The hydrophobic nature of a novel membrane interface regulates the enzyme activity of a voltage-sensing phosphatase

Structural change of the cytoplasmic N-terminus and S1 segment of voltage-sensing phosphatase reported by Anap

BACKGROUND

Voltage-sensing phosphatase contains a structurally conserved S1-S4-based voltage-sensor domain, which undergoes a conformational transition in response to membrane potential change. Unlike that of channels, it is functional even in isolation and is therefore advantageous for studying the transition mechanism, but its nature has not yet been fully elucidated. This study aimed to address whether the cytoplasmic N-terminus and S1 exhibit structural change.

METHODS

Anap, an environment-sensitive unnatural fluorescent amino acid, was site-specifically introduced to the voltage sensor domain to probe local structural changes by using oocyte voltage clamp and photometry. Tetramethylrhodamine was also used to probe some extracellularly accessible positions. In total, 51 positions were investigated.

RESULTS

We detected robust voltage-dependent signals from widely distributed positions including N-terminus and S1. In addition, response to hyperpolarization was observed at the extracellular end of S1, reflecting the local structure flexibility of the voltage-sensor domain in the down-state. We also found that the mechanical coupling between the voltage-sensor and phosphatase domains affects the depolarization-induced optical signals but not the hyperpolarization-induced signals.

CONCLUSIONS

These results fill a gap between the previous interpretations from the structural and biophysical approaches and should provide important insights into the mechanisms of the voltage-sensor domain transition as well as its coupling with the effector.

Sugar binding of sodium-glucose cotransporters analyzed by voltage-clamp fluorometry

Sugar absorption is crucial for life and relies on glucose transporters, including sodium-glucose cotransporters (SGLTs). Although the structure of SGLTs has been resolved, the substrate selectivity of SGLTs across diverse isoforms has not been determined owing to the complex substrate-recognition processes and limited analysis methods. Therefore, this study used voltage-clamp fluorometry (VCF) to explore the substrate-binding affinities of human SGLT1 in Xenopus oocytes. VCF analysis revealed high-affinity binding of D-glucose and D-galactose, which are known transported substrates. D-fructose, which is not a transported substrate, also bound to SGLT1, suggesting potential recognition despite the lack of transport activity. VCF analysis using the T287N mutant of the substrate-binding pocket, which has reduced D-glucose transport capacity, showed that its D-galactose-binding affinity exceeded its D-glucose-binding affinity. This suggests that the change in the VCF signal was due to substrate binding to the binding pocket. Both D-fructose and L-sorbose showed similar binding affinities, indicating that SGLT1 preferentially binds to pyranose-form sugars, including D-fructopyranose. Electrophysiological analysis confirmed that D-fructose binding did not affect the SGLT1 transport function. The significance of the VCF assay lies in its ability to measure sugar-protein interactions in living cells, thereby bridging the gap between structural analyses and functional characterizations of sugar transporters. Our findings also provide insights into SGLT substrate selectivity and the potential for developing medicines with reduced side effects by targeting non-glucose sugars with low bioreactivity.

Role of K364 next to the active site cysteine in voltage-dependent phosphatase activity of Ci-VSP

Voltage-sensing phosphatase (VSP) consists of the voltage sensor domain (VSD) similar to that of voltage-gated ion channels and the cytoplasmic phosphatase region with remarkable similarity to the phosphatase and tensin homolog deleted on chromosome 10 (PTEN). Membrane depolarization activates VSD, leading to dephosphorylation of three species of phosphoinositides (phosphatidylinositol phosphates (PIPs)), PI(3,4,5)P₃, PI(4,5)P₂, and PI(3,4)P₂. VSP dephosphorylates 3- and 5-phosphate of PIPs, unlike PTEN, which shows rigid 3-phosphate specificity. In this study, a bioinformatics search showed that some mammals have VSP orthologs with amino acid diversity in the active center motif, Cx₅R, which is highly conserved among protein tyrosine phosphatases and PTEN-related phosphatases; lysine next to the active site cysteine in the Cx₅R motif was substituted for methionine in VSP orthologs of Tasmanian devil, koala, and prairie deer mouse, and leucine in opossum. Since lysine at the corresponding site in PTEN is known to be critical for enzyme activities, we attempted to address the significance of amino acid diversity among VSP orthologs at this site. K364 was changed to different amino acids in sea squirt VSP (Ci-VSP), and voltage-dependent phosphatase activity in Xenopus oocyte was studied using fluorescent probes for PI(4,5)P₂ and PI(3,4)P₂. All mutants retained both 5-phosphatase and 3-phosphatase activity, indicating that lysine at this site is dispensable for 3-phosphatase activity, unlike PTEN. Notably, K364M mutant showed increased activity both of 5-phosphatase and 3-phosphatase compared with the wild type (WT). It also showed slower kinetics of voltage sensor motion. Malachite green assay of K364M mutant did not show significant difference of phosphatase activity from WT, suggesting tighter interaction between substrate binding and voltage sensing. Mutation corresponding to K364M in the zebrafish VSP led to enhanced voltage-dependent dephosphorylation of PI(4,5)P₂. Further studies will provide clues to understanding of substrate preference in PIPs phosphatases as well as to customization of a molecular tool.

Interaction between S4 and the phosphatase domain mediates electrochemical coupling in voltage-sensing phosphatase (VSP)

Voltage-sensing phosphatase (VSP) consists of a voltage sensor domain (VSD) and a cytoplasmic catalytic region (CCR), which is similar to phosphatase and tensin homolog (PTEN). How the VSD regulates the innate enzyme component of VSP remains unclear. Here, we took a combined approach that entailed the use of electrophysiology, fluorometry, and structural modeling to study the electrochemical coupling in Ciona intestinalis VSP. We found that two hydrophobic residues at the lowest part of S4 play an essential role in the later transition of VSD-CCR coupling. Voltage clamp fluorometry and disulfide bond locking indicated that S4 and its neighboring linker move as one helix (S4-linker helix) and approach the hydrophobic spine in the CCR, a structure located near the cell membrane and also conserved in PTEN. We propose that the hydrophobic spine operates as a hub for translating an electrical signal into a chemical one in VSP.

Engineering an enhanced voltage-sensing phosphatase

Voltage-sensing phosphatases (VSP) consist of a membrane-spanning voltage sensor domain and a cytoplasmic region that has enzymatic activity toward phosphoinositides (PIs). VSP enzyme activity is regulated by membrane potential, and its activation leads to rapid and reversible alteration of cellular PIP levels. These properties enable VSPs to be used as a tool for studying the effects of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂) binding to ion channels and transporters. For example, by applying simple changes in the membrane potential, Danio rerio VSP (Dr-VSP) has been used effectively to manipulate PI(4,5)P₂ in mammalian cells with few, if any, side effects. In the present study, we report an enhanced version of Dr-VSP as an improved molecular tool for depleting PI(4,5)P₂ from cultured mammalian cells. We modified Dr-VSP in two ways. Its voltage-dependent phosphatase activity was enhanced by introducing an aromatic residue at the position of Leu-223 within a membrane-interacting region of the phosphatase domain called the hydrophobic spine. In addition, selective plasma membrane targeting of Dr-VSP was facilitated by fusion with the N-terminal region of Ciona intestinalis VSP. This modified Dr-VSP (CiDr-VSP^mChe L223F, or what we call eVSP) induced more drastic voltage-evoked changes in PI(4,5)P₂ levels, using the activities of Kir2.1, KCNQ2/3, and TRPC6 channels as functional readouts. eVSP is thus an improved molecular tool for evaluating the PI(4,5)P₂ sensitivity of ion channels in living cells.

ANAP: A versatile, fluorescent probe of ion channel gating and regulation

Fluorescence spectroscopy and microscopy are non-destructive methods that provide real-time measurements of ion channel structural dynamics. As such, they constitute a direct path linking the high-resolution structural models from X-ray crystallography and cryo-electron microscopy with the high-resolution functional data from ionic current measurements. The utility of fluorescence as a reporter of channel structure is limited by the palette of available fluorophores. Thiol-reactive fluorophores are small and bright, but are restricted in terms of the positions on a protein that can be labeled and present significant issues with background incorporation. Genetically encoded fluorescent protein tags are specific to a protein of interest, but are very large and usually only used to label the free N- and C-termini of proteins. L-3-(6-acetylnaphthalen-2-ylamino)-2-aminopropionic acid (ANAP) is a fluorescent amino acid that can be specifically incorporated into virtually any site on a protein of interest using amber stop-codon suppression. Due to its environmental sensitivity and potential as a donor in fluorescence resonance energy transfer experiments, it has been adopted by numerous investigators to study voltage, ligand, and temperature-dependent activation of a host of ion channels. Simultaneous measurements of ionic currents and ANAP fluorescence yield exceptional mechanistic insights into channel function. In this chapter, I will summarize the current literature regarding ANAP and ion channels and discuss the practical aspects of using ANAP, including potential pitfalls and confounds.

Reinterpretation of the substrate specificity of the voltage-sensing phosphatase during dimerization

Ciona intestinalis voltage-sensing phosphatase (VSP) has lipid 5- and 3-phosphatase activity, but 3-phosphatase is evident only at high VSP concentrations. Using kinetic modeling including endogenous lipid metabolizing enzymes and VSP phosphatase activities, Kruse et al. show how apparent activation of 3-phosphatase at high concentrations arises.

Voltage-sensing phosphatases (VSPs) cleave both 3- and 5-phosphates from inositol phospholipids in response to membrane depolarization. When low concentrations of Ciona intestinalis VSP are expressed in Xenopus laevis oocytes, the 5-phosphatase reaction can be observed during large membrane depolarizations. When higher concentrations are expressed, the 5-phosphatase activity is observed with smaller depolarizations, and the 3-phosphatase activity is revealed with strong depolarization. Here we ask whether this apparent induction of 3-phosphatase activity is attributable to the dimerization that has been reported when VSP is expressed at higher concentrations. Using a simple kinetic model, we show that these enzymatic phenomena can be understood as an emergent property of a voltage-dependent enzyme with invariant substrate selectivity operating in the context of endogenous lipid-metabolizing enzymes present in oocytes. Thus, a switch of substrate specificity with dimerization need not be invoked to explain the appearance of 3-phosphatase activity at high VSP concentrations.

Molecular mechanisms of coupling to voltage sensors in voltage-evoked cellular signals

The voltage sensor domain (VSD) has long been studied as a unique domain intrinsic to voltage-gated ion channels (VGICs). Within VGICs, the VSD is tightly coupled to the pore-gate domain (PGD) in diverse ways suitable for its specific function in each physiological context, including action potential generation, muscle contraction and relaxation, hormone and neurotransmitter secretion, and cardiac pacemaking. However, some VSD-containing proteins lack a PGD. Voltage-sensing phosphatase contains a cytoplasmic phosphoinositide phosphatase with similarity to phosphatase and tensin homolog (PTEN). H_v1, a voltage-gated proton channel, also lacks a PGD. Within H_v1, the VSD operates as a voltage sensor, gate, and pore for both proton sensing and permeation. H_v1 has a C-terminal coiled coil that mediates dimerization for cooperative gating. Recent progress in the structural biology of VGICs and VSD proteins provides insights into the principles of VSD coupling conserved among these proteins as well as the hierarchy of protein organization for voltage-evoked cell signaling.