CHARACTERIZATION OF THE OMEGA-CONOTOXIN TARGET - EVIDENCE FOR TISSUE-SPECIFIC HETEROGENEITY IN CALCIUM-CHANNEL TYPES

.omega.-Conotoxin GVIA (.omega.-CgTx-VIA) is a 27 amino acid peptide from the venom of the fish-hunting snail, Conus geographus, that blocks voltage-activated Ca channels. The characterization of a biologically active, homogeneous 125I-labeled monoiodinated Tyr22 derivative of .omega.-conotoxin GVIA and its use in binding and cross-linking studies are described. The 125-labeled toxin is specifically cross-linked to a receptor protein with an apparent Mr of 135,000. The stoichiometry between .omega.-conotoxin and nitrendipine binding sites in different chick tissue swas determined. Skeletal muscle has a high concentration of [3H]nitrendipine binding sites (> 1000 fmol/mg) but no detectable .omega.-conotoxin sites (< 7 fmol/mg). Brain microsomes have both binding sites, but .omega. conotoxin targets are in excess. These results, combined with recent electrophysiological studies (E. W. McCleskey, A. P. Fox, D. Feldman, L. J. Bruz, B. M. Olivera, R. W. Tsien, and D. Yoshikami, unpublished results), defined four types of Ca channels in chick tissues, N. T. Ln (.omega. sensitive), and Lm (.omega.-insensitive), and are consistent with the hypothesis that the .alpha.-subunits of certain neuronal Ca2+ channels (Ln, N) are the molecular targets of .omega.-conotoxin GVIA.