Gating of single Shaker potassium channels in Drosophila muscle and in Xenopus oocytes injected with Shaker mRNA.

The voltage-dependent gating mechanism of single A-type potassium channels coded for by the Shaker locus of Drosophila was studied by single-channel recording. A-type channels expressed in Xenopus oocytes injected with Shaker B and Shaker D mRNA exhibited gating and voltage dependence that were qualitatively similar to those of the native Shaker A-types channels from embryonic myotubes. In all three channel types the molecular transition rates leading to the first opening were voltage-dependent, whereas all transitions after the first opening, including inactivation, were independent of voltage. While these channels exhibit some quantitative differences in their transition rates that account for the observed differences in macroscopic currents, in all three cases the voltage dependence of the macroscopic currents is determined by a voltage dependence in the time to first opening. This gating mechanism is similar to that of the vertebrate voltage-gated sodium channel and, together with the sequence similarities in the S4 region of the proteins, suggests a conserved mechanism for activation and inactivation.