Metal ion determinants of conantokin dimerization as revealed in the X-ray crystallographic structure of the Cd²⁺/Mg²⁺–con-T[K7γ] complex

Abstract

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.