Molecular basis for peroxisomal localization of tetrameric carbonyl reductase

Deciphering the structure of a multi-drug resistant Acinetobacter baumannii short-chain dehydrogenase reductase

The rapidly increasing threat of multi-drug-resistant Acinetobacter baumannii infections globally, encompassing a range of clinical manifestations from skin and soft tissue infections to life-threatening conditions like meningitis and pneumonia, underscores an urgent need for novel therapeutic strategies. These infections, prevalent in both hospital and community settings, present a formidable challenge to the healthcare system due to the bacterium's widespread nature and dwindling effective treatment options. Against this backdrop, the exploration of bacterial short-chain dehydrogenase reductases (SDRs) emerges as a promising avenue. These enzymes play pivotal roles in various critical bacterial processes, including fatty acid synthesis, homeostasis, metabolism, and contributing to drug resistance mechanisms. In this study, we present the first examination of the X-ray crystallographic structure of an uncharacterized SDR enzyme from A. baumannii. The tertiary structure of this SDR is distinguished by a central parallel β-sheet, consisting of seven strands, which is flanked by eight α-helices. This configuration exhibits structural parallels with other enzymes in the SDR family, underscoring a conserved architectural theme within this enzyme class. Despite the current ambiguity regarding the enzyme's natural substrate, the importance of many SDR enzymes as targets in anti-bacterial agent design is well-established. Therefore, the detailed structural insights provided in this study open new pathways for the in-silico design of therapeutic agents. By offering a structural blueprint, our findings may provide a platform for future research aimed at developing targeted treatments against this and other multi-drug-resistant infections.

Studying the interaction between PEX5 and its full-length cargo proteins in living cells by a novel Försteŕs resonance energy transfer-based competition assay

The import of the majority of soluble peroxisomal proteins is initiated by the interaction between type-1 peroxisomal targeting signals (PTS1) and their receptor PEX5. PTS1 motifs reside at the extreme C-terminus of proteins and consist of a characteristic tripeptide and a modulatory upstream region. Various PTS1-PEX5 interactions have been studied by biophysical methods using isolated proteins or in heterologous systems such as two-hybrid assays, but a recently established approach based on Försters resonance energy transfer (FRET) allows a quantifying investigation in living cells. FRET is the radiation-free energy transfer between two fluorophores in close proximity and can be used to estimate the fraction of acceptor molecules bound to a donor molecule. For PTS1-PEX5 this method relies on the measurement of FRET-efficiency between the PTS1-binding TPR-domain of PEX5 tagged with mCherry and EGFP fused to a PTS1 peptide. However, this method is less suitable for binding partners with low affinity and protein complexes involving large proteins such as the interaction between full-length PTS1-carrying cargo proteins and PEX5. To overcome this limitation, we introduce a life-cell competition assay based on the same FRET approach but including a fusion protein of Cerulean with the protein of interest as a competitor. After implementing the mathematical description of competitive binding experiments into a fitting algorithm, we demonstrate the functionality of this approach using known interaction partners, its ability to circumvent previous limitations of FRET-measurements and its ability to study the interaction between PEX5 and its full-length cargo proteins. We find that some proteins (SCP2 and AGXT) bind PEX5 with higher affinity than their PTS1-peptides alone, but other proteins (ACOX3, DAO, PerCR-SRL) bind with lower but reasonable affinity, whereas GSTK1 binds with very low affinity. This binding strength was not increased upon elongating the PEX5 TPR-domain at its N-terminus, PEX5(N-TPR), although it interacts specifically with the N-terminal domain of PEX14. Finally, we demonstrate that the latter reduces the interaction strength between PEX5(N-TPR) and PTS1 by a dose-dependent but apparently non-competitive mechanism. Altogether, this demonstrates the power of this novel FRET-based competition approach for studying cargo recognition by PEX5 and protein complexes including large proteins in general.

Translating the Arabidopsis thaliana Peroxisome Proteome Insights to Solanum lycopersicum: Consensus Versus Diversity

Peroxisomes are small, single-membrane specialized organelles present in all eukaryotic organisms. The peroxisome is one of the nodal centers of reactive oxygen species homeostasis in plants, which are generated in a high amount due to various stress conditions. Over the past decade, there has been extensive study on peroxisomal proteins and their signaling pathways in the model plant Arabidopsis thaliana, and a lot has been deciphered. However, not much impetus has been given to studying the peroxisome proteome of economically important crops. Owing to the significance of peroxisomes in the physiology of plants during normal and stress conditions, understating its proteome is of much importance. Hence, in this paper, we have made a snapshot of putative peroxisomal matrix proteins in the economically important vegetable crop tomato (Solanum lycopersicum, (L.) family Solanaceae). First, a reference peroxisomal matrix proteome map was generated for Arabidopsis thaliana using the available proteomic and localization studies, and proteins were categorized into various groups as per their annotations. This was used to create the putative peroxisomal matrix proteome map for S. lycopersicum. The putative peroxisome proteome in S. lycopersicum retains the basic framework: the bulk of proteins had peroxisomal targeting signal (PTS) type 1, a minor group had PTS2, and the catalase family retained its characteristic internal PTS. Apart from these, a considerable number of S. lycopersicum orthologs did not contain any “obvious” PTS. The number of PTS2 isoforms was found to be reduced in S. lycopersicum. We further investigated the PTS1s in the case of both the plant species and generated a pattern for canonical and non-canonical PTS1s. The number of canonical PTS1 proteins was comparatively lesser in S. lycopersicum. The non-canonical PTS1s were found to be comparable in both the plant species; however, S. lycopersicum showed greater diversity in the composition of the signal tripeptide. Finally, we have tried to address the lacunas and probable strategies to fill those gaps.

Identification of Molecular Correlations Between DHRS4 and Progressive Neurodegeneration in Amyotrophic Lateral Sclerosis By Gene Co-Expression Network Analysis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease, and its candidate biomarkers have not yet been fully elucidated in previous studies. Therefore, with the present study, we aim to define and verify effective biomarkers of ALS by bioinformatics. Here, we employed differentially expressed gene (DEG) analysis, weighted gene co-expression network analysis (WGCNA), enrichment analysis, immune infiltration analysis, and protein-protein interaction (PPI) to identify biomarkers of ALS. To validate the biomarkers, we isolated the lumbar spinal cord from mice and characterized them using Western blotting and immunofluorescence. The results showed that Dhrs4 expression in the spinal cord was upregulated with the progression of SOD1G93A mice, and the upregulation of DHRS4 and its synergistic DHRS3 might be primarily associated with the activation of the complement cascade in the immune system (C1QA, C1QB, C1QC, C3, and ITGB2), which might be a novel mechanism that induces spinal neurodegeneration in ALS. We propose that DHRS4 and its synergistic DHRS3 are promising molecular markers for detecting ALS progression.

The Activation Parameters of a Cold-Adapted Short Chain Dehydrogenase Are Insensitive to Enzyme Oligomerization

The structural principles of enzyme cold adaptation are of fundamental interest both for understanding protein evolution and for biotechnological applications. It has become clear in recent years that structural flexibility plays a major role in tuning enzyme activity at low temperatures, which is reflected by characteristic changes in the thermodynamic activation parameters for psychrophilic enzymes, compared to those of mesophilic and thermophilic ones. Hence, increased flexibility of the enzyme surface has been shown to lead to a lower enthalpy and a more negative entropy of activation, which leads to higher activity in the cold. This immediately raises the question of how enzyme oligomerization affects the temperature dependence of catalysis. Here, we address this issue by computer simulations of the catalytic reaction of a cold-adapted bacterial short chain dehydrogenase in different oligomeric states. Reaction free energy profiles are calculated at different temperatures for the tetrameric, dimeric, and monomeric states of the enzyme, and activation parameters are obtained from the corresponding computational Arrhenius plots. The results show that the activation free energy, enthalpy, and entropy are remarkably insensitive to the oligomeric state, leading to the conclusion that assembly of the subunit interfaces does not compromise cold adaptation, even though the mobilities of interfacial residues are indeed affected.

Converting the 3-quinuclidinone reductase from Agrobacterium tumefaciens into the ethyl 4-chloroacetoacetate reductase by site-directed mutagenesis

In this study, the 3-quinuclidinone reductase from Agrobacterium tumefaciens (AtQR) was modified by site-directed mutagenesis. And we further obtained a saturation mutant library in which the residue 197 was mutated. A single-point mutation converted the wild enzyme that originally had no catalytic activity in reduction of ethyl 4-chloroacetoacetate (COBE) into an enzyme with catalytic activity. The results of enzyme activity assays showed that the seven variants could asymmetrically reduce COBE to ethyl (S)-4-chloro-3-hydroxybutyrate ((S)-CHBE) with NADH as coenzyme. In the library, the variant E197N showed higher catalytic efficiency than others. The E197N was optimally active at pH 6.0 and 40°C, and the catalytic efficiency (k_cat /K_m ) for COBE was 51.36 s^-1 ·mM^-1 . This study showed that the substrate specificity of AtQR could be changed through site-directed mutagenesis at the residue 197.

The intrinsically disordered nature of the peroxisomal protein translocation machinery

Despite having a membrane that is impermeable to all but the smallest of metabolites, peroxisomes acquire their newly synthesized (cytosolic) matrix proteins in an already folded conformation. In some cases, even oligomeric proteins have been reported to translocate the organelle membrane. The protein sorting machinery that accomplishes this feat must be rather flexible and, unsurprisingly, several of its key components have large intrinsically disordered domains. Here, we provide an overview on these domains and their interactions trying to infer their functional roles in this protein sorting pathway.

Crystal structure and catalytic characterization of the dehydrogenase/reductase SDR family member 4 (DHRS4) from Caenorhabditis elegans

The human dehydrogenase/reductase SDR family member 4 (DHRS4) is a tetrameric protein that is involved in the metabolism of several aromatic carbonyl compounds, steroids, and bile acids. The only invertebrate DHRS4 that has been characterized to date is that from the model organism Caenorhabditis elegans. We have previously cloned and initially characterized this protein that was recently annotated as DHRS4_CAEEL in the UniProtKB database. Crystallization and X-ray diffraction studies of the full-length DHRS4_CAEEL protein in complex with diacetyl revealed its tetrameric structure and showed that two subunits are connected via an intermolecular disulfide bridge that is formed by N-terminal cysteine residues (Cys5) of each protein chain, which increases the enzymatic activity. A more detailed biochemical and catalytic characterization shows that DHRS4_CAEEL shares some properties with human DHRS4 such as relatively low substrate affinities with aliphatic α-diketones and a preference for aromatic dicarbonyls such as isatin, with a 30-fold lower Km value compared with the human enzyme. Moreover, DHRS4_CAEEL is active with aliphatic aldehydes (e.g. hexanal), while human DHRS4 is not. Dehydrogenase activity with alcohols was only observed with aromatic alcohols. Protein thermal shift assay revealed a stabilizing effect of phosphate buffer that was accompanied by an increase in catalytic activity of more than two-fold. The study of DHRS4 homologs in simple lineages such as C. elegans may contribute to our understanding of the original function of this protein that has been shaped by evolutionary processes in the course of the development from invertebrates to higher mammalian species.

DATABASE

Structural data are available in the PDB under the accession numbers 5OJG and 5OJI.

Pnc1 piggy-back import into peroxisomes relies on Gpd1 homodimerisation

Peroxisomes are eukaryotic organelles that posttranslationally import proteins via one of two conserved peroxisomal targeting signal (PTS1 or 2) mediated pathways. Oligomeric proteins can be imported via these pathways but evidence is accumulating that at least some PTS1-containing monomers enter peroxisomes before they assemble into oligomers. Some proteins lacking a PTS are imported by piggy-backing onto PTS-containing proteins. One of these proteins is the nicotinamidase Pnc1, that is co-imported with the PTS2-containing enzyme Glycerol-3-phosphate dehydrogenase 1, Gpd1. Here we show that Pnc1 co-import requires Gpd1 to form homodimers. A mutation that interferes with Gpd1 homodimerisation does not prevent Gpd1 import but prevents Pnc1 co-import. A suppressor mutation that restores Gpd1 homodimerisation also restores Pnc1 co-import. In line with this, Pnc1 interacts with Gpd1 in vivo only when Gpd1 can form dimers. Redirection of Gpd1 from the PTS2 import pathway to the PTS1 import pathway supports Gpd1 monomer import but not Gpd1 homodimer import and Pnc1 co-import. Our results support a model whereby Gpd1 may be imported as a monomer or a dimer but only the Gpd1 dimer facilitates co-transport of Pnc1 into peroxisomes.

Characterization, prediction and evolution of plant peroxisomal targeting signals type 1 (PTS1s)

Our knowledge of the proteome of plant peroxisomes and their functional plasticity is far from being complete, primarily due to major technical challenges in experimental proteome research of the fragile cell organelle. Several unexpected novel plant peroxisome functions, for instance in biotin and phylloquinone biosynthesis, have been uncovered recently. Nevertheless, very few regulatory and membrane proteins of plant peroxisomes have been identified and functionally described up to now. To define the matrix proteome of plant peroxisomes, computational methods have emerged as important powerful tools. Novel prediction approaches of high sensitivity and specificity have been developed for peroxisome targeting signals type 1 (PTS1) and have been validated by in vivo subcellular targeting analyses and thermodynamic binding studies with the cytosolic receptor, PEX5. Accordingly, the algorithms allow the correct prediction of many novel peroxisome-targeted proteins from plant genome sequences and the discovery of additional organelle functions. In this review, we provide an overview of methodologies, capabilities and accuracies of available prediction algorithms for PTS1 carrying proteins. We also summarize and discuss recent quantitative, structural and mechanistic information of the interaction of PEX5 with PTS1 carrying proteins in relation to in vivo import efficiency. With this knowledge, we develop a model of how proteins likely evolved peroxisomal targeting signals in the past and still nowadays, in which order the two import pathways might have evolved in the ancient eukaryotic cell, and how the secondary loss of the PTS2 pathway probably happened in specific organismal groups.

The peroxisomal protein import machinery displays a preference for monomeric substrates

Peroxisomal matrix proteins are synthesized on cytosolic ribosomes and transported by the shuttling receptor PEX5 to the peroxisomal membrane docking/translocation machinery, where they are translocated into the organelle matrix. Under certain experimental conditions this protein import machinery has the remarkable capacity to accept already oligomerized proteins, a property that has heavily influenced current models on the mechanism of peroxisomal protein import. However, whether or not oligomeric proteins are really the best and most frequent clients of this machinery remain unclear. In this work, we present three lines of evidence suggesting that the peroxisomal import machinery displays a preference for monomeric proteins. First, in agreement with previous findings on catalase, we show that PEX5 binds newly synthesized (monomeric) acyl-CoA oxidase 1 (ACOX1) and urate oxidase (UOX), potently inhibiting their oligomerization. Second, in vitro import experiments suggest that monomeric ACOX1 and UOX are better peroxisomal import substrates than the corresponding oligomeric forms. Finally, we provide data strongly suggesting that although ACOX1 lacking a peroxisomal targeting signal can be imported into peroxisomes when co-expressed with ACOX1 containing its targeting signal, this import pathway is inefficient.

The first minutes in the life of a peroxisomal matrix protein

In the field of intracellular protein sorting, peroxisomes are most famous by their capacity to import oligomeric proteins. The data supporting this remarkable property are abundant and, understandably, have inspired a variety of hypothetical models on how newly synthesized (cytosolic) proteins reach the peroxisome matrix. However, there is also accumulating evidence suggesting that many peroxisomal oligomeric proteins actually arrive at the peroxisome still as monomers. In support of this idea, recent data suggest that PEX5, the shuttling receptor for peroxisomal matrix proteins, is also a chaperone/holdase, binding newly synthesized peroxisomal proteins in the cytosol and blocking their oligomerization. Here we review the data behind these two different perspectives and discuss their mechanistic implications on this protein sorting pathway.

A novel NAD(P)H-dependent carbonyl reductase specifically expressed in the thyroidectomized chicken fatty liver: catalytic properties and crystal structure

A gene encoding a functionally unknown protein that is specifically expressed in the thyroidectomized chicken fatty liver and has a predicted amino acid sequence similar to that of NAD(P)H-dependent carbonyl reductase was overexpressed in Escherichia coli; its product was purified and characterized. The expressed enzyme was an NAD(P)H-dependent broad substrate specificity carbonyl reductase and was inhibited by arachidonic acid at 1.5 μm. Enzymological characterization indicated that the enzyme could be classified as a cytosolic-type carbonyl reductase. The enzyme's 3D structure was determined using the molecular replacement method at 1.98 Å resolution in the presence of NADPH and ethylene glycol. The asymmetric unit consisted of two subunits, and a noncrystallographic twofold axis generated the functional dimer. The structures of the subunits, A and B, differed from each other. In subunit A, the active site contained an ethylene glycol molecule absent in subunit B. Consequently, Tyr172 in subunit A rotated by 103.7° in comparison with subunit B, which leads to active site closure in subunit A. In Y172A mutant, the Km value for 9,10-phenanthrenequinone (model substrate) was 12.5 times higher than that for the wild-type enzyme, indicating that Tyr172 plays a key role in substrate binding in this carbonyl reductase. Because the Tyr172-containing active site lid structure (Ile164-Gln174) is not conserved in all known carbonyl reductases, our results provide new insights into substrate binding of carbonyl reductase. The catalytic properties and crystal structure revealed that thyroidectomized chicken fatty liver carbonyl reductase is a novel enzyme.

Multiple pathways for protein transport to peroxisomes

Peroxisomes are unique among the organelles of the endomembrane system. Unlike other organelles that derive most if not all of their proteins from the ER (endoplasmic reticulum), peroxisomes contain dedicated machineries for import of matrix proteins and insertion of membrane proteins. However, peroxisomes are also able to import a subset of their membrane proteins from the ER. One aspect of peroxisome biology that has remained ill defined is the role the various import pathways play in peroxisome maintenance. In this review, we discuss the available data on matrix and membrane protein import into peroxisomes.

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Peroxisomal membrane and matrix proteins require distinct factors for their transport.

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Matrix proteins fold in the cytosol prior to their import.

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Loaded targeting receptors form part of the matrix protein translocation pore.

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Many membrane proteins are directly inserted into the peroxisomal membrane.

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Some peroxisomal membrane proteins are transported via the ER to peroxisomes.

Rabbit 3-hydroxyhexobarbital dehydrogenase is a NADPH-preferring reductase with broad substrate specificity for ketosteroids, prostaglandin D₂, and other endogenous and xenobiotic carbonyl compounds

3-Hydroxyhexobarbital dehydrogenase (3HBD) catalyzes NAD(P)⁺-linked oxidation of 3-hydroxyhexobarbital into 3-oxohexobarbital. The enzyme has been thought to act as a dehydrogenase for xenobiotic alcohols and some hydroxysteroids, but its physiological function remains unknown. We have purified rabbit 3HBD, isolated its cDNA, and examined its specificity for coenzymes and substrates, reaction directionality and tissue distribution. 3HBD is a member (AKR1C29) of the aldo-keto reductase (AKR) superfamily, and exhibited high preference for NADP(H) over NAD(H) at a physiological pH of 7.4. In the NADPH-linked reduction, 3HBD showed broad substrate specificity for a variety of quinones, ketones and aldehydes, including 3-, 17- and 20-ketosteroids and prostaglandin D₂, which were converted to 3α-, 17β- and 20α-hydroxysteroids and 9α,11β-prostaglandin F₂, respectively. Especially, α-diketones (such as isatin and diacetyl) and lipid peroxidation-derived aldehydes (such as 4-oxo- and 4-hydroxy-2-nonenals) were excellent substrates showing low K(m) values (0.1-5.9 μM). In 3HBD-overexpressed cells, 3-oxohexobarbital and 5β-androstan-3α-ol-17-one were metabolized into 3-hydroxyhexobarbital and 5β-androstane-3α,17β-diol, respectively, but the reverse reactions did not proceed. The overexpression of the enzyme in the cells decreased the cytotoxicity of 4-oxo-2-nonenal. The mRNA for 3HBD was ubiquitously expressed in rabbit tissues. The results suggest that 3HBD is an NADPH-preferring reductase, and plays roles in the metabolisms of steroids, prostaglandin D₂, carbohydrates and xenobiotics, as well as a defense system, protecting against reactive carbonyl compounds.

Characterization of rabbit morphine 6-dehydrogenase and two NAD(+)-dependent 3α(17β)-hydroxysteroid dehydrogenases

Mammalian morphine 6-dehydrogenase (M6DH)(1) converts morphine into a reactive electrophile, morphinone. M6DH belongs to the aldo-keto reductase (AKR) superfamily, but its endogenous substrates and entire amino acid sequence remain unknown. A recent rabbit genomic sequencing predicts three genes for novel AKRs (1C26, 1C27 and 1C28) that share >87% amino acid sequence identity and are similar to the partial sequence of rabbit liver M6DH. We isolated cDNAs for the three AKRs, and compared the properties of their recombinant enzymes. Like M6DH, only AKR1C26 that shares the highest sequence identity with hepatic M6DH oxidized morphine. The three AKRs showed NAD(+)-dependent dehydrogenase activity towards other non-steroidal alicyclic alcohols and 3α/17β-hydroxy-C(18)/C(19)/C(21)-steroids, and their mRNAs were ubiquitously expressed in rabbit tissues. The kinetic constants for the substrates suggest that at least AKR1C26 and AKR1C28 act as NAD(+)-dependent 3α/17β-hydroxysteroid dehydrogenases. AKR1C27 differed from AKR1C28 in its high K(m) values for the substrates and low sensitivity towards competitive inhibitors (ikarisoside A, hinokitiol, hexestrol and zearalenone), despite their 95% sequence identity. The site-directed mutagenesis of Tyr118 and Phe310 in AKR1C27 to the corresponding residues (Phe and Ile, respectively) in AKR1C28 produced an enzyme that was similar to AKR1C28, suggesting their key roles in ligand binding.

Expression, purification, crystallization and preliminary X-ray analysis of NAD(P)H-dependent carbonyl reductase specifically expressed in thyroidectomized chicken fatty liver

An NAD(P)H-dependent carbonyl reductase specifically expressed in thyroidectomized chicken fatty liver was crystallized using the sitting-drop vapour-diffusion method with polyethylene glycol 300 as the precipitant. The crystals belonged to the monoclinic space group C2, with unit-cell parameters a=104.26, b=81.32, c=77.27 Å, β=119.43°, and diffracted to 1.86 Å resolution on beamline NE3A at the Photon Factory. The overall Rmerge was 5.4% and the data completeness was 99.4%.

Molecular and functional evolution of human DHRS2 and DHRS4 duplicated genes

Human DHRS2 and DHRS4 genes code for similar NADP-dependent short-chain carbonyl-reductase enzymes having different substrate specificity. Human DHRS2 and DHRS4 enzymes share several common sequence motives including residues responsible for coenzyme binding as well as for the intimate catalytic oxido-reductase mechanism, while their substrate-binding sequences have very low similarity. We found that DHRS2 and DHRS4 genes are syntenic outparalogues originated from a duplication of the DHRS4 gene that took place before the formation of the mammalian clade. DHRS2 gene evolved more rapidly and underwent positive selection on more sites than the DHRS4 gene. DHRS2 sites under positive selection were mainly located on the enzyme active site thus showing that substrate specificity drove the divergence from the DHRS4 enzyme. Rapid divergent evolution brought the human DHRS2 enzyme to have subcellular localization, synthesis regulation and specialized cellular functions very different from those of the human DHRS4 enzyme.

Molecular requirements for peroxisomal targeting of alanine-glyoxylate aminotransferase as an essential determinant in primary hyperoxaluria type 1

Alanine-glyoxylate aminotransferase is a peroxisomal enzyme, of which various missense mutations lead to irreversible kidney damage via primary hyperoxaluria type 1, in part caused by improper peroxisomal targeting. To unravel the molecular mechanism of its recognition by the peroxisomal receptor Pex5p, we have determined the crystal structure of the respective cargo-receptor complex. It shows an extensive protein/protein interface, with contributions from residues of the peroxisomal targeting signal 1 and additional loops of the C-terminal domain of the cargo. Sequence segments that are crucial for receptor recognition and hydrophobic core interactions within alanine-glyoxylate aminotransferase are overlapping, explaining why receptor recognition highly depends on a properly folded protein. We subsequently characterized several enzyme variants in vitro and in vivo and show that even minor protein fold perturbations are sufficient to impair Pex5p receptor recognition. We discuss how the knowledge of the molecular parameters for alanine-glyoxylate aminotransferase required for peroxisomal translocation could become useful for improved hyperoxaluria type 1 treatment.

Identification of a novel isoform of DHRS4 protein with a nuclear localization signal

The DHRS4 gene encodes an NADP(H)-dependent retinol dehydrogenase/reductase (NRDR) and plays an important role in regulating the synthesis of retinoic acid. In the present study, we identified a novel splice RNA variant, designated NRDRA2, of the human DHRS4 gene by RT-PCR, 3' RACE, and 5' RACE. NRDRA2 mRNA lacked exons 4 and 6, and had a shift in the reading frame when compared to DHRS4 mRNA, resulting in loss of the peroxisomal targeting signal of NRDR and gain of a nuclear localization signal in the predicted NRDRA2 protein. Endogenous NRDRA2 protein was identified in the human cervical carcinoma cell line HeLa by immunoprecipitation and mass spectrometric assay. A green fluorescent protein reporter assay showed that NRDRA2 protein mainly localized to the nuclei, confirming the sequence at its C-terminus as a legitimate nuclear localization signal sequence. This study identifies the alternative transcript variant NRDRA2 encoding a subcellular nuclear localized NRDRA2 protein.

PEX5 protein binds monomeric catalase blocking its tetramerization and releases it upon binding the N-terminal domain of PEX14

Newly synthesized peroxisomal matrix proteins are targeted to the organelle by PEX5. PEX5 has a dual role in this process. First, it acts as a soluble receptor recognizing these proteins in the cytosol. Subsequently, at the peroxisomal docking/translocation machinery, PEX5 promotes their translocation across the organelle membrane. Despite significant advances made in recent years, several aspects of this pathway remain unclear. Two important ones regard the formation and disruption of the PEX5-cargo protein interaction in the cytosol and at the docking/translocation machinery, respectively. Here, we provide data on the interaction of PEX5 with catalase, a homotetrameric enzyme in its native state. We found that PEX5 interacts with monomeric catalase yielding a stable protein complex; no such complex was detected with tetrameric catalase. Binding of PEX5 to monomeric catalase potently inhibits its tetramerization, a property that depends on domains present in both the N- and C-terminal halves of PEX5. Interestingly, the PEX5-catalase interaction is disrupted by the N-terminal domain of PEX14, a component of the docking/translocation machinery. One or two of the seven PEX14-binding diaromatic motifs present in the N-terminal half of PEX5 are probably involved in this phenomenon. These results suggest the following: 1) catalase domain(s) involved in the interaction with PEX5 are no longer accessible upon tetramerization of the enzyme; 2) the catalase-binding interface in PEX5 is not restricted to its C-terminal peroxisomal targeting sequence type 1-binding domain and also involves PEX5 N-terminal domain(s); and 3) PEX14 participates in the cargo protein release step.

Rat aldose reductase-like protein (AKR1B14) efficiently reduces the lipid peroxidation product 4-oxo-2-nonenal

In this study, we examined the substrate specificity, inhibitor sensitivity and kinetic mechanism of a rat aldose reductase-like protein, which is named AKR1B14 in the aldo-keto reductase (AKR) superfamily. AKR1B14 catalyzed the nicotinamide adenine dinucleotide phosphate reduced form (NADPH)-dependent reduction of carbonyl compounds (derived from lipid peroxidation and glycation), xenobiotic aromatic aldehydes and some aromatic ketones. 4-Oxo-2-nonenal, the best substrate showing a K(m) value of 0.16 µM, was reduced into less reactive 4-oxo-2-nonenol, and its cytotoxicity was attenuated by the overexpression of the enzyme in cultured cells. The enzyme also showed low K(m) values (0.9-10 µM) for medium-chain aliphatic aldehydes (such as 4-hydroxynonenal, 1-hexenal and farnesal) and 3-deoxyglucosone, although the K(m) values for short-chain substrates (such as isocaproaldehyde, acrolein and methylglyoxal) were high (16-600 µM). In the reverse reaction, aliphatic and aromatic alcohols were oxidized by AKR1B14 at low rates. AKR1B14 was inhibited by aldose reductase inhibitors such as tolrestat and epalrestat, and their inhibition patterns were noncompetitive versus the aldehyde substrate and competitive with respect to the alcohol substrate. Kinetic analyses of the oxidoreduction and dead-end inhibition suggest that the reaction follows an ordered sequential mechanism.

Role of thiamine pyrophosphate in oligomerisation, functioning and import of peroxisomal 2-hydroxyacyl-CoA lyase

During peroxisomal α-oxidation, the CoA-esters of phytanic acid and 2-hydroxylated straight chain fatty acids are cleaved into a (n-1) fatty aldehyde and formyl-CoA by 2-hydroxyacyl-CoA lyase (HACL1). HACL1 is imported into peroxisomes via the PEX5/PTS1 pathway, and so far, it is the only known peroxisomal TPP-dependent enzyme in mammals. In this study, the effect of mutations in the TPP-binding domain of HACL1 on enzyme activity, subcellular localisation and oligomerisation was investigated. Mutations of the aspartate 455 and serine 456 residues within the TPP binding domain of the human HACL1 did not affect the targeting upon expression in transfected CHO cells, although enzyme activity was abolished. Gel filtration of native and mutated N-His(6)-fusions, expressed in yeast, revealed that the mutations did not influence the oligomerisation of the (apo)enzyme. Subcellular fractionation of yeast cells expressing HACL1 showed that the lyase activity sedimented at high density in a Nycodenz gradient. In these fractions TPP could be measured, but not when mutated HACL1 was expressed, although the recombinant enzyme was still targeted to peroxisomes. These findings indicate that the binding of TPP is not required for peroxisomal targeting and correct folding of HACL1, in contrast to other TPP-dependent enzymes, and suggest that transport of TPP into peroxisomes is dependent on HACL1 import, without requirement of a specific solute transporter.

Protective effect of rat aldo-keto reductase (AKR1C15) on endothelial cell damage elicited by 4-hydroxy-2-nonenal

4-Hydroxy-2-nonenal (HNE), a major reactive product of lipid peroxidation, is believed to play a central role in atherogenic actions triggered by oxidized lipoproteins. An aldo-keto reductase (AKR) 1C15 efficiently reduces HNE and is distributed in many rat tissues including endothelial cells. In this study, we investigated whether AKR1C15 acts as a protective factor against endothelial damage elicited by HNE and oxidized lipoproteins. Treatment of rat endothelial cells with HNE provoked apoptosis through reactive oxygen species (ROS) formation, mitochondrial dysfunction and caspase activation in the cells. AKR1C15 converted HNE into less toxic 1,4-dihydroxy-2-nonene, and its overexpression markedly decreased the susceptibility of the cells to HNE. The forced expression of AKR1C15 also significantly suppressed the loss of cell viability caused by oxidized low-density lipoprotein and its lipidic fraction. Furthermore, the treatment of the cells with sublethal concentrations of HNE resulted in up-regulation of AKR1C15, which was partially abrogated by the ROS inhibitors. Collectively, these data indicate an anti-atherogenic function of AKR1C15 through the protection of endothelial cells from damage elicited by toxic lipids such as HNE.

Getting a camel through the eye of a needle: the import of folded proteins by peroxisomes

Peroxisomes are a family of organelles which have many unusual features. They can arise de novo from the endoplasmic reticulum by a still poorly characterized process, yet possess a unique machinery for the import of their matrix proteins. As peroxisomes lack DNA, their function, which is highly variable and dependent on developmental and/or environmental conditions, is determined by the post-translational import of specific metabolic enzymes in folded or oligomeric states. The two classes of matrix targeting signals for peroxisomal proteins [PTS1 (peroxisomal targeting signal 1) and PTS2] are recognized by cytosolic receptors [PEX5 (peroxin 5) and PEX7 respectively] which escort their cargo proteins to, or possibly across, the peroxisome membrane. Although the membrane translocation mechanism remains unclear, it appears to be driven by thermodynamically favourable binding interactions. Recycling of the receptors from the peroxisome membrane requires ATP hydrolysis for two linked processes: ubiquitination of PEX5 (and the PEX7 co-receptors in yeast) and the function of two peroxisome-associated AAA (ATPase associated with various cellular activities) ATPases, which play a role in recycling or turnover of the ubiquitinated receptors. This review summarizes and integrates recent findings on peroxisome matrix protein import from yeast, plant and mammalian model systems, and discusses some of the gaps in our understanding of this remarkable protein transport system.

Molecular determinants for the stereospecific reduction of 3-ketosteroids and reactivity towards all-trans-retinal of a short-chain dehydrogenase/reductase (DHRS4)

DHRS4, a member of the short-chain dehydrogenase/reductase superfamily, reduces all-trans-retinal and xenobiotic carbonyl compounds. Human DHRS4 differs from other animal enzymes in kinetic constants for the substrates, particularly in its low reactivity to retinoids. We have found that pig, rabbit and dog DHRS4s reduce benzil and 3-ketosteroids into S-benzoin and 3alpha-hydroxysteroids, respectively, in contrast to the stereoselectivity of human DHRS4 which produces R-benzoin and 3beta-hydroxysteroids. Among substrate-binding residues predicted from the crystal structure of pig DHRS4, F158 and L161 in the animal DHRS4 are serine and phenylalanine, respectively, in the human enzyme. Double mutation (F158S/L161F) of pig DHRS4 led to an effective switch of its substrate affinity and stereochemistry into those similar to human DHRS4. The roles of the two residues in determining the stereospecificity in 3-ketosteroid reduction were confirmed by reverse mutation (S158F/F161L) in the human enzyme. The stereochemical control was evaluated by comparison of the 3D models of pig wild-type and mutant DHRS4s with the modeled substrates. Additional mutation of T177N into the human S158F/F161L mutant resulted in almost complete kinetic conversion into a pig DHRS4-type form, suggesting a role of N177 in forming the substrate-binding cavity through an intersubunit interaction in pig and other animal DHRS4s, and explaining why the human enzyme shows low reactivity towards retinoids.

[Structure and function of peroxisomal tetrameric carbonyl reductase]

In this paper, the structure and function of a new tetrameric carbonyl reductase (TCR) is reviewed. TCRs were purified from rabbit and pig heart, using 4-benzoylpyridine as a substrate. Partial peptide sequencing and cDNA cloning of rabbit and pig TCRs revealed that both enzymes belonged to the short-chain dehydrogenase/reductase family and that their subunits consisted of 260 amino acid residues. Rabbit and pig TCRs catalyzed the reduction of alkyl phenyl ketones, alpha-dicarbonyl compounds, quinones and retinals. Both enzymes were potently inhibited by flavonoids and fatty acids. 9,10-Phenanthrenequinone, which is efficiently reduced by rabbit and pig TCRs, mediated the formation of superoxide radical through its redox cycling in pig heart. The C-terminal sequences of rabbit and pig TCRs comprised a type 1 peroxisomal targeting signal (PTS1) Ser-Arg-Leu, suggesting that the enzymes are localized in the peroxisome. In fact, pig TCR was targeted into the peroxisomal matrix, in the case of transfection of HeLa cells with vectors expressing the enzyme. However, when the recombinant pig TCR was directly introduced into HeLa cells, the enzyme was not targeted into the peroxisomal matrix. The crystal structure of recombinant pig TCR demonstrated that the C-terminal PTS1 of each subunit of the enzyme was buried in the interior of the tetrameric molecule. These findings indicate that pig TCR is imported into the peroxisome as a monomer and then forms an active tetramer within this organelle.

Characterization of human DHRS4: an inducible short-chain dehydrogenase/reductase enzyme with 3beta-hydroxysteroid dehydrogenase activity

Human DHRS4 is a peroxisomal member of the short-chain dehydrogenase/reductase superfamily, but its enzymatic properties, except for displaying NADP(H)-dependent retinol dehydrogenase/reductase activity, are unknown. We show that the human enzyme, a tetramer composed of 27kDa subunits, is inactivated at low temperature without dissociation into subunits. The cold inactivation was prevented by a mutation of Thr177 with the corresponding residue, Asn, in cold-stable pig DHRS4, where this residue is hydrogen-bonded to Asn165 in a substrate-binding loop of other subunit. Human DHRS4 reduced various aromatic ketones and alpha-dicarbonyl compounds including cytotoxic 9,10-phenanthrenequinone. The overexpression of the peroxisomal enzyme in cultured cells did not increase the cytotoxicity of 9,10-phenanthrenequinone. While its activity towards all-trans-retinal was low, human DHRS4 efficiently reduced 3-keto-C(19)/C(21)-steroids into 3beta-hydroxysteroids. The stereospecific conversion to 3beta-hydroxysteroids was observed in endothelial cells transfected with vectors expressing the enzyme. The mRNA for the enzyme was ubiquitously expressed in human tissues and several cancer cells, and the enzyme in HepG2 cells was induced by peroxisome-proliferator-activated receptor alpha ligands. The results suggest a novel mechanism of cold inactivation and role of the inducible human DHRS4 in 3beta-hydroxysteroid synthesis and xenobiotic carbonyl metabolism.