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. 2016 Oct 6;167(2):471-483.e10.
doi: 10.1016/j.cell.2016.09.003. Epub 2016 Sep 29.

Mitochondrial Protein Synthesis Adapts to Influx of Nuclear-Encoded Protein

Ricarda Richter-Dennerlein  1 , Silke Oeljeklaus  2 , Isotta Lorenzi  1 , Christin Ronsör  1 , Bettina Bareth  1 , Alexander Benjamin Schendzielorz  1 , Cong Wang  1 , Bettina Warscheid  3 , Peter Rehling  4 , Sven Dennerlein  1
Affiliations

Mitochondrial Protein Synthesis Adapts to Influx of Nuclear-Encoded Protein

"V体育平台登录" Ricarda Richter-Dennerlein et al. Cell. .

Abstract

Mitochondrial ribosomes translate membrane integral core subunits of the oxidative phosphorylation system encoded by mtDNA. These translation products associate with nuclear-encoded, imported proteins to form enzyme complexes that produce ATP. Here, we show that human mitochondrial ribosomes display translational plasticity to cope with the supply of imported nuclear-encoded subunits. Ribosomes expressing mitochondrial-encoded COX1 mRNA selectively engage with cytochrome c oxidase assembly factors in the inner membrane VSports手机版. Assembly defects of the cytochrome c oxidase arrest mitochondrial translation in a ribosome nascent chain complex with a partially membrane-inserted COX1 translation product. This complex represents a primed state of the translation product that can be retrieved for assembly. These findings establish a mammalian translational plasticity pathway in mitochondria that enables adaptation of mitochondrial protein synthesis to the influx of nuclear-encoded subunits. .

Keywords: C12ORF62; COX1; MITRAC; OXPHOS; assembly; cytochrome c oxidase; mitochondrial ribosome; mitochondrial translation; translation regulation; translational plasticity. V体育安卓版.

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Figures

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Graphical abstract
Figure 1
Figure 1
C12ORF62 Interacts with Translating Mitochondrial Ribosomes (A) [35S]methionine-labeled mitochondrial translation products were subjected to immunoprecipitation with indicated antibodies. Total, 5%; eluates, 100%. (B) Equal amounts of isolated mitochondria from differentially SILAC-labeled HEK293T wild-type cells or cells expressing C12ORF62FLAG were subjected to FLAG-immunoisolation. Purified protein complexes were natively eluted and analyzed by liquid chromatography-mass spectrometry (LC-MS) (n = 4). Thresholds: p < 0.05; mean ratios >5 or >10 (dashed lines). (C) Isolated mitochondria from HEK293T cells expressing C12ORF62FLAG were treated with thiamphenicol (TAP) (50 μg/mL) for 24 hr where indicated and subjected to FLAG-immunoprecipitation. Total, 3%; eluates, 100%. (D) FLAG-immunoprecipitation experiments were performed from isolated mitochondria purified from HEK293T cells expressing FLAG tagged mL62, C12ORF62, MITRAC12, MITRAC15, or MITRAC7. Total, 3%; eluates, 100%. See also Figure S1 and Table S1.
Figure 2
Figure 2
C12ORF62-Associated Mitochondrial Ribosomes Are Committed to COX1 Synthesis (A–C) Immunoprecipitation from indicated cell lines was performed as in Figure 1D. Isolated RNA samples of total and eluates were quantified by real-time PCR (A) and NanoString technology (B and C). Levels of copurified RNA were calculated relative to mL62 (A) or total (B and C), respectively (mean ± SEM; n = 3).
Figure 3
Figure 3
C12ORF62 Associates with COX1 during Synthesis (A and B) [35S]methionine-labeled mitochondrial translation products from 143B wild-type (WT) and COX1G6930A mutant (G6930A) cell lines were subjected to immunoisolation using indicated antibodies. Total, 5%; eluates, 100%. (C and D) HEK293T cells were incubated in [35S]methionine-containing medium for 30 min and treated with puromycin (Pur) (2 μg/mL) where indicated for 10 or 20 min. Cell lysates were subjected to C12ORF62 immunoisolation. Total, 5%; eluates, 100%. COX1 fragments (COX1f1–f3), indicated by asterisks, were quantified in (D) (mean ± SEM; n = 3). (E) HEK293T cells were labeled and treated as in (C). Puromycin was added for 20 min where indicated prior to immunoisolation using C12ORF62 and MITRAC12 antibodies. Samples were analyzed as in (C). (F and G) Experiments were performed as in (C). Radiolabeled samples were subjected to immunoisolation using antibodies against MITRAC7 (F) and TIM21 (G), in comparison to C12ORF62. See also Figure S2.
Figure 4
Figure 4
Membrane Topology and Interactions of COX1 Fragments (A and B) Mitochondrial translation products were labeled with [35S]methionine in 143B wild-type (WT) and COX1G6930A mutant cells, treated with puromycin (Pur) (2 μg/mL) where indicated and subjected to C12ORF62 immunoprecipitation. Total, 5%; eluates, 100%. Asterisks and a red arrowhead indicate puromycin-released COX1 fragments COX1f1–f3 and mutant COX11–342, respectively. (C) Schematic presentation of COX1 topology. The position of premature stop codons for in vitro COX1 synthesis, as in (D), are indicated by arrowheads. Relative position of mutation of COX1G6930A in transmembrane domain nine is indicated by a red arrow. N, amino-terminal; C, C-terminal; IMS, intermembrane space; IMM, inner mitochondrial membrane. (D) HEK293T cells, 143B wild-type (WT) and the COX1G6930A mutant were treated and radiolabeled as in (A). COX1 peptides of defined lengths (indicated by the number of amino acids) were synthesized and radiolabeled in vitro using rabbit reticulocyte lysates. Puromycin (Pur)-released COX1 fragments are indicated by asterisks and aligned with in vitro synthesized COX1 peptides (asterisk). Mutant COX11–342 protein in the COXG6930A cell line is marked by red arrowheads. (E) Schematic presentation of discontinuously synthesized fragments in relation to COX1 membrane topology.
Figure 5
Figure 5
C12ORF62 Acts as the Initial COX1 Maturation Protein (A and B) Depletion efficiency and specificity of C12ORF62 siRNA-mediated downregulation. HEK293T cells were treated with siRNA oligonucleotides against C12orf62 or with control siRNA for 72 hr followed by western blot analysis (A). Expression of a siC12orf62-1-resistant form of FLAG tagged C12ORF62 restores the growth phenotype upon loss of endogenous C12ORF62. Cells inducibly expressing scC12ORF62FLAG were treated with siC12orf62-1 for 72 hr in the presence of tetracycline (Tet) where indicated. Dashed line represents the starting cell number (mean ± SEM, n = 6) (B). (C–E) COX1 synthesis and stability in C12ORF62-deficient cells. C12ORF62 was depleted from HEK293T cells for 72 hr followed by radiolabeling for 1 hr (C). Levels of newly synthesized COX1 were quantified, internally standardized to ATP6, and normalized to non-targeting control (dashed line) in C12ORF62- and MITRAC12-deficient cells (mean ± SEM, n = 3) (D). COX1 stability was determined after 24 hr chase (mean ± SEM, n = 3) (E). (F) FLAG immunoisolation of C12ORF62 and MITRAC12. HEK293T cells were [35S]methionine labeled, treated with puromycin (Pur) where indicated, and subjected to immunoprecipitation. Protein complexes were natively eluted and analyzed by SDS-PAGE (left panel) and BN-PAGE (right panel). COX1 fragments are indicated by asterisks and pre-complexes of MITRAC are labeled with MITRACPre1 and MITRACPre2 (# undefined complex). (G) COX1 translation intermediates can be retrieved for translation. HEK293T cells were incubated with [35S]methionine and chased after media replacement for indicated time points with cold methionine. All samples were treated with puromycin for 20 min prior to MITRAC12FLAG immunoisolations (left panel). COX1 fragments and full-length COX1 isolated by MITRAC12FLAG were quantified (mean ± SEM, n = 4) (right panels). See also Figure S3.
Figure 6
Figure 6
COX4 Interacts with Premature COX1 (A) Structure of COX1 and COX4 in the mature bovine complex IV. PDB: 3ASN; COX4, red; COX1, green with TM1, blue; TM12, orange (Suga et al., 2011). (B) Two-dimensional BN/SDS-PAGE analysis of COX4. RCS, respiratory chain supercomplexes; IV, complex IV. (C) HEK293T cells were treated with puromycin (Pur) for 20 min where indicated. Mitochondria were isolated and [35S]methionine-labeled COX4 was imported for 45 min. Samples were subjected to FLAG immunoprecipitation followed by BN-PAGE (upper panel) and SDS-PAGE (lower panel) analyses. (D) Isolated mitochondria from HEK293T cells expressing COX4FLAG were subjected to FLAG-immunoprecipitation. Total, 1%; eluates, 100%. (E) Mitochondrial translation products were radiolabeled in HEK293T cells. Cell lysates were subjected to immunoisolation using indicated antibodies. Mitochondrial translation was terminated with puromycin (Pur) prior to isolation where indicated. #, undefined mitochondrial translation product. Total, 5%; eluates, 100%. (F) Mitochondrial translation products were labeled with [35S]methionine in 143B wild-type (WT) and COX1G6930A mutant cells. Samples were subjected to immunoisolation and analyzed as in (E). Total, 5%; eluates, 100%. See also Figure S4.
Figure 7
Figure 7
Nascent Chain Ribosome Complexes Accumulate on Early Assembly Factors upon Block of Complex IV Assembly (A and B) MITRAC12 was depleted in HEK293T cells by siRNA treatment for 72 hr prior to [35S]methionine labeling and immunoisolation using antibodies as indicated (A). COX1 fragments are labeled by asterisks. Total, 5%; eluates, 100%. (B) Quantification of COX1 fragments upon MITRAC12 depletion (mean ± SEM; n = 3). (C) Cell lysates (50 μg for MITRAC12, C12ORF62, SDHA, and TIM44; 25 μg for all other proteins) were analyzed by SDS-PAGE and western blotting. Quantification of COX4, COX1, and C12ORF62 protein levels (mean ± SEM, n = 4) after 72 hr knock-down of COX4. (D) Quantification of newly synthesized COX1 upon COX5B and COX4 depletion. Level of COX1 synthesis (standardized to ATP6) were calculated and normalized to non-targeting control (mean ± SEM; n = 3). (E) COX4 was ablated in HEK293T cells prior to radiolabeling of mitochondrial translation products. Samples were subjected to immunoprecipitation using indicated antibodies. Radiolabeled protein bands were quantified along the gel lanes and the differential profile (Δintensity) plotted along the gel section of interest. Bottom graph, increase of COX1f1/f2 fragments relative to the control (mean ± SEM; n = 4). (F) COX4 and MITRAC12 were depleted in HEK293T cells expressing C12ORF62FLAG by siRNA treatment for 72 hr prior to [35S]methionine labeling and FLAG-immunoisolation. Upper panel: western blot analysis of cell lysates. Lower panel: BN-PAGE analysis of natively eluted immunoisolation samples. As a control, one sample was treated with puromycin prior to isolation omitting siRNA treatment. See also Figure S5.
Figure S1
Figure S1
Yeast Cox14 and Coa3 Assembly Factors Do Not Associate with Mitochondrial Ribosomes, Related to Figure 1 Mitochondria from wild-type (WT) and Mba1ProtA expressing strains were solubilized in digitonin buffer and protein complexes isolated via immunoisolation (IgG chromatography or Cox14- or Coa3-specific antiserum). Eluates were separated by SDS-PAGE and analyzed by western-blotting. Total 1%, eluate 100%.
Figure S2
Figure S2
Late COX Biogenesis Factors Do Not Interact with Nascent Chain Fragments as well as Yeast Coa3, Related to Figure 3 (A and B) Longer exposures of experiments shown in Figures 3F and 3G. (C) Yeast Coa3 does not interact with Cox1 nascent chain. [35S]methionine labeling of mitochondrial translation products was carried out in organello mitochondria. Mitochondrial translational products were labeled for 25 min. Samples were treated with 0.1 mM puromycin (Pur) for 15 min and mitochondria solubilized in digitonin buffer and subjected to immunoisolation using Coa3-specific antiserum. Samples were analyzed by SDS-PAGE and digital autoradiography or western blot. Total 10%, eluate 100%.
Figure S3
Figure S3
Mitochondrial Translation Is Affected upon Reduction of C12ORF62 and MITRAC12, Related to Figure 5 (A) Downregulation of the m-AAA protease does not restore COX1 levels. AFG3L2 was depleted by siRNA application for 72 hr and COX1 quantified after SDS-PAGE and digital autoradiography (mean ± SEM; n = 3). (B) Steady-state levels of mitochondrial proteins after siRNA-mediated downregulation of MITRAC12. HEK293T cells were treated with siRNA molecules against Mitrac12 for 72 hr followed by western blot analysis. (C and D) COX1 synthesis and stability in C12ORF62- and MITRAC12-deficient cells. C12ORF62 and MITRAC12 were depleted from HEK293T cells for 72 hr followed by radiolabeling for 1 hr. The cells were either harvested (C) or after media replacement, chased for indicated time points (D). Samples were analyzed by SDS-PAGE and digital autoradiography.
Figure S4
Figure S4
Interaction Interface Map of COX4 to COX1 in the Mature Bovine Complex IV, Related to Figure 6 Highlighted are COX4 (red), COX1 TM2 and TM11 (green), COX1 TM1 (blue) and COX1 TM12 (orange). PDB: 3ASN (Suga et al., 2011).
Figure S5
Figure S5
Reduction of COX6C, SCO2, and SURF1 Does Not Lead to an Accumulation of Nascent Chain Ribosome Complexes, Related to Figure 7 (A) Depletion of COX4 leads to cytochrome c oxidase deficiency. Isolated mitochondria from siRNA mediated COX4 ablated HEK293T cells (72 h) were subjected to BN-PAGE and activity staining. (B) Steady state of mitochondrial proteins after siRNA-mediated downregulation (72 h) of COX5B, COX6C, SCO2 and in Surf1−/− cells. (C) Mitochondrial protein synthesis in COX6C- and SCO2-depleted cells, and in Surf1−/− HEK293T cells. Mitochondrial translation products were radiolabeled for 1 hr in HEK293T cells treated with siRNAs against Cox6C and Sco2 (72 h) and in Surf1−/− cells. Cells were harvested and samples analyzed by SDS-PAGE and digital autoradiography. (D) Nascent COX1 does not accumulate in a C12ORF62 complex upon loss of COX6C, SCO2 or SURF1. COX6C and SCO2 were ablated in HEK293T cells by siRNA application for 72h prior to radiolabeling of mitochondrial translation products. Samples were subjected to immunoprecipitation using indicated antibodies and eluates analyzed by SDS-PAGE and autoradiography. Total, 5%; eluates, 100%
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