Abstract
In our systematic survey of channel-like genes from the whole genome sequence of Ciona intestinalis, one of the ascidians, we found a gene with weak homology to a voltage-gated sodium channel. Its cDNA was cloned by RT-PCR from young adult. This protein contains an enzyme-like region instead of pore-forming region following the four transmembrane regions. The fourth transmembrane segment of this protein showed significant homology to the S4-segment of the voltage-gated channels, which is known to contain several positively charged amino acid residues periodically located at every third position and move across the membrane according to the membrane potential change. Therefore, we named this Ciona protein CiVSP (voltage-sensor containing protein). To test if the S4-like region works as voltage sensor in CiVSP, cRNA of CiVSP was injected to Xenopus oocyte and gating current was tried to be recorded using the two electrode voltage-clamp technique and the cut-open oocyte voltage-clamp technique. Robust gating current with voltage dependency was detected. The G-V curve of ON-currents showed similar voltage-dependency and the magnitude to those of OFF-currents. The truncated protein, lacking the C-terminal enzyme domain, still showed gating current. Further, when two of four positive charged amino acid residues in the S4-like segment were mutated to neutral ones, gating current was completely lost. These results suggest that voltage sensor of CiVSP operates in a similar way as that of voltage-gated channels. [Jpn J Physiol 54 Suppl:S130 (2004)]
