Structure, dynamics, and selectivity of the sodium channel blocker mu-conotoxin SIIIA

mu-SIIIA, a novel mu-conotoxin from Conus striatus, appeared to be a selective blocker of tetrodotoxin-resistant sodium channels in frog preparations. It also exhibited potent analgesic activity in mice, although its selectivity profile against mammalian sodium channels remains unknown. We have determined the structure of mu-SIIIA in aqueous solution and characterized its backbone dynamics by NMR and its functional properties electrophysiologically. Consistent with the absence of hydroxyprolines, mu-SIIIA adopts a single conformation with all peptide bonds in the trans conformation. The C-terminal region contains a well-defined helix encompassing residues 11-16, while residues 3-5 in the N-terminal region form a helix-like turn resembling 3 10-helix. The Trp12 and His16 side chains are close together, as in the related conotoxin mu-SmIIIA, but Asn2 is more distant. Dynamics measurements show that the N-terminus and Ser9 have larger-magnitude motions on the subnanosecond time scale, while the C-terminus is more rigid. Cys4, Trp12, and Cys13 undergo significant conformational exchange on microsecond to millisecond time scales. mu-SIIIA is a potent, nearly irreversible blocker of Na V1.2 but also blocks Na V1.4 and Na V1.6 with submicromolar potency. The selectivity profile of mu-SIIIA, including poor activity against the cardiac sodium channel, Na V1.5, is similar to that of the closely related mu-KIIIA, suggesting that the C-terminal regions of both are critical for blocking neuronal Na V1.2. The structural and functional characterization described in this paper of an analgesic mu-conotoxin that targets neuronal subtypes of mammalian sodium channels provides a basis for the design of novel analogues with an improved selectivity profile.