Discovery and characterization of the short kappaA-conotoxins: a novel subfamily of excitatory conotoxins.

We have characterized the defining members of a novel subfamily of excitatory conotoxins, the short kappaA-conotoxins (kappaA(S)-conotoxins). kappaA-conotoxins PIVE and PIVF (kappaA-PIVE and kappaA-PIVF) were purified from Conus purpurascens venom. Both peptides elicited excitatory activity upon injection into fish. kappaA-PIVE was synthesized for further characterization. The excitatory effects of kappaA-PIVE in vivo were dose dependent, causing hyperactivity at low doses and rapid immobilization at high doses, symptomatic of a type of excitotoxic shock. Consistent with these observations, kappaA-PIVE caused repetitive action potentials in frog motor axons in vitro. Similar results have been reported for other structurally distinct conotoxin families; such peptides appear to be required by most fish-hunting cone snails for the rapid immobilization of prey. Unexpected structure-function relationships were revealed between these peptides and two families of homologous conotoxins: the alphaA-conotoxins (muscle nAChR antagonists) and kappaA-conotoxins (excitotoxins), which all share a common arrangement of cysteine residues (CC-C-C-C-C). Biochemically, the kappaA(S)-conotoxins more closely resemble the alphaA(S)-conotoxins than the other kappaA-conotoxin subfamily, the long kappaA-conotoxins (kappaA(L)-conotoxins); however, kappaA(S)- and alphaA(S)-conotoxins produce different physiological effects. In contrast, the kappaA(S)-and kappaA(L)-conotoxins that diverge in several biochemical characteristics are clearly more similar in their physiological effects.