Multiple sequence alignment of AChBP with nAChR monomers. The alignment was divided into subunit 1 monomers (alpha3, alpha4, alpha7) (top) and subunit 2 monomers (beta2, beta4, and alpha7) (bottom), in order to preserve separate residue numbering consistent with human alpha4 and beta2 nAChR. Side-chain residues contacting docked acetylcholine and nicotine are colored gold in subunit 1 and cyan in subunit 2. Additional residues forming the ligand binding pocket boundary but further from the ligands are shown in bold. Non-conserved residues that can be used to design isotype selective agonists are underlined, and marked by a "!" in the numbering line. AChBP residues contacting HEPES in the crystal structure of the complex  have a shaded background. Secondary structure elements are shown under the sequences : H=alpha helix, E = beta-strand, B=beta-bridge.