Redistribution of Terbium Ions Across Acetylcholine Receptor-Enriched Membranes Induced by Agonist Desensitization

Using small-angle x-ray diffraction from centrifugally oriented acetylcholine receptor (AChR) enriched membranes coupled with anomalous scattering from terbium ions (Tb 3+) titrated into presumed Ca 2+ binding sites, we have mapped the distribution of Tb 3+ perpendicular to the membrane plane using a heavy atom refinement algorithm. We have compared the distribution of Tb 3+ in the closed resting state with that in the carbamylcholine-desensitized state. In the closed resting state we find 45 Tb 3+ ions distributed in 10 narrow peaks perpendicular to the membrane plane. Applying the same refinement procedure to the data from carbamylcholine desensitized AChR we find 18 fewer Tb 3+ ions in eight peaks, and slight rearrangements of Tb 3+ density in the peaks near the ends of the AChR ion channel pore. These agonist dependent changes in the Tb 3+ stoichiometry and distribution suggest a likely role for multivalent cations in stabilizing the different functional states of the AChR, and the changes in the Tb 3+ distribution at the two ends of the pore suggest a potential role for multivalent cations in the gating of the ion channel.