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Results for “Biochemistry”

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bioRxiv2018

Supply and consumption of glucose 6-phosphate in the chloroplast stroma

Fructose 6-phosphate is an intermediate in the Calvin-Benson cycle and can be acted on by phosphoglucoisomerase to make glucose 6-phosphate (G6P) for starch synthesis. A high concentration of G6P is favorable for starch synthesis but can also stimulate G6P dehydrogenase initiating the glucose-6-phosphate shunt an alternative pathway around the Calvin-Benson cycle. A low concentration of glucose 6-phosphate will limit this futile cycle. In order to understand the biochemical regulation of plastidic glucose 6-phosphate supply and consumption, we characterized biochemical parameters of two key enzymes, phosphoglucoisomerase (PGI) and G6P dehydrogenase (G6PDH). We have found that the plastidic PGI in has a higher Km for G6P compared to that for fructose 6-phosphate. The Km of G6PDH isoform 1 is increased under reducing conditions. The other two isoforms exhibit less redox regulation; isoform 2 is the most inhibited by NADPH. Our results support the conclusion that PGI restricts stromal G6P synthesis limiting futile cycling via G6PDH. It also acts like a one-way valve, allowing carbon to leave the Calvin-Benson cycle but not reenter. We found flexible redox regulation of G6PDH that could regulate the glucose-6-phosphate shunt.\n\nHighlightGlucose 6-phosphate stimulates glucose-6-phosphate dehydrogenase. This enzyme is less active during the day but retains significant activity that is very sensitive to the concentration of glucose 6-phopshate.

biochemistry↗

bioRxiv2018

Rapid covalent-probe discovery by electrophile fragment screening

Covalent probes can display unmatched potency, selectivity and duration of action, however, their discovery is challenging. In principle, fragments that can irreversibly bind their target can overcome the low affinity that limits reversible fragment screening. Such electrophilic fragments were considered non-selective and were rarely screened. We hypothesized that mild electrophiles might overcome the selectivity challenge, and constructed a library of 993 mildly electrophilic fragments. We characterized this library by a new high-throughput thiol-reactivity assay and screened them against ten cysteine-containing proteins. Highly reactive and promiscuous fragments were rare and could be easily eliminated. By contrast, we found selective hits for most targets. Combination with high-throughput crystallography allowed rapid progression to potent and selective probes for two enzymes, the deubiquitinase OTUB2, and the pyrophosphatase NUDT7. No inhibitors were previously known for either. This study highlights the potential of electrophile fragment screening as a practical and efficient tool for covalent ligand discovery.

biochemistry↗

bioRxiv2018

Dissection of protonation sites for antibacterial recognition and transport in QacA, a multi-drug efflux transporter.

QacA is a drug:H+ antiporter (DHA2) with 14 transmembrane helices, that renders antibacterial resistance to methicillin-resistant Staphylococcus aureus (MRSA) strains, with homologues in other pathogenic organisms. It is a highly promiscuous antiporter, capable of H+- driven efflux of a wide array of cationic antibacterial compounds and dyes. Our study, using a homology model of QacA, reveals a group of six protonatable residues in its vestibule. Systematic mutagenesis resulted in identification of D34 (TM1), and a cluster of acidic residues in TM13 including E407 and D411 and D323 in TM10, as being crucial for substrate recognition and transport of monovalent and divalent cationic antibacterial compounds. The transport and binding properties of QacA and its mutants were explored using whole cells, inside-out vesicles, substrate-induced H+ release and microscale thermophoresis. We identify two sites, D34 and D411 as vital players in substrate recognition while E407 facilitates substrate efflux as a protonation site. We also observe that E407 plays a moonlighting role as a substrate recognition site for dequalinium transport. These observations rationalize the promiscuity of QacA for diverse substrates. The study unravels the role of acidic residues in QacA with implications for substrate recognition, promiscuity and processive transport in multidrug efflux transporters, related to QacA.\n\nHighlightsO_LIA homology model of QacA with 14 TM helices was used to test the importance of acidic residues within the vestibule.\nC_LIO_LIMono and Divalent cationic substrate recognition requires two sites D34 (TM1) and D411 (TM13).\nC_LIO_LIE407 (TM13) is important for protonation driven efflux.\nC_LIO_LISubstrate recognition of a divalent substrate, dequalinium occurs at E407 instead of D411 providing glimpses into the promiscuity of substrate recognition in QacA.\nC_LI

biochemistry↗

bioRxiv2018

Mitochondria-Associated Membranes (MAMs) are involved in Bax mitochondrial localization and cytochrome c release

The distribution of the pro-apoptotic protein Bax in the outer mitochondrial membrane (OMM) is a central point of regulation of apoptosis. It is now widely recognized that parts of the endoplasmic reticulum (ER) are closely associated to the OMM, and are actively involved in different signalling processes. We adressed a possible role of these domains, called Mitochondria-Associated Membranes (MAMs) in Bax localization and fonction, by expressing the human protein in a yeast mutant deleted of MDM34, a ERMES component (ER-Mitochondria Encounter Structure). By affecting MAMs stability, the deletion of MDM34 altered Bax mitochondrial localization, and decreased its capacity to release cytochrome c. Furthermore, the deletion of MDM34 decreased the size of an uncompletely released, MAMs-associated pool of cytochrome c.

biochemistry↗

bioRxiv2018

Electrophilic PROTACs that degrade nuclear proteins by engaging DCAF16

Ligand-dependent protein degradation has emerged as a compelling strategy to pharmacologically control the protein content of cells. So far, only a limited number of E3 ligases have been found to support this process. Here, we use a chemical proteomic strategy to discover that DCAF16 - a poorly characterized substrate recognition component of CUL4-DDB1 E3 ubiquitin ligases - promotes nuclear-restricted protein degradation upon modification by cysteine-directed heterobifunctional electrophilic compounds.

biochemistry↗

bioRxiv2018

Bidirectional control of mitochondrial respiration and permeability transition pore by nitric oxide donors and L-arginine. Implication of mitochondrial NO/cGMP/PKG - signaling system

The role of mitochondrial calcium-dependent NO synthase in the control of respiration and mitochondrial permeability transition pore (MPTP) opening, as well as possible involvement of mitochondrial NO synthase/guanylate cyclase/kinase G-signaling system (mtNOS-SS) in the regulation of these processes are not sufficiently studied. In this work, using rat liver mitochondria, we applied specific inhibitors of the enzymes of this signaling system to evaluate its role in the control of respiration and MPTP. The respiration was supported by pyruvate and glutamate or succinate in the presence of hexokinase, glucose and ADP. The results indicate that L-arginine and NO donors SNP and SNAP produce bidirectional concentration-dependent effects on the respiration and MPTP opening evoked by calcium ions or D,L-palmitoylcarnitine. Maximal activation of respiration was observed at 20 {micro}M of L-arginine or SNP. At low concentrations, L-arginine (to 500 {micro}M) and NO donors (to 50 {micro}M) increased the threshold concentrations of calcium and D,L-palmitoylcarnitine required for the dissipation of the mitochondrial membrane potential and pore opening. The application of the inhibitors of NO synthase, guanylate cyclase, and kinase G eliminated both effects. These data indicate the involvement of mtNOS-SS in the activation of respiration and deceleration of MPTP opening. At high concentrations, L-arginine and NO donors inhibited the respiration and promoted pore opening, indicating that the inhibition induced by NO excess dominates over the protection caused by mtNOS-SS. These results demonstrate that the functioning of mtNOS-SS might provide a feedforward activation of respiration and a lowering of MPTP sensitivity to calcium and palmitoylcarnitine overload.

biochemistry↗

bioRxiv2018

MxB restricts HIV-1 by targeting the tri-hexamer interface of the viral capsid

Myxovirus resistance protein B (MxB) is an interferon-inducible restriction factor of HIV-1 that blocks nuclear import of the viral genome. Evidence suggests that MxB recognizes higher-order interfaces of the HIV capsid lattice, but the mechanistic details of this interaction are not known. Previous studies have mapped the restriction activity of MxB to its N-terminus encompassing a triple arginine motif 11RRR13. Here we demonstrate a direct and specific interaction between the MxB N-terminus and helical assemblies of HIV-1 capsid protein (CA) using highly purified recombinant proteins. We performed thorough mutagenesis to establish the detailed molecular requirements for the CA interaction with MxB. The results map MxB binding to the interface of three CA hexamers, specifically interactions between positively charged MxB N-terminal residues and negatively charged CA residues. Our crystal structures show that the CA mutations affecting MxB interaction and restriction do not alter the conformation of capsid assembly. In addition, 30 microsecond long all-atom molecular dynamics (MD) simulations of the complex between the MxB N-terminus and the HIV CA tri-hexamer interface show persistent MxB binding and identify a MxB-binding pocket surrounded by three CA hexamers. These results establish the molecular details of the binding of a lattice-sensing host factor onto HIV capsid, and provide insight into how MxB recognizes HIV capsid for the restriction of HIV-1 infection.\n\nAuthor summaryThe human antiviral protein MxB is a restriction factor that fights HIV infection. Previous experiments have demonstrated that MxB targets the HIV capsid, a protein shell that protects the viral genome. To make the conical shaped capsid, HIV CA proteins are organized into a lattice composed of hexamer and pentamer building blocks, providing many interfaces for host proteins to recognize. Through extensive biochemical and biophysical studies and molecular dynamics simulations, we show that MxB is targeting the HIV capsid by recognizing the region created at the intersection of three CA hexamers. We are further able to map this interaction to a few CA residues, located in a negatively-charged well at the interface between the three CA hexamers. This work provides detailed residue-level mapping of the targeted capsid interface and how MxB interacts. This information could inspire the development of capsid-targeting therapies for HIV.

biochemistry↗

bioRxiv2018

Tightly-orchestrated rearrangements govern catalytic center assembly of the ribosome

The catalytic activity of the ribosome is mediated by RNA, yet proteins are essential for the function of the peptidyl transferase center (PTC). In eukaryotes, final assembly of the PTC occurs in the cytoplasm by insertion of the ribosomal protein Rpl10. We determine structures of six intermediates in late nuclear and cytoplasmic maturation of the large subunit that reveal a tightly-choreographed sequence of protein and RNA rearrangements controlling the insertion of Rpl10. We also determine the structure of the biogenesis factor Yvh1 and show how it promotes assembly of the P stalk, a critical element for recruitment of GTPases that drive translation. Together, our structures provide a blueprint for final assembly of a functional ribosome.\n\nOne Sentence SummaryCryo-EM structures of six novel intermediates in the assembly of the large ribosomal subunit reveal mechanism of creating the catalytic center.

biochemistry↗

bioRxiv2018

DNA replication initiation in Bacillus subtilis; Structural and functional characterisation of the essential DnaA-DnaD interaction

The homotetrameric DnaD protein is essential in low G+C content gram positive bacteria and is involved in replication initiation at oriC and re-start of collapsed replication forks. It interacts with the ubiquitously conserved bacterial master replication initiation protein DnaA at the oriC but structural and functional details of this interaction are lacking, thus contributing to our incomplete understanding of the molecular details that underpin replication initiation in bacteria. DnaD comprises N-terminal (DDBH1) and C-terminal (DDBH2) domains, with contradicting bacterial two-hybrid and yeast two-hybrid studies suggesting that either the former or the latter interact with DnaA, respectively. Using Nuclear Magnetic Resonance (NMR) we show that both DDBH1 and DDBH2 interact with the N-terminal domain I of DnaA and studied the DDBH2 interaction in structural detail by NMR. We revealed two families of conformations for the DDBH2-DnaA domain I complex and showed that the DnaA-interaction patch of DnaD is distinct from the DNA-interaction patch, suggesting that DnaD can bind simultaneously DNA and DnaA. Using sensitive single-molecule FRET techniques we revealed that DnaD remodels DnaA-DNA filaments consistent with stretching and/or untwisting. Furthermore, the DNA binding activity of DnaD is redundant for this filament remodelling. This in turn suggests that DnaA and DnaD are working collaboratively in the oriC to locally melt the DNA duplex during replication initiation.

biochemistry↗

bioRxiv2018

Cryo-EM structure of cardiac amyloid fibrils from an immunoglobulin light chain (AL) amyloidosis patient

Systemic light chain (AL) amyloidosis is a life-threatening disease caused by aggregation and deposition of monoclonal immunoglobulin light chains (LC) in target organs. Severity of heart involvement is the most important factor determining prognosis. Here, we report the 4.0 [A] resolution cryo-electron microscopy (cryo-EM) map and structural model of amyloid fibrils extracted from the heart of an AL patient affected by severe amyloid cardiomyopathy. The fibrils are composed of one asymmetric protofilament, showing typical 4.9 [A] stacking and parallel cross-{beta} architecture. Two distinct polypeptide stretches belonging to the LC variable domain (Vl) could be modelled in the density (total of 77 residues), stressing the role of the Vl domain in fibril assembly and LC aggregation. Despite high levels of Vl sequence variability, residues stabilising the observed fibril core are conserved through several Vl domains, highlighting structural motifs that may be common to misfolded LCs. Our data shed first light on the architecture of life-threatening LC amyloid deposits, and provide a rationale for correlating LC amino acid sequences and fibril structures.

biochemistry↗

bioRxiv2018

The intrinsic and regulated proteomes of barley seeds in response to fungal infection

Barley is an important cereal grain used for beer brewing, animal feed, and human food consumption. Fungal disease can impact barley production, as it causes substantial yield loss and lowers seed quality. We used sequential window acquisition of all theoretical ions mass spectrometry (SWATH-MS) to measure and quantify the relative abundance of proteins within seeds of different barley varieties under various fungal pathogen burdens. Fungal burden in the leaves and stems of barley resulted in changes to the seed proteome. However, these changes were minimal and showed substantial variation among barley samples infected with different pathogens. The limited effect of intrinsic disease resistance on the seed proteome is consistent with the main mediators of disease resistance being present in the leaves and stems of the plant. The seeds of barley varieties accredited for use as malt had higher levels of proteins associated with starch synthesis and beer quality. The proteomic workflows developed and implemented here have potential application in quality control, breeding and processing of barley, and other agricultural products.

biochemistry↗

bioRxiv2018

The structural basis of lipid scrambling and inactivation in the endoplasmic reticulum scramblase TMEM16K

Membranes in cells have defined distributions of lipids in each leaflet, controlled by lipid scramblases and flip/floppases. However, for some intracellular membranes such as the endoplasmic reticulum the scramblases have not been identified. Members of the TMEM16 family have either lipid scramblase and ion channel activity, or specific chloride channel activity. Although TMEM16K is widely distributed and associated with the neurological disorder autosomal recessive spinocerebellar ataxia type 10 (SCAR10), its location in cells, function and structure are largely uncharacterised. Here we show that TMEM16K is an ER-resident calcium-regulated lipid scramblase. Our crystal structures of TMEM16K show a scramblase fold, with an open lipid transporting groove. Additional structures solved by cryo-EM reveal extensive conformational changes extending from the cytoplasmic to the ER side of the membrane, giving a state with a closed lipid permeation pathway. Molecular dynamics simulations showed that the open-groove conformation is necessary for scramblase activity. Our results suggest mechanisms by which missense variants of TMEM16K could cause SCAR10 ataxia, providing new hypotheses to explore for therapy.

biochemistry↗

bioRxiv2018

PBiochemical and physiological investigations on adenosine 5’ monophosphate deaminase from Plasmodium spp

AbbreviationsHGXPRT - Hypoxanthine-guanine-xanthine phosphoribosyltransferase, ADSS - Adenylosuccinate synthetase, ASL - Adenylosuccinate lyase, GMPS - Guonosine monophosphate synthetase, IMPDH - Inosine monophosphate dehydrogenase, ISN1 - Inosine monophosphate specific nucleotidase, PNP - Purine nucleoside phosphorylase\n\nSummaryInterplay between ATP generating and utilizing pathways in a cell is responsible for maintaining cellular ATP/energy homeostasis that is reflected by Adenylate Energy Charge (AEC) ratio. Adenylate kinase (AK), that catalyzes inter-conversion of ADP, ATP and AMP, plays a major role in maintaining AEC, and is regulated by cellular AMP levels. Hence, the enzymes AMP deaminase (AMPD) and nucleotidases, which catabolize AMP, indirectly regulate AK activity and in-turn affect AEC. Here, we present the first report on AMPD from Plasmodium, the causative agent of malaria. The recombinant enzyme expressed in Saccharomyces cerevisiae was studied using functional complementation assay and residues vital for enzyme activity have been identified. Similarities and differences between Plasmodium falciparum AMPD (PfAMPD) and its homologs from yeast, Arabidopsis and humans are also discussed. The AMPD gene was deleted in the murine malaria parasite P. berghei and was found to be non-essential for intra-erythrocytic growth of the knockout parasites. However, when episomal expression was attempted, viable parasites were not obtained, suggesting that perturbing AMP homeostasis by over-expressing AMPD might be lethal. As AMPD is known to be allosterically modulated by ATP, GTP and phosphate, allosteric activators of PfAMPD could be developed as anti-parasitic agents.

biochemistry↗

bioRxiv2018

Protein aggregation capture on microparticles enables multi-purpose proteomics sample preparation

Universal proteomics sample preparation is challenging due to the high heterogeneity of biological samples. Here we describe a novel mechanism that exploits the inherent instability of denatured proteins for non-specific immobilization on microparticles by protein aggregation capture. To demonstrate the general applicability of this mechanism, we analyzed phosphoproteomes, tissue proteomes, and interaction proteomes as well as dilute secretomes. The findings presents a practical, sensitive and cost-effective proteomics sample preparation method.

biochemistry↗

bioRxiv2018

Proteomics reveals profound metabolic changes in the alcohol use disorder brain

Changes in brain metabolism are a hallmark of Alcohol Use Disorder (AUD). Determining how AUD changes the brain proteome is critical for understanding the effects of alcohol consumption on biochemical processes in the brain. We used data-independent acquisition mass spectrometry proteomics to study differences in the abundance of proteins associated with AUD in pre-frontal lobe and motor cortex from autopsy brain. AUD had a substantial effect on the overall brain proteome exceeding the inherent differences between brain regions. Proteins associated with glycolysis, trafficking, the cytoskeleton, and excitotoxicity were altered in abundance in AUD. We observed extensive changes in the abundance of key metabolic enzymes, consistent with a switch from glucose to acetate utilization in the AUD brain. We propose that metabolic adaptations allowing efficient acetate utilization contribute to ethanol dependence in AUD.

biochemistry↗

bioRxiv2018

LRRK2 exonic variants associated with Parkinson’s disease augment phosphorylation levels for LRRK2-Ser1292 and Rab10-Thr73

Leucine-rich repeat kinase 2 (LRRK2) is associated to Parkinsons disease (PD). The most common form of LRRK2 PD is caused by the G2019S variant. Besides G2019S, eight other LRRK2 variants causing familial PD also have amino acid substitutions located in a LRRK2 enzymatic domainsuggesting that enzymatic activity is at the core of mechanisms underlying disease risk. Common LRRK2 polymorphic risk variations such as G2385R, A419V, R1628 and M1646T all reside in other LRRK2 domains. Prior knowledge is limited on how these variants influence LRRK2 function. To investigate the impact on enzymatic function of both rare and common LRRK2 variation a comprehensive profiling of nineteen LRRK2 exonic variants was pursued. Six LRRK2 phosphorylation sites were identified by mass spectrometry. Besides already known phosphorylation sites such as Ser910, Ser935, Ser955, Ser973 and Ser1292 also Thr826 was confirmed by a targeted MRM assay as a LRRK2 phosphorylation site in mammalian cells. Phosphorylation site occupancy for all six LRRK2 sites was obtained but no obvious correlation to risk of disease was found. Instead, application of phospho-specific antibodies targeting LRRK2 phosphorylation sites confirmed that autophosphorylation at Ser1292 was significantly increased for all disease-causing variants whereas no significant differences could be observed for the common intermediate risk variants. Recently, Rab10 and Rab12 have been shown to be bona fide LRRK2 substrates and we find that both rare and common LRRK2 exonic variants augment the phosphorylation of Rab10. This was not observed with Rab12. Furthermore, the protective variant N551K has reduced Rab10 phosphorylation compared to LRRK2 WT. This was not observed with the protective variant R1398H. Our findings support the hypothesis that increased LRRK2 kinase function is associated with increased PD risk but also highlights the need for more sensitive tools for detection of increases in kinase activity in carriers of LRRK2 PD risk variants.\n\nAbbreviations

biochemistry↗

bioRxiv2018

The three dimensional structure of Bovine Salivary Protein 30b (BSP30b) and its interaction with specific rumen bacteria

Bovine Salivary Protein 30b (BSP30b) is a member of the tubular lipid-binding (TULIP) superfamily that includes the human bactericidal/permeability-increasing proteins (BPI), lipopolysaccharide binding proteins (LBP) and palate, lung, and nasal epithelium carcinoma-21 associated proteins (PLUNC). BSP30b is most closely related to the PLUNC family and is predominantly found in bovine saliva. There are four BSP30 isoforms (BSP30a-d) and collectively, they are the most abundant protein component of bovine saliva. The PLUNC family members are proposed to be lipid binding proteins, although in most cases their lipid ligands are unknown. Here, we present the X-ray crystal structure of BSP30b at 2.0 [A] resolution. We used a double methionine mutant and Se-Met SAD phasing to solve the structure. The structure adopts a curved cylindrical form with a hydrophobic channel formed by an /{beta} wrap, which is consistent with the TULIP superfamily. The structure of BSP30b in complex with oleic acid is also presented where the ligand is accommodated within the hydrophobic channel. The electron density for oleic acid suggests that the ligand is only partially occupied in the binding site implying that oleic acid may not be the preferred ligand. GFP-tagged BSP30b binds to the surface of olive oil droplets, as observed under fluorescent microscopy, and acts as a surfactant consistent with its association with decreased susceptibility to bloat in cattle. Bacteria extracted directly from bovine rumen contents indicate that the GFP_BSP30b fusion protein binds to a small number of selected bacterial species in vivo. These results suggest that BSP30b may bind to bacterial lipids from specific species and that this abundant protein may have important biological roles via interacting with rumen bacteria during feeding and rumination.

biochemistry↗

bioRxiv2018

Macrophage migration inhibitory factor (MIF) of Syrian golden hamster (Mesocricetus auratus) has similar structure and function as human MIF and promotes pancreatic tumor growth in vivo

Macrophage migration inhibitory factor (MIF) is a pleiotropic cytokine that increasingly is being studied in cancers and inflammatory diseases. Though murine models have been instrumental in understanding the functional role of MIF in different pathological conditions, the information obtained from these models is biased towards a specific species. In experimental science, results obtained from multiple clinically relevant animal models always provide convincing data that might recapitulate in humans. Syrian golden hamster (Mesocricetus auratus), is a clinically relevant animal model for multiple human diseases. Hence, the major objectives of this study were to characterize structure and function of hamster MIF, and finally evaluate its effect on pancreatic tumor growth in vivo. Initially, the recombinant hamster MIF (rha-MIF) was cloned, expressed and purified in bacterial expression system. The rha-MIF primary sequence, biochemical properties and crystal structure analysis showed a greater similarity with human MIF. The crystal structure of hamster MIF illustrates that it forms a homotrimer as known in human and mouse. However, hamster MIF exhibits some minor structural variations when compared to human and mouse MIF. The in vitro functional studies show that rha-MIF has tautomerase activity and enhances activation and migration of hamster peripheral blood mononuclear cells (PBMCs). Interestingly, injection of rha-MIF into HapT1 pancreatic tumor bearing hamsters significantly enhanced the tumor growth and tumor associated angiogenesis. Together, the current study shows a structural and functional similarity between hamster and human MIF. Moreover, it has demonstrated that a high-level of circulating MIF originating from non-tumor cells might also promote pancreatic tumor growth in vivo.

biochemistry↗