Fyn kinase regulates translation in mammalian mitochondria.

BACKGROUND: Mitochondrial translation machinery solely exists for the synthesis of 13 mitochondrially-encoded subunits of the oxidative phosphorylation (OXPHOS) complexes in mammals. Therefore, it plays a critical role in mitochondrial energy production. However, regulation of the mitochondrial translation machinery is still poorly understood. In comprehensive proteomics studies with normal and diseased tissues and cell lines, we and others have found the majority of mitochondrial ribosomal proteins (MRPs) to be phosphorylated. Neither the kinases for these phosphorylation events nor their specific roles in mitochondrial translation are known. METHODS: Mitochondrial kinases are responsible for phosphorylation of MRPs enriched from bovine mitoplasts by strong cation-exchange chromatography and identified by mass spectrometry-based proteomics analyses of kinase rich fractions. Phosphorylation of recombinant MRPs and 55S ribosomes was assessed by in vitro phosphorylation assays using the kinase-rich fractions. The effect of identified kinase on OXPHOS and mitochondrial translation was assessed by various cell biological and immunoblotting approaches. RESULTS: Here, we provide the first evidence for the association of Fyn kinase, a Src family kinase, with mitochondrial translation components and its involvement in phosphorylation of 55S ribosomal proteins in vitro. Modulation of Fyn expression in human cell lines has provided a link between mitochondrial translation and energy metabolism, which was evident by the changes in 13 mitochondrially encoded subunits of OXPHOS complexes. CONCLUSIONS AND GENERAL SIGNIFICANCE: Our findings suggest that Fyn kinase is part of a complex mechanism that regulates protein synthesis and OXPHOS possibly by tyrosine phosphorylation of translation components in mammalian mitochondria.

Mitochondrial ribosomal protein L12 selectively associates with human mitochondrial RNA polymerase to activate transcription.

Basal transcription of human mitochondrial DNA (mtDNA) in vitro requires the single-subunit, bacteriophage-related RNA polymerase, POLRMT, and transcription factor h-mtTFB2. This two-component system is activated differentially at mtDNA promoters by human mitochondrial transcription factor A (h-mtTFA). Mitochondrial ribosomal protein L7/L12 (MRPL12) binds directly to POLRMT, but whether it does so in the context of the ribosome or as a "free" protein in the matrix is unknown. Furthermore, existing evidence that MRPL12 activates mitochondrial transcription derives from overexpression studies in cultured cells and transcription experiments using crude mitochondrial lysates, precluding direct effects of MRPL12 on transcription to be assigned. Here, we report that depletion of MRPL12 from HeLa cells by shRNA results in decreased steady-state levels of mitochondrial transcripts, which are not accounted for by changes in RNA stability. We also show that a significant "free" pool of MRPL12 exists in human mitochondria not associated with ribosomes. "Free" MRPL12 binds selectively to POLRMT in vivo in a complex distinct from those containing h-mtTFB2. Finally, using a fully recombinant mitochondrial transcription system, we demonstrate that MRPL12 stimulates promoter-dependent and promoter-independent transcription directly in vitro. Based on these results, we propose that, when not associated with ribosomes, MRPL12 has a second function in transcription, perhaps acting to facilitate the transition from initiation to elongation. We speculate that this is one mechanism to coordinate mitochondrial ribosome biogenesis and transcription in human mitochondria, where transcription of rRNAs from the mtDNA presumably needs to be adjusted in accordance with the rate of import and assembly of the nucleus-encoded MRPs into ribosomes.

Regulation of mammalian mitochondrial translation by post-translational modifications.

Mitochondria are responsible for the production of over 90% of the energy in eukaryotes through oxidative phosphorylation performed by electron transfer and ATP synthase complexes. Mitochondrial translation machinery is responsible for the synthesis of 13 essential proteins of these complexes encoded by the mitochondrial genome. Emerging data suggest that acetyl-CoA, NAD(+), and ATP are involved in regulation of this machinery through post-translational modifications of its protein components. Recent high-throughput proteomics analyses and mapping studies have provided further evidence for phosphorylation and acetylation of ribosomal proteins and translation factors. Here, we will review our current knowledge related to these modifications and their possible role(s) in the regulation of mitochondrial protein synthesis using the homology between mitochondrial and bacterial translation machineries. However, we have yet to determine the effects of phosphorylation and acetylation of translation components in mammalian mitochondrial biogenesis. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.

Phosphorylation of mammalian mitochondrial EF-Tu by Fyn and c-Src kinases.

Src Family Kinases (SFKs) are tyrosine kinases known to regulate glucose and fatty acid metabolism as well as oxidative phosphorylation (OXPHOS) in mammalian mitochondria. We and others discovered the association of the SFK kinases Fyn and c-Src with mitochondrial translation components. This translational system is responsible for the synthesis of 13 mitochondrial (mt)-encoded subunits of the OXPHOS complexes and is, thus, essential for energy generation. Mitochondrial ribosomal proteins and various translation elongation factors including Tu (EF-Tumt) have been identified as possible Fyn and c-Src kinase targets. However, the phosphorylation of specific residues in EF-Tumt by these kinases and their roles in the regulation of protein synthesis are yet to be explored. In this study, we report the association of EF-Tumt with cSrc kinase and mapping of phosphorylated Tyr (pTyr) residues by these kinases. We determined that a specific Tyr residue in EF-Tumt at position 266 (EF-Tumt-Y266), located in a highly conserved c-Src consensus motif is one of the major phosphorylation sites. The potential role of EF-Tumt-Y266 phosphorylation in regulation of mitochondrial translation investigated by site-directed mutagenesis. Its phosphomimetic to Glu residue (EF-Tumt-E266) inhibited ternary complex (EF-Tumt•GTP•aatRNA) formation and translation in vitro. Our findings along with data mining analysis of the c-Src knock out (KO) mice proteome suggest that the SFKs have possible roles for regulation of mitochondrial protein synthesis and oxidative energy metabolism in animals.

Evaluation of mitochondrial biogenesis and ROS generation in high-grade serous ovarian cancer.

Introduction: Ovarian cancer is one of the leading causes of death for women with cancer worldwide. Energy requirements for tumor growth in epithelial high-grade serous ovarian cancer (HGSOC) are fulfilled by a combination of aerobic glycolysis and oxidative phosphorylation (OXPHOS). Although reduced OXPHOS activity has emerged as one of the significant contributors to tumor aggressiveness and chemoresistance, up-regulation of mitochondrial antioxidant capacity is required for matrix detachment and colonization into the peritoneal cavity to form malignant ascites in HGSOC patients. However, limited information is available about the mitochondrial biogenesis regulating OXPHOS capacity and generation of mitochondrial reactive oxygen species (mtROS) in HGSOC. Methods: To evaluate the modulation of OXPHOS in HGSOC tumor samples and ovarian cancer cell lines, we performed proteomic analyses of proteins involved in mitochondrial energy metabolism and biogenesis and formation of mtROS by immunoblotting and flow cytometry, respectively. Results and discussion: We determined that the increased steady-state expression levels of mitochondrial- and nuclear-encoded OXPHOS subunits were associated with increased mitochondrial biogenesis in HGSOC tumors and ovarian cancer cell lines. The more prominent increase in MT-COII expression was in agreement with significant increase in mitochondrial translation factors, TUFM and DARS2. On the other hand, the ovarian cancer cell lines with reduced OXPHOS subunit expression and mitochondrial translation generated the highest levels of mtROS and significantly reduced SOD2 expression. Evaluation of mitochondrial biogenesis suggested that therapies directed against mitochondrial targets, such as those involved in transcription and translation machineries, should be considered in addition to the conventional chemotherapies in HGSOC treatment.

Contacts between mammalian mitochondrial translational initiation factor 3 and ribosomal proteins in the small subunit.

Mammalian mitochondrial translational initiation factor 3 (IF3(mt)) binds to the small subunit of the ribosome displacing the large subunit during the initiation of protein biosynthesis. About half of the proteins in mitochondrial ribosomes have homologs in bacteria while the remainder are unique to the mitochondrion. To obtain information on the ribosomal proteins located near the IF3(mt) binding site, cross-linking studies were carried out followed by identification of the cross-linked proteins by mass spectrometry. IF3(mt) cross-links to mammalian mitochondrial homologs of the bacterial ribosomal proteins S5, S9, S10, and S18-2 and to unique mitochondrial ribosomal proteins MRPS29, MRPS32, MRPS36 and PTCD3 (Pet309) which has now been identified as a small subunit ribosomal protein. IF3(mt) has extensions on both the N- and C-termini compared to the bacterial factors. Cross-linking of a truncated derivative lacking these extensions gives the same hits as the full length IF3(mt) except that no cross-links were observed to MRPS36. IF3 consists of two domains separated by a flexible linker. Cross-linking of the isolated N- and C-domains was observed to a range of ribosomal proteins particularly with the C-domain carrying the linker which showed significant cross-linking to several ribosomal proteins not found in prokaryotes.

Role of mitochondrial translation in remodeling of energy metabolism in ER/PR(+) breast cancer.

Remodeling of mitochondrial energy metabolism is essential for the survival of tumor cells in limited nutrient availability and hypoxic conditions. Defects in oxidative phosphorylation (OXPHOS) and mitochondrial biogenesis also cause a switch in energy metabolism from oxidative to aerobic glycolysis contributing to the tumor heterogeneity in cancer. Specifically, the aberrant expressions of mitochondrial translation components such as ribosomal proteins (MRPs) and translation factors have been increasingly associated with many different cancers including breast cancer. The mitochondrial translation is responsible for the synthesis 13 of mitochondrial-encoded OXPHOS subunits of complexes. In this study, we investigated the contribution of mitochondrial translation in the remodeling of oxidative energy metabolism through altered expression of OXPHOS subunits in 26 ER/PR(+) breast tumors. We observed a significant correlation between the changes in the expression of mitochondrial translation-related proteins and OXPHOS subunits in the majority of the ER/PR(+) breast tumors and breast cancer cell lines. The reduced expression of OXPHOS and mitochondrial translation components also correlated well with the changes in epithelial-mesenchymal transition (EMT) markers, E-cadherin (CHD1), and vimentin (VIM) in the ER/PR(+) tumor biopsies. Data mining analysis of the Clinical Proteomic Tumor Analysis Consortium (CPTAC) breast cancer proteome further supported the correlation between the reduced OXPHOS subunit expression and increased EMT and metastatic marker expression in the majority of the ER/PR(+) tumors. Therefore, understanding the role of MRPs in the remodeling of energy metabolism will be essential in the characterization of heterogeneity at the molecular level and serve as diagnostic and prognostic markers in breast cancer.

Properties of the C-terminal tail of human mitochondrial inner membrane protein Oxa1L and its interactions with mammalian mitochondrial ribosomes.

In humans the mitochondrial inner membrane protein Oxa1L is involved in the biogenesis of membrane proteins and facilitates the insertion of both mitochondrial- and nuclear-encoded proteins from the mitochondrial matrix into the inner membrane. The C-terminal approximately 100-amino acid tail of Oxa1L (Oxa1L-CTT) binds to mitochondrial ribosomes and plays a role in the co-translational insertion of mitochondria-synthesized proteins into the inner membrane. Contrary to suggestions made for yeast Oxa1p, our results indicate that the C-terminal tail of human Oxa1L does not form a coiled-coil helical structure in solution. The Oxa1L-CTT exists primarily as a monomer in solution but forms dimers and tetramers at high salt concentrations. The binding of Oxa1L-CTT to mitochondrial ribosomes is an enthalpy-driven process with a K(d) of 0.3-0.8 microM and a stoichiometry of 2. Oxa1L-CTT cross-links to mammalian mitochondrial homologs of the bacterial ribosomal proteins L13, L20, and L28 and to mammalian mitochondrial specific ribosomal proteins MRPL48, MRPL49, and MRPL51. Oxa1L-CTT does not cross-link to proteins decorating the conventional exit tunnel of the bacterial large ribosomal subunit (L22, L23, L24, and L29).

NAD+-dependent deacetylase SIRT3 regulates mitochondrial protein synthesis by deacetylation of the ribosomal protein MRPL10.

A member of the sirtuin family of NAD(+)-dependent deacetylases, SIRT3, is located in mammalian mitochondria and is important for regulation of mitochondrial metabolism, cell survival, and longevity. In this study, MRPL10 (mitochondrial ribosomal protein L10) was identified as the major acetylated protein in the mitochondrial ribosome. Ribosome-associated SIRT3 was found to be responsible for deacetylation of MRPL10 in an NAD(+)-dependent manner. We mapped the acetylated Lys residues by tandem mass spectrometry and determined the role of these residues in acetylation of MRPL10 by site-directed mutagenesis. Furthermore, we observed that the increased acetylation of MRPL10 led to an increase in translational activity of mitochondrial ribosomes in Sirt3(-/-) mice. In a similar manner, ectopic expression and knockdown of SIRT3 in C2C12 cells resulted in the suppression and enhancement of mitochondrial protein synthesis, respectively. Our findings constitute the first evidence for the regulation of mitochondrial protein synthesis by the reversible acetylation of the mitochondrial ribosome and characterize MRPL10 as a novel substrate of the NAD(+)-dependent deacetylase, SIRT3.

Purification of human mitochondrial ribosomal L7/L12 stalk proteins and reconstitution of functional hybrid ribosomes in Escherichia coli.

Bacterial ribosomal L7/L12 stalk is formed by L10, L11, and multiple copies of L7/L12, which plays an essential role in recruiting initiation and elongation factors during translation. The homologs of these proteins, MRPL10, MRPL11, and MRPL12, are present in human mitochondrial ribosomes. To evaluate the role of MRPL10, MRPL11, and MRPL12 in translation, we over-expressed and purified components of the human mitochondrial L7/L12 stalk proteins in Escherichia coli. Here, we designed a construct to co-express MRPL10 and MRPL12 using a duet expression system to form a functional MRPL10-MRPL12 complex. The goal is to demonstrate the homology between the mitochondrial and bacterial L7/L12 stalk proteins and to reconstitute a hybrid ribosome to be used in structural and functional studies of the mitochondrial stalk.

Regulation of succinate dehydrogenase activity by SIRT3 in mammalian mitochondria.

A member of the sirtuin family of NAD(+)-dependent deacetylases, SIRT3, is identified as one of the major mitochondrial deacetylases located in mammalian mitochondria responsible for deacetylation of several metabolic enzymes and components of oxidative phosphorylation. Regulation of protein deacetylation by SIRT3 is important for mitochondrial metabolism, cell survival, and longevity. In this study, we identified one of the Complex II subunits, succinate dehydrogenase flavoprotein (SdhA) subunit, as a novel SIRT3 substrate in SIRT3 knockout mice. Several acetylated Lys residues were mapped by tandem mass spectrometry, and we determined the role of acetylation in Complex II activity in SIRT3 knockout mice. In agreement with SIRT3-dependent activation of Complex I, we observed that deacetylation of the SdhA subunit increased the Complex II activity in wild-type mice. In addition, we treated K562 cell lines with nicotinamide and kaempferol to inhibit deacetylase activity of SIRT3 and stimulate SIRT3 expression, respectively. Stimulation of SIRT3 expression decreased the level of acetylation of the SdhA subunit and increased Complex II activity in kaempherol-treated cells compared to control and nicotinamide-treated cells. Evaluation of acetylated residues in the SdhA crystal structure from porcine and chicken suggests that acetylation of the hydrophilic surface of SdhA may control the entry of the substrate into the active site of the protein and regulate the enzyme activity. Our findings constitute the first evidence of the regulation of Complex II activity by the reversible acetylation of the SdhA subunit as a novel substrate of the NAD(+)-dependent deacetylase, SIRT3.

c-Src kinase impairs the expression of mitochondrial OXPHOS complexes in liver cancer.

Src family kinases (SFKs) play a crucial role in the regulation of multiple cellular pathways, including mitochondrial oxidative phosphorylation (OXPHOS). Aberrant activities of one of the most predominant SFKs, c-Src, was identified as a fundamental cause for dysfunctional cell signaling and implicated in cancer development and metastasis, especially in human hepatocellular carcinoma (HCC). Recent work in our laboratory revealed that c-Src is implicated in the regulation of mitochondrial energy metabolism in cancer. In this study, we investigated the effect of c-Src expression on mitochondrial energy metabolism by examining changes in the expression and activities of OXPHOS complexes in liver cancer biopsies and cell lines. An increased expression of c-Src was correlated with an impaired expression of nuclear- and mitochondrial-encoded subunits of OXPHOS complexes I and IV, respectively, in metastatic biopsies and cell lines. Additionally, we observed a similar association between high c-Src and reduced OXPHOS complex expression and activity in mouse embryonic fibroblast (MEF) cell lines. Interestingly, the inhibition of c-Src kinase activity with the SFK inhibitor PP2 and c-Src siRNA stimulated the expression of complex I and IV subunits and increased their enzymatic activities in both cancer and normal cells. Evidence provided in this study reveals that c-Src impairs the expression and function of mitochondrial OXPHOS complexes, resulting in a significant defect in mitochondrial energy metabolism, which can be a contributing factor to the development and progression of liver cancer. Furthermore, our findings strongly suggest that SFK inhibitors should be used in the treatment of HCC and other cancers with aberrant c-Src kinase activity to improve mitochondrial energy metabolism.

Phosphorylated proteins of the mammalian mitochondrial ribosome: implications in protein synthesis.

Mitochondria, the powerhouse of eukaryotic cells, have their own translation machinery that is solely responsible for synthesis of 13 mitochondrially encoded protein subunits of oxidative phosphorylation complexes. Phosphorylation is a well-known post-translational modification in regulation of many processes in mammalian mitochondria including oxidative phosphorylation. However, there is still very limited knowledge on phosphorylation of mitochondrial ribosomal proteins and their role(s) in ribosome function. In this study, we have identified the mitochondrial ribosomal proteins that are phosphorylated at serine, threonine or tyrosine residues. Twenty-four phosphorylated proteins were visualized by phosphorylation-specific techniques including in vitro radiolabeling, residue specific antibodies for phosphorylated residues, or ProQ phospho dye and identified by tandem mass spectrometry. Translation assays with isolated ribosomes that were phosphorylated in vitro by kinases PKA, PKCdelta, or Abl Tyr showed up to 30% inhibition due to phosphorylation. Findings from this study should serve as the framework for future studies addressing the regulation mechanisms of mitochondrial translation machinery by phosphorylation and other post-translational modifications.

Mitochondrial oxidative phosphorylation is impaired in TALLYHO mice, a new obesity and type 2 diabetes animal model.

Type 2 diabetes has become an epidemic disease largely explained by the dramatic increase in obesity in recent years. Mitochondrial dysfunction is suggested as an underlying factor in obesity and type 2 diabetes. In this study, we evaluated changes in oxidative phosphorylation and mitochondrial biogenesis in a new human obesity and type 2 diabetes model, TALLYHO/Jng mice. We hypothesized that the sequence variants identified in the whole genome analysis of TALLYHO/Jng mice would affect oxidative phosphorylation and contribute to obesity and insulin resistant phenotypes. To test this hypothesis, we investigated differences in the expression and activity of oxidative phosphorylation complexes, including the transcription and translation of nuclear- and mitochondrial-encoded subunits and enzymatic activities, in the liver and kidney of TALLYHO/Jng and C57BL/6 J mice. A significant decrease was observed in the expression of nuclear- and mitochondrial-encoded subunits of complex I and IV, respectively, in TALLYHO/Jng mice, which coincided with significant reductions in their enzymatic activities. Furthermore, sequence variants were identified in oxidative phosphorylation complex subunits, a mitochondrial tRNA synthetase, and mitochondrial ribosomal proteins. Our data suggested that the lower expression and activity of oxidative phosphorylation complexes results in the diminished energy metabolism observed in TALLYHO/Jng mice. Sequence variants identified in mitochondrial proteins accentuated a defect in mitochondrial protein synthesis which also contributes to impaired biogenesis and oxidative phosphorylation in TALLYHO/Jng mice. These results demonstrated that the identification of factors contributing to mitochondrial dysfunction will allow us to improve the disease prognosis and treatment of obesity and type 2 diabetes in humans.

Identification of phosphorylation sites in mammalian mitochondrial ribosomal protein DAP3.

Mammalian mitochondrial ribosomes synthesize 13 proteins that are essential for oxidative phosphorylation. In addition to their role in protein synthesis, some of the mitochondrial ribosomal proteins have acquired functions in other cellular processes such as apoptosis. Death-associated protein 3 (DAP3), also referred to as mitochondrial ribosomal protein S29 (MRP-S29), is a GTP-binding pro-apoptotic protein located in the small subunit of the ribosome. Previous studies have shown that phosphorylation is one of the most likely regulatory mechanisms for DAP3 function in apoptosis and may be in protein synthesis; however, no phosphorylation sites were identified. In this study, we have investigated the phosphorylation status of ribosomal DAP3 and mapped the phosphorylation sites by tandem mass spectrometry. Mitochondrial ribosomal DAP3 is phosphorylated at Ser215 or Thr216, Ser220, Ser251 or Ser252, and Ser280. In addition, phosphorylation of recombinant DAP3 by Protein kinase A and Protein kinase Cdelta at residues that are endogenously phosphorylated in ribosomal DAP3 suggests both of these kinases as potential candidates responsible for the in vivo phosphorylation of DAP3 in mammalian mitochondria. Interestingly, the majority of the phosphorylation sites detected in our study are clustered around the highly conserved GTP-binding motifs, speculating on the significance of these residues on protein conformation and activity. Site-directed mutagenesis studies on selected phosphorylation sites were performed to determine the effect of phosphorylation on cell proliferation and PARP cleavage as indication of caspase activation. Overall, our findings suggest DAP3, a mitochondrial ribosomal small subunit protein, is a novel phosphorylated target.

HSP90 Inhibition and Cellular Stress Elicits Phenotypic Plasticity in Hematopoietic Differentiation.

Cancer cells exist in a state of Darwinian selection using mechanisms that produce changes in gene expression through genetic and epigenetic alteration to facilitate their survival. Cellular plasticity, or the ability to alter cellular phenotype, can assist in survival of premalignant cells as they progress to full malignancy by providing another mechanism of adaptation. The connection between cellular stress and the progression of cancer has been established, although the details of the mechanisms have yet to be fully elucidated. The molecular chaperone HSP90 is often upregulated in cancers as they progress, presumably to allow cancer cells to deal with misfolded proteins and cellular stress associated with transformation. The objective of this work is to test the hypothesis that inhibition of HSP90 results in increased cell plasticity in mammalian systems that can confer a greater adaptability to selective pressures. The approach used is a murine in vitro model system of hematopoietic differentiation that utilizes a murine hematopoietic stem cell line, erythroid myeloid lymphoid (EML) clone 1, during their maturation from stem cells to granulocytic progenitors. During the differentiation protocol, 80%-90% of the cells die when placed in medium where the major growth factor is granulocyte-macrophage-colony stimulating factor. Using this selection point model, EML cells exhibit increases in cellular plasticity when they are better able to adapt to this medium and survive. Increases in cellular plasticity were found to occur upon exposure to geldanamycin to inhibit HSP90, when subjected to various forms of cellular stress, or inhibition of histone acetylation. Furthermore, we provide evidence that the cellular plasticity associated with inhibition of HSP90 in this model involves epigenetic mechanisms and is dependent upon high levels of stem cell factor signaling. This work provides evidence for a role of HSP90 and cellular stress in inducing phenotypic plasticity in mammalian systems that has new implications for cellular stress in progression and evolution of cancer.

Impaired mitochondrial protein synthesis in head and neck squamous cell carcinoma.

Human head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer type worldwide, possibly due to the significant role of alcohol and tobacco use in its development. Underlying most cancers are defects in mitochondrial functions such as energy metabolism and apoptosis. In fact, the mutations in mitochondrial DNA (mtDNA), which encode proteins for oxidative phosphorylation (OXPHOS), have been associated with human head and neck cancers. Here, we investigated the changes in the expression of OXPHOS complexes and the contribution of the defects in mitochondrial translation in the progression of HNSCC. Western blot analyses of the several stage IVA HNSCC primary tumors have shown reduction in the expression of COII and ATP5A of the OXPHOS complexes IV and V subunits, respectively. On the other hand, expression of the majority of the OXPHOS subunits, except complex II SDHB subunit, was impaired in a patient with a stage IV tumor with a regional lymph node. Interestingly, an overall reduction in one of the mitochondrial-encoded subunits of the complex IV, COII, accentuated a possible defect in mitochondrial translation machinery in two of the stage IVA tumors. Evidence provided in this study suggests for the first time that the mitochondrial translation defect(s) could be due to a decrease in the expression of one of the essential mitochondrial ribosomal proteins, MRPL11, in head and neck tumor biopsies. We also observed an acquired mitochondrial translation deficiency in the HN8 cell line derived from a lymph node metastasis but not in the HN22 cells derived from the primary tumor of the same patient. These seminal observations suggest that the mitochondrial translation machinery deserves further investigation for accurate molecular assessment and treatment of HNSCC.

Identification and characterization of CHCHD1, AURKAIP1, and CRIF1 as new members of the mammalian mitochondrial ribosome.

Defects in mitochondrial ribosomal proteins (MRPs) cause various diseases in humans. Because of the essential role of MRPs in synthesizing the essential subunits of oxidative phosphorylation (OXPHOS) complexes, identifying all of the protein components involved in the mitochondrial translational machinery is critical. Initially, we identified 79 MRPs; however, identifying MRPs with no clear homologs in bacteria and yeast mitochondria was challenging, due to limited availability of expressed sequence tags (ESTs) in the databases available at that time. With the improvement in genome sequencing and increased sensitivity of mass spectrometry (MS)-based technologies, we have established four previously known proteins as MRPs and have confirmed the identification of ICT1 (MRP58) as a ribosomal protein. The newly identified MRPs are MRPS37 (Coiled-coil-helix-coiled-coil-helix domain containing protein 1-CHCHD1), MRPS38 (Aurora kinase A interacting protein1, AURKAIP1), MRPS39 (Pentatricopeptide repeat-containing protein 3, PTCD3), in the small subunit and MRPL59 (CR-6 interacting factor 1, CRIF1) in the large subunit. Furthermore, we have demonstrated the essential roles of CHCHD1, AURKAIP1, and CRIF1in mitochondrial protein synthesis by siRNA knock-down studies, which had significant effects on the expression of mitochondrially encoded proteins.

Regulation of mitochondrial ribosomal protein S29 (MRPS29) expression by a 5'-upstream open reading frame.

Mitochondrial ribosomal protein S29 (MRPS29) is a mitochondrial pro-apoptotic protein also known as death associated protein 3 (DAP3). Over-expression of MRPS29 has been reported to induce apoptosis in several different human cell lines while conferring resistance in glioma and Ataxia telangiectasia cells. These two contradictory reports led us to investigate the MRPS29-induced apoptosis further. Cyber searches of the EST databases revealed the presence of a splice variant of MRPS29 mRNA containing an upstream open reading frame (uORF) at the 5' untranslated region (UTR). In this study, we confirmed the presence of this uORF using real-time RT-PCR and investigated its role in MRPS29 expression.