A Single Kinase Generates the Majority of the Secreted Phosphoproteome.

The existence of extracellular phosphoproteins has been acknowledged for over a century. However, research in this area has been undeveloped largely because the kinases that phosphorylate secreted proteins have escaped identification. Fam20C is a kinase that phosphorylates S-x-E/pS motifs on proteins in milk and in the extracellular matrix of bones and teeth. Here, we show that Fam20C generates the majority of the extracellular phosphoproteome. Using CRISPR/Cas9 genome editing, mass spectrometry, and biochemistry, we identify more than 100 secreted phosphoproteins as genuine Fam20C substrates. Further, we show that Fam20C exhibits broader substrate specificity than previously appreciated. Functional annotations of Fam20C substrates suggest roles for the kinase beyond biomineralization, including lipid homeostasis, wound healing, and cell migration and adhesion. Our results establish Fam20C as the major secretory pathway protein kinase and serve as a foundation for new areas of investigation into the role of secreted protein phosphorylation in human biology and disease.

Reversible phosphorylation of Rpn1 regulates 26S proteasome assembly and function.

The fundamental importance of the 26S proteasome in health and disease suggests that its function must be finely controlled, and yet our knowledge about proteasome regulation remains limited. Posttranslational modifications, especially phosphorylation, of proteasome subunits have been shown to impact proteasome function through different mechanisms, although the vast majority of proteasome phosphorylation events have not been studied. Here, we have characterized 1 of the most frequently detected proteasome phosphosites, namely Ser361 of Rpn1, a base subunit of the 19S regulatory particle. Using a variety of approaches including CRISPR/Cas9-mediated gene editing and quantitative mass spectrometry, we found that loss of Rpn1-S361 phosphorylation reduces proteasome activity, impairs cell proliferation, and causes oxidative stress as well as mitochondrial dysfunction. A screen of the human kinome identified several kinases including PIM1/2/3 that catalyze S361 phosphorylation, while its level is reversibly controlled by the proteasome-resident phosphatase, UBLCP1. Mechanistically, Rpn1-S361 phosphorylation is required for proper assembly of the 26S proteasome, and we have utilized a genetic code expansion system to directly demonstrate that S361-phosphorylated Rpn1 more readily forms a precursor complex with Rpt2, 1 of the first steps of 19S base assembly. These findings have revealed a prevalent and biologically important mechanism governing proteasome formation and function.

Inhibition of dual-specificity tyrosine phosphorylation-regulated kinase 2 perturbs 26S proteasome-addicted neoplastic progression.

Dependence on the 26S proteasome is an Achilles' heel for triple-negative breast cancer (TNBC) and multiple myeloma (MM). The therapeutic proteasome inhibitor, bortezomib, successfully targets MM but often leads to drug-resistant disease relapse and fails in breast cancer. Here we show that a 26S proteasome-regulating kinase, DYRK2, is a therapeutic target for both MM and TNBC. Genome editing or small-molecule mediated inhibition of DYRK2 significantly reduces 26S proteasome activity, bypasses bortezomib resistance, and dramatically delays in vivo tumor growth in MM and TNBC thereby promoting survival. We further characterized the ability of LDN192960, a potent and selective DYRK2-inhibitor, to alleviate tumor burden in vivo. The drug docks into the active site of DYRK2 and partially inhibits all 3 core peptidase activities of the proteasome. Our results suggest that targeting 26S proteasome regulators will pave the way for therapeutic strategies in MM and TNBC.

Phosphorylation of serine96 of histidine-rich calcium-binding protein by the Fam20C kinase functions to prevent cardiac arrhythmia.

Precise Ca cycling through the sarcoplasmic reticulum (SR), a Ca storage organelle, is critical for proper cardiac muscle function. This cycling initially involves SR release of Ca via the ryanodine receptor, which is regulated by its interacting proteins junctin and triadin. The sarco/endoplasmic reticulum Ca ATPase (SERCA) pump then refills SR Ca stores. Histidine-rich Ca-binding protein (HRC) resides in the lumen of the SR, where it contributes to the regulation of Ca cycling by protecting stressed or failing hearts. The common Ser96Ala human genetic variant of HRC strongly correlates with life-threatening ventricular arrhythmias in patients with idiopathic dilated cardiomyopathy. However, the underlying molecular pathways of this disease remain undefined. Here, we demonstrate that family with sequence similarity 20C (Fam20C), a recently characterized protein kinase in the secretory pathway, phosphorylates HRC on Ser96. HRC Ser96 phosphorylation was confirmed in cells and human hearts. Furthermore, a Ser96Asp HRC variant, which mimics constitutive phosphorylation of Ser96, diminished delayed aftercontractions in HRC null cardiac myocytes. This HRC phosphomimetic variant was also able to rescue the aftercontractions elicited by the Ser96Ala variant, demonstrating that phosphorylation of Ser96 is critical for the cardioprotective function of HRC. Phosphorylation of HRC on Ser96 regulated the interactions of HRC with both triadin and SERCA2a, suggesting a unique mechanism for regulation of SR Ca homeostasis. This demonstration of the role of Fam20C-dependent phosphorylation in heart disease will open new avenues for potential therapeutic approaches against arrhythmias.

Enzymatic Phosphorylation of Ser in a Type I Collagen Peptide.

Phosphoproteomics studies have reported phosphorylation at multiple sites within collagen, raising the possibility that these post-translational modifications regulate the physical or biological properties of collagen. In this study, molecular dynamics simulations and experimental studies were carried out on model peptides to establish foundational principles of phosphorylation of Ser residues in collagen. A (Gly-Xaa-Yaa)11 peptide was designed to include a Ser-containing sequence from type I collagen that was reported to be phosphorylated. The physiological kinase involved in collagen phosphorylation is not known. In vitro studies showed that a model kinase ERK1 (extracellular signal-regulated protein kinase 1) would phosphorylate Ser within the consensus sequence if the collagen-like peptide is in the denatured state but not in the triple-helical state. The peptide was not a substrate for FAM20C, a kinase present in the secretory pathway, which has been shown to phosphorylate many extracellular matrix proteins. The unfolded single chain (Gly-Xaa-Yaa)11 peptide containing phosphoSer was able to refold to form a stable triple helix but at a reduced folding rate and with a small decrease in thermal stability relative to the nonphosphorylated peptide at neutral pH. These biophysical studies on model peptides provide a basis for investigations into the physiological consequences of collagen phosphorylation and the application of phosphorylation to regulate the properties of collagen biomaterials.

Dynamic regulation of FGF23 by Fam20C phosphorylation, GalNAc-T3 glycosylation, and furin proteolysis.

The family with sequence similarity 20, member C (Fam20C) has recently been identified as the Golgi casein kinase. Fam20C phosphorylates secreted proteins on Ser-x-Glu/pSer motifs and loss-of-function mutations in the kinase cause Raine syndrome, an often-fatal osteosclerotic bone dysplasia. Fam20C is potentially an upstream regulator of the phosphate-regulating hormone fibroblast growth factor 23 (FGF23), because humans with FAM20C mutations and Fam20C KO mice develop hypophosphatemia due to an increase in full-length, biologically active FGF23. However, the mechanism by which Fam20C regulates FGF23 is unknown. Here we show that Fam20C directly phosphorylates FGF23 on Ser(180), within the FGF23 R(176)XXR(179)/S(180)AE subtilisin-like proprotein convertase motif. This phosphorylation event inhibits O-glycosylation of FGF23 by polypeptide N-acetylgalactosaminyltransferase 3 (GalNAc-T3), and promotes FGF23 cleavage and inactivation by the subtilisin-like proprotein convertase furin. Collectively, our results provide a molecular mechanism by which FGF23 is dynamically regulated by phosphorylation, glycosylation, and proteolysis. Furthermore, our findings suggest that cross-talk between phosphorylation and O-glycosylation of proteins in the secretory pathway may be an important mechanism by which secreted proteins are regulated.

Thiazolidinediones are acute, specific inhibitors of the mitochondrial pyruvate carrier.

Facilitated pyruvate transport across the mitochondrial inner membrane is a critical step in carbohydrate, amino acid, and lipid metabolism. We report that clinically relevant concentrations of thiazolidinediones (TZDs), a widely used class of insulin sensitizers, acutely and specifically inhibit mitochondrial pyruvate carrier (MPC) activity in a variety of cell types. Respiratory inhibition was overcome with methyl pyruvate, localizing the effect to facilitated pyruvate transport, and knockdown of either paralog, MPC1 or MPC2, decreased the EC50 for respiratory inhibition by TZDs. Acute MPC inhibition significantly enhanced glucose uptake in human skeletal muscle myocytes after 2 h. These data (i) report that clinically used TZDs inhibit the MPC, (ii) validate that MPC1 and MPC2 are obligatory components of facilitated pyruvate transport in mammalian cells, (iii) indicate that the acute effect of TZDs may be related to insulin sensitization, and (iv) establish mitochondrial pyruvate uptake as a potential therapeutic target for diseases rooted in metabolic dysfunction.

Enhancing anti-AML activity of venetoclax by isoflavone ME-344 through suppression of OXPHOS and/or purine biosynthesis in vitro.

Venetoclax (VEN), in combination with low dose cytarabine (AraC) or a hypomethylating agent, is FDA approved to treat acute myeloid leukemia (AML) in patients who are over the age of 75 or cannot tolerate standard chemotherapy. Despite high response rates to these therapies, most patients succumb to the disease due to relapse and/or drug resistance, providing an unmet clinical need for novel therapies to improve AML patient survival. ME-344 is a potent isoflavone with demonstrated inhibitory activity toward oxidative phosphorylation (OXPHOS) and clinical activity in solid tumors. Given that OXPHOS inhibition enhances VEN antileukemic activity against AML, we hypothesized that ME-344 could enhance the anti-AML activity of VEN. Here we report that ME-344 enhanced VEN to target AML cell lines and primary patient samples while sparing normal hematopoietic cells. Cooperative suppression of OXPHOS was detected in a subset of AML cell lines and primary patient samples. Metabolomics analysis revealed a significant reduction of purine biosynthesis metabolites by ME-344. Further, lometrexol, a purine biosynthesis inhibitor, synergistically enhanced VEN-induced apoptosis in AML cell lines. Interestingly, AML cells with acquired AraC resistance showed significantly increased purine biosynthesis metabolites and sensitivities to ME-344. Furthermore, synergy between ME-344 and VEN was preserved in these AraC-resistant AML cells. In vivo studies revealed significantly prolonged survival upon combination therapy of ME-344 and VEN in NSGS mice bearing parental or AraC-resistant MV4-11 leukemia compared to the vehicle control. This study demonstrates that ME-344 enhances VEN antileukemic activity against preclinical models of AML by suppressing OXPHOS and/or purine biosynthesis.

Dual inhibition of HSF1 and DYRK2 impedes cancer progression.

Preserving proteostasis is a major survival mechanism for cancer. Dual specificity tyrosine phosphorylation-regulated kinase 2 (DYRK2) is a key oncogenic kinase that directly activates the transcription factor heat-shock factor 1 (HSF1) and the 26S proteasome. Targeting DYRK2 has proven to be a tractable strategy to target cancers sensitive to proteotoxic stress; however, the development of HSF1 inhibitors remains in its infancy. Importantly, multiple other kinases have been shown to redundantly activate HSF1 that promoted ideas to directly target HSF1. The eventual development of direct HSF1 inhibitor KRIBB11 suggests that the transcription factor is indeed a druggable target. The current study establishes that concurrent targeting of HSF1 and DYRK2 can indeed impede cancer by inducing apoptosis faster than individual targetting. Furthermore, targeting the DYRK2-HSF1 axis induces death in proteasome inhibitor-resistant cells and reduces triple-negative breast cancer (TNBC) burden in ectopic and orthotopic xenograft models. Together the data indicate that cotargeting of kinase DYRK2 and its substrate HSF1 could prove to be a beneficial strategy in perturbing neoplastic malignancies.

Enhancing anti-AML activity of venetoclax by isoflavone ME-344 through suppression of OXPHOS and/or purine biosynthesis.

Venetoclax (VEN), in combination with low dose cytarabine (AraC) or a hypomethylating agent, is FDA approved to treat acute myeloid leukemia (AML) in patients who are over the age of 75 or cannot tolerate standard chemotherapy. Despite high response rates to these combination therapies, most patients succumb to the disease due to relapse and/or drug resistance, providing an unmet clinical need for novel therapies to improve AML patient survival. ME-344 is a potent isoflavone with demonstrated inhibitory activity toward oxidative phosphorylation (OXPHOS) and clinical activity in solid tumors. Given that OXPHOS inhibition enhances VEN antileukemic activity against AML, we hypothesized that ME-344 could enhance the anti-AML activity of VEN. Here we report that ME-344 synergized with VEN to target AML cell lines and primary patient samples while sparing normal hematopoietic cells. Cooperative suppression of OXPHOS was detected in a subset of AML cell lines and primary patient samples. Metabolomics analysis revealed a significant reduction of purine biosynthesis metabolites by ME-344. Further, lometrexol, an inhibitor of purine biosynthesis, synergistically enhanced VEN-induced apoptosis in AML cell lines. Interestingly, AML cells with acquired resistance to AraC showed significantly increased purine biosynthesis metabolites and sensitivities to ME-344. Furthermore, synergy between ME-344 and VEN was preserved in these AraC-resistant AML cells. These results translated into significantly prolonged survival upon combination of ME-344 and VEN in NSGS mice bearing parental or AraC-resistant MV4-11 leukemia. This study demonstrates that ME-344 enhances VEN antileukemic activity against preclinical models of AML by suppressing OXPHOS and/or purine biosynthesis.

Monitoring phosphorylation of the pyruvate dehydrogenase complex.

The pyruvate dehydrogenase multienzyme complex (PDC) is a key regulatory point in cellular metabolism linking glycolysis to the citric acid cycle and lipogenesis. Reversible phosphorylation of the pyruvate dehydrogenase enzyme is a critical regulatory mechanism and an important point for monitoring metabolic activity. To directly determine the regulation of the PDC by phosphorylation, we developed a complete set of phospho-antibodies against the three known phosphorylation sites on the E1 alpha subunit of pyruvate dehydrogenase (PDHE1alpha). We demonstrate phospho-site specificity of each antibody in a variety of cultured cells and tissue extracts. In addition, we show sensitivity of these antibodies to PDH activity using the pyruvate dehydrogenase kinase-specific inhibitor dichloroacetate. We go on to use these antibodies to assess PDH phosphorylation in a patient suffering from Leigh's syndrome. Finally, we observe changes in individual phosphorylation states following a small molecule screen, demonstrating that these reagents should be useful for monitoring phosphorylation of PDHE1alpha and, therefore, overall metabolism in the disease state as well as in response to a myriad of physiological and pharmacological stimuli.

A secretory pathway kinase regulates sarcoplasmic reticulum Ca2+ homeostasis and protects against heart failure.

Ca2+ signaling is important for many cellular and physiological processes, including cardiac function. Although sarcoplasmic reticulum (SR) proteins involved in Ca2+ signaling have been shown to be phosphorylated, the biochemical and physiological roles of protein phosphorylation within the lumen of the SR remain essentially uncharacterized. Our laboratory recently identified an atypical protein kinase, Fam20C, which is uniquely localized to the secretory pathway lumen. Here, we show that Fam20C phosphorylates several SR proteins involved in Ca2+ signaling, including calsequestrin2 and Stim1, whose biochemical activities are dramatically regulated by Fam20C mediated phosphorylation. Notably, phosphorylation of Stim1 by Fam20C enhances Stim1 activation and store-operated Ca2+ entry. Physiologically, mice with Fam20c ablated in cardiomyocytes develop heart failure following either aging or induced pressure overload. We extended these observations to show that non-muscle cells lacking Fam20C display altered ER Ca2+ signaling. Overall, we show that Fam20C plays an overarching role in ER/SR Ca2+ homeostasis and cardiac pathophysiology.

Intravenous (-)-epicatechin reduces myocardial ischemic injury by protecting mitochondrial function.

BACKGROUND: Targeting the mitochondria during ischemia/reperfusion (IR) can confer cardioprotection leading to improved clinical outcomes. The cardioprotective potential of (-)-epicatechin (EPI) during IR via modulation of mitochondrial function was evaluated. METHODS AND RESULTS: Ischemia was induced in rats via a 45 min occlusion of the left anterior descending coronary artery followed by 1 h, 48 h, or 3 week reperfusion. EPI (10 mg/kg) was administered IV 15 min prior to reperfusion for the single dose group and again 12 h later for the double dose group. Controls received water. Experiments also utilized cultured neonatal rat ventricular myocytes (NRVM) and myoblasts. A single dose of EPI reduced infarct size by 27% at 48 h and 28% at 3 week. Double dose treatment further decreased infarct size by 80% at 48 h, and 52% by 3 weeks. The protective effect of EPI on mitochondrial function was evident after 1h of reperfusion when mitochondria demonstrated less respiratory inhibition, lower mitochondrial Ca2+ load, and a preserved pool of NADH that correlated with higher tissue ATP levels. Mechanistic studies in NRVM revealed that EPI acutely stimulated maximal rates of respiration, an effect that was blocked by inhibitors of the mitochondrial pyruvate carrier, nitric oxide synthase, or soluble guanylyl cyclase. In myoblasts, knockdown of components of the mitochondrial pyruvate carrier blocked EPI-induced respiratory stimulation. CONCLUSIONS: IV EPI confers cardioprotection via preservation of mitochondrial function potentially through enhanced substrate provision. These provocative results document a novel mechanism of a natural product with potential clinical utility.

Wolfram Syndrome protein, Miner1, regulates sulphydryl redox status, the unfolded protein response, and Ca2+ homeostasis.

Miner1 is a redox-active 2Fe2S cluster protein. Mutations in Miner1 result in Wolfram Syndrome, a metabolic disease associated with diabetes, blindness, deafness, and a shortened lifespan. Embryonic fibroblasts from Miner1(-/-) mice displayed ER stress and showed hallmarks of the unfolded protein response. In addition, loss of Miner1 caused a depletion of ER Ca(2+) stores, a dramatic increase in mitochondrial Ca(2+) load, increased reactive oxygen and nitrogen species, an increase in the GSSG/GSH and NAD(+)/NADH ratios, and an increase in the ADP/ATP ratio consistent with enhanced ATP utilization. Furthermore, mitochondria in fibroblasts lacking Miner1 displayed ultrastructural alterations, such as increased cristae density and punctate morphology, and an increase in O2 consumption. Treatment with the sulphydryl anti-oxidant N-acetylcysteine reversed the abnormalities in the Miner1 deficient cells, suggesting that sulphydryl reducing agents should be explored as a treatment for this rare genetic disease.

Identification of a mitochondrial target of thiazolidinedione insulin sensitizers (mTOT)--relationship to newly identified mitochondrial pyruvate carrier proteins.

Thiazolidinedione (TZD) insulin sensitizers have the potential to effectively treat a number of human diseases, however the currently available agents have dose-limiting side effects that are mediated via activation of the transcription factor PPARγ. We have recently shown PPARγ-independent actions of TZD insulin sensitizers, but the molecular target of these molecules remained to be identified. Here we use a photo-catalyzable drug analog probe and mass spectrometry-based proteomics to identify a previously uncharacterized mitochondrial complex that specifically recognizes TZDs. These studies identify two well-conserved proteins previously known as brain protein 44 (BRP44) and BRP44 Like (BRP44L), which recently have been renamed Mpc2 and Mpc1 to signify their function as a mitochondrial pyruvate carrier complex. Knockdown of Mpc1 or Mpc2 in Drosophila melanogaster or pre-incubation with UK5099, an inhibitor of pyruvate transport, blocks the crosslinking of mitochondrial membranes by the TZD probe. Knockdown of these proteins in Drosophila also led to increased hemolymph glucose and blocked drug action. In isolated brown adipose tissue (BAT) cells, MSDC-0602, a PPARγ-sparing TZD, altered the incorporation of (13)C-labeled carbon from glucose into acetyl CoA. These results identify Mpc1 and Mpc2 as components of the mitochondrial target of TZDs (mTOT) and suggest that understanding the modulation of this complex, which appears to regulate pyruvate entry into the mitochondria, may provide a viable target for insulin sensitizing pharmacology.

Secreted kinase phosphorylates extracellular proteins that regulate biomineralization.

Protein phosphorylation is a fundamental mechanism regulating nearly every aspect of cellular life. Several secreted proteins are phosphorylated, but the kinases responsible are unknown. We identified a family of atypical protein kinases that localize within the Golgi apparatus and are secreted. Fam20C appears to be the Golgi casein kinase that phosphorylates secretory pathway proteins within S-x-E motifs. Fam20C phosphorylates the caseins and several secreted proteins implicated in biomineralization, including the small integrin-binding ligand, N-linked glycoproteins (SIBLINGs). Consequently, mutations in Fam20C cause an osteosclerotic bone dysplasia in humans known as Raine syndrome. Fam20C is thus a protein kinase dedicated to the phosphorylation of extracellular proteins.

Mitochondrial phosphatase PTPMT1 is essential for cardiolipin biosynthesis.

PTPMT1 was the first protein tyrosine phosphatase found localized to the mitochondria, but its biological function was unknown. Herein, we demonstrate that whole body deletion of Ptpmt1 in mice leads to embryonic lethality, suggesting an indispensable role for PTPMT1 during development. Ptpmt1 deficiency in mouse embryonic fibroblasts compromises mitochondrial respiration and results in abnormal mitochondrial morphology. Lipid analysis of Ptpmt1-deficient fibroblasts reveals an accumulation of phosphatidylglycerophosphate (PGP) along with a concomitant decrease in phosphatidylglycerol. PGP is an essential intermediate in the biosynthetic pathway of cardiolipin, a mitochondrial-specific phospholipid regulating the membrane integrity and activities of the organelle. We further demonstrate that PTPMT1 specifically dephosphorylates PGP in vitro. Loss of PTPMT1 leads to dramatic diminution of cardiolipin, which can be partially reversed by the expression of catalytic active PTPMT1. Our study identifies PTPMT1 as the mammalian PGP phosphatase and points to its role as a regulator of cardiolipin biosynthesis.

Chapter 13 Localization and function of the 2Fe-2S outer mitochondrial membrane protein mitoNEET.

MitoNEET is an integral protein of the outer mitochondrial membrane and is the flagship of a small family of proteins whose hallmark is the presence of a CDGSH domain. Initially annotated as a zinc finger, the CDGSH domain actually binds a redox-active 2Fe-2S cluster, giving mitoNEET the distinction of being the first 2Fe-2S protein identified in the outer membrane of mitochondria. This chapter describes methods for isolating mitochondrial membrane fractions that are enriched in mitoNEET, generating constructs for the expression of recombinant mitoNEET protein and analyzing the 2Fe-2S cluster of mitoNEET in vitro.

Dual specificity phosphatases 18 and 21 target to opposing sides of the mitochondrial inner membrane.

Although large-scale approaches have identified numerous mitochondrial phosphoproteins, little is known about the mitochondrial kinases and phosphatases that regulate these phosphoproteins. Here, we identify two members of the atypical dual specificity phosphatases (DSP), DSP18 and DSP21, that are localized in mitochondria. Although DSP18 is widely expressed in several mammalian tissues, DSP21 is selectively expressed in the testes. We demonstrate that DSP18 and DSP21 are targeted to mitochondria by cryptic internal localization signals. Subfractionation of mitochondria demonstrated that DSP18 is located in the intermembrane space as a peripheral membrane protein of the inner membrane. In contrast, subfractionation of rat testis mitochondria revealed DSP21 is localized to the matrix as a peripheral membrane protein of the inner membrane. Moreover, we demonstrate that a previously reported substrate for DSP18, the stress-activated protein kinase, does not localize to mitochondria in several different tissues, making it an unlikely substrate for DSP18. Finally, we show that induction of apoptosis by treatment with staurosporine causes translocation of DSP18 from the intermembrane space into the cytosol similar to other apoptogenic factors such as cytochrome c. This work rigorously demonstrates the unique location of two highly similar DSPs on opposing sides of the mitochondrial inner membrane.

MitoNEET is an iron-containing outer mitochondrial membrane protein that regulates oxidative capacity.

Members of the thiazolidinedione (TZD) class of insulin-sensitizing drugs are extensively used in the treatment of type 2 diabetes. Pioglitazone, a member of the TZD family, has been shown to bind specifically to a protein named mitoNEET [Colca JR, McDonald WG, Waldon DJ, Leone JW, Lull JM, Bannow CA, Lund ET, Mathews WR (2004) Am J Physiol 286:E252-E260]. Bioinformatic analysis reveals that mitoNEET is a member of a small family of proteins containing a domain annotated as a CDGSH-type zinc finger. Although annotated as a zinc finger protein, mitoNEET contains no zinc, but instead contains 1.6 mol of Fe per mole of protein. The conserved sequence C-X-C-X(2)-(S/T)-X(3)-P-X-C-D-G-(S/A/T)-H is a defining feature of this unique family of proteins and is likely involved in iron binding. Localization studies demonstrate that mitoNEET is an integral protein present in the outer mitochondrial membrane. An amino-terminal anchor sequence tethers the protein to the outer membrane with the CDGSH domain oriented toward the cytoplasm. Cardiac mitochondria isolated from mitoNEET-null mice demonstrate a reduced oxidative capacity, suggesting that mito- NEET is an important iron-containing protein involved in the control of maximal mitochondrial respiratory rates.