Abstract
When we surveyed ion channel genes from the genome sequence of a tunicate, Ciona intestinalis, we discovered a novel putative membrane protein which has a voltage sensor motif with significant homology to voltage-gated channels, and a phosphatase domain just downstream of the transmembrane region. We termed it CiVSP (Ciona Voltage Sensor-containing Phosphatase). In fact, VSP showed phosphoinositide phosphatase activity and gating currents. We examined whether the voltage sensor functionally couples with the phosphatase domain of CiVSP. cRNAs encoding CiVSP and several types of phosphoinositide-sensitive ion channels were coinjected into Xenopus oocyte and the two-electrode voltage clamp recording was performed. Changes of the phosphatase activity with membrane potentials were detected by monitoring changes of ion currents through exogenously expressed channels. Channel activities were increased after hyperpolarization and were decreased by depolarization. It suggests that phosphatase activity of CiVSP is higher in a hyperpolarized condition than in a depolarized condition. These phenomena were never observed in oocytes injected only with channel cRNAs and in oocytes coinjected with a mutant CiVSP in which the phosphatase activity was eliminated. These results strongly suggest that CiVSP is a Voltage-Sensitive Phosphatase. CiVSP is the first report of a membrane protein which can transduce electrical signal into chemical signal without requiring ionic flow through membrane. [Jpn J Physiol 54 Suppl:S130 (2004)]
