Phosphorylation of mitochondrial proteins in bovine heart. Characterization of kinases and substrates

The basics of mitochondrial cAMP signalling: Where, when, why

Cytosolic cAMP signalling in live cells has been extensively investigated in the past, while only in the last decade the existence of an intramitochondrial autonomous cAMP homeostatic system began to emerge. Thanks to the development of novel tools to investigate cAMP dynamics and cAMP/PKA-dependent phosphorylation within the matrix and in other mitochondrial compartments, it is now possible to address directly and in intact living cells a series of questions that until now could be addressed only by indirect approaches, in isolated organelles or through subcellular fractionation studies. In this contribution we discuss the mechanisms that regulate cAMP dynamics at the surface and inside mitochondria, and its crosstalk with organelle Ca²⁺ handling. We then address a series of still unsolved questions, such as the intramitochondrial localization of key elements of the cAMP signaling toolkit, e.g., adenylate cyclases, phosphodiesterases, protein kinase A (PKA) and Epac. Finally, we discuss the evidence for and against the existence of an intramitochondrial PKA pool and the functional role of cAMP increases within the organelle matrix.

The organochlorine pesticide toxaphene reduces non-mitochondrial respiration and induces heat shock protein 70 expression in early-staged zebrafish (Danio rerio)

Toxaphene is a restricted-use pesticide produced by reacting chlorine gas with camphene. It was heavily used as a pesticide for agricultural purposes in the 1960-1970s, but despite being banned >30 years ago, it can remain elevated in the soil due to its resistance to metabolic degradation; this has led to longstanding concerns about elevated levels of toxaphene and other organochlorine pesticides (OCPs) in the environment. The objective of this study were to determine the effects of waterborne exposure to toxaphene on early life stages of zebrafish. Based on the LC₅₀, zebrafish embryos were exposed to control (embryo rearing media or DMSO) or to one dose of toxaphene ranging between 0.011 and 111.1 μg/mL from 6 h post fertilization (hpf) up to 120 hpf. Significant mortality and hatch time delays were observed in embryos exposed to toxaphene (at or above 0.11 and 1.11 μg/mL, depending on the assay). Higher prevalence of deformities was noted at higher doses (≥0.011 μg/mL), and these included pericardial edema and skeletal deformities. As energy production is important for normal development, mitochondrial bioenergetics were assessed in embryos following toxaphene exposure. Embryos exposed to 11.1 or 111 μg/mL toxaphene for 24 h showed lower non-mitochondrial respiration (~30%) compared to both solvent and no treatment controls. Expression of transcripts related to oxidative damage responses and apoptosis were measured and heat shock protein 70 was significantly increased with 111 μg/mL toxaphene (14.5 fold), while the expression levels of caspase 3, caspase 9, and superoxide dismutase 1 were not changed. These data demonstrate that developmental deformities induced by toxaphene include pericardial edema and skeletal deformity, and that toxaphene can affect oxidative phosphorylation in early staged zebrafish.

Regulation of mitochondrial functions by protein phosphorylation and dephosphorylation

The mitochondria are double membrane-bound organelles found in most eukaryotic cells. They generate most of the cell’s energy supply of adenosine triphosphate (ATP). Protein phosphorylation and dephosphorylation are critical mechanisms in the regulation of cell signaling networks and are essential for almost all the cellular functions. For many decades, mitochondria were considered autonomous organelles merely functioning to generate energy for cells to survive and proliferate, and were thought to be independent of the cellular signaling networks. Consequently, phosphorylation and dephosphorylation processes of mitochondrial kinases and phosphatases were largely neglected. However, evidence accumulated in recent years on mitochondria-localized kinases/phosphatases has changed this longstanding view. Mitochondria are increasingly recognized as a hub for cell signaling, and many kinases and phosphatases have been reported to localize in mitochondria and play important functions. However, the strength of the evidence on mitochondrial localization and the activities of the reported kinases and phosphatases vary greatly, and the detailed mechanisms on how these kinases/phosphatases translocate to mitochondria, their subsequent function, and the physiological and pathological implications of their localization are still poorly understood. Here, we provide an updated perspective on the recent advancement in this area, with an emphasis on the implications of mitochondrial kinases/phosphatases in cancer and several other diseases.

Protein Kinase A Governs Oxidative Phosphorylation Kinetics and Oxidant Emitting Potential at Complex I

The mitochondrial electron transport system (ETS) is responsible for setting and maintaining both the energy and redox charges throughout the cell. Reversible phosphorylation of mitochondrial proteins, particularly via the soluble adenylyl cyclase (sAC)/cyclic AMP (cAMP)/Protein kinase A (PKA) axis, has recently been revealed as a potential mechanism regulating the ETS. However, the governance of cAMP/PKA signaling and its implications on ETS function are incompletely understood. In contrast to prior reports using exogenous bicarbonate, we provide evidence that endogenous CO₂ produced by increased tricarboxylic acid (TCA) cycle flux is insufficient to increase mitochondrial cAMP levels, and that exogenous addition of membrane permeant 8Br-cAMP does not enhance mitochondrial respiratory capacity. We also report important non-specific effects of commonly used inhibitors of sAC which preclude their use in studies of mitochondrial function. In isolated liver mitochondria, inhibition of PKA reduced complex I-, but not complex II-supported respiratory capacity. In permeabilized myofibers, inhibition of PKA lowered both the K_m and V_max for complex I-supported respiration as well as succinate-supported H₂O₂ emitting potential. In summary, the data provided here improve our understanding of how mitochondrial cAMP production is regulated, illustrate a need for better tools to examine the impact of sAC activity on mitochondrial biology, and suggest that cAMP/PKA signaling contributes to the governance of electron flow through complex I of the ETS.

Role of soluble adenylyl cyclase in mitochondria

The soluble adenylyl cyclase (sAC) catalyzes the conversion of ATP into cyclic AMP (cAMP). Recent studies have shed new light on the role of sAC localized in mitochondria and its product cAMP, which drives mitochondrial protein phosphorylation and regulation of the oxidative phosphorylation system and other metabolic enzymes, presumably through the activation of intra-mitochondrial PKA. In this review article, we summarize recent findings on mitochondrial sAC activation by bicarbonate (HCO(3)(-)) and calcium (Ca²⁺) and the effects on mitochondrial metabolism. We also discuss putative mechanisms whereby sAC-mediated mitochondrial protein phosphorylation regulates mitochondrial metabolism. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

cAMP and mitochondria

Phosphorylation of mitochondrial proteins has emerged as a major regulatory mechanism for metabolic adaptation. cAMP signaling and PKA phosphorylation of mitochondrial proteins have just started to be investigated, and the presence of cAMP-generating enzymes and PKA inside mitochondria is still controversial. Here, we discuss the role of cAMP in regulating mitochondrial bioenergetics through protein phosphorylation and the evidence for soluble adenylyl cyclase as the source of cAMP inside mitochondria.

Mitochondria: A mirror into cellular dysfunction in heart disease

Cardiovascular (CV) disease is the single most significant cause of morbidity and mortality worldwide. The emerging global impact of CV disease means that the goals of early diagnosis and a wider range of treatment options are now increasingly pertinent. As such, there is a greater need to understand the molecular mechanisms involved and potential targets for intervention. Mitochondrial function is important for physiological maintenance of the cell, and when this function is altered, the cell can begin to suffer. Given the broad range and significant impacts of the cellular processes regulated by the mitochondria, it becomes important to understand the roles of the proteins associated with this organelle. Proteomic investigations of the mitochondria are hampered by the intrinsic properties of the organelle, including hydrophobic mitochondrial membranes; high proportion of basic proteins (pI greater than 8.0); and the relative dynamic range issues of the mitochondria. For these reasons, many proteomic studies investigate the mitochondria as a discrete subproteome. Once this has been achieved, the alterations that result in functional changes with CV disease can be observed. Those alterations that lead to changes in mitochondrial function, signaling and morphology, which have significant implications for the cardiomyocyte in the development of CV disease, are discussed.

Cardiac mitochondrial matrix and respiratory complex protein phosphorylation

It has become appreciated over the last several years that protein phosphorylation within the cardiac mitochondrial matrix and respiratory complexes is extensive. Given the importance of oxidative phosphorylation and the balance of energy metabolism in the heart, the potential regulatory effect of these classical signaling events on mitochondrial function is of interest. However, the functional impact of protein phosphorylation and the kinase/phosphatase system responsible for it are relatively unknown. Exceptions include the well-characterized pyruvate dehydrogenase and branched chain α-ketoacid dehydrogenase regulatory system. The first task of this review is to update the current status of protein phosphorylation detection primarily in the matrix and evaluate evidence linking these events with enzymatic function or protein processing. To manage the scope of this effort, we have focused on the pathways involved in energy metabolism. The high sensitivity of modern methods of detecting protein phosphorylation and the low specificity of many kinases suggests that detection of protein phosphorylation sites without information on the mole fraction of phosphorylation is difficult to interpret, especially in metabolic enzymes, and is likely irrelevant to function. However, several systems including protein translocation, adenine nucleotide translocase, cytochrome c, and complex IV protein phosphorylation have been well correlated with enzymatic function along with the classical dehydrogenase systems. The second task is to review the current understanding of the kinase/phosphatase system within the matrix. Though it is clear that protein phosphorylation occurs within the matrix, based on (32)P incorporation and quantitative mass spectrometry measures, the kinase/phosphatase system responsible for this process is ill-defined. An argument is presented that remnants of the much more labile bacterial protein phosphoryl transfer system may be present in the matrix and that the evaluation of this possibility will require the application of approaches developed for bacterial cell signaling to the mitochondria.

Respiratory chain complex I, a main regulatory target of the cAMP/PKA pathway is defective in different human diseases

In mammals, complex I (NADH-ubiquinone oxidoreductase) of the mitochondrial respiratory chain has 31 supernumerary subunits in addition to the 14 conserved from prokaryotes to humans. Multiplicity of structural protein components, as well as of biogenesis factors, makes complex I a sensible pace-maker of mitochondrial respiration. The work reviewed here shows that the cAMP/PKA pathway regulates the biogenesis, assembly and catalytic activity of complex I and mitochondrial oxygen superoxide production. The structural, functional and regulatory complexity of complex I, renders it particularly vulnerable to genetic and sporadic pathological factors. Complex I dysfunction has, indeed, been found, to be associated with several human diseases. Knowledge of the pathogenetic mechanisms of these diseases can help to develop new therapeutic strategies.

Progesterone synthesis by human placental mitochondria is sensitive to PKA inhibition by H89

The transfer of cholesterol to mitochondria, which might involve the phosphorylation of proteins, is the rate-limiting step in human placental steroidogenesis. Protein kinase A (PKA) activity and its role in progesterone synthesis by human placental mitochondria were assessed in this study. The results showed that PKA and phosphotyrosine phosphatase D1 are associated with syncytiotrophoblast mitochondrial membrane by an anchoring kinase cAMP protein-121. The ³²P-labeled of four major proteins was analyzed. The specific inhibitor of PKA, H89, decreased progesterone synthesis in mitochondria while in mitochondrial steroidogenic contact sites protein-phosphorylation was diminished, suggesting that PKA plays a role in placental hormone synthesis. In isolated mitochondria, PKA activity was unaffected by the addition of cAMP suggesting a constant activity of this kinase in the syncytiotrophoblast. The presence of PKA and phosphotyrosine phosphatase D1 anchored to mitochondria by an anchoring kinase cAMP protein-121 indicated that syncytiotrophoblast mitochondria contain a full phosphorylation/dephosphorylation system.

Cardiac mitochondria in heart failure: normal cardiolipin profile and increased threonine phosphorylation of complex IV

Mitochondrial dysfunction is a major contributor in heart failure (HF). We investigated whether the decrease in respirasome organization reported by us previously in cardiac mitochondria in HF is due to changes in the phospholipids of the mitochondrial inner membrane or modifications of the subunits of the electron transport chain (ETC) complexes. The contents of the main phospholipid species, including cardiolipin, as well as the molecular species of cardiolipin were unchanged in cardiac mitochondria in HF. Oxidized cardiolipin molecular species were not observed. In heart mitochondria isolated from HF, complex IV not incorporated into respirasomes exhibits increased threonine phosphorylation. Since HF is associated with increased adrenergic drive to cardiomyocytes, this increased protein phosphorylation might be explained by the involvement of cAMP-activated protein kinase. Does the preservation of cAMP-induced phosphorylation changes of mitochondrial proteins or the addition of exogenous cAMP have similar effects on oxidative phosphorylation? The usage of phosphatase inhibitors revealed a specific decrease in complex I-supported respiration with glutamate. In saponin-permeabilized cardiac fibers, pre-incubation with cAMP decreases oxidative phosphorylation due to a defect localized at complex IV of the ETC inter alia. We propose that phosphorylation of specific complex IV subunits decreases oxidative phosphorylation either by limiting the incorporation of complex IV in supercomplexes or by decreasing supercomplex stability.

Mitochondria in heart failure

This review focuses on the evidence accumulated in humans and animal models to the effect that mitochondria are key players in the progression of heart failure (HF). Mitochondria are the primary source of energy in the form of adenosine triphosphate that fuels the contractile apparatus, and are thus essential for the pumping activity of the heart. We evaluate changes in mitochondrial morphology and alterations in the main components of mitochondrial energetics, such as substrate utilization and oxidative phosphorylation coupled with the level of respirasomes, in the context of their contribution to the chronic energy deficit and mechanical dysfunction in HF.

In vitro and in vivo studies of F(0)F(1)ATP synthase regulation by inhibitor protein IF(1) in goat heart

A method has been developed to allow the level of F(0)F(1)ATP synthase capacity and the quantity of IF(1) bound to this enzyme be measured in single biopsy samples of goat heart. ATP synthase capacity was determined from the maximal mitochondrial ATP hydrolysis rate and IF(1) content was determined by detergent extraction followed by blue native gel electrophoresis, two-dimensional SDS-PAGE and immunoblotting with anti-IF(1) antibodies. Anaesthetized open-chest goats were subjected to ischemic preconditioning and/or sudden increases of coronary blood flow (CBF) (reactive hyperemia). When hyperemia was induced before ischemic preconditioning, a steep increase in synthase capacity, followed by a deep decrease, was observed. In contrast, hyperemia did not affect synthase capacity when applied after ischemic preconditioning. Similar effects could be produced in vitro by treatment of heart biopsy samples with anoxia (down-regulation of the ATP synthase) or high-salt or high-pH buffers (up-regulation). We show that both in vitro and in vivo the same close inverse correlation exists between enzyme activity and IF(1) content, demonstrating that under all conditions tested the only significant modulator of the enzyme activity was IF(1). In addition, both in vivo and in vitro, 1.3-1.4 mol of IF(1) was predicted to fully inactivate 1 mol of synthase, thus excluding the existence of significant numbers of non-inhibitory binding sites for IF(1) in the F(0) sector.

The phosphorylation of subunits of complex I from bovine heart mitochondria

In bovine heart mitochondria and in submitochondrial particles, membrane-associated proteins with apparent molecular masses of 18 and 10 kDa become strongly radiolabeled by [(32)P]ATP in a cAMP-dependent manner. The 18-kDa phosphorylated protein is subunit ESSS from complex I and not as previously reported the 18 k subunit (with the N-terminal sequence AQDQ). The phosphorylated residue in subunit ESSS is serine 20. In the 10 kDa band, the complex I subunit MWFE was phosphorylated on serine 55. In the presence of protein kinase A and cAMP, the same subunits of purified complex I were phosphorylated by [(32)P]ATP at the same sites. Subunits ESSS and MWFE both contribute to the membrane arm of complex I. Each has a single hydrophobic region probably folded into a membrane spanning alpha-helix. It is likely that the phosphorylation site of subunit ESSS lies in the mitochondrial matrix and that the site in subunit MWFE is in the intermembrane space. Subunit ESSS has no known role, but subunit MWFE is required for assembly into complex I of seven hydrophobic subunits encoded in the mitochondrial genome. The possible effects of phosphorylation of these subunits on the activity and/or the assembly of complex I remain to be explored.

A Cryptic Targeting Signal Induces Isoform-specific Localization of p46Shc to Mitochondria

The human Src homology and collagen (Shc) gene encodes three protein isoforms of 46, 52, and 66 kDa that belong to a family of molecular adapters involved in several signal transduction pathways. Recently, the 66-kDa isoform has been shown to play a central role in controlling reactive oxygen species metabolism and life span in mammals. Despite the large amount of information available on the biology and biochemistry of Shc proteins, very little is known regarding the regulation of their subcellular localization. Here we demonstrate the specific and selective localization of p46Shc to the mitochondrial matrix. Through deletion mapping experiments, we show that targeting of p46Shc to mitochondria is mediated by its first 32 amino acids, which behave as a bona fide mitochondrial targeting sequence. We further demonstrate that the N-terminal location of the signal peptide is critical for its function. This accounts for the observation that p52Shc and p66Shc, containing the same sequence but more internally located, display a remarkably different subcellular localization. These findings indicate that p46Shc may exert a non-redundant biological function in signal transduction pathways involving mitochondria.

Physiological Ca2+ level and Ca2+-induced Permeability Transition Pore control protein phosphorylation in rat brain mitochondria

Phosphorylation of several low molecular mass proteins (3.5, 17, 23 and 29kDa) was observed in rat brain mitochondria (RBM) at ATP concentration close to that in the mitochondrial matrix. Furthermore, regulatory effects of Ca2+ on phosphorylation of these proteins were investigated. Protein phosphorylation was found to be modulated by Ca2+ in the physiological concentration range (10(-8) to 10(-6)M free Ca2+). Incorporation of 32P from [gamma-32P]ATP into the 17kDa protein was dramatically increased within the 10(-7) to 10(-6)M free Ca2+ range, whereas an opposite effect was observed for the 3.5kDa polypeptide. Strong de-phosphorylation of the 3.5kDa polypeptide and enhanced 32P-incorporation into the 17 and 23kDa proteins were found with supra-threshold Ca2+ loads and these effects were eliminated or reduced in the presence of cyclosporin A, an inhibitor of Permeability Transition Pore (PTP) opening. In the presence of calmidazolium (Cmz), a calmodulin antagonist, enhanced levels of phosphorylation of the 17 and 3.5kDa polypeptides were observed and the 17kDa protein phosphorylation was suppressed by H-8, a protein kinase A inhibitor. It is concluded that Ca2+ in physiological concentrations, as a second messenger, can control phosphorylation of the low molecular mass phospoproteins in RBM, in addition to well known regulation of some Krebs cycle dehydrogenases by Ca2+. The protein phosphorylation was strongly dependent on the Ca2+-induced PTP opening.

Phosphorylation of formate dehydrogenase in potato tuber mitochondria

Two highly phosphorylated proteins were detected after two-dimensional (blue native/SDS-PAGE) gel electrophoretic separation of the matrix fraction isolated from potato tuber mitochondria. These two phosphoproteins were identified by mass spectrometry as formate dehydrogenase (FDH) and the E1alpha-subunit of pyruvate dehydrogenase (PDH). Isoelectric focusing/SDS-PAGE two-dimensional gels separated FDH and PDH and resolved several different phosphorylated forms of FDH. By using combinations of matrix-assisted laser desorption/ionization mass spectrometry and electrospray ionization tandem mass spectrometry, several phosphorylation sites were identified for the first time in FDH and PDH. FDH was phosphorylated on Thr76 and Thr333, whereas PDH was phosphorylated on Ser294. Both Thr76 and Thr333 in FDH were accessible to protein kinases, as demonstrated by protein structure homology modeling. The extent of phosphorylation of both FDH and PDH was strongly decreased by NAD+, formate, and pyruvate, indicating that reversible phosphorylation of FDH and PDHs was regulated in a similar fashion. At low oxygen concentrations inside the intact potato tubers, FDH activity was strongly increased relative to cytochrome c oxidase activity pointing to a possible involvement of FDH in hypoxic metabolism. Computational sequence analysis indicated that a conserved local sequence motif of pyruvate formate-lyase is found in the Arabidopsis thaliana genome, and this enzyme might be the source of formate for FDH in plants.

Identification of 14 new phosphoproteins involved in important plant mitochondrial processes

Protein phosphorylation is a very important posttranslational modification the role of which is practically unexplored in mitochondria. Using two-dimensional gel electrophoresis followed by mass spectrometry, 14 new phosphoproteins are identified in potato tuber mitochondria, all household proteins also present in mammalian and fungal mitochondria. Seven of the new phosphoproteins are involved in the tricarboxylic acid cycle or associated reactions, four are subunits of respiratory complexes and involved in electron transport, ATP synthesis and protein processing, two are heat shock proteins and one is involved in defence against oxidative stress. These findings open up entirely new possibilities for the regulation and signal integration of mitochondrial processes.

Minor participation of cAMP on the protein kinase phosphorylation of mitochondrial and cytosolic fractions from Ascaris suum: a comparative study with porcine heart muscle

In contrast to porcine heart muscle in which cAMP effectively activated the phosphorylation of cytosolic proteins, cAMP exerted a minor effect on the phosphorylation of proteins from the soluble fraction of Ascaris suum muscle. Similarly, cAMP did not enhance the kinase activity in the mitochondrial membranes from porcine heart and A. suum, although major differences in protein phosphorylation were observed between both fractions. However, cAMP-dependent protein kinases (PKA) were evidenced in the parasitic soluble mitochondrial fraction, since the phosphorylation of histone IIA and kemptide was augmented in this fraction, in the presence of cAMP. An increase in the phosphorylation of exogenously added A. suum phosphofructokinase was also obtained when cAMP was added to the parasite soluble mitochondrial fraction. The phosphorylation of phosphofructokinase by this fraction was inhibited when kemptide and cAMP were included in the reaction mixture, suggesting substrate competition for the same PKA. Although PKI (6-22), a reported inhibitor of the catalytic subunit of mammalian cAMP-dependent PKAs, did not affect the endogenous phosphorylation of proteins in the various A. suum fractions, an inhibition on the phosphorylation of exogenously added kemptide and phosphofructokinase was observed when PKI (6-22) was incubated with the parasite mitochondrial soluble fraction.

Characterization and location of Src-dependent tyrosine phosphorylation in rat brain mitochondria

Analysis of protein phosphorylation in highly purified rat brain mitochondria revealed the presence of several alkali-stable phosphoproteins whose phosphorylation markedly increases upon treatment with peroxovanadate and Mn(2+), a property indicating tyrosine phosphorylation. These include three prominent bands, with apparent sizes of 50, 60, and 75 kDa, which are detectable by anti-phosphotyrosine. Tyrosine phosphorylation disappears when mitochondria are treated with PP2, an inhibitor of the Src kinase family, suggesting the presence of members of this family in rat brain mitochondria. Immunoblotting and immunoprecipitation assays of mitochondrial lysates confirmed the presence of Fyn, Src and Lyn kinases, as well as Csk, a protein kinase which negatively controls the activity of the Src kinase family. Results show that tyrosine-phosphorylated proteins are membrane-bound and that they are located on the inner surface of the outer membrane and/or the external surface of the inner membrane. Instead, Src tyrosine kinases are mainly located in the intermembrane space - in particular, as revealed by immunogold experiments for Lyn kinase, in the cristal lumen. Rat brain mitochondria were also found to possess a marked level of tyrosine phosphatase activity, strongly inhibited by peroxovanadate.

Control of oxygen free radical formation from mitochondrial complex I: roles for protein kinase A and pyruvate dehydrogenase kinase

Human NADH CoQ oxidoreductase is composed of a total of 43 subunits and has been demonstrated to be a major site for the production of superoxide by mitochondria. Incubation of rat heart mitochondria with ATP resulted in the phosphorylation of two mitochondrial membrane proteins, one with a M(r) of 6 kDa consistent with the NDUFA1 (MWFE), and one at 18kDa consistent with either NDUFS4 (AQDQ) or NDUFB7 (B18). Phosphorylation of both subunits was enhanced by cAMP derivatives and protein kinase A (PKA) and was inhibited by PKA inhibitors (PKAi). When mitochondrial membranes were incubated with pyruvate dehydrogenase kinase, phosphorylation of an 18kDa protein but not a 6kDa protein was observed. NADH cytochrome c reductase activity was decreased and superoxide production rates with NADH as substrate were increased. On the other hand, with protein kinase A-driven phosphorylation, NADH cytochrome c reductase was increased and superoxide production decreased. Overall there was a 4-fold variation in electron transport rates observable at the extremes of these phosphorylation events. This suggests that electron flow through complex I and the production of oxygen free radicals can be regulated by phosphorylation events. In light of these observations we discuss a potential model for the dual regulation of complex I and the production of oxygen free radicals by both PKA and PDH kinase.

Cyclic adenosine monophosphate-dependent phosphorylation of mammalian mitochondrial proteins: enzyme and substrate characterization and functional role

A study is presented on cyclic adenosine monophosphate- (cAMP-) dependent phosphorylation of mammalian mitochondrial proteins. Immunodetection with specific antibodies reveals the presence of the catalytic and the regulatory subunits of cAMP-dependent protein kinase (PKA) in the inner membrane and matrix of bovine heart mitochondria. The mitochondrial cAMP-dependent protein kinase phosphorylates mitochondrial proteins of 29, 18, and 6.5 kDa. With added histone as substrate, PKA exhibits affinities for ATP and cAMP and pH optimum comparable to those of the cytosolic PKA. Among the mitochondrial proteins phosphorylated by PKA, one is the nuclear-encoded (NDUFS4 gene) 18 kDa subunit of complex I, which has phosphorylation consensus sites in the C terminus and in the presequence. cAMP promotes phosphorylation of the 18 kDa subunit of complex I in myoblasts in culture and in their isolated mitoplast fraction. In both cases cAMP-dependent phosphorylation of the 18 kDa subunit of complex I is accompanied by enhancement of the activity of the complex. These results, and the finding of mutations in the NDUFS4 gene in patients with complex I deficiency, provide evidence showing that cAMP-dependent phosphorylation of the 18 kDa subunit of complex I plays a major role in the control of the mitochondrial respiratory activity.

Protein phosphorylation/dephosphorylation in the inner membrane of potato tuber mitochondria

Inside-out inner mitochondrial membranes free of matrix proteins were isolated from purified potato tuber (Solanum tuberosum L.) mitochondria and incubated with ¿gamma-(32)PATP. Proteins were separated by SDS-PAGE and visualized by autoradiography. Phosphorylation of inner membrane proteins, including ATPase subunits, was strongly inhibited by the phosphoprotein phosphatase inhibitor NaF. We propose that an inner membrane phosphoprotein phosphatase is required for activation of the inner membrane protein kinase. When prelabelled inner membranes were incubated in the absence of ¿gamma-(32)PATP, there was no phosphoprotein dephosphorylation unless a soluble matrix fraction was added. This dephosphorylation was inhibited by NaF, but not by okadaic acid. We conclude that the mitochondrial matrix contains a phosphoprotein phosphatase that is responsible for dephosphorylation of inner membrane phosphoproteins.

cAMP-dependent protein kinase and phosphoproteins in mammalian mitochondria. An extension of the cAMP-mediated intracellular signal transduction

Evidence has been obtained for the occurrence of a cAMP-dependent serine protein kinase associated with the inner membrane/matrix of mammalian mitochondria. The catalytic site of this kinase is localized at the inner side of the inner membrane, where it phosphorylates a number of mitochondrial proteins. One of these has been identified as the AQDQ subunit of complex I. cAMP-dependent phosphorylation of this protein promotes the activity of complex I and mitochondrial respiration. A 5 bp duplication in the nuclear gene encoding this protein has been found in a human patient, which eliminates the phosphorylation site. PKA anchoring proteins have recently been identified in the outer membrane of mammalian mitochondria, which could direct phosphorylation of proteins at contact sites with other cell structures.

Increased phosphorylation of specific rat cardiac and retinal proteins in taurine-depleted animals: isolation and identification of the phosphoproteins

Partial depletion of the tissue levels of taurine in the alive animal stimulates in vitro phosphorylation of both an approximately 20 kDa (94%) and an approximately 44 kDa (85%) protein present in subcellular fractions of the rat retina and heart. Tissue levels of taurine were reduced by feeding the animals 1.5% guanidinoethanesulfonic acid (GES) in their drinking water for 6 weeks. The increase in the in vitro phosphorylation of both the approximately 20 kDa and approximately 44 kDa proteins was reversed when the animals were subsequently given 1.5% taurine in their drinking water for an additional 6 weeks. Isolation, purification, tryptic digestion, and peptide sequence analysis of the retinal approximately 20 kDa phosphoprotein suggest that it is histone H2B. However, the results are ambiguous due to an impurity (< 25%) in the retinal preparation. Sequence analysis of the approximately 44 kDa phosphoprotein indicates that it is pyruvate dehydrogenase.

Two subunits of the F0F1-ATPase are phosphorylated in the inner mitochondrial membrane

Inside-out submitochondrial particles from potato tuber mitochondria were incubated with [gamma-32P]ATP. More than 16 phosphorylated polypeptides were detected by autoradiography on an SDS-gel. Two phosphoproteins, migrating at 22 and 28 kDa, were excised from the SDS-gel, electroeluted, and purified further by anion chromatography. The phosphoproteins were N-terminally sequenced. Over the regions sequenced, the 22 and 28 kDa phosphoproteins had 100% sequence identity with potato proteins identified as the delta'-subunit of the F1-ATPase and the b-subunit of the F0-ATPase, respectively. We suggest that phosphorylation of these proteins may control the interaction between F1 and F0 and regulate energy coupling in oxidative phosphorylation.

Phosphorylation of mitochondrial telomere binding protein of Candida parapsilosis by camp-dependent protein kinase

Mitochondrial telomere-binding protein (mtTBP) of Candida parapsilosis binds with high affinity to 5' single-stranded overhang of the linear mitochondrial DNA of this yeast (Tomáska, L'., Nosek, J., and Fukuhara, H. (1997) J. Biol. Chem. 272, 3049-3056). Here it is reported that mtTBP is phosphorylated by catalytic subunit of cAMP-dependent protein kinase in vitro. Phosphorylated mtTBP has dramatically reduced ability to bind telomeric oligonucleotide in the gel-mobility retardation assay without affecting the oligomerization of mtTBP in vitro. MtTBP is one of the few mitochondrial proteins and the first mitochondrial single-strand DNA binding proteins that was demonstrated to serve as a substrate for cAMP-dependent protein kinase.

Mitochondrial cytochrome c oxidase subunit IV is phosphorylated by an endogenous kinase

This study was undertaken to identify novel mitochondrial membrane proteins that are potential targets for phosphorylation. Mitochondrial membranes were incubated in the presence of [gamma-32P]ATP and the Triton X-114 extractable protein was subjected to ion-exchange and polyacrylamide gel chromatography to purify a major phosphorylated protein of approximately 17000 Da. The determined peptide sequence of the purified phosphoprotein corresponded to a segment of cytochrome c oxidase subunit IV, an inner membrane protein of 17160 Da. The identity of the phosphoprotein was confirmed by two-dimensional electrophoresis and Western blotting. The results identify mitochondrial cytochrome c oxidase subunit IV as a protein which is phosphorylated by an endogenous kinase.

Topology of the mitochondrial cAMP-dependent protein kinase and its substrates

In intact bovine heart mitochondria, cAMP-dependent phosphorylation of 42, 29, 18 and 6.5 kDa proteins was inhibited by carboxyatractyloside. This shows that both mitochondrial cAMP-dependent protein kinase (mtPKA) and its protein substrates are localized at the matrix side of the inner mitochondrial membrane. Proteins of 42, 29, 18, and 6.5 kDa were also bound at the outer surface of mitochondria where they were phosphorylated by the added purified catalytic subunit of PKA. In the cytosol from bovine heart proteins of the above molecular weights were phosphorylated by the cytosolic PKA.

The nuclear-encoded 18 kDa (IP) AQDQ subunit of bovine heart complex I is phosphorylated by the mitochondrial cAMP-dependent protein kinase

In bovine heart mitochondria a protein of M(r) 18 kDa, phosphorylated by mtPKA, is associated to the NADH-ubiquinone oxidoreductase in the inner membrane and is present in purified preparation of this complex. The 18 kDa phosphoprotein has now been isolated and sequenced. It is identified as the 18 kDa (IP) AQDQ subunit of complex I, a protein of 133 amino acids with a phosphorylation consensus site RVS at position 129-131.

Characterization of proteins phosphorylated by the cAMP-dependent protein kinase of bovine heart mitochondria

Characterization of two mitochondrial proteins of M(r) 42 and 18 kDa, respectively, phosphorylated by the cAMP-dependent protein kinase of bovine heart mitochondria (mtPKA), is presented. A 42 kDa protein is found to be loosely associated to complexes I, III and IV of the respiratory chain and complex V (ATP synthase) in the inner mitochondrial membrane. An 18 kDa protein is associated to complex I in the inner membrane and in a purified preparation of this complex where it can be phosphorylated by the isolated catalytic subunit of PKA.

Histidine and tyrosine phosphorylation in pea mitochondria: evidence for protein phosphorylation in respiratory redox signalling

A 37 kDa protein in pea mitochondria was found to contain phosphorylated residues. Phosphorylation was acid-labile but stable in alkali solution, a unique property of phosphorylation on histidine, indicating that a signal transduction pathway with homology to bacterial two-component systems might exist in plant mitochondria. We also describe the first example of tyrosine phosphorylation in plant organelles and the first indication of protein phosphorylation as part of a redox signalling mechanism in mitochondria. Labelling of three proteins (28, 27 and 12 kDa) was found to be dependent on the redox state of the reaction medium. Their phospho-groups were resistant to alkali as well as acid treatment and labelling was inhibited by the tyrosine kinase inhibitor genistein.

cAMP-dependent protein phosphorylation in mitochondria of bovine heart

A study is presented of the cAMP-dependent phosphorylation in bovine heart mitochondria of three proteins of 42, 16 and 6.5 kDa associated to the inner membrane. These proteins are also phosphorylated by the cytosolic cAMP-dependent protein kinase and by the purified catalytic subunit of this enzyme. In the cytosol, proteins of 16 and 6.5 kDa are phosphorylated by the cAMP-dependent kinase. It is possible that cytosolic and mitochondrial cAMP-dependent kinases phosphorylate the same proteins in the two compartments.

The outer membrane of plant mitochondria contains a calcium-dependent protein kinase and multiple phosphoproteins

Highly purified mitochondria from potato (Solanum tuberosum L. cv. Bintje) tubers were subfractionated into a matrix fraction, an inner membrane fraction and an outer membrane fraction with minimal cross-contamination. When the matrix and inner membrane fractions were incubated with [gamma-32P]ATP only one and three prominent phosphoproteins were detected after SDS-PAGE and autoradiography, respectively. In contrast, more than 20 phosphoproteins could be labelled in the outer membrane fraction, the main ones at 12, 18, 26, 43, 58, 60, 65, 74 and 110 kDa. Only one band, at 18 kDa, was detectable when the labelling was done in the presence of EGTA. We conclude that the outer membrane of plant mitochondria contains at least one Ca(2+)-dependent protein kinase and more than 20 endogenous substrates.