Voltage-dependent anion channel

Crystal Structure of the Human Voltage-Dependent Anion Channel. The arrows denote the antiparallel beta sheets that form the characteristic beta-barrel

Voltage-dependent anion channels are a class of porin ion channel located on the outer mitochondrial membrane.^[1] There is debate as to whether or not this channel is expressed in the cell surface membrane.^{[2] [3][4]}

This major protein of the outer mitochondrial membrane of eukaryotes forms a voltage-dependent anion-selective channel (VDAC) that behaves as a general diffusion pore for small hydrophilic molecules.^[5][6][7][8] The channel adopts an open conformation at low or zero membrane potential and a closed conformation at potentials above 30-40 mV. VDAC facilitates the exchange of ions and molecules between mitochondria and cytosol and is regulated by the interactions with other proteins and small molecules.^[9]

Structure

This protein contains about 280 amino acids and forms a beta barrel which spans span the mitochondrial outer membrane.^[10][11]

Since its discovery in 1976, extensive function and structure analysis of VDAC proteins has been conducted. A prominent feature of the pore emerged: when reconstituted into planar lipid bilayers, there is a voltage-dependent switch between an anion-selective high-conductance state with high metabolite flux and a cation-selective low-conductance state with limited passage of metabolites.

More than 30 years after its initial discovery, in 2008, three independent structural projects of VDAC-1 were completed. The first was solved by multi-dimensional NMR spectroscopy. The second applied a hybrid approach using crystallographic data. The third was for mouse VDAC-1 crystals determined by X-ray crystallographic techniques. The three projects of the 3D structures of VDAC-1 revealed many structural features. First, VDAC-1 represents a new structural class of outer membrane β-barrel proteins with an odd number of strands. Another aspect is that the negatively charged side chain of residue E73 is oriented towards the hydrophobic membrane environment. The 19-stranded 3D structure obtained under different experimental sources by three different laboratories fits the EM and AFM data from native membrane sources and represents a biologically relevant state of VDAC-1.^[9]

Mechanism

At membrane potentials exceeding 30 mV (positive or negative), VDAC assumes a closed state, and transitions to its open state once the voltage drops below this threshold. Although both states allow passage of simple salts, VDAC is much more stringent with organic anions, a category into which most metabolites fall.^[12] The precise mechanism for coupling voltage changes to conformational changes within the protein has not yet been worked out, but studies by Thomas et al. suggest that when the protein transitions to the closed form, voltage changes lead to the removal of a large section of the protein from the channel and decrease effective pore radius.^[13] Several lysine residues, as well as Glu-152, have been implicated as especially important sensor residues within the protein.^[14]

Biological Function

The voltage-dependent ion channel plays a key role in regulating metabolic and energetic flux across the outer mitochondrial membrane. It is involved in the transport of ATP, ADP, pyruvate, malate, and other metabolites, and thus communicates extensively with enzymes from metabolic pathways.^[12] The ATP-dependent cytosolic enzymes hexokinase, glucokinase, and glycerol kinase, as well as the mitochondrial enzyme creatine kinase, have all been found to bind to VDAC. This binding puts them in close proximity to ATP released from the mitochondria. In particular, the binding of hexokinase is presumed to play a key role in coupling glycolysis to oxidative phosphorylation.^[13] Additionally, VDAC is an important regulator of Ca²⁺ transport in and out of the mitochondria. Because Ca²⁺ is a cofactor for metabolic enzymes such as pyruvate dehydrogenase and isocitrate dehydrogenase, energetic production and homeostasis are both affected by VDAC’s permeability to Ca²⁺.^[15]

Disease Relevance

VDAC has also been shown to play a role in apoptosis.^[16] During apoptosis, increased permeability of VDAC allows for the release of apoptogenic factors such as cytochrome c. Although cyt. c plays an essential role in oxidative phosphorylation within the mitochondrion, in the cytosol it activates proteolytic enzymes called caspases, which play a major role in cell death.^[17] Although the mechanism for VDAC-facilitated cyt. c release has not yet been fully elucidated, some research suggests that oligomerization between individual subunits may create a large flexible pore through which cyt. c can pass.^[18] A more important factor is that release of cyt c. is also regulated by the Bcl-2 protein family: Bax interacts directly with VDAC to increase pore size and promote cyt. c release, while anti-apoptotic Bcl-xL produces the exact opposite effect.^[19] In fact, it has been shown that antibodies that inhibit VDAC also interfere with Bax-mediated cyt. c release in both isolated mitchondria and whole cells.^[20] This key role in apoptosis suggests VDAC as a potential target for chemotherapeutic drugs.

Examples

Yeast contains two members of this family (genes POR1 and POR2); vertebrates have at least three members (genes VDAC1, VDAC2 and VDAC3).^[10]

Humans, like most higher eukaryotes, encode three different VDACs; VDAC1, VDAC2, and VDAC3. Together with TOMM40 and TOMM40L they represent a family of evolutionarily related β-barrels.^[21]

Plants have the largest number of VDACs. Arabidopsis encode four different VDACs but this number can be larger in other species.^[22]

References

External links

• Voltage-Dependent Anion Channels at the US National Library of Medicine Medical Subject Headings (MeSH)

  Membrane transport protein: ion channels (TC 1A)
  Ca2+: Calcium channel Ligand-gated Inositol trisphosphate receptor 1 2 3 Ryanodine receptor 1 2 3 Voltage-gated L-type/Cavα 1.1 1.2 1.3 1.4 N-type/Cavα2.2 P-type/Cavα 2.1 Q-type/Cavα2.1 R-type/Cavα2.3 T-type/Cavα 3.1 3.2 3.3 α2δ-subunits 1 2 β-subunits β1 β2 β3 β4 γ-subunits γ1 γ2 γ3 γ4 Cation channels of sperm 1 2 3 4 Two-pore channel 1 2
  Na+: Sodium channel Constitutively active Epithelial sodium channel α β γ δ Proton-gated Amiloride-sensitive cation channel 1 2 3 4 Voltage-gated Navα 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 7A Navβ 1 2 3 4
  K+: Potassium channel Calcium-activated BK channel α1 β1 β2 β3 β4 SK channel SK1 SK2 SK3 IK channel IK1 KCa 1.1 2.1 2.2 2.3 3.1 4.1 4.2 5.1 Inward-rectifier KATP Kir 1.1 2.1 2.2 2.3 2.4 2.6 GIRK/Kir 3.1 3.2 3.3 3.4 Kir 4.1 4.2 5.1 6.1 6.2 7.1 Tandem pore domain K2P 1 2 3 4 5 6 7 9 10 12 13 15 16 17 18 Voltage-gated Kvα1-6 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 2.1 2.2 3.1 3.2 3.3 3.4 4.1 4.2 4.3 5.1 6.1 6.2 6.3 6.4 Kvα7-12 7.1 7.2 7.3 7.4 7.5 8.1 8.2 9.1 9.2 9.3 10.1 10.2 11.1/hERG 11.2 11.3 12.1 12.2 12.3 Kvβ 1 2 3 KCNIP 1 2 3 4 minK/ISK minK/ISK-like MiRP 1 2 3 Shaker gene
  Miscellaneous Cl−: Chloride channel Calcium-activated chloride channels Anoctamin ANO1 Bestrophin 1 2 Chloride Channel Accessory 1 2 3 4 CFTR CLCN 1 2 3 4 5 6 7 KA KB CLIC 1 2 3 4 5 6 L1 CLNS 1A 1B H+: Proton channel HVCN1 M+: CNG cation channel α 1 2 3 4 β 1 2 3 HCN FC 1 2 3 4 M+: TRP cation channel TRPA (1) TRPC 1 2 3 4 4AP 5 6 7 TRPM 1 2 3 4 5 6 7 8 TRPML 1 2 3 TRPN TRPP 1 2 TRPV 1 2 3 4 5 6 H2O (+ solutes): Porin Aquaporin 0 1 2 3 4 5 6 7 8 9 Voltage-dependent anion channel 1 2 3 General bacterial porin family Cytoplasm: Gap junction Connexin: A GJA1 GJA3 GJA4 GJA5 GJA8 GJA9 GJA10 B GJB1 GJB2 GJB3 GJB4 GJB5 GJB6 GJB7 C GJC1 GJC2 GJC3 D GJD2 GJD3 GJD4) Innexin
  By gating mechanism Ion channel class Ligand-gated Light-gated Voltage-gated Stretch-activated
  see also disorders

This article incorporates text from the public domain Pfam and InterPro IPR001925