The metal-free and calcium-bound structures of a γ-carboxyglutamic acid-containing contryphan from Conus marmoreus, glacontryphan-M

Glacontryphan-M, a novel calcium-dependent inhibitor of L-type voltage-gated Ca2+ channels expressed in mouse pancreatic beta-cells, was recently isolated from the venom of the cone snail Conus marmoreus (Hansson, K., Ma, X., Eliasson, L., Czerwiec, E., Furie, B., Furie, B. C., Rorsman, P., and Stenflo, J. ( 2004) J. Biol. Chem. 278, 32453-32463). The conserved disulfide-bonded loop of the contryphan family of conotoxins including a D-Trp is present; however, unique to glacontryphan-M is a histidine within the intercysteine-loop and two gamma-carboxy-glutamic acid (Gla) residues, formed by post-translational modification of glutamic acid. The two calcium-binding Gla residues are located in a four residue N-terminal extension of this contryphan. To better understand the structural and functional significance of these residues, we have determined the structure of glacontryphan-M using two-dimensional H-1 NMR spectroscopy in the absence and presence of calcium. Comparisons of the glacontryphan-M structures reveal that calcium binding induces structural perturbations within the Gla-containing N terminus and the Cys(11)-Cys(5)-Pro(6) region of the intercysteine loop. The backbone of N-terminal residues perturbed by calcium, Gla(2) and Ser(3), moves away from the His(8) and Trp(10) aromatic rings and the alignment of the D-Trp(7) and His(8) aromatic rings with respect to the Trp(10) rings is altered. The blockage of L-type voltage-gated Ca2+ channel currents by glacontryphan-M requires calcium binding to N-terminal Gla residues, where presumably histidine and tryptophan may be accessible for interaction with the channel. The backbone Calpha conformation of the intercysteine loop of calcium-bound glacontryphan-M superimposes on known structures of contryphan-R and Vn (0.83 and 0.66 Angstrom, respectively). Taken together these data identify that glacontryphan-M possesses the canonical contryphan intercysteine loop structure, yet possesses critical determinants necessary for a calcium-induced functionally required conformation.