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 (T1+, K+, Ba2+, Mg2+ or without ions). From the difference electron density profiles, we found a pair of symmetrically located ion-binding sites for T1- at 9.6 (+/- 0.3) A and for Ba2+ at 13.0 (+/- 0.2) A from the midpoint of the gramicidin channel. The location of Ba(2+)-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 T1(+)-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 beta 6.3 helix and hydrogen-bonded head-to-head at their N termini.) But our experiment shows that the T1(+)-binding site is either near the bottom of or below the first helix turn.
Olah
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Physics Department, Rice University, Houston, TX 77251.
Huang
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Liu
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Wu
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D001665:Binding Sites; D002942:Circular Dichroism; D006096:Gramicidin / Q000737:chemistry* / Q000378:metabolism; D006860:Hydrogen Bonding; D007473:Ion Channels / Q000737:chemistry* / Q000378:metabolism; D007477:Ions; D014961:X-Ray Diffraction
D007473:Ion Channels; D007477:Ions; D006096:Gramicidin