Differential action on coregulator interaction defines inverse retinoid agonists and neutral antagonists

Retinoic acid receptors (RARs) are ligand-dependent transcription factors that control a plethora of physiological processes. RARs exert their functions by regulating gene networks controlling cell growth, differentiation, survival, and death. Uncovering the molecular details by which synthetic ligands direct specificity and functionality of nuclear receptors is key to rational drug development. Here we define the molecular basis for (E)-4-[2-[5,6-Dihydro-5,5-dimethyl-8-(2-phenylethynyl)naphthalen-2-yl]ethen-1-yl]benzoic acid (BMS204,493) acting as the inverse pan-RAR agonist and define 4-[5,6-Dihydro-5,5-dimethyl-8-(quinolin-3-yl)naphthalen-2-carboxamido]benzoic acid (BMS195,614) as the neutral RARalpha-selective antagonist. We reveal the details of the differential coregulator interactions imposed on the receptor by the ligands and show that the anchoring of H12 is fundamentally distinct in the presence of the two ligands, thus accounting for the observed effects on coactivator and corepressor interactions. These ligands will facilitate studies on the role of the constitutive activity of RARs, particularly of the tumor suppressor RARbeta, whose specific functions relative to other RARs have remained elusive.

Detection and structural features of the betaB2-B3-crystallin heterodimer by radical probe mass spectrometry (RP-MS)

The predilection of the beta-crystallin B2 subunit to interact with the betaB3 subunit rather than self associate is evident by the detection of the betaB2-B3-crystallin heterodimer by native gel electrophoresis and electrospray ionisation time-of-flight (ESI-TOF) mass spectrometry under non denaturing conditions. The complex has been detected for the first time and its molecular mass is measured to be 47,450 +/- 1 Da. Radical probe mass spectrometry (RP-MS) was subsequently applied to investigate the nature of the heterodimer through the limited oxidation of the subunits in the complex. Two peptide segments of the betaB2 subunit and six of the betaB3 subunit were found to oxidise, with far greater oxidation observed within the betaB3 versus the betaB2 subunit. This, and the observation that the oxidation data of betaB2 subunit is inconsistent with the structure of the betaB2 monomer, demonstrates that the protection of betaB2 is conferred by its association with betaB3 subunit within the heterodimer where only the residues of, and towards, its N-terminal domain remain exposed to solvent. The results suggest that the betaB2 subunit adopts a more compacted form than in its monomeric form in order for much of its structure to be enveloped by the betaB3 subunit within the heterodimer.

An optimized MALDI mass spectrometry method for improved detection of lysine/arginine/histidine free peptides

Transcription factors and their regulators possess "basic amino acid free domains" which modulate transcriptional gene activation. We aimed at optimizing a MALDI mass spectrometry (MS) analytical method for the characterization of such domains after protein enzymatic digestion. A panel of recombinant transcription factors with different basic residue contents was proteolytically digested with the Asp-N endoprotease and resulting peptide mixtures were analyzed by MALDI-MS with alpha-cyano-4-hydroxy-cinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB) as matrix. We found that peptides without lysine, arginine, histidine (Lys/Arg/His free peptides) were efficiently detected in the positive ion mode only when using DHB. These findings proved to be very useful for two different targeted proteomic applications. Indeed, the MALDI-MS/MS identification of the CARM1 proteolytic cleavage site, which happens in a Lys/Arg/His free domain, could only be achieved using the DHB matrix. Moreover, in routine proteomic analyses, the detection efficiency of Lys/Arg/His free C-terminal peptides of two-dimensional gel separated proteins was strongly enhanced when DHB was used instead of CHCA.

Modulation of E-cadherin monomer folding by cooperative binding of calcium ions

Classical cadherins are transmembrane glycoproteins involved in calcium-dependent cell-cell adhesion. Calcium ions are coordinated at the interface between successive modules of the cadherin ectodomain and are thought to regulate the adhesive interactions of cadherins when present at millimolar concentrations. It is widely accepted that calcium plays a critical role in cadherin-mediated cell-cell adhesion, but the nature of cadherin-calcium binding remains a matter of debate. We investigated the parameters of noncovalent cadherin-calcium binding, using the two N-terminal modules of E-cadherin (E/EC12) with a native N-terminal end and nondenaturing electrospray ionization mass spectrometry. By directly visualizing the molecular complexes, we demonstrated that E/EC12 binds three calcium ions, with an average KD of 20 +/- 0.7 microM. These calcium ions bound cooperatively to E/EC12 in its monomeric state, and these properties were not modified by an N-terminal extension consisting of a single methionine residue. This binding induced specific structural changes, as shown by assessments of protease sensitivity, circular dichroism, and mass spectrometry. Furthermore, the D103A mutation (a residue involved in E-cadherin adhesive function) modified calcium binding and led to a loss of cooperativity and the absence of structural changes, despite calcium binding. As the amino acids involved in calcium binding are found within the cadherin consensus motif, our findings may be relevant to other members of the cadherin family.

Endogenous Plasma Peptide Detection and Identification in the Rat by a Combination of Fractionation Methods and Mass Spectrometry

Mass spectrometry-based analyses are essential tools in the field of biomarker research. However, detection and characterization of plasma low abundance and/or low molecular weight peptides is challenged by the presence of highly abundant proteins, salts and lipids. Numerous strategies have already been tested to reduce the complexity of plasma samples. The aim of this study was to enrich the low molecular weight fraction of rat plasma. To this end, we developed and compared simple protocols based on membrane filtration, solid phase extraction, and a combination of both. As assessed by UV absorbance, an albumin depletion >99% was obtained. The multistep fractionation strategy (including reverse phase HPLC) allowed detection, in a reproducible manner (CV < 30%-35%), of more than 450 peaks below 3000 Da by MALDI-TOF/MS. A MALDI-TOF/MS-determined LOD as low as 1 fmol/μL was obtained, thus allowing nanoLC-Chip/MS/MS identification of spiked peptides representing ~10–6% of total proteins, by weight. Signal peptide recovery ranged between 5%-100% according to the spiked peptide considered. Tens of peptide sequence tags from endogenous plasma peptides were also obtained and high confidence identifications of low abundance fibrinopeptide A and B are reported here to show the efficiency of the protocol. It is concluded that the fractionation protocol presented would be of particular interest for future differential (high throughput) analyses of the plasma low molecular weight fraction.

Oligomerization behavior of the archaeal Sm2-type protein from Archaeoglobus fulgidus

As part of a functional analysis of archaeal Sm-related proteins, we have studied the oligomerization behavior of the Sm-2 type protein from the euryarchaeon Archaeoglobus fulgidus using gel filtration chromatography and noncovalent mass spectrometry. Our experiments show that the oligomeric state of the protein depends on the pH and presence of RNA. The protein forms a hexamer at acidic pH in the absence of RNA. The addition of RNA (oligo U10) induces the formation of a heptamer over the whole pH range studied. The stability of both the hexamer and the RNA-bound heptamer increases at lower pH.

Monitoring ligand-mediated nuclear receptor-coregulator interactions by noncovalent mass spectrometry

Retinoid receptors are ligand-dependent transcription factors belonging to the nuclear receptor superfamily. Retinoic acid (RARalpha, beta, gamma) and retinoid X (RXRalpha, beta, gamma) receptors mediate the retinoid/rexinoid signal to the transcriptional machineries by interacting at the first level with coactivators or corepressors, which leads to the recruitment of enzymatically active noncovalent complexes at target gene promoters. It has been shown that the interaction of corepressors with nuclear receptors involves conserved LXXI/HIXXXI/L consensus sequences termed corepressor nuclear receptor (CoRNR) boxes. Here we describe the use of nondenaturing electrospray ionization mass spectrometry (ESI-MS) to determine the characteristics of CoRNR box peptide binding to the ligand binding domains of the RARalpha-RXRalpha heterodimer. The stability of the RARalpha-RXRalpha-CoRNR ternary complexes was monitored in the presence of different types of agonists or antagonists for the two receptors, including inverse agonists. These results show unambiguously the differential impact of distinct retinoids on corepressor binding. We show that ESI-MS is a powerful technique that complements classical methods and allows one to: (a) obtain direct evidence for the formation of noncovalent NR complexes; (b) determine ligand binding stoichiometries and (c) monitor ligand effects on these complexes.

Respiratory adaptations to the deep-sea hydrothermal vent environment: the case of Segonzacia mesatlantica, a crab from the Mid-Atlantic Ridge

Segonzacia mesatlantica (Crustacea; Decapoda; Brachyura) is the only endemic crab species known from the Mid-Atlantic Ridge (MAR) hydrothermal vents. Known from all explored sites in the Atlantic, its wide distribution makes this species a model to study physiological adaptation, and specifically respiratory strategies. Native haemocyanin (Hc) comprises four non-covalent associations in equilibrium formed by monomers, hexamers, dodecamers and octadecamers made up of approximately 75 kDa polypeptide chains. Four different amino acid chains are observed with a molecular mass ranging from 75,234 to 75,972 Da. Experiments carried-out under pressure suggested that the percentage of monomer increased in the haemolymph under hypoxic condition. We have also observed a shift of the proportion of the two dodecamer series, suggesting a rapid modification of the Hc phenotype between hypoxic and hyperoxic conditions. Native Hc possesses a high oxygen affinity ( P50 = 2.2 Torr at 15 degrees C and pH 7.5), a large Bohr effect (Deltalog P50 / DeltapH approximately -2.7) and a slightly reverse temperature effect (DeltaH = +17.19 kJ mol(-1). The composition of Segonzacia haemolymph is similar to that of other littoral species except for the large enrichment in free copper and zinc. As for other species from hydrothermal vent sites, Segonzacia haemolymph possesses a higher buffer capacity than littoral species. Moreover, species from the hydrothermal vent decapods from Pacific hydrothermal vent that encounter higher CO2 content in their environment have a higher buffer capacity than Atlantic vent species. The results presented are discussed in relation with the physico-chemical characteristics of the hydrothermal vent environment.

All-trans retinoic acid is a ligand for the orphan nuclear receptor ROR beta

Retinoids regulate gene expression through binding to the nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). In contrast, no ligands for the retinoic acid receptor-related orphan receptors beta and gamma (ROR beta and gamma) have been identified, yet structural data and structure-function analyses indicate that ROR beta is a ligand-regulated nuclear receptor. Using nondenaturing mass spectrometry and scintillation proximity assays we found that all-trans retinoic acid (ATRA) and several retinoids bind to the ROR beta ligand-binding domain (LBD). The crystal structures of the complex with ATRA and with the synthetic analog ALRT 1550 reveal the binding modes of these ligands. ATRA and related retinoids inhibit ROR beta but not ROR alpha transcriptional activity suggesting that high-affinity, subtype-specific ligands could be designed for the identification of ROR beta target genes. Our results identify ROR beta as a retinoid-regulated nuclear receptor, providing a novel pathway for retinoid action.

Unfolding of rabbit muscle creatine kinase induced by acid. A study using electrospray ionization mass spectrometry, isothermal titration calorimetry, and fluorescence spectroscopy

Electrospray ionization mass spectrometry, isothermal titration calorimetry (ITC), fluorescence spectroscopy, and glutaraldehyde cross-linking SDS-PAGE have been used to study the unfolding of rabbit muscle creatine kinase (MM-CK) induced by acid. The mass spectrometric experiments show that MM-CK is unfolded gradually when titrated with acid. MM-CK is a dimer (the native state) at pH 7.0 and becomes an equilibrium mixture of the dimer and a partially folded monomer (the intermediate) between pH 6.7 and 5.0. The dimeric protein becomes an equilibrium mixture of the intermediate and an unfolded monomer (the unfolded state) between pH 5.0 and 3.0 and is almost fully unfolded at pH 3.0 reached. The results from a "phase diagram" method of fluorescence show that the conformational transition between the native state and the intermediate of MM-CK occurs in the pH range of 7.0-5.2, and the transition between the intermediate and the unfolded state of the protein occurs between pH 5.2 and 3.0. The intrinsic molar enthalpy changes for formation of the unfolded state of MM-CK induced by acid at 15.0, 25.0, 30.0, and 37.0 degrees C have been determined by ITC. A large positive molar heat capacity change of the unfolding, 8.78 kcal mol-1 K-1, at all temperatures examined indicates that hydrophobic interaction is the dominant driving force stabilizing the native structure of MM-CK. Combining the results from these four methods, we conclude that the acid-induced unfolding of MM-CK follows a "three-state" model and that the intermediate state of the protein is a partially folded monomer.

Comparative ESI-MS study of approximately 2.2 MDa native hemocyanins from deep-sea and shore crabs: from protein oligomeric state to biotope

In the past years, the potential of electrospray ionization mass spectrometry (ESI-MS) for the observation of intact weak interactions, such as non-covalent protein-ligand, protein-protein, protein-DNA complexes, has spread out. The coupling of ESI with time-of-flight (TOF) and quadrupole-time-of-flight (Q-TOF) analyzers has even enabled the detection of larger complexes with molecular weights greatly higher than 200 kDa. In this paper, we report a comparative ESI-MS study on the protein quaternary structure of native hemocyanins (Hc) from crabs living in different biotopes: a shore crab (Carcinus maenas) and two deep-sea crabs (Segonzacia mesatlantica and Bythograea thermydron). Hc is an extracellular blood protein, composed of several protein chains which can associate in large multimers. The goal of this study is to point out that the oligomerization state of native Hcs is biotope-dependent. Depending on the crab, ESI-MS analyses under non-denaturing conditions reveal different oligomeric forms present in equilibrium in solution. Molecular weights up to 2,235 kDa were measured for the associations of 30 subunits of the Bythograea thermydron Hc. Thanks to ESI-MS analyses, it could be concluded for the first time that the oligomerization state of native Hcs is dependent on the crab environment. The investigation of these different non-covalent self-assemblies is very important for the life history of crabs, since they are directly related with different oxygen binding abilities and thus, with their ability to colonize habitats with different oxygen contents.

Properties and regulation of the bifunctional enzyme HPr kinase/phosphatase in Bacillus subtilis

The bifunctional allosteric enzyme HPr kinase/phosphatase (HPrK/P) from Bacillus subtilis is a key enzyme in the main mechanism of carbon catabolite repression/activation (i.e. a means for the bacteria to adapt rapidly to environmental changes in carbon sources). In this regulation system, the enzyme can phosphorylate and dephosphorylate two proteins, HPr/HPr(Ser(P)) and Crh/Crh(Ser(P)), sensing the metabolic state of the cell. To acquire further insight into the properties of HPrK/P, electrospray ionization mass spectrometry, dynamic light scattering, and BIACORE were used to determine the oligomeric state of the protein under native conditions, revealing that the enzyme exists as a hexamer at pH 6.8 and as a monomer and dimer at pH 9.5. Using an in vitro radioactive assay, the influence of divalent cations, pH, temperature, and different glycolytic intermediates on the activity as well as kinetic parameters were investigated. The presence of divalent cations was found to be essential for both opposing activities of the enzyme. Furthermore, pH values equal to the internal pH of vegetative cells seem to favor the kinase activity, whereas lower pH values increased the phosphatase activity. Among the glycolytic intermediates evaluated, fructose 1,6-diphosphate and fructose 2,6-diphosphate were found to be allosteric activators in the kinase assay, whereas high concentrations inhibited the phosphatase activity, except for fructose 1,6-diphosphate in the case of HPr(Ser(P)). Phosphatase activity was induced by inorganic phosphate as well as acetyl phosphate and glyceraldehyde 3-phosphate. Kinetic parameters indicate a preference for binding of HPr compared with Crh to the enzyme and supported a strong positive cooperativity. This work suggests that the oligomeric state of the enzyme is influenced by several effectors and is correlated to the kinase or phosphatase activity. The phosphatase activity is mainly supported by the hexameric form.

Structural and functional evidence for ligand-independent transcriptional activation by the estrogen-related receptor 3

The crystal structure of the ligand binding domain (LBD) of the estrogen-related receptor 3 (ERR3) complexed with a steroid receptor coactivator-1 (SRC-1) peptide reveals a transcriptionally active conformation in absence of any ligand. The structure explains why estradiol does not bind ERRs with significant affinity. Docking of the previously reported ERR antagonists, diethylstilbestrol and 4-hydroxytamoxifen, requires structural rearrangements enlarging the ligand binding pocket that can only be accommodated with an antagonist LBD conformation. Mutant receptors in which the ligand binding cavity is filled up by bulkier side chains still interact with SRC-1 in vitro and are transcriptionally active in vivo, but are no longer efficiently inactivated by diethylstilbestrol or 4-hydroxytamoxifen. These results provide structural and functional evidence for ligand-independent transcriptional activation by ERR3.

Biochemical characterization of the helper component of Cauliflower mosaic virus

The helper component of Cauliflower mosaic virus is encoded by viral gene II. This protein (P2) is dispensable for virus replication but required for aphid transmission. The purification of P2 has never been reported, and hence its biochemical properties are largely unknown. We produced the P2 protein via a recombinant baculovirus with a His tag fused at the N terminus. The fusion protein was purified by affinity chromatography in a soluble and biologically active form. Matrix-assisted laser desorption time-of-flight mass spectrometry demonstrated that P2 is not posttranslationally modified. UV circular dichroism revealed the secondary structure of P2 to be 23% alpha-helical. Most alpha-helices are suggested to be located in the C-terminal domain. Using size exclusion chromatography and aphid transmission testing, we established that the active form of P2 assembles as a huge soluble oligomer containing 200 to 300 subunits. We further showed that P2 can also polymerize as long paracrystalline filaments. We mapped P2 domains involved in P2 self-interaction, presumably through coiled-coil structures, one of which is proposed to form a parallel trimer. These regions have previously been reported to also interact with viral P3, another protein involved in aphid transmission. Possible interference between the two types of interaction is discussed with regard to the biological activity of P2.

Mass spectrometry as a novel approach to probe cooperativity in multimeric enzymatic systems

Investigating cooperativity in multimeric enzymes is of utmost interest to improve our understanding of the mechanism of enzymatic regulation. In the present article, we propose a novel approach based on mass spectrometry to probe cooperativity in the binding of a ligand to a multisubunit enzyme. This approach presents the selective advantage of giving a direct insight into all the subsequent ligation states that are formed in solution as the ligand is added to the enzyme. A quantitative interpretation of the electrospray ionization (ESI) mass spectra gives the relative abundance of all the distinct enzymatic species, which allows one to directly deduce the cooperativity of the system. The overall method is described for the addition of the oxidized cofactor nicotinamide adenine dinucleotide (NAD(+)) to a dimeric mutant of Bacillus stearothermophilus glyceraldehyde-3-phosphate dehydrogenase (GPDH). It is then applied to four tetrameric enzymes: sturgeon muscle GPDH, wild type and S48G mutant of GPDH from B. stearothermophilus, and alcohol dehydrogenase (ADH) from Bakers yeast. The results illustrate the possibilities offered by this new technique. First, mass spectrometry allows a control of the enzymes before the addition of NAD(+). Second, the cooperative behavior can be drawn from one single ESI mass spectrum, which makes the method highly attractive in terms of the amount of biological material required. Above all, the major benefit lies in the direct visualization of all the enzymatic species that are in equilibrium in solution. The direct measurement of cooperativity readily resolve the inconvenience of the classical approaches employed in this field, which all need to model the experimental data in order to get the cooperative behavior of the system.