A conotoxin from Conus textile with unusual posttranslational modifications reduces presynaptic Ca2+ influx

Cone snails are gastropod mollusks of the genus Conus that live in tropical marine habitats. They are predators that paralyze their prey by injection of venom containing a plethora of small, conformationally constrained pepti des (conotoxins), We report the identification, characterization, and struc ture of a gamma-carboxyglutamic acid-containing peptide, conotoxin epsilon- TxIX, isolated from the venom of the molluscivorous cone snail, Conus texti le. The disulfide bonding pattern of the four cysteine residues, an unparal leled degree of posttranslational processing including bromination, hydroxy lation, and glycosylation define a family of conotoxins that may target pre synaptic Ca2+ channels or act on G protein-coupled presynaptic receptors vi a another mechanism. This conotoxin selectively reduces neurotransmitter re lease at an Aplysia cholinergic synapse by reducing the presynaptic influx of Ca2+ in a slow and reversible fashion, The three-dimensional structure, determined by two-dimensional H-1 NMR spectroscopy, identifies an electrone gative patch created by the side chains of two gamma-carboxyglutamic acid r esidues that extend outward from a cavernous cleft. The glycosylated threon ine and hydroxylated proline enclose a localized hydrophobic region centere d on the brominated tryptophan residue within the constrained intercysteine region.