Voltage-dependent anion-selective channel 1 (VDAC-1) is a beta barrel protein that in humans is encoded by the VDAC1 gene located on chromosome 5. [4] [5] It forms an ion channel in the outer mitochondrial membrane (OMM) and also the outer cell membrane. In the OMM, it allows ATP to diffuse out of the mitochondria into the cytoplasm. In the cell membrane, it is involved in volume regulation. Within all eukaryotic cells, mitochondria are responsible for synthesis of ATP among other metabolite needed for cell survival. VDAC1 therefore allows for communication between the mitochondrion and the cell mediating the balance between cell metabolism and cell death. Besides metabolic permeation, VDAC1 also acts as a scaffold for proteins such as hexokinase that can in turn regulate metabolism. [6]
This protein is a voltage-dependent anion channel and shares high structural homology with the other VDAC isoforms (VDAC2 and VDAC3), which are involved in the regulation of cell metabolism, mitochondrial apoptosis, and spermatogenesis. [7] [8] [9] [10] Over expression and misregulation of this pore could lead to apoptosis in the cell leading to a variety of diseases within the body. In particular, since VDAC1 is the major calcium ion transport channel, its dysfunction is implicated in cancer, Parkinson's (PD), and Alzheimer's disease. [11] [12] [13] In addition, recent studies have shown that an over expression within the VDAC1 protein is linked to Type 2 Diabetes. Lund University released a study that demonstrated the effects of blocking VDAC1 over expression can prevent the spread of Type 2 Diabetes. [14]
The three VDAC isoforms (VDAC1, VDAC2, and VDAC3) have highly conserved DNA sequences as well as 3D structures forming a wide β-barrel structure, inside of which the alpha helical N-terminal segment resides to partially close the pore. [15] VDAC1's structure was solved by 3 independent labs by x-ray crystallography, Nuclear Magnetic Resonance (NMR) spectroscopy, or a combination of both. Two of these structural studies were used to determine human VDAC1 (hVDAC1) structure while X-ray crystallography was used to solve murine VDAC1 (mVDAC1) structure that differs from hVDAC1 by only two residues. [16] [17] [18] These determined structures aligned with earlier circular dichroism studies that predicted the presence of alpha helix and β-strand domains. [19]
Structural analysis of mVDAC1's structure showed a barrel-like channel composed of 19 amphipathic β-strands, with the N-terminus and C-terminus both facing towards the inter membrane space of the mitochondrion. [20] [21] β-strands are connected via loops and are arranged in an anti-parallel pattern with the exception of β-strands 1 and 19 which are parallel. [18] The pore has a height of 40 Ẳ, spans a distance of 27 Ẳ by 20 Ẳ at the openings and tapers down to 20 Ẳ by 14 Ẳ at the N-terminal α-helix segment in the open state. [22] The closed state conformation has yet to be isolated and determined. Additionally, the N-terminus has an alpha helical segment that is held to the inside wall of the pore by hydrophobic interactions with residues on β-sheets 8-18. [18] This N-terminus can serve as a scaffold for the movement of ions or attachment of proteins. One such example is seen as it is the docking site for HK1 binding. [6] A significant residue to point out is the glutamate located at the 73rd residue on the amino acid chain (E73). This residue is found in VDAC1 and VDAC2 but not VDAC3. The side chain of this charged residue points into the phospholipid bilayer which would normally cause repulsive forces to occur. E73 however, has been implicated in VDAC1 function and interaction. [23]
VDAC1 belongs to the mitochondrial porin family and is expected to share similar biological functions to the other VDAC isoforms. [24] Of the three isoforms, VDAC1 is the main calcium ion transport channel in mitochondria and the most abundantly transcribed. [12] [25] VDAC1 is involved in cell metabolism by transporting ATP and other small metabolites across the outer mitochondrial membrane (OMM) allowing regulation of the TCA cycle and, by extension, reactive oxygen species (ROS) production. [11] In yeast cells, ROS accumulate under conditions of oxidative stress, which results in impaired mitochondrial function and a “petite” phenotype. However, petite yeast cells exhibit a longer lifespan than wild-type cells and indicate a protective function by VDAC1 in similar circumstances, such as aging. [6] [25]
VDAC1 allows for the conductance of molecules into and out of the mitochondrion. Its permeability is dependent on VDAC1's conformational state which is determined by voltage. At low voltage (10mV), the pore is in an "open" state where the channel is weakly anion selective and allows for a greater flux of metabolites. Because of the large pore size, metabolic gating under saturated ATP conditions reveal a transport of 2,000,000 ATP/second and a transport of 10,000 ATP under physiological conditions. [26] At a higher voltage in the positive or negative direction (>30mV), the pore is in a "closed" state and is weakly cation selective allowing for less metabolites to be transported. [18] The flux of metabolites can be seen as negligible. This change in states is mediated by a conformational change in the protein that has yet to be discovered. Since the alpha helical N-terminus segment is located in the center of the pore, it is ideally situated for metabolic gating. This lead researchers to believe that the Alpha helix was a key contributor to determining the conformational states. However, more recent studies have shown the N-terminal is unnecessary for proper voltage gating and therefore suggest the flexible beta barrel as the mechanism of conformational change. [22]
Atomic Force Microscopy (AFM) revealed the presence of VDAC1 monomers as well as dimers and larger oligomers showcasing the interaction of the pore with itself, however, dimers are more frequent. [27] hVDAC1 in particular has been shown to arrange in parallel dimers leading to increased permeability of the pore. [16] The glutamate located at the 73rd position on VDAC1 has also been shown to play a role in oligomerization when in the presence of calcium. [23] VDACs can also oligomerize to form part of the mitochondrial permeability transition pore (MPTP) and, thus, facilitate cytochrome C release, leading to apoptosis. VDACs have also been observed to interact with pro- or antiapoptotic proteins, such as Bcl-2 family proteins and kinases, and so may contribute to apoptosis independently from the MPTP. [24]
The voltage dependent anion channels all function in ion and metabolite transport although their physiological roles are different. Because of their role, dysfunction of the channels can lead to various diseases. VDAC1 has been implicated in cancer through its interactions with the antiapoptotic family of proteins, Bcl-2 proteins, particularly Bcl-xl, and Mcl-1, which are overexpressed during cancer. These two Bcl-2 proteins interact with VDAC1 to regulate calcium ion transport across the OMM and, ultimately, ROS production. While high levels of ROS induce cell death, non-lethal levels interfere with signal transduction pathways that can then promote cell proliferation, migration, and invasion in cancer cells. [11] Moreover, VDAC1 overexpression has been associated with increased apoptotic response and anti-cancer drugs and treatment efficacy, further supporting VDAC1 as a therapeutic target for cancer treatment. [11] [28]
VDAC1's function in calcium ion transport has also been linked to neurodegenerative diseases. In PD, VDAC1 increases calcium ion levels within the mitochondria, resulting in increased mitochondrial permeability, disrupted mitochondrial membrane potential, elevated ROS production, cell death, and neuronal degeneration. [12] VDAC1 has been shown to interact with Amyloid β (Aβ) leading to increased conductance of the channel and eventually apoptosis of the cell. [13]
VDAC1 acts as a scaffold for many proteins as well as allows for the flux of ions and metabolites through interactions within the pore.
A major metabolite that moves through this channel is ATP. A low affinity binding site used for fast transport of this molecule was discovered by the Markov state modeling approach. It was shown that ATP binds to multiple basic residues within the pore sequentially, in essence moving through the channel. [29]
VDAC1 has also been shown to interact with:
A mitochondrion (pl. mitochondria) is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used throughout the cell as a source of chemical energy. They were discovered by Albert von Kölliker in 1857 in the voluntary muscles of insects. Meaning a thread-like granule, the term mitochondrion was coined by Carl Benda in 1898. The mitochondrion is popularly nicknamed the "powerhouse of the cell", a phrase popularized by Philip Siekevitz in a 1957 Scientific American article of the same name.
Porins are beta barrel proteins that cross a cellular membrane and act as a pore, through which molecules can diffuse. Unlike other membrane transport proteins, porins are large enough to allow passive diffusion, i.e., they act as channels that are specific to different types of molecules. They are present in the outer membrane of gram-negative bacteria and some gram-positive mycobacteria, the outer membrane of mitochondria, and the outer chloroplast membrane.
The apoptosome is a large quaternary protein structure formed in the process of apoptosis. Its formation is triggered by the release of cytochrome c from the mitochondria in response to an internal (intrinsic) or external (extrinsic) cell death stimulus. Stimuli can vary from DNA damage and viral infection to developmental cues such as those leading to the degradation of a tadpole's tail.
The mitochondrial permeability transition pore is a protein that is formed in the inner membrane of the mitochondria under certain pathological conditions such as traumatic brain injury and stroke. Opening allows increase in the permeability of the mitochondrial membranes to molecules of less than 1500 daltons in molecular weight. Induction of the permeability transition pore, mitochondrial membrane permeability transition, can lead to mitochondrial swelling and cell death through apoptosis or necrosis depending on the particular biological setting.
Apoptosis regulator BAX, also known as bcl-2-like protein 4, is a protein that in humans is encoded by the BAX gene. BAX is a member of the Bcl-2 gene family. BCL2 family members form hetero- or homodimers and act as anti- or pro-apoptotic regulators that are involved in a wide variety of cellular activities. This protein forms a heterodimer with BCL2, and functions as an apoptotic activator. This protein is reported to interact with, and increase the opening of, the mitochondrial voltage-dependent anion channel (VDAC), which leads to the loss in membrane potential and the release of cytochrome c. The expression of this gene is regulated by the tumor suppressor P53 and has been shown to be involved in P53-mediated apoptosis.
The BH3 interacting-domain death agonist, or BID, gene is a pro-apoptotic member of the Bcl-2 protein family. Bcl-2 family members share one or more of the four characteristic domains of homology entitled the Bcl-2 homology (BH) domains, and can form hetero- or homodimers. Bcl-2 proteins act as anti- or pro-apoptotic regulators that are involved in a wide variety of cellular activities.
Bcl-2 homologous antagonist/killer is a protein which in humans is encoded by the BAK1 gene on chromosome 6. It belongs to the BCL2 protein family. BCL2 family members form oligomers or heterodimers and act as anti- or pro-apoptotic regulators that are involved in a wide variety of cellular activities. This protein localizes to mitochondria, and functions to induce apoptosis. It interacts with and accelerates the opening of the mitochondrial voltage-dependent anion channel, which leads to a loss in membrane potential and the release of cytochrome c. This protein also interacts with the tumor suppressor P53 after exposure to cell stress.
Mitochondrial membrane transport proteins, also known as mitochondrial carrier proteins, are proteins which exist in the membranes of mitochondria. They serve to transport molecules and other factors, such as ions, into or out of the organelles. Mitochondria contain both an inner and outer membrane, separated by the inter-membrane space, or inner boundary membrane. The outer membrane is porous, whereas the inner membrane restricts the movement of all molecules. The two membranes also vary in membrane potential and pH. These factors play a role in the function of mitochondrial membrane transport proteins. There are 53 discovered human mitochondrial membrane transporters, with many others that are known to still need discovered.
Voltage-dependent anion channels, or mitochondrial porins, are a class of porin ion channel located on the outer mitochondrial membrane. There is debate as to whether or not this channel is expressed in the cell surface membrane.
Protein kinase C epsilon type (PKCε) is an enzyme that in humans is encoded by the PRKCE gene. PKCε is an isoform of the large PKC family of protein kinases that play many roles in different tissues. In cardiac muscle cells, PKCε regulates muscle contraction through its actions at sarcomeric proteins, and PKCε modulates cardiac cell metabolism through its actions at mitochondria. PKCε is clinically significant in that it is a central player in cardioprotection against ischemic injury and in the development of cardiac hypertrophy.
Bcl-2-like protein 1 is a protein encoded in humans by the BCL2L1 gene. Through alternative splicing, the gene encodes both of the human proteins Bcl-xL and Bcl-xS.
Hexokinase-1 (HK1) is an enzyme that in humans is encoded by the HK1 gene on chromosome 10. Hexokinases phosphorylate glucose to produce glucose-6-phosphate (G6P), the first step in most glucose metabolism pathways. This gene encodes a ubiquitous form of hexokinase which localizes to the outer membrane of mitochondria. Mutations in this gene have been associated with hemolytic anemia due to hexokinase deficiency. Alternative splicing of this gene results in five transcript variants which encode different isoforms, some of which are tissue-specific. Each isoform has a distinct N-terminus; the remainder of the protein is identical among all the isoforms. A sixth transcript variant has been described, but due to the presence of several stop codons, it is not thought to encode a protein. [provided by RefSeq, Apr 2009]
ADP/ATP translocase 4 (ANT4) is an enzyme that in humans is encoded by the SLC25A31 gene on chromosome 4. This enzyme inhibits apoptosis by catalyzing ADP/ATP exchange across the mitochondrial membranes and regulating membrane potential. In particular, ANT4 is essential to spermatogenesis, as it imports ATP into sperm mitochondria to support their development and survival. Outside this role, the SLC25AC31 gene has not been implicated in any human disease.
Voltage-dependent anion-selective channel protein 2 is a protein that in humans is encoded by the VDAC2 gene on chromosome 10. This protein is a voltage-dependent anion channel and shares high structural homology with the other VDAC isoforms. VDACs are generally involved in the regulation of cell metabolism, mitochondrial apoptosis, and spermatogenesis. Additionally, VDAC2 participates in cardiac contractions and pulmonary circulation, which implicate it in cardiopulmonary diseases. VDAC2 also mediates immune response to infectious bursal disease (IBD).
Voltage-dependent anion-selective channel protein 3 (VDAC3) is a protein that in humans is encoded by the VDAC3 gene on chromosome 8. The protein encoded by this gene is a voltage-dependent anion channel and shares high structural homology with the other VDAC isoforms. Nonetheless, VDAC3 demonstrates limited pore-forming ability and, instead, interacts with other proteins to perform its biological functions, including sperm flagella assembly and centriole assembly. Mutations in VDAC3 have been linked to male infertility, as well as Parkinson's disease.
Hexokinase 2 also known as HK2 is an enzyme which in humans is encoded by the HK2 gene on chromosome 2. Hexokinases phosphorylate glucose to produce glucose-6-phosphate (G6P), the first step in most glucose metabolism pathways. This gene encodes hexokinase 2, the predominant form found in skeletal muscle. It localizes to the outer membrane of mitochondria. Expression of this gene is insulin-responsive, and studies in rat suggest that it is involved in the increased rate of glycolysis seen in rapidly growing cancer cells. [provided by RefSeq, Apr 2009]
The Bcl-2 family consists of a number of evolutionarily-conserved proteins that share Bcl-2 homology (BH) domains. The Bcl-2 family is most notable for their regulation of apoptosis, a form of programmed cell death, at the mitochondrion. The Bcl-2 family proteins consists of members that either promote or inhibit apoptosis, and control apoptosis by governing mitochondrial outer membrane permeabilization (MOMP), which is a key step in the intrinsic pathway of apoptosis. A total of 25 genes in the Bcl-2 family were identified by 2008.
Tryptophan-rich sensory proteins (TspO) are a family of proteins that are involved in transmembrane signalling. In either prokaryotes or mitochondria they are localized to the outer membrane, and have been shown to bind and transport dicarboxylic tetrapyrrole intermediates of the haem biosynthetic pathway. They are associated with the major outer membrane porins and with the voltage-dependent anion channel.
ADP/ATP translocase 2 is a protein that in humans is encoded by the SLC25A5 gene on the X chromosome.
Mitochondria-associated membranes (MAMs) represent regions of the endoplasmic reticulum (ER) which are reversibly tethered to mitochondria. These membranes are involved in import of certain lipids from the ER to mitochondria and in regulation of calcium homeostasis, mitochondrial function, autophagy and apoptosis. They also play a role in development of neurodegenerative diseases and glucose homeostasis.