Predatory sea snails from the Conus family produce a variety of venomous small helical peptides called conantokins that are rich in γ-carboxyglutamic acid (Gla) residues. As potent and selective antagonists of the N-methyl-d-aspartate receptor, these peptides are potential therapeutic agents for a variety of neurological conditions. The two most studied members of this family of peptides are con-G and con-T. Con-G has Gla residues at sequence positions 3, 4, 7, 10, and 14, and requires divalent cation binding to adopt a helical conformation. Although both Ca2+ and Mg2+ can fulfill this role, Ca2+ induces dimerization of con-G, whereas the Mg2+-complexed peptide remains monomeric. A variant of con-T, con-T[K7γ] (γ is Gla), contains Gla residues at the same five positions as in con-G and behaves very similarly with respect to metal ion binding and dimerization; each peptide binds two Ca2+ ions and two Mg2+ ions per helix. To understand the difference in metal ion selectivity, affinity, and the dependence on Ca2+ for dimer formation, we report here the structure of the monomeric Cd2+/Mg2+–con-T[K7γ] complex, and, by comparison with the previously published con-T[K7γ]/Ca2+ dimer structure, we suggest explanations for both metal ion binding site specificity and metal-ion-dependent dimerization.
We thank the following for financial support: GR-179 (085P100549) from the Michigan Economic Development Corporation (to J.H.G.), NIH Grant HL019982 (to F.J.C.), and NIH Grant GM0638947 (to J.H.G.). CCDC-667723 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.