Voltage-sensing phosphatase: actions and potentials

Abstract

Voltage sensors have been well studied in voltage-gated ion channels for neuronal excitation and muscle contraction. The recent discovery of a voltage-sensing phosphatase, VSP, has changed the idea that voltage sensors are unique to ion flux through membranes. Recent findings on mechanisms and potential applications of VSP are reviewed.

Figure 1. Topology of voltage-sensing phosphatase (VSP) and charge movements of the voltage sensor

A, the S4 segment in the voltage sensor (dotted box) contains multiple positively charged residues (red). Amino acids from ascidian and teleost VSP (Ci-VSP and Dr-VSP) are compared with those of the Drosophila Shaker potassium channel. B, ‘gating’ or sensing currents evoked at 160 mV under whole-cell patch clamp from tsA201 cells transfected with Ci-VSP (blue) and Dr-VSP (red). C, charge–voltage (Q–V) relations of Ci-VSP (blue) and Dr-VSP (red) (modified from Hossain et al. 2008). Normalized values of Off-charges are shown as mean and standard deviations, collected from 6 and 5 cells for Ci-VSP and Dr-VSP, respectively. Maximum Off-charges were 2.89 ± 0.3 pC pF⁻¹ and 0.11 ± 0.1 pC pF⁻¹ for Ci-VSP and Dr-VSP, respectively.

Figure 2. Phosphoinositide consensus sequence and substrate specificity

A, amino acid alignment of the active centres of PTENs and VSPs. Human has two orthologue genes of VSP, TPTE and TPIP. Both genes give rise to several alternatively spliced variants (Walker et al. 2001). Mouse genome has one VSP-like gene, mTPTE, that gives rise to at least three alternatively spliced variants including TPTE-A. There is one amino acid difference between VSP and PTEN (glycine for VSP and alanine, for PTEN). B, comparison of enzyme activities of Ci-VSP and human PTEN using a malachite green assay (from Iwasaki et al. 2008). Activities toward a panel of synthetic di-C₁₆-phosphoinositides were determined. The relative activities of Ci-VSP and PTEN for the panel of phospholipids were calculated from their specific activities divided by the specific activities against PI(3,4,5)P₃.

Figure 3. KCNQ2/3 current decay induced by VSP expressed in oocyte and tsA201 cell

A, schematic diagram of regulation of K⁺ channel activity by changes in the concentration of PI(4,5)P₂ induced by the dephosphorylating activity of VSP. B and C, two examples of voltage-induced change of VSP enzyme activities reported by KCNQ2/3 channels are shown. B, traces of KCNQ2/3 currents from a Xenopus oocyte coexpressing KCNQ2/3 and Ci-VSP (Murata et al. 2005). Voltage was stepped in 10 mV increments ranging from −20 to +60 mV from a holding potential of −60 mV. Note that the decay becomes sharper as the voltage increases. C, traces of KCNQ2/3 currents from HEK293 cells coexpressing KCNQ2/3 and Dr-VSP (Hossain et al. 2008). Voltage was stepped in 20 mV increments ranging from −60 to +100 mV from a holding potential of −80 mV.