Probing the Redox States of Sodium Channel Cysteines at the Binding Site of μO-Conotoxin GVIIJ

mu O -Conotoxin GVIIJ is a 35-amino acid peptide that readily blocks six of eight tested Na(V)1 subunit isoforms of voltage-gated sodium channels. mu O -GVIIJ is unusual in having an S-cysteinylated cysteine (at residue 24). A proposed reaction scheme involves the peptidechannel complex stabilized by a disulfide bond formed via thioldisulfide exchange between Cys24 of the peptide and a Cys residue at neurotoxin receptor site 8 in the pore module of the channel (specifically, Cys910 of rat Na(V)1.2). To examine this model, we synthesized seven derivatives of mu O -GVIIJ in which Cys24 was disulfide-bonded to various thiols (or SR groups) and tested them on voltage-clamped Xenopus laevis oocytes expressing Na(V)1.2. In the proposed model, the SR moiety is a leaving group that is no longer present in the final peptidechannel complex; thus, the same koff value should be obtained regardless of the SR group. We observed that all seven derivatives, whose kon values varied over a 30-fold range, had the same koff value. Concordant results were observed with Na(V)1.6, for which the koff was 17-fold larger. Additionally, we tested two mu O -GVIIJ derivatives (where SR was glutathione or a free thiol) on two Na(V)1.2 Cys replacement mutants (Na(V)1.2[C912A] and Na(V)1.2[C918A]) without and with reduction of channel disulfides by dithiothreitol. The results indicate that Cys910 in wild-type Na(V)1.2 has a free thiol and conversely suggest that in Na(V)1.2[C912A] and Na(V)1.2[C918A], Cys910 is disulfide-bonded to Cys918 and Cys912, respectively. Redox states of extracellular cysteines of sodium channels have hitherto received scant attention, and further experiments with GVIIJ may help fill this void.