calcium channels. These studies of a simple but apparently ubiquitous prokaryotic channel should provide insights into the general mechanisms of ion transfer across bilayers. The channels may be a model for a 'minimalist' calcium channel that was the progenitor during evolution of the protein calcium channels of eukaryotes.
