title: Structural basis for cooperative interactions of substituted 2-aminopyrimidines with the acetylcholine binding protein.
abstract: The nicotinic acetylcholine receptor (nAChR) and the acetylcholine binding protein (AChBP) are pentameric oligomers in which binding sites for nicotinic agonists and competitive antagonists are found at selected subunit interfaces. The nAChR spontaneously exists in multiple conformations associated with its activation and desensitization steps, and conformations are selectively stabilized by binding of agonists and antagonists. In the nAChR, agonist binding and the associated conformational changes accompanying activation and desensitization are cooperative. AChBP, which lacks the transmembrane spanning and cytoplasmic domains, serves as a homology model of the extracellular domain of the nAChRs. We identified unique cooperative binding behavior of a number of 4,6-disubstituted 2-aminopyrimidines to Lymnaea AChBP, with different molecular variants exhibiting positive, nH > 1.0, and negative cooperativity, nH < 1.0. Therefore, for a distinctive set of ligands, the extracellular domain of a nAChR surrogate suffices to accommodate cooperative interactions. X-ray crystal structures of AChBP complexes with examples of each allowed the identification of structural features in the ligands that confer differences in cooperative behavior. Both sets of molecules bind at the agonist-antagonist site, as expected from their competition with epibatidine. An analysis of AChBP quaternary structure shows that cooperative ligand binding is associated with a blooming or flare conformation, a structural change not observed with the classical, noncooperative, nicotinic ligands. Positively and negatively cooperative ligands exhibited unique features in the detailed binding determinants and poses of the complexes.
authors.affiliation: Department of Pharmacology, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, La Jolla, CA 92093;
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Kaczanowska
authors.affiliation: Department of Pharmacology, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, La Jolla, CA 92093;Department of Structural Biology, Weizmann Institute of Science, Rehovot 76100, Israel;
authors.forename: Michal
authors.initials: M
authors.lastname: Harel
authors.affiliation: Department of Pharmacology, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, La Jolla, CA 92093;
authors.forename: Zoran
authors.initials: Z
authors.lastname: Radić
authors.affiliation: Department of Pharmacology, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, La Jolla, CA 92093;Centre National de la Recherche Scientifique, Unité de Recherche Associée 2182, Institut Pasteur F-75015 Paris and Collège de France, F-75005 Paris, France; and changeux@noos.fr pwtaylor@ucsd.edu.
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Changeux
authors.affiliation: School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332.
authors.forename: M G
authors.initials: MG
authors.lastname: Finn
authors.affiliation: Department of Pharmacology, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, La Jolla, CA 92093; changeux@noos.fr pwtaylor@ucsd.edu.
authors.forename: Palmer
authors.initials: P
authors.lastname: Taylor
mesh_terms: D000109:Acetylcholine; D000818:Animals; D001667:Binding, Competitive; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000737:chemistry / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D011725:Pyridines / Q000737:chemistry / Q000378:metabolism; D011743:Pyrimidines / Q000737:chemistry* / Q000378:metabolism*; D014316:Tritium
chemical_list: C428944:AChBP protein, Lymnaea; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002352:Carrier Proteins; D011725:Pyridines; D011743:Pyrimidines; D014316:Tritium; C012180:2-aminopyrimidine; C082748:epibatidine; D000109:Acetylcholine
keywords: allostery; crystallography; nicotinic receptor