THE STRUCTURE OF THE POTASSIUM CHANNEL - MOLECULAR-BASIS OF K+ CONDUCTION AND SELECTIVITY

The potassium channel from Streptomyces lividans is an integral membra ne protein with sequence similarity to all known K+ channels, particul arly in the pore region. X-ray analysis with data to 3.2 angstroms rev eals that four identical subunits create an inverted teepee, or cone, cradling the selectivity filter of the pore in its outer end, The narr ow selectivity filter is only 12 angstroms long, whereas the remainder of the pore is wider and lined with hydrophobic amino acids, A large water-filled cavity and helix dipoles are positioned so as to overcome electrostatic destabilization of an ion in the pore at the center of the bilayer. Main chain carbonyl oxygen atoms from the K+ channel sign ature sequence line the selectivity filter, which is held open by stru ctural constraints to coordinate K+ ions but not smaller Na+ ions. The selectivity filter contains two K+ ions about 7.5 angstroms apart. Th is configuration promotes ion conduction by exploiting electrostatic r epulsive forces to overcome attractive forces between K+ ions and the selectivity filler. The architecture oi the pore establishes the physi cal principles underlying selective K+ conduction.