Abstract

The potassium channel from Streptomyces lividans is an integral membrane protein with sequence similarity to all known K1 channels, particularly in the pore region. X-ray analysis with data to 3.2 angstroms reveals that four identical subunits create an inverted teepee, or cone, cradling the selectivity filter of the pore in its outer end. The narrow 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 K1 channel signature sequence line the selectivity filter, which is held open by structural constraints to co-ordinate K1 ions but not smaller Na1 ions. The selectivity filter contains two K1 ions about 7.5 angstroms apart. This configuration promotes ion conduction by exploiting electro-static repulsive forces to overcome attractive forces between K1 ions and the selectivity filter. The architecture of the pore establishes the physical principles underlying selective K1 conduction. Potassium ions diffuse rapidly across cell