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Potassium channel with seven potassium ions bound This potassium channel, composed of 4 identical subunits, from the bacterium Streptomyces lividans, was crystallized as a complex with a monoclonal antibody (Fab fragments, not shown). That enhances the hydrophilic surfaces, and promotes crystallization, rather than aggregation and precipitation. Each identical subunit has two transmembrane alpha-helices, and a shorter third helix that dips into the central channel. A loop from the short helix interacts with 6 of the potassium ions shown. The seventh ion is in the center of the channel. The 6 ions are bound at the selectivity filter. Only 2 subunits are shown for better visibility of the ions. These ions are stripped of water and are solvated by oxygen atoms from the protein (shown in white). Ions that are too large will not fit, of course, but atoms that are too small (e.g. sodium) will not enter because they cannot be solvated by these fixed oxygen atoms. The seventh is solvated by molecules of water (not shown).1k4c (PDB) Zhou Y, Morais-Cabral JH, Kaufman A, MacKinnon R. Zhou Y, MacKinnon R. Zhou Y, MacKinnon R. |