Defining the mechanism of galectin-3-mediated TGF-ß1 activation and its role in lung fibrosis.

Integrin-mediated activation of the profibrotic mediator transforming growth factor-ß1 (TGF-ß1), plays a critical role in idiopathic pulmonary fibrosis (IPF) pathogenesis. Galectin-3 is believed to contribute to the pathological wound healing seen in IPF, although its mechanism of action is not precisely defined. We hypothesized that galectin-3 potentiates TGF-ß1 activation and/or signaling in the lung to promote fibrogenesis. We show that galectin-3 induces TGF-ß1 activation in human lung fibroblasts (HLFs) and specifically that extracellular galectin-3 promotes oleoyl-L-α-lysophosphatidic acid sodium salt-induced integrin-mediated TGF-ß1 activation. Surface plasmon resonance analysis confirmed that galectin-3 binds to αv integrins, αvß1, αvß5, and αvß6, and to the TGFßRII subunit in a glycosylation-dependent manner. This binding is heterogeneous and not a 1:1 binding stoichiometry. Binding interactions were blocked by small molecule inhibitors of galectin-3, which target the carbohydrate recognition domain. Galectin-3 binding to ß1 integrin was validated in vitro by coimmunoprecipitation in HLFs. Proximity ligation assays indicated that galectin-3 and ß1 integrin colocalize closely (≤40 nm) on the cell surface and that colocalization is increased by TGF-ß1 treatment and blocked by galectin-3 inhibitors. In the absence of TGF-ß1 stimulation, colocalization was detectable only in HLFs from IPF patients, suggesting the proteins are inherently more closely associated in the disease state. Galectin-3 inhibitor treatment of precision cut lung slices from IPF patients' reduced Col1a1, TIMP1, and hyaluronan secretion to a similar degree as TGF-ß type I receptor inhibitor. These data suggest that galectin-3 promotes TGF-ß1 signaling and may induce fibrogenesis by interacting directly with components of the TGF-ß1 signaling cascade.

The adaptability of the ion-binding site by the Ag(I)/Cu(I) periplasmic chaperone SilF.

The periplasmic chaperone SilF has been identified as part of an Ag(I) detoxification system in Gram-negative bacteria. Sil proteins also bind Cu(I) but with reported weaker affinity, therefore leading to the designation of a specific detoxification system for Ag(I). Using isothermal titration calorimetry, we show that binding of both ions is not only tighter than previously thought but of very similar affinities. We investigated the structural origins of ion binding using molecular dynamics and QM/MM simulations underpinned by structural and biophysical experiments. The results of this analysis showed that the binding site adapts to accommodate either ion, with key interactions with the solvent in the case of Cu(I). The implications of this are that Gram-negative bacteria do not appear to have evolved a specific Ag(I) efflux system but take advantage of the existing Cu(I) detoxification system. Therefore, there are consequences for how we define a particular metal resistance mechanism and understand its evolution in the environment.

Structure-based development of new RAS-effector inhibitors from a combination of active and inactive RAS-binding compounds.

The RAS gene family is frequently mutated in human cancers, and the quest for compounds that bind to mutant RAS remains a major goal, as it also does for inhibitors of protein-protein interactions. We have refined crystallization conditions for KRAS169Q61H-yielding crystals suitable for soaking with compounds and exploited this to assess new RAS-binding compounds selected by screening a protein-protein interaction-focused compound library using surface plasmon resonance. Two compounds, referred to as PPIN-1 and PPIN-2, with related structures from 30 initial RAS binders showed binding to a pocket where compounds had been previously developed, including RAS effector protein-protein interaction inhibitors selected using an intracellular antibody fragment (called Abd compounds). Unlike the Abd series of RAS binders, PPIN-1 and PPIN-2 compounds were not competed by the inhibitory anti-RAS intracellular antibody fragment and did not show any RAS-effector inhibition properties. By fusing the common, anchoring part from the two new compounds with the inhibitory substituents of the Abd series, we have created a set of compounds that inhibit RAS-effector interactions with increased potency. These fused compounds add to the growing catalog of RAS protein-protein inhibitors and show that building a chemical series by crossing over two chemical series is a strategy to create RAS-binding small molecules.

Aerobic Photocatalytic H2 Production by a [NiFe] Hydrogenase Engineered to Place a Silver Nanocluster in the Electron Relay.

Hydrogenase-1 (Hyd-1) from E. coli poses a conundrum regarding the properties of electrocatalytic reversibility and associated bidirectionality now established for many redox enzymes. Its excellent H2-oxidizing activity begins only once a substantial overpotential is applied, and it cannot produce H2. A major reason for its unidirectional behavior is that the reduction potentials of its electron-relaying FeS clusters are too positive relative to the 2H+/H2 couple at neutral pH; consequently, electrons held within the enzyme lack the energy to drive H2 production. However, Hyd-1 is O2-tolerant and even functions in air. Changing a tyrosine (Y) or threonine (T), located on the protein surface within 10 Å of the distal [4Fe-4S] and medial [3Fe-4S] clusters, to cysteine (C), allows site-selective attachment of a silver nanocluster (AgNC), the reduced or photoexcited state of which is a powerful reductant. The AgNC provides a new additional redox site, capturing externally supplied electrons with sufficiently high energy to drive H2 production. Assemblies of Y'227C (or T'225C) with AgNCs/PMAA (PMAA = polymethyl acrylate templating several AgNC) are also electroactive for H2 production at a TiO2 electrode. A colloidal system for visible-light photo-H2 generation is made by building the hybrid enzyme into a heterostructure with TiO2 and graphitic carbon nitride (g-C3N4), the resulting scaffold promoting uptake of electrons excited at the AgNC. Each hydrogenase produces 40 molecules of H2 per second and sustains 20% activity in air.

The structure of hydrogenase-2 from Escherichia coli: implications for H2-driven proton pumping.

Under anaerobic conditions, Escherichia coli is able to metabolize molecular hydrogen via the action of several [NiFe]-hydrogenase enzymes. Hydrogenase-2, which is typically present in cells at low levels during anaerobic respiration, is a periplasmic-facing membrane-bound complex that functions as a proton pump to convert energy from hydrogen (H2) oxidation into a proton gradient; consequently, its structure is of great interest. Empirically, the complex consists of a tightly bound core catalytic module, comprising large (HybC) and small (HybO) subunits, which is attached to an Fe-S protein (HybA) and an integral membrane protein (HybB). To date, efforts to gain a more detailed picture have been thwarted by low native expression levels of Hydrogenase-2 and the labile interaction between HybOC and HybA/HybB subunits. In the present paper, we describe a new overexpression system that has facilitated the determination of high-resolution crystal structures of HybOC and, hence, a prediction of the quaternary structure of the HybOCAB complex.

Mechanistic Exploitation of a Self-Repairing, Blocked Proton Transfer Pathway in an O2-Tolerant [NiFe]-Hydrogenase.

Catalytic long-range proton transfer in [NiFe]-hydrogenases has long been associated with a highly conserved glutamate (E) situated within 4 Å of the active site. Substituting for glutamine (Q) in the O2-tolerant [NiFe]-hydrogenase-1 from Escherichia coli produces a variant (E28Q) with unique properties that have been investigated using protein film electrochemistry, protein film infrared electrochemistry, and X-ray crystallography. At pH 7 and moderate potential, E28Q displays approximately 1% of the activity of the native enzyme, high enough to allow detailed infrared measurements under steady-state conditions. Atomic-level crystal structures reveal partial displacement of the amide side chain by a hydroxide ion, the occupancy of which increases with pH or under oxidizing conditions supporting formation of the superoxidized state of the unusual proximal [4Fe-3S] cluster located nearby. Under these special conditions, the essential exit pathway for at least one of the H+ ions produced by H2 oxidation, and assumed to be blocked in the E28Q variant, is partially repaired. During steady-state H2 oxidation at neutral pH (i.e., when the barrier to H+ exit via Q28 is almost totally closed), the catalytic cycle is dominated by the reduced states "Nia-R" and "Nia-C", even under highly oxidizing conditions. Hence, E28 is not involved in the initial activation/deprotonation of H2, but facilitates H+ exit later in the catalytic cycle to regenerate the initial oxidized active state, assumed to be Nia-SI. Accordingly, the oxidized inactive resting state, "Ni-B", is not produced by E28Q in the presence of H2 at high potential because Nia-SI (the precursor for Ni-B) cannot accumulate. The results have important implications for understanding the catalytic mechanism of [NiFe]-hydrogenases and the control of long-range proton-coupled electron transfer in hydrogenases and other enzymes.

Mechanism of hydrogen activation by [NiFe] hydrogenases.

The active site of [NiFe] hydrogenases contains a strictly conserved arginine that suspends a guanidine nitrogen atom <4.5 Å above the nickel and iron atoms. The guanidine headgroup interacts with the side chains of two conserved aspartic acid residues to complete an outer-shell canopy that has thus far proved intractable to investigation by site-directed mutagenesis. Using hydrogenase-1 from Escherichia coli, the strictly conserved residues R509 and D574 have been replaced by lysine (R509K) and asparagine (D574N) and the highly conserved D118 has been replaced by alanine (D118A) or asparagine (D118N/D574N). Each enzyme variant is stable, and their [(RS)2Niµ(SR)2Fe(CO)(CN)2] inner coordination shells are virtually unchanged. The R509K variant had >100-fold lower activity than native enzyme. Conversely, the variants D574N, D118A and D118N/D574N, in which the position of the guanidine headgroup is retained, showed 83%, 26% and 20% activity, respectively. The special kinetic requirement for R509 implicates the suspended guanidine group as the general base in H2 activation by [NiFe] hydrogenases.

Structures of replication initiation proteins from staphylococcal antibiotic resistance plasmids reveal protein asymmetry and flexibility are necessary for replication.

Antibiotic resistance in pathogenic bacteria is a continual threat to human health, often residing in extrachromosomal plasmid DNA. Plasmids of the pT181 family are widespread and confer various antibiotic resistances to Staphylococcus aureus. They replicate via a rolling circle mechanism that requires a multi-functional, plasmid-encoded replication protein to initiate replication, recruit a helicase to the site of initiation and terminate replication after DNA synthesis is complete. We present the first atomic resolution structures of three such replication proteins that reveal distinct, functionally relevant conformations. The proteins possess a unique active site and have been shown to contain a catalytically essential metal ion that is bound in a manner distinct from that of any other rolling circle replication proteins. These structures are the first examples of the Rep_trans Pfam family providing insights into the replication of numerous antibiotic resistance plasmids from Gram-positive bacteria, Gram-negative phage and the mobilisation of DNA by conjugative transposons.

Importance of the Active Site "Canopy" Residues in an O2-Tolerant [NiFe]-Hydrogenase.

The active site of Hyd-1, an oxygen-tolerant membrane-bound [NiFe]-hydrogenase from Escherichia coli, contains four highly conserved residues that form a "canopy" above the bimetallic center, closest to the site at which exogenous agents CO and O2 interact, substrate H2 binds, and a hydrido intermediate is stabilized. Genetic modification of the Hyd-1 canopy has allowed the first systematic and detailed kinetic and structural investigation of the influence of the immediate outer coordination shell on H2 activation. The central canopy residue, arginine 509, suspends a guanidine/guanidinium side chain at close range above the open coordination site lying between the Ni and Fe atoms (N-metal distance of 4.4 Å): its replacement with lysine lowers the H2 oxidation rate by nearly 2 orders of magnitude and markedly decreases the H2/D2 kinetic isotope effect. Importantly, this collapse in rate constant can now be ascribed to a very unfavorable activation entropy (easily overriding the more favorable activation enthalpy of the R509K variant). The second most important canopy residue for H2 oxidation is aspartate 118, which forms a salt bridge to the arginine 509 headgroup: its mutation to alanine greatly decreases the H2 oxidation efficiency, observed as a 10-fold increase in the potential-dependent Michaelis constant. Mutations of aspartate 574 (also salt-bridged to R509) to asparagine and proline 508 to alanine have much smaller effects on kinetic properties. None of the mutations significantly increase sensitivity to CO, but neutralizing the expected negative charges from D118 and D574 decreases O2 tolerance by stabilizing the oxidized resting NiIII-OH state ("Ni-B"). An extensive model of the catalytic importance of residues close to the active site now emerges, whereby a conserved gas channel culminates in the arginine headgroup suspended above the Ni and Fe.

Hydrogen activation by [NiFe]-hydrogenases.

Hydrogenase-1 (Hyd-1) from Escherichia coli is a membrane-bound enzyme that catalyses the reversible oxidation of molecular H2 The active site contains one Fe and one Ni atom and several conserved amino acids including an arginine (Arg(509)), which interacts with two conserved aspartate residues (Asp(118) and Asp(574)) forming an outer shell canopy over the metals. There is also a highly conserved glutamate (Glu(28)) positioned on the opposite side of the active site to the canopy. The mechanism of hydrogen activation has been dissected by site-directed mutagenesis to identify the catalytic base responsible for splitting molecular hydrogen and possible proton transfer pathways to/from the active site. Previous reported attempts to mutate residues in the canopy were unsuccessful, leading to an assumption of a purely structural role. Recent discoveries, however, suggest a catalytic requirement, for example replacing the arginine with lysine (R509K) leaves the structure virtually unchanged, but catalytic activity falls by more than 100-fold. Variants containing amino acid substitutions at either or both, aspartates retain significant activity. We now propose a new mechanism: heterolytic H2 cleavage is via a mechanism akin to that of a frustrated Lewis pair (FLP), where H2 is polarized by simultaneous binding to the metal(s) (the acid) and a nitrogen from Arg(509) (the base).

Structural basis of high-order oligomerization of the cullin-3 adaptor SPOP.

Protein ubiquitination in eukaryotic cells is mediated by diverse E3 ligase enzymes that each target specific substrates. The cullin E3 ligase complexes are the most abundant class of E3 ligases; they contain various cullin components that serve as scaffolds for interaction with substrate-recruiting adaptor proteins. SPOP is a BTB-domain adaptor of the cullin-3 E3 ligase complexes; it selectively recruits substrates via its N-terminal MATH domain, whereas its BTB domain mediates dimerization and interactions with cullin-3. It has recently been recognized that the high-order oligomerization of SPOP enhances the ubiquitination of substrates. Here, a dimerization interface in the SPOP C-terminus is identified and it is shown that the dimerization interfaces of the BTB domain and of the C-terminus act independently and in tandem to generate high-order SPOP oligomers. The crystal structure of the dimeric SPOP C-terminal domain is reported at 1.5 Šresolution and it is shown that Tyr353 plays a critical role in high-order oligomerization. A model of the high-order SPOP oligomer is presented that depicts a helical organization that could enhance the efficiency of substrate ubiquitination.

Identification, characterization and preliminary X-ray diffraction analysis of the rolling-circle replication initiator protein from plasmid pSTK1.

Antibiotic resistance in bacterial pathogens poses an ever-increasing risk to human health. In antibiotic-resistant strains of Staphylococcus aureus this resistance often resides in extra-chromosomal plasmids, such as those of the pT181 family, which replicate via a rolling-circle mechanism mediated by a plasmid-encoded replication initiation protein. Currently, there is no structural information available for the pT181-family Rep proteins. Here, the crystallization of a catalytically active fragment of a homologous replication initiation protein from the thermophile Geobacillus stearothermophilus responsible for the replication of plasmid pSTK1 is reported. Crystals of the RepSTK1 fragment diffracted to a resolution of 2.5 Šand belonged to space group P212121.

The structural basis of Holliday junction resolution by T7 endonuclease I.

The four-way (Holliday) DNA junction is the central intermediate in homologous recombination, a ubiquitous process that is important in DNA repair and generation of genetic diversity. The penultimate stage of recombination requires resolution of the DNA junction into nicked-duplex species by the action of a junction-resolving enzyme, examples of which have been identified in a wide variety of organisms. These enzymes are nucleases that are highly selective for the structure of branched DNA. The mechanism of this selectivity has, however, been unclear in the absence of structural data. Here we present the crystal structure of the junction-resolving enzyme phage T7 endonuclease I in complex with a synthetic four-way DNA junction. Although the enzyme is structure-selective, significant induced fit occurs in the interaction, with changes in the structure of both the protein and the junction. The dimeric enzyme presents two binding channels that contact the backbones of the junction's helical arms over seven nucleotides. These interactions effectively measure the relative orientations and positions of the arms of the junction, thereby ensuring that binding is selective for branched DNA that can achieve this geometry.

Comprehensive structural, infrared spectroscopic and kinetic investigations of the roles of the active-site arginine in bidirectional hydrogen activation by the [NiFe]-hydrogenase 'Hyd-2' from Escherichia coli.

The active site of [NiFe]-hydrogenases contains a strictly-conserved pendant arginine, the guanidine head group of which is suspended immediately above the Ni and Fe atoms. Replacement of this arginine (R479) in hydrogenase-2 from E. coli results in an enzyme that is isolated with a very tightly-bound diatomic ligand attached end-on to the Ni and stabilised by hydrogen bonding to the Nζ atom of the pendant lysine and one of the three additional water molecules located in the active site of the variant. The diatomic ligand is bound under oxidising conditions and is removed only after a prolonged period of reduction with H2 and reduced methyl viologen. Once freed of the diatomic ligand, the R479K variant catalyses both H2 oxidation and evolution but with greatly decreased rates compared to the native enzyme. Key kinetic characteristics are revealed by protein film electrochemistry: most importantly, a very low activation energy for H2 oxidation that is not linked to an increased H/D isotope effect. Native electrocatalytic reversibility is retained. The results show that the sluggish kinetics observed for the lysine variant arise most obviously because the advantage of a more favourable low-energy pathway is massively offset by an extremely unfavourable activation entropy. Extensive efforts to establish the identity of the diatomic ligand, the tight binding of which is an unexpected further consequence of replacing the pendant arginine, prove inconclusive.

The structure of the Bach2 POZ-domain dimer reveals an intersubunit disulfide bond.

Bach2 is a transcriptional repressor that is expressed during specific stages of B-cell development and in neuronal cells. It plays a critical role in modulating class-switch recombination during the differentiation of mature B cells to antibody-secreting plasma cells and it is also an important regulator of apoptotic responses to oxidative stress. Bach2 has been implicated both as an oncogene and as a tumour suppressor in human malignancy. The interaction of Bach2 with its target genes is mediated via its basic leucine-zipper region, whereas the N-terminal POZ domain recruits transcriptional co-repressors and class II histone deacetylases. Here, the crystal structure of the human Bach2 POZ domain is reported at 2.1 Å resolution. The Bach2 POZ-domain dimer resembles the POZ-domain dimers of the POZ zinc finger transcription factors and dimerization is independent of an N-terminal region that has previously been implicated in the dimerization of the POZ basic leucine-zipper protein Bach1. The Bach2 POZ domain crystallized in two forms which differed by the presence of an intersubunit disulfide bond. The intersubunit disulfide bond is present both in bacterially expressed Bach2 POZ domain in solution and in protein expressed in transfected eukaryotic cells. These crystal structures will be relevant for understanding the regulation of Bach2 in response to oxidative stress and for the design of therapeutics that target the Bach2 POZ domain in human malignancy.

Structure of New Delhi metallo-ß-lactamase 1 (NDM-1).

Antibiotic resistance in bacterial pathogens poses a serious threat to human health and the metallo-ß-lactamase (MBL) enzymes are responsible for much of this resistance. The recently identified New Delhi MBL 1 (NDM-1) is a novel member of this family that is capable of hydrolysing a wide variety of clinically important antibiotics. Here, the crystal structure of NDM-1 from Klebsiella pneumoniae is reported and its structure and active site are discussed in the context of other recently deposited coordinates of NDM-1.

The crystalline state as a dynamic system: IR microspectroscopy under electrochemical control for a [NiFe] hydrogenase.

Controlled formation of catalytically-relevant states within crystals of complex metalloenzymes represents a significant challenge to structure-function studies. Here we show how electrochemical control over single crystals of [NiFe] hydrogenase 1 (Hyd1) from Escherichia coli makes it possible to navigate through the full array of active site states previously observed in solution. Electrochemical control is combined with synchrotron infrared microspectroscopy, which enables us to measure high signal-to-noise IR spectra in situ from a small area of crystal. The output reports on active site speciation via the vibrational stretching band positions of the endogenous CO and CN- ligands at the hydrogenase active site. Variation of pH further demonstrates how equilibria between catalytically-relevant protonation states can be deliberately perturbed in the crystals, generating a map of electrochemical potential and pH conditions which lead to enrichment of specific states. Comparison of in crystallo redox titrations with measurements in solution or of electrode-immobilised Hyd1 confirms the integrity of the proton transfer and redox environment around the active site of the enzyme in crystals. Slowed proton-transfer equilibria in the hydrogenase in crystallo reveals transitions which are only usually observable by ultrafast methods in solution. This study therefore demonstrates the possibilities of electrochemical control over single metalloenzyme crystals in stabilising specific states for further study, and extends mechanistic understanding of proton transfer during the [NiFe] hydrogenase catalytic cycle.

Anaerobic fixed-target serial crystallography.

Cryogenic X-ray diffraction is a powerful tool for crystallographic studies on enzymes including oxygenases and oxidases. Amongst the benefits that cryo-conditions (usually employing a nitro-gen cryo-stream at 100 K) enable, is data collection of di-oxy-gen-sensitive samples. Although not strictly anaerobic, at low temperatures the vitreous ice conditions severely restrict O2 diffusion into and/or through the protein crystal. Cryo-conditions limit chemical reactivity, including reactions that require significant conformational changes. By contrast, data collection at room temperature imposes fewer restrictions on diffusion and reactivity; room-temperature serial methods are thus becoming common at synchrotrons and XFELs. However, maintaining an anaerobic environment for di-oxy-gen-dependent enzymes has not been explored for serial room-temperature data collection at synchrotron light sources. This work describes a methodology that employs an adaptation of the 'sheet-on-sheet' sample mount, which is suitable for the low-dose room-temperature data collection of anaerobic samples at synchrotron light sources. The method is characterized by easy sample preparation in an anaerobic glovebox, gentle handling of crystals, low sample consumption and preservation of a localized anaerobic environment over the timescale of the experiment (<5 min). The utility of the method is highlighted by studies with three X-ray-radiation-sensitive Fe(II)-containing model enzymes: the 2-oxoglutarate-dependent l-arginine hy-droxy-lase VioC and the DNA repair enzyme AlkB, as well as the oxidase isopenicillin N synthase (IPNS), which is involved in the biosynthesis of all penicillin and cephalosporin antibiotics.

Structure of the human Nac1 POZ domain.

Nac1 is a POZ-domain transcription factor that is involved in the self-renewal of embryonic stem cells. It is overexpressed in ovarian serous carcinoma and targeting the interactions of its POZ domain is a potential therapeutic strategy. Nac1 lacks a zinc-finger DNA-binding domain and thereby differs from most other POZ-domain transcription factors. Here, the crystal structure of the Nac1 POZ domain at 2.1 A resolution is reported. The Nac1 POZ domain crystallized as a dimer in which the interaction interfaces between subunits resemble those found in the POZ-zinc finger transcription factors. The organization of the Nac1 POZ-domain core resembles reported POZ-domain structures, whereas the C-terminus differs markedly. The C-terminal alpha-helix of the Nac1 POZ domain is shorter than that observed in most other POZ-domain transcription factors; variation in the organization of this region may be a general feature of POZ-domain structures.

Structure of an Escherichia coli N-acetyl-D-neuraminic acid lyase mutant, E192N, in complex with pyruvate at 1.45 angstrom resolution.

The structure of a mutant variant of Escherichia coli N-acetyl-d-neuraminic acid lyase (NAL), E192N, in complex with pyruvate has been determined in a new crystal form. It crystallized in space group P2(1)2(1)2(1), with unit-cell parameters a = 78.3, b = 108.5, c = 148.3 angstrom. Pyruvate has been trapped in the active site as a Schiff base with the catalytic lysine (Lys165) without the need for reduction. Unlike the previously published crystallization conditions for the wild-type enzyme, in which a mother-liquor-derived sulfate ion is strongly bound in the catalytic pocket, the low-salt conditions described here will facilitate the determination of further E. coli NAL structures in complex with other activesite ligands.