Changes in membrane potential affect ion channels and transporters, which then alter intracellular chemical conditions. Other signalling pathways coupled to membrane potential have been suggested but their underlying mechanisms are unknown. Here we describe a novel protein from the ascidian Ciona intestinalis that has a transmembrane voltage-sensing domain homologous to the S1-S4 segments of voltage-gated channels and a cytoplasmic domain similar to phosphatase and tensin homologue. This protein, named C. intestinalis voltage-sensor-containing phosphatase (Ci-VSP), displays channel-like 'gating' currents and directly translates changes in membrane potential into the turnover of phosphoinositides. The activity of the phosphoinositide phosphatase in Ci-VSP is tuned within a physiological range of membrane potential. Immunocytochemical studies show that Ci-VSP is expressed in Ciona sperm tail membranes, indicating a possible role in sperm function or morphology. Our data demonstrate that voltage sensing can function beyond channel proteins and thus more ubiquitously than previously realized.

Voltage-evoked signals play critical roles in neural activities, muscle contraction and exocytosis. Ciona voltage-sensor containing phosphatase (Ci-VSP) consists of the transmembrane voltage sensor domain (VSD) and a cytoplasmic domain of phosphoinositide phosphatase, homologous to phosphatase and tensin homologue deleted on chromosome 10 (PTEN). Previous experiments utilizing potassium channels as the sensor for phosphoinositides have demonstrated that phosphatase activities of Ci-VSP are voltage dependent. However, it still remained unclear whether enzyme activity is activated by depolarization or hyperpolarization. Further, a large gap in voltage dependency was found between the charge movement of the VSD and potassium channel-reporting phosphatase activities. In this study, voltage-dependent dynamics of phosphoinositides mediated by Ci-VSP were examined by confocal imaging and electrical measurements in Xenopus oocytes. Imaging of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P 2 ) using green fluorescent protein (GFP)-tagged pleckstrin homology (PH) domains from phospholipase C δ subunit (PLC-δ) showed that PtdIns(4,5)P 2 concentration is reduced during depolarization. In the presence of Ci-VSP, IRK1 channels with higher sensitivity to phosphoinositide than GIRK2 channels decreased their magnitude during depolarization over 0 mV, indicating that the PtdIns(4,5)P 2 level is reduced upon depolarization. KCNQ2/3 channels coexpressed with Ci-VSP exhibited voltage-dependent decay of the outward current that became sharper with higher depolarization in a voltage range up to 100 mV. These results indicate that Ci-VSP has an activity that depletes PtdIns(4,5)P 2 unlike PTEN and that depolarization-activated voltage sensor movement is translated into activation of phosphatase activity.