MT-ATP8

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 F_o subunit 8 (or subunit A6L). This subunit belongs to the F_o 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.

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 F₁F_o 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 F_o 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 F_o 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.

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 F_o region causes rotation of F₁, which has a water-soluble component that hydrolyzes ATP and together, the F₁F_o 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.

The nomenclature of the enzyme has a long history. The F₁ fraction derives its name from the term "Fraction 1" and F_o (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 F_o unit of ATP synthase. The F_o 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.

Mutations to MT-ATP8 and other genes affecting oxidative phosphorylation in the mitochondria have been associated with a variety of neurodegenerative and cardiovascular disorders, including mitochondrial complex V deficiency, Leber's hereditary optic neuropathy (LHON), mitochondrial encephalomyopathy with stroke-like episodes (MELAS), Leigh syndrome, and NARP syndrome. Most of the body's cells contain thousands of mitochondria, each with one or more copies of mitochondrial DNA. The severity of some mitochondrial disorders is associated with the percentage of mitochondria in each cell that has a particular genetic change. People with Leigh syndrome due to a MT-ATP6 gene mutation tend to have a very high percentage of mitochondria with the mutation (from more than 90 percent to 95 percent). The less-severe features of NARP result from a lower percentage of mitochondria with the mutation, typically 70 percent to 90 percent. Because these two conditions result from the same genetic changes and can occur in different members of a single family, researchers believe that they may represent a spectrum of overlapping features instead of two distinct syndromes.

Mitochondrial complex V deficiency presents with heterogeneous clinical manifestations including neuropathy, ataxia, hypertrophic cardiomyopathy. Hypertrophic cardiomyopathy can present with negligible to extreme hypertrophy, minimal to extensive fibrosis and myocyte disarray, absent to severe left ventricular outflow tract obstruction, and distinct septal contours/morphologies with extremely varying clinical course.