Gramicidin A crystals contain two cation binding sites per channel

Abstract

GRAMICIDIN A, a pentadecapeptide antibiotic from Bacillus brevis, dimerises to form transmembrane channels in biological and synthetic membranes^1–4. These channels are permeable to alkali cations and protons, impermeable to anions and blocked by calcium ions^5–8. The high transport rate (∼10⁷ ions s⁻¹), moderate selectivity and simplicity of the gramicidin channel make it an attractive model for elucidating the mechanism of ion transport in biological membranes. Our previous X-ray crystallographic study showed that crystals of native gramicidin and of its complex with caesium ion contain cylindrical channels which are formed from helical dimers of gramicidin⁹. We also found that the binding of caesium leads to a large conformational change: the length of the channel decreases from 32 to 26 Å, whereas the diameter of the cylinder formed by the peptide main chain atoms increases from about 5 to 6.8 Å. We report here X-ray studies showing that the same conformational change is induced by the binding of potassium ion. Furthermore, the isomorphism of the K⁺-complex and Cs⁺-complex crystals enabled us to calculate difference Fourier projections and define the location of the ion-binding sites.