Voltage sensitivity and conformational change of isolated S4L45 fragments from sodium channels are tuned to proline

Peptide fragments reproducing the sequences of S4 segments extended with L45 linkers from the four homologous domains of the electric eel sodium channel were chemically synthesized and purified to allow circular dichroism studies in various solvents and conductance assays in planar lipid bilayers. Repeats III (with proline) and IV (lacking proline) present the lowest and highest helicities, respectively. The conformational transition (from helix to beta-strand) shown to occur on an increase of solvent dielectric constant is broader with repeat III. Analytical ultracentrifugation (interference fringe pattern) is consistent with a monodispersion of the peptide. In macroscopic conductance experiments, the proline containing peptides (repeats I, II and especially III) display higher voltage-sensitivities than repeat IV. The apparent and averaged number of monomers per intramembrane conducting aggregate is 4-5. The influence of proline is confirmed in similar experiments carried out on homologous S4 segments of repeat IV of the human skeletal muscle sodium channel comparing the wild type and an analogue where the fourth arginine was substituted with a proline. Thus, both conformational switching and voltage-sensitivity appear correlated to the presence and position of a single proline residue. Since voltage sensors are likely to experience different polarity environments in the channel open and closed states, our results suggest an alternative gating mechanism, i.e. a voltage-driven conformational change of S4L45s. The data also implies a plausible functional asymmetry, namely a "three- or four-stroke" activation sequentially involving the four domains of the sodium channel.