Mitochondrial membrane proteins, mutations

P, Ferrero I, Mootha VK, Tiranti V, Zeviani M. MPV17 encodes an inner mitochondrial membrane protein and is mutated in infantile hepatic mitochondrial DNA depletion. Nat Genet. 

An essential part of the rationale presented above under (a) consists of the identification of altered products of mitochondrial protein synthesis as a result of the mutation. Although this is not a sufficient criterion for mitochondrial specification (since an altered protein might arise as a result of a mutational alteration in a component of the mitochondrial protein-synthesizing machinery, i.e., one of the mt r- or tRNAs, (as in poky Neu-rospora), it is a necessary one. We have therefore devoted considerable effort to demonstrating the capability of the mitochondria in the mutant to perform some form of protein synthesis. We did this by showing that they were capable of incorporating (1) labeled formate into formylmethionyl-puromycin as a measure of mitochondrial polypeptide-chain initiations (see also next section) (2) labeled leucine into mitochondrial membrane proteins in a reaction that is insensitive to cycloheximide (CHX), but sensitive to chloramphenicol (CAP) and (3) that continued exposure of cells to the latter led to their conversion to petite phenocopies, s5 of characteristic aspects of their phenotype, such as the presence of cytochrome b, and that this change was reversed on removal of the inhibitor (see also Table I). 

A subsequent study in 2002 of 27 families with a condition known as multiminicore disease (MmD) also linked mutations in SEPNl to disease pathology. Multiple mutations were identified in exons 1, 5, 7, 8, 10, and 11, and the authors also mentioned that this region (RSMD) had been previously linked to MmD. Minicores are lesions by histochemistry of mitochondrial depletion within muscle tissue. The first biochemical study of selenoprotein N aimed to identify the protein localization by immunohistochemistry and found that the primary protein product of several identified mRNAs (splice variants) was a 70 kDa protein present in the endoplasmic reticulum. Two potential ER targeting domains were shown to be present and the peptide expressed from the first exon was shown to be required for localization into the ER. This study also revealed that selenoprotein N was an integral membrane protein that is N-glycosylated. Expression analysis showed pronounced levels in embryonic tissue with a reduction after development and differentiation. 

Citrulline is exchanged for ornithine across the inner mitochondrial membrane by ORNT-1. Ornithine is produced in the cytosol as the final step in the urea cycle and must be returned to the mitochondrial matrix for transcarbamoyla-tion by OTC. A second ornithine-citrulline antiporter (ORNT-2) is also expressed in the liver mitochondria and may attenuate the severity of disease in patients with HHH (Hyperammonemia, Hyperornithinemia, Homocitrullinuria) disease due to ORNT-1 deficiency. This disorder typically manifests later in life with intermittent hyperammonemic encephalopathy and protein aversion. Intramitochondrial ornithine deficiency causes both hyperammonemia and hyperornithinemia due to a lack of substrate for OTC. Homocitrullinuria occurs due to the use of lysine by OTC as an alternate substrate. The diagnosis is confirmed by mutation analysis. 

Coxl7, an 8.1-kDa cysteine-rich protein, was the first copper chaperone to be identified. Saccharomyces cerevisiae harboring mutations in coxl 7 are respiratory deficient, a phenotype resulting from their inability to assemble a functional cytochrome c oxidase complex (Glerum et al., 1996a). coxl7 mutant yeast are, however, able to express all the subunits of the cytochrome c oxidase complex, indicating that the lesion must lie in a posttranslational step that is essential for assembly of the functional complex in the mitochondrial membrane. Unlike other cytochrome c... 

Petite mutants spontaneous mutants, chiefly yeasts, with chemical or physical defects in the respiratory chain. P. m. grow very slowly and form small ( petite ) colonies on nutrient agar. The same phenotype can be produced by a chromosomal mutation (segregational petite), or a mutation in the mitochondrial DNA (vegetative or neutral petite). In the latter case, mitochondrial structure is considerably altered, largely due to changes in the amino acid composition of the structural proteins of the inner mitochondrial membrane. Since these structural proteins are important for the correct arrangement and conformation of the respiratory chain enzymes, the effect of petite mutation on the respiratory chain is probably secondary. 

The products of the mitochondrial protein-synthesizing system are all found in the inner mitochondrial membrane and are hydrophobic in nature. This hydrophobic nature makes it particularly difficult to identify mitochondrial products as part of a functional entity, since on extraction they are dissociated from other proteins of the entity and are often denatured. A more productive approach has been to eliminate the mitochondrial contribution by the administration of antibiotics or by mutation of the mitochondrial DNA, and assay the function in situ for altered properties. To date, possibly four inner membrane-associated functions have been implicated as containing products of the mitochondrial synthetic systems. In all cases they are multicomponent complexes containing both mitochon-drially and cytoplasmically synthesized components. These are the cytochrome oxidase complex, the oligomycin-sensitive ATPase, the cytochrome b complex, and the mitochondrial ribosomes. All of the other inner membrane functions which have so far been studied are synthesized entirely in the cytoplasm. 

Defects of nuclear DNA also cause mitochondrial diseases. As mentioned above, the vast majority of mitochondrial proteins are encoded by nDNA, synthesized in the cytoplasm and imported into the mitochondria through a complex series of steps. Diseases can be due to mutations in genes encoding respiratory chain subunits, ancillary proteins controlling the proper assembly of the respiratory chain complexes, proteins controlling the importation machinery, or proteins controlling the lipid composition of the inner membrane. All these disorders will be transmitted by mendelian inheritance. From a biochemical point of view, all areas of mitochondrial metabolism can be affected (see below). 

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