Clockwise domain arrangement of the sodium channel revealed by mu-conotoxin (GIIIA) docking orientation

mu -Conotoxins (mu -CTXs) specifically inhibit Na+ flux by occluding the po re of voltage-gated Na+ channels. Although the three-dimensional structures of mu -CTXs are well defined, the molecular configuration of the channel r eceptor is much less certain; even the fundamental question of whether the four homologous Nat channel domains are arranged in a clockwise or counter- clockwise configuration remains unanswered. Residues Asp(762) and Glu(765) from domain II and Asp(1241) from domain III of rat skeletal muscle Naf cha nnels are known to be critical for Ec-CTX binding. We probed toxin-channel interactions by determining the potency of block of wild-type, D762K, E765K , and D1241C channels by wildtype and point-mutated mu -CTXs (R1A, Q14D, K1 1A, K16A, and R19A). Individual interaction energies for different toxin-ch annel pairs were quantified from the half-blocking concentrations using mut ant cycle analysis. We find that Asp(762) and Glu(765) interact strongly wi th Gln(14) and Arg(19) but not Arg(1) and that Asp(1241) is tightly coupled to Lys(16) but not Arg(1) or Lys(11). These newly identified toxin-channel interactions within adjacent domains, interpreted in light of the known as ymmetric toxin structure, fix the orientation of the toxin with respect to the channel and reveal that the four internal domains of Na+ channels age a rranged in a clockwise configuration as viewed from the extracellular surfa ce.