Analysis of AChBP structures. Cluster analysis of the distances between the αC atoms of C190 (at the tip of loop C) and TrpB in Aplysia AChBPs. The distances were fitted by k clusters. (A) Example AChBP structures. Dashed line is the αC–αC distance. (Bottom) Putative relationship between structures and the catch-and-hold scheme for AChRs. (Inset) Unliganded Torpedo AChR (Protein Data Bank accession no. 2bg9; Unwin, 2005); a transmitter-binding site region is boxed. (B) Scatter plot of the distances, with each structure assigned to one of three populations. The mean ± SD for each population was 17.4 ± 0.5 (white), 13.6 ± 0.7 (blue), and 11.9 ± 0.3 (red) Å. The ligands were (from left to right): 4OH-DMXBA, anabasine, apo- (arrow), cocaine, compound 31, compound 35, d-tubocurarine, DMXBA, epibatidine, HEPES, imidacloprid, methylcaconitine, strychnine, sulfate, thiacloprid, tropisetron, Y53C-MMTS-Apo, Y53C-MMTS-ACh, and lobeline (see Materials and methods for Protein Data Bank accession nos.). (C) Model selection (number of populations) using the AICc goodness-of-fit index. The distribution of distances was optimally described by two or three populations.