LOCATION OF ION-BINDING SITES IN THE GRAMICIDIN CHANNEL BY X-RAY-DIFFRACTION

We report the first X-ray diffraction on gramicidin in its membrane-active form by using uniformly aligned multilayer samples of membranes containing gramicidin and ions (Tl+, K+, Ba2+, Mg2+ or without ions). Prom the difference electron density profiles, we found a pair of symmetrically located ion-binding sites for Tl+ at 9.6(±0.3) Å and for Ba2+ at 13.0(±0.2) Å from the midpoint of the gramicidin channel. The location of Ba2+-binding sites is near the ends of the channel, consistent with the experimental observation that divalent cations do not permeate but block the channel. The location of Tl+-binding sites is somewhat of a surprise. It was generally thought that monovalent cations bind to the first turn of the helix from the mouth of the channel. (It is now generally accepted that the gramicidin channel is a cylindrical pore formed by two monomers, each a single-stranded β6.3 helix and hydrogen-bonded head-to-head at their N termini.) But our experiment shows that the Tl+-binding site is either near the bottom of or below the first helix turn. © 1991.