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We report the discovery and functional characterization of αM-Conotoxin MIIIJ, a peptide from the venom of the fish-hunting cone snail  Conus magus . Injections of αM-MIIIJ induced paralysis in goldfish ( Carassius auratus ) but not mice. Intracellular recording from skeletal muscles of fish ( C. auratus ) and frog ( Xenopus laevis ) revealed that αM-MIIIJ inhibited postsynaptic nicotinic acetylcholine receptors (nAChRs) with an IC 50  of ~0.1 μM. With comparable potency, αM-MIIIJ reversibly blocked ACh-gated currents (I ACh ) of voltage-clamped  X. laevis  oocytes exogenously expressing nAChRs cloned from zebrafish ( Danio rerio ) muscle. αM-MIIIJ also protected against slowly-reversible block of I ACh  by α-bungarotoxin (α-BgTX, a snake neurotoxin) and α-conotoxin EI (α-EI, from  Conus ermineus  another fish hunter) that competitively block nAChRs at the ACh binding site. Furthermore, assessment by fluorescence microscopy showed that αM-MIIIJ inhibited the binding of fluorescently-tagged α-BgTX at neuromuscular junctions of  X. laevis,   C. auratus , and  D. rerio . (Note, we observed that αM-MIIIJ can block adult mouse and human muscle nAChRs exogenously expressed in  X. laevis  oocytes, but with IC 50 s ~100-times higher than those of zebrafish nAChRs.) Taken together, these results indicate that αM-MIIIJ inhibits muscle nAChRs and furthermore apparently does so by interfering with the binding of ACh to its receptor. Comparative alignments with homologous sequences identified in other fish hunters revealed that αM-MIIIJ defines a new class of muscle nAChR inhibitors from cone snails.

αM-Conotoxin MIIIJ Blocks Nicotinic Acetylcholine Receptors at Neuromuscular Junctions of Frog and Fish