mu-Conotoxin GIIIA interactions with the voltage-gated Na+ channel predicta clockwise arrangement of the domains

Voltage-gated Na+ channels underlie the electrical activity of most excitab le cells, and these channels are the targets of many antiarrhythmic, antico nvulsant, and local anesthetic drugs. The channel pore is formed by a singl e polypeptide chain, containing four different, but homologous domains that are thought to arrange themselves circumferentially to form the ion permea tion pathway. Although several structural models have been proposed, there has been no agreement concerning whether the four domains are arranged in a clockwise or a counterclockwise pattern around the pore, which is a fundam ental question about the tertiary structure of the channel. We have probed the local architecture of the rat adult skeletal muscle Na+ channel (mu1) o uter vestibule and selectivity filter using mu -conotoxin GIIIA (mu -CTX), a neurotoxin of known structure that binds in this region. Interactions bet ween the pore-forming loops from three different domains and four toxin res idues were distinguished by mutant cycle analysis. Three of these residues, Gln-14, Hydroxyproline-17 (Hyp-17), and Lys-16 are arranged approximately at right angles to each other in a plane above the critical Arg-13 that bin ds directly in the ion permeation pathway. Interaction points were identifi ed between Hyp-17 and channel residue Met-1240 of domain III and between Ly s-16 and Glu-403 of domain I and Asp-1532 of domain IV. These interactions were estimated to contribute -1.0 +/- 0.1, -0.9 +/- 0.3, and -1.4 +/- 0.1 k cal/mol of coupling energy to the native toxin-channel complex, respectivel y. mu -CTX residues Gln-14 and Arg-1, both on the same side of the toxin mo lecule, interacted with Thr-759 of domain II. Three analytical approaches t o the pattern of interactions predict that the channel domains most probabl y are arranged in a clockwise configuration around the pore as viewed from the extracellular surface.