Allotopic expression of yeast mitochondrial maturase to study mitochondrial import of hydrophobic proteins

Identification of a mitochondrial-targeting secretory protein from Nocardia seriolae which induces apoptosis in fathead minnow cells

Nocardia seriolae is the main pathogen responsible for fish nocardiosis. A mitochondrial-targeting secretory protein (MTSP) 3141 with an N-terminal transit peptide (TP) from N. seriolae was predicted by bioinformatic analysis based on the genomic sequence of the N. seriolae strain ZJ0503. However, the function of the MTSP3141 and its homologs remains totally unknown. In this study, mass spectrometry analysis of the extracellular products from N. seriolae proved that MTSP3141 was a secretory protein, subcellular localization research showed the MTSP3141-GFP fusion protein co-localized with mitochondria in fathead minnow (FHM) cells, the TP played an important role in mitochondria targeting, and only the TP located at N-terminus but not C-terminus can lead to mitochondria directing. Moreover, quantitative assays of mitochondrial membrane potential (ΔΨm) value, caspase-3 activity and apoptosis-related gene (Bcl-2, Bax, Bad, Bid and p53) mRNA expression suggested that cell apoptosis was induced in FHM cells by the overexpression of both MTSP3141 and MTSP3141ΔTP (with the N-terminal TP deleted) proteins. Taken together, the results of this study indicated that the MTSP3141 of N. seriolae was a secretory protein, might target mitochondria, induce apoptosis in host cells and function as a virulence factor.

Allotopic mRNA Localization to the Mitochondrial Surface Rescues Respiratory Chain Defects in Fibroblasts Harboring Mitochondrial DNA Mutations Affecting Complex I or V Subunits

The possibility of synthesizing mitochondrial DNA (mtDNA)-coded proteins in the cytosolic compartment, called allotopic expression, provides an attractive option for genetic treatment of human diseases caused by mutations of the corresponding genes. However, it is now appreciated that the high hydrophobicity of proteins encoded by the mitochondrial genome represents a strong limitation on their mitochondrial import when translated in the cytosol. Recently, we optimized the allotopic expression of a recoded ATP6 gene in human cells, by forcing its mRNA to localize to the mitochondrial surface. In this study, we show that this approach leads to a long-lasting and complete rescue of mitochondrial dysfunction of fibroblasts harboring the neurogenic muscle weakness, ataxia and retinitis Pigmentosa T8993G ATP6 mutation or the Leber hereditary optic neuropathy G11778A ND4 mutation. The recoded ATP6 gene was associated with the cis-acting elements of SOD2, while the ND4 gene was associated with the cis-acting elements of COX10. Both ATP6 and ND4 gene products were efficiently translocated into the mitochondria and functional within their respective respiratory chain complexes. Indeed, the abilities to grow in galactose and to produce adenosine triphosphate (ATP) in vitro were both completely restored in fibroblasts allotopically expressing either ATP6 or ND4. Notably, in fibroblasts harboring the ATP6 mutation, allotopic expression of ATP6 led to the recovery of complex V enzymatic activity. Therefore, mRNA sorting to the mitochondrial surface represents a powerful strategy that could ultimately be applied in human therapy and become available for an array of devastating disorders caused by mtDNA mutations.

Genetic Correction of Mitochondrial Diseases: Using the Natural Migration of Mitochondrial Genes to the Nucleus in Chlorophyte Algae as a Model System

Mitochondrial diseases display great diversity in clinical symptoms and biochemical characteristics. Although mtDNA mutations have been identified in many patients, there are currently no effective treatments. A number of human diseases result from mutations in mtDNA-encoded proteins, a group of proteins that are hydrophobic and have multiple membrane-spanning regions. One method that has great potential for overcoming the pathogenic consequences of these mutations is to place a wild-type copy of the affected gene in the nucleus, and target the expressed protein to the mitochondrion to function in place of the defective protein. Several respiratory chain subunit genes, which are typically mtDNA encoded, are nucleus encoded in the chlorophyte algae Chlamydomonas reinhardtii and Polytomella sp. Analysis of these genes has revealed adaptations that facilitated their expression from the nucleus. The nucleus-encoded proteins exhibited diminished physical constraints for import as compared to their mtDNA-encoded homologues. The hydrophobicity of the nucleus-encoded proteins is diminished in those regions that are not involved in subunit-subunit interactions or that contain amino acids critical for enzymatic reactions of the proteins. In addition, these proteins have unusually large mitochondrial targeting sequences. Information derived from these studies should be applicable toward the development of genetic therapies for human diseases resulting from mutations in mtDNA-encoded polypeptides.

An algal nucleus-encoded subunit of mitochondrial ATP synthase rescues a defect in the analogous human mitochondrial-encoded subunit

Unlike most organisms, the mitochondrial DNA (mtDNA) of Chlamydomonas reinhardtii, a green alga, does not encode subunit 6 of F(0)F(1)-ATP synthase. We hypothesized that C. reinhardtii ATPase 6 is nucleus encoded and identified cDNAs and a single-copy nuclear gene specifying this subunit (CrATP6, with eight exons, four of which encode a mitochondrial targeting signal). Although the algal and human ATP6 genes are in different subcellular compartments and the encoded polypeptides are highly diverged, their secondary structures are remarkably similar. When CrATP6 was expressed in human cells, a significant amount of the precursor polypeptide was targeted to mitochondria, the mitochondrial targeting signal was cleaved within the organelle, and the mature polypeptide was assembled into human ATP synthase. In spite of the evolutionary distance between algae and mammals, C. reinhardtii ATPase 6 functioned in human cells, because deficiencies in both cell viability and ATP synthesis in transmitochondrial cell lines harboring a pathogenic mutation in the human mtDNA-encoded ATP6 gene were overcome by expression of CrATP6. The ability to express a nucleus-encoded version of a mammalian mtDNA-encoded protein may provide a way to import other highly hydrophobic proteins into mitochondria and could serve as the basis for a gene therapy approach to treat human mitochondrial diseases.

Overexpression of yeast karyopherin Pse1p/Kap121p stimulates the mitochondrial import of hydrophobic proteins in vivo

During evolution, cellular processes leading to the transfer of genetic information failed to send all the mitochondrial genes into the nuclear genome. Two mitochondrial genes are still exclusively located in the mitochondrial genome of all living organisms. They code for two highly hydrophobic proteins: the apocytochrome b and the subunit I of cytochrome oxidase. Assuming that the translocation machinery could not efficiently transport long hydrophobic fragments, we searched for multicopy suppressors of this physical blockage. We demonstrated that overexpression of Pse1p/Kap121p or Kap123p, which belong to the superfamily of karyopherin beta proteins, facilitates the translocation of chimeric proteins containing several stretches of apocytochrome b fused to a reporter mitochondrial gene. The effect of PSE1/KAP121 overexpression (in which PSE1 is protein secretion enhancer 1) on mitochondrial import of the chimera is correlated with an enrichment of the corresponding transcript in cytoplasmic ribosomes associated with mitochondria. PSE1/KAP121 overexpression also improves the import of the hydrophobic protein Atm1p, an ABC transporter of the mitochondrial inner membrane. These results suggest that in vivo PSE1/KAP121 overexpression facilitates, either directly or indirectly, the co-translational import of hydrophobic proteins into mitochondria.

Computational method to predict mitochondrially imported proteins and their targeting sequences

Most of the proteins that are used in mitochondria are imported through the double membrane of the organelle. The information that guides the protein to mitochondria is contained in its sequence and structure, although no direct evidence can be obtained. In this article, discriminant analysis has been performed with 47 parameters and a large set of mitochondrial proteins extracted from the SwissProt database. A computational method that facilitates the analysis and objective prediction of mitochondrially imported proteins has been developed. If only the amino acid sequence is considered, 75-97% of the mitochondrial proteins studied have been predicted to be imported into mitochondria. Moreover, the existence of mitochondrial-targeting sequences is predicted in 76-94% of the analyzed mitochondrial precursor proteins. As a practical application, the number of unknown yeast open reading frames that might be mitochondrial proteins has been predicted, which revealed that many of them are clustered.