MT-ATP8 (or ATP8) is a mitochondrial gene with the full name 'mitochondrially encoded ATP synthase membrane subunit 8' that encodes a subunit of mitochondrial ATP synthase, ATP synthase Fo subunit 8 (or subunit A6L). This subunit belongs to the Fo complex of the large, transmembrane F-type ATP synthase. This enzyme, which is also known as complex V, is responsible for the final step of oxidative phosphorylation in the electron transport chain. Specifically, one segment of ATP synthase allows positively charged ions, called protons, to flow across a specialized membrane inside mitochondria. Another segment of the enzyme uses the energy created by this proton flow to convert a molecule called adenosine diphosphate (ADP) to ATP. Subunit 8 differs in sequence between Metazoa, plants and Fungi.
Structure The ATP synthase protein 8 of human and other mammals is encoded in the mitochondrial genome by the MT-ATP8 gene. When the complete human mitochondrial genome was first published, the MT-ATP8 gene was described as the unidentified reading frame URF A6L. An unusual feature of the MT-ATP8 gene is its 46-nucleotide overlap with the MT-ATP6 gene. With respect to the reading frame (+1) of MT-ATP8, the MT-ATP6 gene starts on the +3 reading frame. The MT-ATP8 protein weighs 8 kDa and is composed of 68 amino acids. The protein is a subunit of the F1Fo ATPase, also known as Complex V, which consists of 14 nuclear- and 2 mitochondrial-encoded subunits. F-type ATPases consist of two structural domains, F1 containing the extramembraneous catalytic core and Fo containing the membrane proton channel, linked together by a central stalk and a peripheral stalk. As an A subunit, MT-ATP8 is contained within the non-catalytic, transmembrane Fo portion of the complex, comprising the proton channel. The catalytic portion of mitochondrial ATP synthase consists of 5 different subunits (alpha, beta, gamma, delta, and epsilon) assembled with a stoichiometry of 3 alpha, 3 beta, and a single representative of the other 3. The proton channel consists of three main subunits (a, b, c). This gene encodes the delta subunit of the catalytic core. Alternatively spliced transcript variants encoding the same isoform have been identified.
Function The MT-ATP8 gene encodes a subunit of mitochondrial ATP synthase, located within the thylakoid membrane and the inner mitochondrial membrane. Mitochondrial ATP synthase catalyzes ATP synthesis, utilizing an electrochemical gradient of protons across the inner membrane during oxidative phosphorylation. The Fo region causes rotation of F1, which has a water-soluble component that hydrolyzes ATP and together, the F1Fo creates a pathway for movement of protons across the membrane. This protein subunit appears to be an integral component of the stator stalk in yeast mitochondrial F-ATPases. The stator stalk is anchored in the membrane, and acts to prevent futile rotation of the ATPase subunits relative to the rotor during coupled ATP synthesis/hydrolysis. This subunit may have an analogous function in Metazoa.
Nomenclature The nomenclature of the enzyme has a long history. The F1 fraction derives its name from the term "Fraction 1" and Fo (written as a subscript letter "o", not "zero") derives its name from being the binding fraction for oligomycin, a type of naturally-derived antibiotic that is able to inhibit the Fo unit of ATP synthase. The Fo region of ATP synthase is a proton pore that is embedded in the mitochondrial membrane. It consists of three main subunits A, B, and C, and (in humans) six additional subunits, d, e, f, g, MT-ATP6 (or F6), and MT-ATP8 (or A6L). 3D structure of E. coli homologue of this subunit was modeled based on electron microscopy data (chain M of PDB: 1c17). It forms a transmembrane 4-α-bundle.
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