THE DIVALENT CATION-BINDING SITES OF GRAMICIDIN-A TRANSMEMBRANE ION-CHANNEL

The conductance of the gramicidin A single channels in glycerolmonooleate membranes is strongly reduced in the presence of Mn2+ cations. The nmr experiments were performed for N-terminal to N-terminal gramicidin A dimer formed by two right-handed single-stranded helixes incorporated into the sodium dodecyl sulfate micelles in the presence of Mn2+ ions. Dependence of the nonselective spin-lattice relaxation rates of the gramicidin A protons on Mn2+ concentration was analyzed to determine coordinates of the divalent cation binding sites. It is inferred that Mn2+ ions are bound at the channel mouths at distances of 6.4, 8.6, and 8.8 angstrom (+/- 2 angstrom) from the oxygen atoms of exposed carbonyl groups of D-Leu 12, 14, and 10, respectively. The bounded Mn2+ retains its hydrate shell, the size of which (almost-equal-to 6 angstrom) exceeds the inner pore diameter (almost-equal-to 4 angstrom). That makes the gramicidin A channel impermeable for divalent cations.