The insertion of monoamine oxidase A into the outer membrane of rat liver mitochondria

A cytochrome c is the natural electron acceptor for nicotine oxidoreductase

Nicotine oxidoreductase (NicA2), a member of the flavin-containing amine oxidase family, is of medical relevance as it shows potential as a therapeutic to aid cessation of smoking due to its ability to oxidize nicotine into a non-psychoactive metabolite. However, the use of NicA2 in this capacity is stymied by its dismal O₂-dependent activity. Unlike other enzymes in the amine oxidase family, NicA2 reacts very slowly with O₂, severely limiting its nicotine-degrading activity. Instead of using O₂ as an oxidant, we discovered that NicA2 donates electrons to a cytochrome c, which means that NicA2 is actually a dehydrogenase. This is surprising, as enzymes of the flavin-containing amine oxidase family were invariably thought to use O₂ as an electron acceptor. Our findings establish new perspectives for engineering this potentially useful therapeutic and prompt a reconsideration of the term 'oxidase' in referring to members of the flavin-containing amine 'oxidase' family.

Ubiquitin Subproteome of Brain Mitochondria and Its Changes Induced by Experimental Parkinsonism and Action of Neuroprotectors

The review summarizes the data of our research and published studies on the ubiquitination of brain mitochondrial proteins and its changes during the development of experimental parkinsonism and administration of the neuroprotector isatin (indole-2,3-dione) with special attention to the mitochondrial ubiquitin-conjugating system and location of ubiquitinated proteins in these organelles. Incubation of brain mitochondrial fraction with biotinylated ubiquitin in vitro resulted in the incorporation of biotinylated ubiquitin in both mitochondrial and mitochondria-associated proteins. According to the interactome analysis, the identified non-ubiquitinated proteins are able to form tight complexes with ubiquitinated proteins or their partners and components of mitochondrial membranes, in which interactions of ubiquitin chains with the ubiquitin-binding protein domains play an important role. The studies of endogenous ubiquitination in the total brain mitochondrial fraction of C57Bl mice performed in different laboratories have shown that mitochondrial proteins represent about 30% of all ubiquitinated proteins. However, comparison of brain subproteomes of mitochondrial ubiquitinated proteins reported in the literature revealed significant differences both in their composition and involvement of identified ubiquitinated proteins in biological processes listed in the Gene Ontology database. The development of experimental parkinsonism in C57Bl mice induced by a single-dose administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) resulted in a decrease in the total number of mitochondrial ubiquitinated proteins and increase in the number of oxidized mitochondrial proteins containing the ubiquitin signature (K-ε-GG). Comparison of ubiquitinated proteins associated with the mouse brain mitochondrial fraction and mouse brain mitochondrial proteins bound to the proteasome ubiquitin receptor (Rpn10 subunit) did not reveal any common proteins. This suggests that ubiquitination of brain mitochondrial proteins is not directly related to their degradation in the proteasomes. Proteomic profiling of brain isatin-binding proteins identified enzymes involved in the ubiquitin-conjugating system functioning. Mapping of the identified isatin-binding proteins to known metabolic pathways indicates their participation in the parkin (E3 ubiquitin ligase)-associated pathway (CH000000947). The functional links involving brain mitochondrial ubiquitinated proteins were found only in the group of animals with the MPTP-induced parkinsonism, but not in animals treated with MPTP/isatin or isatin only. This suggests that the neuroprotective effect of isatin may be associated with the impaired functional relationships of proteins targeted to subsequent degradation.

The PINK1 kinase-driven ubiquitin ligase Parkin promotes mitochondrial protein import through the presequence pathway in living cells

Most of over a thousand mitochondrial proteins are encoded by nuclear genes and must be imported from the cytosol. Little is known about the cytosolic events regulating mitochondrial protein import, partly due to the lack of appropriate tools for its assessment in living cells. We engineered an inducible biosensor for monitoring the main presequence-mediated import pathway with a quantitative, luminescence-based readout. This tool was used to explore the regulation of mitochondrial import by the PINK1 kinase-driven Parkin ubiquitin ligase, which is dysfunctional in autosomal recessive Parkinson's disease. We show that mitochondrial import was stimulated by Parkin, but not by disease-causing Parkin variants. This effect was dependent on Parkin activation by PINK1 and accompanied by an increase in the abundance of K11 ubiquitin chains on mitochondria and by ubiquitylation of subunits of the translocase of outer mitochondrial membrane. Mitochondrial import efficiency was abnormally low in cells from patients with PINK1- and PARK2-linked Parkinson's disease and was restored by phosphomimetic ubiquitin in cells with residual Parkin activity. Altogether, these findings uncover a role of ubiquitylation in mitochondrial import regulation and suggest that loss of this regulatory loop may underlie the pathophysiology of Parkinson's disease, providing novel opportunities for therapeutic intervention.

Membrane Reconstitution of Monoamine Oxidase Enzymes on Supported Lipid Bilayers

Monoamine oxidase A and B (MAO-A and B) are mitochondrial outer membrane enzymes that are implicated in a number of human diseases, and the pharmacological inhibition of these enzymes is a promising therapeutic strategy to alleviate disease symptoms. It has been suggested that optimal levels of enzymatic activity occur in the membrane-associated state, although details of the membrane association process remain to be understood. Herein, we have developed a supported lipid bilayer platform to study MAO-A and B binding and evaluate the effects of known pharmacological inhibitors on the membrane association process. By utilizing the quartz crystal microbalance-dissipation (QCM-D) technique, it was determined that both MAOs exhibit tight binding to negatively and positively charged bilayers with distinct concentration-dependent binding profiles while only transiently binding to neutral bilayers. Importantly, in the presence of known inhibitors, the MAOs showed increased binding to negatively charged bilayers, although there was no effect of inhibitor treatment on binding to positively charged bilayers. Taken together, our findings establish that the membrane association of MAOs is highly dependent on membrane surface charge, and we outline an experimental platform to support the in vitro reconstitution of monoamine oxidases on synthetic membranes, including the evaluation of pharmacological drug candidates.

90 years of monoamine oxidase: some progress and some confusion

It would not be practical to attempt to deal with all the advances that have informed our understanding of the behavior and functions of this enzyme over the past 90 years. This account concentrates key advances that explain why the monoamine oxidases remain of pharmacological and biochemical interest and on some areas of continuing uncertainty. Some issues that remain to be understood or are in need of further clarification are highlighted.

Privileged scaffolds as MAO inhibitors: Retrospect and prospects

This review aims to be a comprehensive, authoritative, critical, and readable review of general interest to the medicinal chemistry community because it focuses on the pharmacological, chemical, structural and computational aspects of diverse chemical categories as monoamine oxidase inhibitors (MAOIs). Monoamine oxidases (MAOs), namely MAO-A and MAO-B represent an enormously valuable class of neuronal enzymes embodying neurobiological origin and functions, serving as potential therapeutic target in neuronal pharmacotherapy, and hence we have coined the term "Neurozymes" which is being introduced for the first time ever. Nowadays, therapeutic attention on MAOIs engrosses two imperative categories; MAO-A inhibitors, in certain mental disorders such as depression and anxiety, and MAO-B inhibitors, in neurodegenerative disorders like Alzheimer's disease (AD) and Parkinson's disease (PD). The use of MAOIs declined due to some potential side effects, food and drug interactions, and introduction of other classes of drugs. However, curiosity in MAOIs is reviving and the recent developments of new generation of highly selective and reversible MAOIs, have renewed the therapeutic prospective of these compounds. The initial section of the review emphasizes on the detailed classification, structural and binding characteristics, therapeutic potential, current status and future challenges of the privileged pharmacophores. However, the chemical prospective of privileged scaffolds such as; aliphatic and aromatic amines, amides, hydrazines, azoles, diazoles, tetrazoles, indoles, azines, diazines, xanthenes, tricyclics, benzopyrones, and more interestingly natural products, along with their conclusive SARs have been discussed in the later segment of review. The last segment of the article encompasses some patents granted in the field of MAOIs, in a simplistic way.

Monoamine oxidase-A knockdown in human neuroblastoma cells reveals protection against mitochondrial toxins

The study examined how the mitochondrial enzyme monoamine oxidase-A (MAO-A), which produces hydrogen peroxide as a catalytic by-product, influences death and survival mechanisms. Targeted microRNA (miRNA) was used to stably knock down MAO-A mRNA, protein, and catalytic activity by 60-70% in SH-SY5Y human neuroblastoma cells. The effects of MAO-A knockdown (KD) on ATP, oxidative stress, electron transport chain, and survival following exposure to mitochondrial toxins were assessed. In control cells, complex I inhibition resulted in caspase-mediated cell death linked with ROS production and reduced ATP, followed by up-regulation of MAO-A mRNA, protein, and enzyme activity levels. Inhibition of complex III and IV resulted in a similar increase in MAO-A expression, while up-regulation of MAO-A was lower following complex II inhibition. MAO-A KD decreased basal reactive oxygen species levels by 50% and increased levels of ATP and reduced glutathione and Bcl-2. MAO-A KD specifically increased the activity of complex I but had no effect on complex II-IV activities. Furthermore, MAO-A KD protected against inhibitors of complex I, III, and IV. In summary, endogenous MAO-A levels influence mitochondrial function, notably complex I activity, and MAO-A may be a target for protection against neurodegenerative conditions that involve oxidative stress and mitochondrial dysfunction as underlying pathogenic factors.

Missense mutations in the human SDHB gene increase protein degradation without altering intrinsic enzymatic function

Mutations of succinate dehydrogenase subunit B (SDHB) play a crucial role in the pathogenesis of the most aggressive and metastatic pheochromocytomas (PHEOs) and paragangliomas (PGLs). Although a variety of missense mutations in the coding sequence of the SDHB gene have been found in PHEOs and PGLs, it has been unclear whether these mutations impair mRNA expression, protein stability, subcellular localization, or intrinsic protein function. RT-PCR and Western blot analysis of SDHB mRNA and protein expression from SDHB-related PHEOs and PGLs demonstrated intact mRNA expression but significantly reduced protein expression compared to non-SDHB PHEOs and PGLs. A pulse-chase assay of common SDHB missense mutations in transfected HeLa cell lines demonstrated that the loss of SDHB function was due to a reduction in mutant protein half-life, whereas colocalization of SDHB with mitochondria and immunoprecipitation with SDHA demonstrated intact subcellular localization and complex formation. The half-life of the SDHB protein increased after treatment with histone deacetylase inhibitors (HDACis), implicating the protein quality control machinery in the degradation of mutant SDHB protein. These findings provide the first direct mechanism of functional loss resulting from SDHB mutations and suggest that reducing protein degradation with HDACis may serve as a novel therapeutic paradigm for preventing the development of SDHB-related tumors.

A monoamine oxidase from scallop Chlamys farreri serving as an immunomodulator in response against bacterial challenge

Monoamine oxidase (MAO) is an essential enzyme in the catabolism of monoamines, and implicated in the immune response of vertebrates. In the present study, the full-length cDNA encoding monoamine oxidase (designated CfMAO) was cloned from Chlamys farreri by using rapid amplification of cDNA ends (RACE) approaches and expression sequence tag (EST) analysis. The open reading frame of CfMAO cDNA encoded 519 amino acids, which shared 73.9% similarity with that from oyster Crassostrea gigas, and 64.5-66.3% similarity with those from vertebrates. A conserved Amino_oxidase domain and a transmembrane domain were identified in the deduced CfMAO protein. The mRNA transcripts of CfMAO could be detected in all the tested tissues, including haemocytes, hepatopancreas, kidney, adductor muscle, mantle, gill and gonad. The mRNA expression of CfMAO was up-regulated significantly in haemocytes of scallops during 6-48 h after bacteria Vibrio anguillarum challenge, and it reached the peak (25.9-fold, P < 0.05) at 12h. The cDNA fragment encoding the mature peptide of CfMAO was expressed in the prokaryotic expression system, and 1mg of the recombinant protein (rCfMAO) could catalyze the deamination of 3665.59 nmol serotonin, 2061.89 nmol norepinephrine, 2104.85 nmol epinephrine or 3040.34 nmol dopamine within 1 min (nmol min⁻¹ mg⁻¹) in vitro. When the reaction mixture was coincubated with 0.1 mmol L⁻¹ MAO inhibitor clorgyline, its catalyzing activity to deaminize serotonin and dopamine was decreased significantly to 1603.69 and 955.39 nmol min⁻¹ mg⁻¹ (P < 0.05) respectively. These results indicated that CfMAO, as the homologue of monoamine oxidase in scallop C. farreri, could modulate the immune response of scallops through the deamination of monoamines.

Ecstasy-induced oxidative stress to adolescent rat brain mitochondria in vivo: influence of monoamine oxidase type A

The administration of a neurotoxic dose of 3,4-methylenedioxymethamphetamine (MDMA; 'ecstasy') to the rat results in mitochondrial oxidative damage in the central nervous system, namely lipid and protein oxidation and mitochondrial DNA deletions with subsequent impairment of the correspondent protein expression. Although these toxic effects were shown to be prevented by monoamine oxidase B inhibition, the role of monoamine oxidase A (MAO-A) in MDMA-mediated mitochondrial damage remains to be evaluated. Thus, the aim of the present study was to clarify the potential interference of a specific inhibition of MAO-A by clorgyline, on the deleterious effects produced by a binge administration of a neurotoxic dose of MDMA (10 mg MDMA/kg of body weight, intraperitoneally, every 2 hours in a total of four administrations) to an adolescent rat model. The parameters evaluated were mitochondrial lipid peroxidation, protein carbonylation and expression of the respiratory chain protein subunits II of reduced nicotinamide adenine dinucleotide dehydrogenase (NDII) and I of cytochrome oxidase (COXI). Considering that hyperthermia has been shown to contribute to the neurotoxic effects of MDMA, another objective of the present study was to evaluate the body temperature changes mediated by MDMA with a MAO-A selective inhibition by clorgyline. The obtained results demonstrated that the administration of a neurotoxic binge dose of MDMA to an adolescent rat model previously treated with the specific MAO-A inhibitor, clorgyline, resulted in synergistic effects on serotonin- (5-HT) mediated behaviour and body temperature, provoking high mortality. Inhibition of MAO-A by clorgyline administration had no protective effect on MDMA-induced alterations on brain mitochondria (increased lipid peroxidation, protein carbonylation and decrease in the expression of the respiratory chain subunits NDII and COXI), although it aggravated MDMA-induced decrease in the expression of COXI. These results reinforce the notion that the concomitant use of MAO-A inhibitors and MDMA is counter indicated because of the resulting severe synergic toxicity.

Acetyl-L-carnitine provides effective in vivo neuroprotection over 3,4-methylenedioximethamphetamine-induced mitochondrial neurotoxicity in the adolescent rat brain

3,4-Methylenedioximethamphetamine (MDMA, ecstasy) is a worldwide abused stimulant drug, with persistent neurotoxic effects and high prevalence among adolescents. The massive release of 5-HT from pre-synaptic storage vesicles induced by MDMA followed by monoamine oxidase B (MAO-B) metabolism, significantly increases oxidative stress at the mitochondrial level. l-Carnitine and its ester, acetyl-l-carnitine (ALC), facilitate the transport of long chain free fatty acids across the mitochondrial membrane enhancing neuronal anti-oxidative defense. Here, we show the potential of ALC against the neurotoxic effects of MDMA exposure. Adolescent male Wistar rats were assigned to four groups: control saline solution, isovolumetric to the MDMA solution, administered i.p.; MDMA (4x10 mg/kg MDMA, i.p.); ALC/MDMA (100 mg/kg 30 min of ALC prior to MDMA, i.p.) and ALC (100 mg/kg, i.p.). Rats were killed 2 weeks after exposure and brains were analyzed for lipid peroxidation, carbonyl formation, mitochondrial DNA (mtDNA) deletion and altered expression of the DNA-encoded subunits of the mitochondrial complexes I (NADH dehydrogenase, NDII) and IV (cytochrome c oxidase, COXI) from the respiratory chain. Levels of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) were also assessed. The present work is the first to successfully demonstrate that pretreatment with ALC exerts effective neuroprotection against the MDMA-induced neurotoxicity at the mitochondrial level, reducing carbonyl formation, decreasing mtDNA deletion, improving the expression of the respiratory chain components and preventing the decrease of 5-HT levels in several regions of the rat brain. These results indicate potential benefits of ALC application in the prevention and treatment of neurodegenerative disorders.

tRNA import into yeast mitochondria is regulated by the ubiquitin-proteasome system

In Saccharomyces cerevisiae, one of two cytosolic lysine-tRNAs is partially imported into mitochondria. We demonstrate that three components of the ubiquitin/26S proteasome system (UPS), Rpn13p, Rpn8p and Doa1p interact with the imported tRNA and with the essential factor of its mitochondrial targeting, pre-Msk1p. Genetic and biochemical assays demonstrate that UPS plays a dual regulatory role, since the overall inhibition of cellular proteasome activity reduces tRNA import, while specific depletion of Rpn13p or Doa1p increases it. This result suggests a functional link between UPS and tRNA mitochondrial import in yeast and indicates on the existence of negative and positive import regulators.

Mitochondrial association of myocilin, product of a glaucoma gene, in human trabecular meshwork cells

The trabecular meshwork (TM), an ocular tissue next to the cornea, is a major site for regulation of the aqueous humor outflow. Malfunctioning of this tissue is believed to be responsible for development of glaucoma, a major blinding disease. Myocilin is a gene directly linked to the most common form of glaucoma. Its protein product has been localized to both intra- and extra-cellular sites in TM cells. This study was to investigate the association of myocilin with mitochondria in TM cells. In vitro mitochondrial import assays showed that myocilin was imported to the TM mitochondria, targeting to mitochondrial membranes and/or the intermembrane space. The targeting was mediated mostly via the amino-terminal region of myocilin. When myocilin expression was induced either by treatment with dexamethasone or transfection with a myocilin construct, the mitochondrial membrane potential in TM cells, as assessed by JC-1 staining, was lowered. Subcellular fractionation and Western blot analyses confirmed that a portion of myocilin sedimented with the mitochondrial fractions. Upon anti-Fas treatment to provoke apoptosis, an increase of myocilin distribution in cytosolic fraction was observed, suggesting that myocilin was partially released from mitochondrial compartments. These results confirmed the association of myocilin with TM cell mitochondria and indicated that myocilin may have a proapoptotic role in TM cells.

Monoamine oxidase and tobacco dependence

Tobacco smoking is a leading cause of preventable death around the world, and there are major public health and research efforts in many countries aimed at reducing its usage. However, the molecular mechanisms underlying tobacco dependence are still not completely understood. Nicotine's action on nicotinic acetylcholine receptors, and the downstream release of dopamine, is believed to be the major pathway underlying tobacco dependence. However there is mounting evidence indicating that non-nicotinic components of tobacco smoke also play a role by inhibiting monoamine oxidase (MAO) and subsequently altering neurotransmitter levels. This article provides a review of the current knowledge of the association between MAO and tobacco dependence and suggests that further research into this topic is likely to lead to more effective pharmacotherapies for smoking cessation.

Mitochondrial protein import and human health and disease

The targeting and assembly of nuclear-encoded mitochondrial proteins are essential processes because the energy supply of humans is dependent upon the proper functioning of mitochondria. Defective import of mitochondrial proteins can arise from mutations in the targeting signals within precursor proteins, from mutations that disrupt the proper functioning of the import machinery, or from deficiencies in the chaperones involved in the proper folding and assembly of proteins once they are imported. Defects in these steps of import have been shown to lead to oxidative stress, neurodegenerative diseases, and metabolic disorders. In addition, protein import into mitochondria has been found to be a dynamically regulated process that varies in response to conditions such as oxidative stress, aging, drug treatment, and exercise. This review focuses on how mitochondrial protein import affects human health and disease.

Ubiquitin-protein ligase parkin and its role in the development of Parkinson’s disease

Parkin is a protein encoded by the corresponding parkin gene. It exhibits ubiquitin-protein ligase activity. In this review, we analyze domain structure, substrate specificity, subcellular localization of parkin, and regulation of its activity. Then we discuss data on the effects of various mutations in the parkin gene on structure and functions of this protein and results obtained with parkin knock-out animals. Better understanding of parkin biochemistry, its compartmentalization, functions, and altered functions would help the development of new approaches for the treatment of both inherited and sporadic cases of Parkinson's disease.

Increased glutathione S-transferase activity rescues dopaminergic neuron loss in a Drosophila model of Parkinson's disease

Loss-of-function mutations of the parkin gene are a major cause of early-onset parkinsonism. To explore the mechanism by which loss of parkin function results in neurodegeneration, we are using a genetic approach in Drosophila. Here, we show that Drosophila parkin mutants display degeneration of a subset of dopaminergic (DA) neurons in the brain. The neurodegenerative phenotype of parkin mutants is enhanced by loss-of-function mutations of the glutathione S-transferase S1 (GstS1) gene, which were identified in an unbiased genetic screen for genes that modify parkin phenotypes. Furthermore, overexpression of GstS1 in DA neurons suppresses neurodegeneration in parkin mutants. Given the previous evidence for altered glutathione metabolism and oxidative stress in sporadic Parkinson's disease (PD), these data suggest that the mechanism of DA neuron loss in Drosophila parkin mutants is similar to the mechanisms underlying sporadic PD. Moreover, these findings identify a potential therapeutic approach in treating PD.

The growth of mdp1/rsp5 mutants of Saccharomyces cerevisiae is affected by mutations in the ATP-binding domain of the plasma membrane H+ -ATPase

Mutations in the PMA1 gene, encoding plasma membrane H+ -ATPase, were isolated that are able to suppress the temperature sensitivity (ts) phenotype of mdp1 mutations located in RSP5, the ubiquitin-protein ligase gene. The mdp1 mutants were previously found to change the mitochondrial/cytosolic distribution of Mod5p-I, the tRNA modifying enzyme, and to affect fluid phase endocytosis. The data presented reveal that mdp1 mutants are also pH sensitive, and hypersensitive to hygromycin B and paromomycin. The ts phenotype, hygromycin B and paromomycin sensitivity are suppressed by pmal-t, but the pH sensitivity, the effect of mdp1 on Mod5p-I cytoplasmic/mitochondrial localization and endocytosis are not. Characterization of pmal-t revealed the substitution of amino acid G(653)V in the ATP-binding domain of the H+ -ATPase. Our results indicate that Rsp5 ubiquitin-protein ligase may also influence, in addition to protein distribution, the functioning of plasma membrane H+ -ATPase and the response of cells to stress.

An unassembled subunit of NAD(+)-dependent isocitrate dehydrogenase is insoluble and covalently modified

The NAD(+)-dependent isocitrate dehydrogenase of Saccharomyces cerevisiae is an octamer composed of four Idh1p subunits and four Idh2p subunits. Isocitrate dehydrogenase functions in the tricarboxylic acid cycle and has also been reported to bind to the 5' nontranslated region of mitochondrially encoded mRNAs. Mutants defective in either or both of these subunits are unable to grow on the nonfermentable carbon source, acetate, but will utilize glycerol or ethanol. Mutant strains lacking Idh2p maintain normal if not elevated levels of mitochondrial Idh1p. In addition to the mature unassembled Idh1p subunit, a complex of bands in the 85- to 170-kDa range (Idh1p-Cpx) is observed using NAD-IDH antiserum. Both Idh1p and Idh1p-Cpx are insoluble within the mitochondrion and are associated with the mitochondrial inner membrane. A histidine-tagged form of Idh1p was expressed in yeast strains. Chemical amounts of the Idh1p-Cpx could be purified from strains lacking Idh2p but not from strains containing normal levels of Idh2p. The data indicate that Idh1p-Cpx is an aggregated and cross-linked form of Idh1p that may be oxidized within the mitochondrion as a consequence of its aborted assembly.

QSAR analysis of indole analogues as monoamine oxidase inhibitors

The quantitative structure-activity relationship (QSAR) analysis with comparative molecular field analysis (CoMFA) of indole derivatives-monoamine oxidase (MAO) inhibitors were done. The pharmacophore model included four features: two hydrophobic rings, one donor atom, and one acceptor site. The predictive values (cross-validated r2) of QSAR analysis for the inhibition of MAO-A and MAO-B were 0.743 and 0.603, respectively. The contributions of steric and electrostatic fields in the interaction between inhibitors and enzymes were equal. The three-dimensional arrangement of these fields for MAO-A and MAO-B suggests that structures of active site for both enzymes are considerably differed from each other.

Characterization of a human ubiquitin-conjugating enzyme gene UBE2L3

Ubiquitin-conjugating enzymes (E2s) are essential components of the post-translational protein ubiquitination pathway, mediating the transfer of activated ubiquitin to substrate proteins. We have identified a human gene, UBE2L3, localized on Chromosome (Chr) 22q11. 2-13.1, encoding an E2 almost identical to that encoded by the recently described human L-UBC (UBE2L1) gene present on Chr 14q24.3. Using chromosome-specific vectorette PCR, we have determined the intron/exon structure of UBE2L3. In contrast to the intronless UBE2L1 gene, the coding sequence of UBE2L3 is interrupted by three large introns. UBE2L3-derived mRNA appears to be the predominant species in most tissues rather than the transcript from UBE2L1 or another homologous gene UBE2L2, which maps to Chr 12q12. We also present additional evidence that these genes are members of a larger multigene family. The primary sequence of the protein encoded by UBE2L3 is identical to partial peptide sequence derived from the rabbit E2 'E2-F1,' suggesting that we have identified the human homolog of this protein. This latter E2 has been demonstrated to participate in transcription factor NF-kappaB maturation, c-fos degradation, and human papilloma virus-mediated p53 degradation in vitro.

Lysine residues at positions 234 and 236 in yeast porin are involved in its assembly into the mitochondrial outer membrane

Various point mutations of lysyl residues in yeast mitochondrial porin (283 residues) were tested for their ability to assemble in vitro into the outer membranes of intact yeast mitochondria. Assembly was evaluated by protection from proteinases. The extent of assembly of two of the mutants, K234E and K236E porins, was much less than for wild-type in either post-translational or co-translational assembly assays. Lysine to glutamate mutants at other positions and K234R porin assembled as well as wild-type, but K234Q porin was poorly inserted. When both Lys-234 and Lys-236 were mutated, K234R/K236R porin was inserted better than K234Q/K236Q porin, which was inserted better than K234E/K236E; however, none of these mutants assembled as well as wild-type porin. It was concluded that optimal assembly of yeast porin depended on the presence of positively charged residues at both positions 234 and 236 and a lysine at one of these positions. After undergoing the assembly reaction, mutants that were vulnerable to proteinase K (i.e. K234E, K234Q, and K236E porins) seemed to be incompletely digested and were, to varying degrees, resistant to extraction by Na2CO3 (pH 11.5). These experiments suggested that these mutants were incompletely inserted into the outer membrane. Both Lys-234 and Lys-236 are included in an internal pentapeptide, VKAKV, that is conserved in porins from protists, plants, and animals, and it is possible that, at least, the lysines in this tract are one of the signals for the membrane assembly of these proteins.

Assembly of the preprotein receptor MOM72/MAS70 into the protein import complex of the outer membrane of mitochondria

Membrane integration and assembly of MOM72 from Neurospora crassa and its yeast homolog MAS70 was studied with isolated mitochondria. After synthesis in vitro, the precursors of MOM72/MAS70 are tightly folded and expose only their N-terminal amino acid residues comprising the targeting and the membrane anchor domain. Insertion of the protein into the mitochondrial outer membrane (MOM) occurs in a time- and temperature-dependent manner and is stimulated by ATP. MOM72/MAS70 is then assembled into the outer membrane MOM complex. Whereas membrane insertion occurred independently of the presence of protease-sensitive surface components, the assembly reaction depended on such components. In the MOM complex MOM72 and MAS70 were found in the neighborhood of different components in yeast and N. crassa mitochondria. MOM72 was found in association with MOM22 in N. crassa mitochondria, whereas MAS70 was in proximity to a 37-kDa component in yeast outer mitochondrial membrane. The interaction with the 37-kDa protein is important for integration of MAS70 into the MOM complex. Thus, the 37-kDa protein plays an important role in the biogenesis of MAS70.

Folding, flavinylation, and mitochondrial import of 6-hydroxy-D-nicotine oxidase fused to the presequence of rat dimethylglycine dehydrogenase

We analyzed the folding, covalent flavinylation, and mitochondrial import of the rabbit reticulocyte lysate-translated bacterial 6-hydroxy-D-nicotine oxidase (6-HDNO) fused to the mitochondrial targeting sequence of rat liver dimethylglycine dehydrogenase. Translation of 6-HDNO in FAD-supplemented reticulocyte lysate resulted in a protein that contained covalently incorporated FAD, exhibited enzyme activity, and was trypsin-resistant, a characteristic of the tight conformation of the holoenzyme. The attached mitochondrial presequence did not prevent folding, binding of FAD, or enzyme activity of the 6-HDNO moiety of the fusion protein (pre-6-HDNO). Pre-6-HDNO was imported into rat liver mitochondria and processed by the mitochondrial processing peptidase. Incubation of the trypsin-resistant pre-holo-6-HDNO protein with deenergized rat liver mitochondria demonstrated that upon contact with mitochondria, the protein was unfolded and became trypsin sensitive. Mitochondrial import assays showed that the unfolded pre-holo-6-HDNO with covalently attached FAD was imported into rat liver mitochondria. Inside the mitochondrion the holo-6-HDNO was refolded into the trypsin-resistant conformation. However, when pre-apo-6-HDNO was imported only part of the protein became trypsin resistant (approximately 20%). Addition of FAD and the allosteric effector glycerol 3-phosphate to apo-6-HDNO containing mitochondrial matrix was required to transform the protein into the trypsin-resistant conformation characteristic of holo-6-HDNO.

Import and insertion of proteins into the mitochondrial outer membrane

Nuclear-encoded proteins destined for insertion into the mitochondrial outer membrane, follow the same general pathway for import as proteins that are translocated to interior compartments within the organelle. This observation is true both for beta-barrel-type proteins and for proteins that contain hydrophobic alpha-helical transmembrane segments. In this review, we describe what is known about the various steps leading to protein insertion into the outer membrane, and discuss the energetics that favor vectorial translocation into and across this membrane. The selection of the outer membrane during import may involve a lateral release of the translocating polypeptide from the import machinery so that the appropriate domains of the protein become embedded in the lipid bilayer. One type of topogenic domain that can guarantee such selection of the outer membrane is a signal-anchor sequence of the type characterized for the bitopic protein Mas70p. It is suggested that a signal-anchor sequence selective for the mitochondrial outer membrane causes abrogation of polypeptide translocation and triggers the release of the transmembrane segment into the surrounding lipid bilayer, prior to any possibility for the commitment of translocation to the interior of the organelle. Specific structural features of the signal-anchor sequence specify its orientation in the membrane, and can confer on this sequence the ability to form homo-oligomers and hetero-oligomers. Strategies other than a signal-anchor sequence may be employed by other classes of proteins for selection of the outer-membrane. Of note is the ability of the outer-membrane import machinery to catalyze integration of the correct set of proteins into the outer-membrane bilayer, while allowing proteins that are destined for integration into the bilayer of the inner membrane to pass through unimpeded. Again, however, different proteins may employ different strategies. One model proposes that this can be accomplished by a combination of a matrix-targeting signal and a distal stop-transfer sequence. In this model, the formation of contact sites, which is triggered when the matrix-targeting signal engages the import machinery of the inner membrane, may prevent the outer-membrane translocon from recognizing and responding to the downstream stop-transfer domain. This allows the transmembrane segment to pass across the outer-membrane, and subsequently integrate into the inner membrane.

Functional expression of C-terminally truncated human monoamine oxidase type A in Saccharomyces cerevisiae

The deduced amino acid sequence of human liver monoamine oxidase type A was analyzed with secondary structure programs. These analyses and comparison to other flavoproteins identified a single potential transmembrane hydrophobic peptide at the C-terminus suggesting that this peptide is a membrane anchor and that the remainder of the protein constitutes a soluble domain. Truncation of the C-terminus by 24 amino acids which are inclusive of the putative transmembrane peptide, however, gave a protein which exhibited solubility properties substantially similar to the wild type enzyme. This result indicates that the hydrophobic behavior of monoamine oxidase type A is due to more complex features than a single transmembrane anchor. The mutant enzyme expressed in yeast appears to form a disulfide bond which reduces catalytic efficiency by up to 90%. Full activity, however, can be recovered by incubation with dithiothreitol, suggesting that in the wild type enzyme the amino acid residues deleted in the mutant protein protect two cysteine residues (those involved in the formation of the disulfide bond in the mutant) from oxidation and that the deleted residues are in close proximity to the active site. The activation experiments indicated that the deleted amino acids do not contribute any catalytic residues to the active site.

A constitutive form of heat-shock protein 70 is located in the outer membranes of mitochondria from rat liver

HSP73, the constitutive form of heat-shock protein 70, has been implicated in the translocation of preproteins across the mitochondrial membranes, being required for maintaining mitochondrial preproteins in an import competent conformation. Here we report that highly purified mitochondrial outer membranes contain a protein indistinguishable from HSP73 as a tightly associated peripheral component of the membrane. This membrane form of HSP73 was photolabelled with [alpha-32P]ATP and could be released from the outer membrane with sodium carbonate, but not after incubation of the membranes with salt or with ATP. A sensitive immunoassay with an anti-HSP73 monoclonal antibody, revealed the association of HSP73 with mitochondrial outer membrane vesicles at a level similar to that of preprotein import receptors.

Enhanced enzymatic degradation of radical damaged mitochondrial membrane components

The location of a protein (soluble or membrane-bound) influences the extent of oxidative damage caused by free radicals. It has been established that after radical attack, soluble proteins can become more susceptible to hydrolysis by individual proteinases than native proteins. We have now examined the hydrolytic susceptibility following radical attack of a protein that is located within a membrane environment, mitochondrial monoamine oxidase (MAO). After exposure to oxygen radicals generated by gamma irradiation, hydrolysis of sub-mitochondrial particles (SMP) containing MAO was increased in three respects. First, the generation of small fragments of MAO by the proteinases elastase and trypsin, was enhanced. Second, the generation by these enzymes and by phospholipase A2 of non-sedimentable membrane fragments containing MAO was also increased. Third, autolysis of SMP was enhanced. Hence, proteins located within membranes may become more susceptible to enzymatic degradation following oxidative damage.

The Pas2 protein essential for peroxisome biogenesis is related to ubiquitin-conjugating enzymes

In the yeast Saccharomyces cerevisiae, PAS genes are essential for the biogenesis and proliferation of peroxisomes. Recently, the first two genes, PAS1 (ref. 3) and PAS3 (ref. 4), have been characterized. Here we report the cloning and sequencing of the PAS2 gene. It encodes a new member of the ubiquitin-conjugating (UBC) protein family and is the first member associated with peroxisomes. The proposed function of the Pas2 protein as a UBC enzyme (UBC10) is supported by the fact that site-directed mutagenesis of a strictly conserved and functionally essential cysteine residue of UBC proteins leads to mutant Pas2 proteins unable to complement pas2 mutant strains. Ubiquitination of proteins is known to play an important part in DNA repair, sporulation, cell cycle control and degradation of abnormal proteins. We provide evidence for a crucial role of the ubiquitin-conjugation pathway in organelle formation.