Exploring Water in the Hydrophobic Cavity of the Bacterial Potassium Ion Channel KcsA

Abstract

In many protein ion channels found in biological membranes, a central cavity is formed between the membrane-spanning subunits of the protein, though which ions flow selectively. There is no general consensus regarding whether nonpolar cavities inside proteins are water filled or empty, or what conditions are necessary and sufficient for these two cases. The potassium channel from the soil bacteria Streptomyces lividans named KcsA, is the first of the potassium-selective channel proteins whose structure has been solved crystallographically (Doyle, D.A. et al., Science 280, 1998). KcsA has become the principal model for the pore domain in voltage-gated K+, Na+, and Ca2+ channels due to highly conserved structural motifs in the pore domain characteristic for these channels. The cavity dimensions for KcsA inferred from crystallographic structures allow lessthan 20 water molecules to be accommodated. Crystallographic maps, however, do not typically reveal the true conformation of the channel in a fluid lipid environment, nor can they reveal water in cavities due to its high thermal disorder and low scattering density. In this report, we present neutron diffraction data on KcSA pore domain incorporated in lipid liquid crystalline phases. Hydrogen to Deuterium substitution is used to determine the amount of water and its distribution across the pore domain of the channel.