The Single Residue K12 Governs the Exceptional Voltage Sensitivity of Mitochondrial Voltage-Dependent Anion Channel Gating.

The voltage-dependent anion channel (VDAC) is a ß-barrel channel of the mitochondrial outer membrane (MOM) that passively transports ions, metabolites, polypeptides, and single-stranded DNA. VDAC responds to a transmembrane potential by "gating," i.e. transitioning to one of a variety of low-conducting states of unknown structure. The gated state results in nearly complete suppression of multivalent mitochondrial metabolite (such as ATP and ADP) transport, while enhancing calcium transport. Voltage gating is a universal property of ß-barrel channels, but VDAC gating is anomalously sensitive to transmembrane potential. Here, we show that a single residue in the pore interior, K12, is responsible for most of VDAC's voltage sensitivity. Using the analysis of over 40 µs of atomistic molecular dynamics (MD) simulations, we explore correlations between motions of charged residues inside the VDAC pore and geometric deformations of the ß-barrel. Residue K12 is bistable; its motions between two widely separated positions along the pore axis enhance the fluctuations of the ß-barrel and augment the likelihood of gating. Single channel electrophysiology of various K12 mutants reveals a dramatic reduction of the voltage-induced gating transitions. The crystal structure of the K12E mutant at a resolution of 2.6 Å indicates a similar architecture of the K12E mutant to the wild type; however, 60 µs of atomistic MD simulations using the K12E mutant show restricted motion of residue 12, due to enhanced connectivity with neighboring residues, and diminished amplitude of barrel motions. We conclude that ß-barrel fluctuations, governed particularly by residue K12, drive VDAC gating transitions.

A lower affinity to cytosolic proteins reveals VDAC3 isoform-specific role in mitochondrial biology.

Voltage-dependent anion channel (VDAC) is the major pathway for the transport of ions and metabolites across the mitochondrial outer membrane. Among the three known mammalian VDAC isoforms, VDAC3 is the least characterized, but unique functional roles have been proposed in cellular and animal models. Yet, a high-sequence similarity between VDAC1 and VDAC3 is indicative of a similar pore-forming structure. Here, we conclusively show that VDAC3 forms stable, highly conductive voltage-gated channels that, much like VDAC1, are weakly anion selective and facilitate metabolite exchange, but exhibit unique properties when interacting with the cytosolic proteins α-synuclein and tubulin. These two proteins are known to be potent regulators of VDAC1 and induce similar characteristic blockages (on the millisecond time scale) of VDAC3, but with 10- to 100-fold reduced on-rates and altered α-synuclein blocking times, indicative of an isoform-specific function. Through cysteine scanning mutagenesis, we found that VDAC3's cysteine residues regulate its interaction with α-synuclein, demonstrating VDAC3-unique functional properties and further highlighting a general molecular mechanism for VDAC isoform-specific regulation of mitochondrial bioenergetics.

Multiple neurosteroid and cholesterol binding sites in voltage-dependent anion channel-1 determined by photo-affinity labeling.

Voltage-dependent anion channel-1 (VDAC1) is a mitochondrial porin that is implicated in cellular metabolism and apoptosis, and modulated by numerous small molecules including lipids. VDAC1 binds sterols, including cholesterol and neurosteroids such as allopregnanolone. Biochemical and computational studies suggest that VDAC1 binds multiple cholesterol molecules, but photolabeling studies have identified only a single cholesterol and neurosteroid binding site at E73. To identify all the binding sites of neurosteroids in VDAC1, we apply photo-affinity labeling using two sterol-based photolabeling reagents with complementary photochemistry: 5α-6-AziP which contains an aliphatic diazirine, and KK200 which contains a trifluoromethyl-phenyldiazirine (TPD) group. 5α-6-AziP and KK200 photolabel multiple residues within an E73 pocket confirming the presence of this site and mapping sterol orientation within this pocket. In addition, KK200 photolabels four other sites consistent with the finding that VDAC1 co-purifies with five cholesterol molecules. Both allopregnanolone and cholesterol competitively prevent photolabeling at E73 and three other sites indicating that these are common sterol binding sites shared by both neurosteroids and cholesterol. Binding at the functionally important residue E73 suggests a possible role for sterols in regulating VDAC1 signaling and interaction with partner proteins.

Assessing the role of residue E73 and lipid headgroup charge in VDAC1 voltage gating.

The voltage-dependent anion channel (VDAC) is the most abundant protein of the mitochondrial outer membrane (MOM) where it regulates transport of ions and metabolites in and out of the organelle. VDAC function is extensively studied in a lipid bilayer system that allows conductance monitoring of reconstituted channels under applied voltage. The process of switching from a high-conductance state, open to metabolites, to a variety of low-conducting states, which excludes metabolite transport, is termed voltage gating and the mechanism remains poorly understood. Recent studies have implicated the involvement of the membrane-solvated residue E73 in the gating process through ß-barrel destabilization. However, there has been no direct experimental evidence of E73 involvement in VDAC1 voltage gating. Here, using electrophysiology measurements, we exclude the involvement of E73 in murine VDAC1 (mVDAC1) voltage gating process. With an established protocol of assessing voltage gating of VDACs reconstituted into planar lipid membranes, we definitively show that mVDAC1 gating properties do not change when E73 is replaced by either a glutamine or an alanine. We further demonstrate that cholesterol has no effect on mVDAC1 gating characteristics, though it was shown that E73 is coordinating residue in the cholesterol binding site. In contrast, we found a pronounced gating effect based on the charge of the phospholipid headgroup, where the positive charge stimulates and negative charge suppresses gating. These findings call for critical evaluation of the existing models of VDAC gating and contribute to our understanding of VDAC's role in control of MOM permeability and regulation of mitochondrial respiration and metabolism.

Protonation state of glutamate 73 regulates the formation of a specific dimeric association of mVDAC1.

The voltage-dependent anion channel (VDAC) is the most abundant protein in the outer mitochondrial membrane and constitutes the primary pathway for the exchange of ions and metabolites between the cytosol and the mitochondria. There is accumulating evidence supporting VDAC's role in mitochondrial metabolic regulation and apoptosis, where VDAC oligomerization has been implicated with these processes. Herein, we report a specific pH-dependent dimerization of murine VDAC1 (mVDAC1) identified by double electron-electron resonance and native mass spectrometry. Intermolecular distances on four singly spin-labeled mVDAC1 mutants were used to generate a model of the low-pH dimer, establishing the presence of residue E73 at the interface. This dimer arrangement is different from any oligomeric state previously described, and it forms as a steep function of pH with an apparent pKa of 7.4. Moreover, the monomer-dimer equilibrium affinity constant was determined using native MS, revealing a nearly eightfold enhancement in dimerization affinity at low pH. Mutation of E73 to either alanine or glutamine severely reduces oligomerization, demonstrating the role of protonated E73 in enhancing dimer formation. Based on these results, and the known importance of E73 in VDAC physiology, VDAC dimerization likely plays a significant role in mitochondrial metabolic regulation and apoptosis in response to cytosolic acidification during cellular stress.

Photoaffinity labeling with cholesterol analogues precisely maps a cholesterol-binding site in voltage-dependent anion channel-1.

Voltage-dependent anion channel-1 (VDAC1) is a highly regulated ß-barrel membrane protein that mediates transport of ions and metabolites between the mitochondria and cytosol of the cell. VDAC1 co-purifies with cholesterol and is functionally regulated by cholesterol, among other endogenous lipids. Molecular modeling studies based on NMR observations have suggested five cholesterol-binding sites in VDAC1, but direct experimental evidence for these sites is lacking. Here, to determine the sites of cholesterol binding, we photolabeled purified mouse VDAC1 (mVDAC1) with photoactivatable cholesterol analogues and analyzed the photolabeled sites with both top-down mass spectrometry (MS), and bottom-up MS paired with a clickable, stable isotope-labeled tag, FLI-tag. Using cholesterol analogues with a diazirine in either the 7 position of the steroid ring (LKM38) or the aliphatic tail (KK174), we mapped a binding pocket in mVDAC1 localized to Thr83 and Glu73, respectively. When Glu73 was mutated to a glutamine, KK174 no longer photolabeled this residue, but instead labeled the nearby Tyr62 within this same binding pocket. The combination of analytical strategies employed in this work permits detailed molecular mapping of a cholesterol-binding site in a protein, including an orientation of the sterol within the site. Our work raises the interesting possibility that cholesterol-mediated regulation of VDAC1 may be facilitated through a specific binding site at the functionally important Glu73 residue.

Structural features and lipid binding domain of tubulin on biomimetic mitochondrial membranes.

Dimeric tubulin, an abundant water-soluble cytosolic protein known primarily for its role in the cytoskeleton, is routinely found to be associated with mitochondrial outer membranes, although the structure and physiological role of mitochondria-bound tubulin are still unknown. There is also no consensus on whether tubulin is a peripheral membrane protein or is integrated into the outer mitochondrial membrane. Here the results of five independent techniques-surface plasmon resonance, electrochemical impedance spectroscopy, bilayer overtone analysis, neutron reflectometry, and molecular dynamics simulations-suggest that α-tubulin's amphipathic helix H10 is responsible for peripheral binding of dimeric tubulin to biomimetic "mitochondrial" membranes in a manner that differentiates between the two primary lipid headgroups found in mitochondrial membranes, phosphatidylethanolamine and phosphatidylcholine. The identification of the tubulin dimer orientation and membrane-binding domain represents an essential step toward our understanding of the complex mechanisms by which tubulin interacts with integral proteins of the mitochondrial outer membrane and is important for the structure-inspired design of tubulin-targeting agents.

Click Chemistry Reagent for Identification of Sites of Covalent Ligand Incorporation in Integral Membrane Proteins.

Identifying sites of protein-ligand interaction is important for structure-based drug discovery and understanding protein structure-function relationships. Mass spectrometry (MS) has emerged as a useful tool for identifying residues covalently modified by ligands. Current methods use database searches that are dependent on acquiring interpretable fragmentation spectra (MS2) of peptide-ligand adducts. This is problematic for identifying sites of hydrophobic ligand incorporation in integral membrane proteins (IMPs), where poor aqueous solubility and ionization of peptide-ligand adducts and collision-induced adduct loss hinder the acquisition of quality MS2 spectra. To address these issues, we developed a fast ligand identification (FLI) tag that can be attached to any alkyne-containing ligand via Cu(I)-catalyzed cycloaddition. The FLI tag adds charge to increase solubility and ionization, and utilizes stable isotope labeling for MS1 level identification of hydrophobic peptide-ligand adducts. The FLI tag was coupled to an alkyne-containing neurosteroid photolabeling reagent and used to identify peptide-steroid adducts in MS1 spectra via the stable heavy isotope pair. Peptide-steroid adducts were not identified in MS2-based database searches because collision-induced adduct loss was the dominant feature of collision-induced dissociation (CID) fragmentation, but targeted analysis of MS1 pairs using electron transfer dissociation (ETD) markedly reduced adduct loss. Using the FLI tag and ETD, we identified Glu73 as the site of photoincorporation of our neurosteroid ligand in the IMP, mouse voltage-dependent anion channel-1 (mVDAC1), and top-down MS confirmed a single site of photolabeling.

From low- to high-potential bioenergetic chains: Thermodynamic constraints of Q-cycle function.

The electrochemical parameters of all cofactors in the supercomplex formed by the Rieske/cytb complex and the SoxM/A-type O2-reductase from the menaquinone-containing Firmicute Geobacillus stearothermophilus were determined by spectroelectrochemistry and EPR redox titrations. All redox midpoint potentials (Em) were found to be lower than those of ubi- or plastoquinone-containing systems by a value comparable to the redox potential difference between the respective quinones. In particular, Em values of +200mV, -360mV, -220mV and -50mV (at pH7) were obtained for the Rieske cluster, heme bL, heme bH and heme ci, respectively. Comparable values of -330mV, -200mV and +120mV for hemes bL, bH and the Rieske cluster were determined for an anaerobic Firmicute, Heliobacterium modesticaldum. Thermodynamic constraints, optimization of proton motive force build-up and the necessity of ROS-avoidance imposed by the rise in atmospheric O2 2.5billionyears ago are discussed as putative evolutionary driving forces resulting in the observed redox upshift. The close conservation of the entire redox landscape between low and high potential systems suggests that operation of the Q-cycle requires the precise electrochemical tuning of enzyme cofactors to the quinone substrate as stipulated in P. Mitchell's hypothesis.