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Microbial transglutaminases (MTGs) catalyze the formation of Gln–Lys isopeptide bonds and are widely used for the cross-linking of proteins and peptides in food and biotechnological applications (e.g. to improve the texture of protein-rich foods or in generating antibody-drug conjugates). Currently used MTGs have low substrate specificity, impeding their biotechnological use as enzymes that do not cross-react with nontarget substrates (i.e. as bio-orthogonal labeling systems). Here, we report the discovery of an MTG from Kutzneria albida (KalbTG), which exhibited no cross-reactivity with known MTG substrates or commonly used target proteins, such as antibodies. KalbTG was produced in Escherichia coli as soluble and active enzyme in the presence of its natural inhibitor ammonium to prevent potentially toxic cross-linking activity. The crystal structure of KalbTG revealed a conserved core similar to other MTGs but very short surface loops, making it the smallest MTG characterized to date. Ultra-dense peptide array technology involving a pool of 1.4 million unique peptides identified specific recognition motifs for KalbTG in these peptides. We determined that the motifs YRYRQ and RYESK are the best Gln and Lys substrates of KalbTG, respectively. By first reacting a bifunctionalized peptide with the more specific KalbTG and in a second step with the less specific MTG from Streptomyces mobaraensis, a successful bio-orthogonal labeling system was demonstrated. Fusing the KalbTG recognition motif to an antibody allowed for site-specific and ratio-controlled labeling using low label excess. Its site specificity, favorable kinetics, ease of use, and cost-effective production render KalbTG an attractive tool for a broad range of applications, including production of therapeutic antibody-drug conjugates.

Dual inhibition of fatty acid binding proteins 4 and 5 (FABP4 and FABP5) is expected to provide beneficial effects on a number of metabolic parameters such as insulin sensitivity and blood glucose levels and should protect against atherosclerosis. Starting from a FABP4 selective focused screening hit, biostructure information was used to modulate the selectivity profile in the desired way and to design potent dual FABP4/5 inhibitors with good selectivity against FABP3. With very good pharmacokinetic properties and no major safety alerts, compound 12 was identified as a suitable tool compound for further in vivo investigations.

Doublecortin is a microtubule-associated protein produced during neurogenesis. The protein stabilizes microtubules and stimulates their polymerization, which allows migration of immature neurons to their designated location in the brain. Mutations in the gene that impair doublecortin function and cause severe brain formation disorders are located on a tandem repeat of two doublecortin domains. The molecular mechanism of action of doublecortin is only incompletely understood. Anti-doublecortin antibodies, such as the rabbit polyclonal Abcam 18732, are widely used as neurogenesis markers. Here, we report the generation and characterization of antibodies that bind to single doublecortin domains. The antibodies were used as tools to obtain structures of both domains. Four independent crystal structures of the N-terminal domain reveal several distinct open and closed conformations of the peptide linking N- and C-terminal domains, which can be related to doublecortin function. An NMR assignment and a crystal structure in complex with a camelid antibody fragment show that the doublecortin C-terminal domain adopts the same well defined ubiquitin-like fold as the N-terminal domain, despite its reported aggregation and molten globule-like properties. The antibodies' unique domain specificity also renders them ideal research tools to better understand the role of individual domains in doublecortin function. A single chain camelid antibody fragment specific for the C-terminal doublecortin domain affected microtubule binding, whereas a monoclonal mouse antibody specific for the N-terminal domain did not. Together with steric considerations, this suggests that the microtubule-interacting doublecortin domain observed in cryo-electron micrographs is the C-terminal domain rather than the N-terminal one.

Type II DNA topoisomerases alter the supercoiling state of DNA in an ATP-dependent fashion that requires large conformational changes. The directionality of DNA strand transfer is controlled by three transient protein interfaces, termed the N-gate, DNA-gate, and C-gate. Bacterial gyrase is a type II DNA topoisomerase of A2B2 composition. The N-gate is formed by the two GyrB subunits and the GyrA subunits form the DNA- and C-gates. In structures of type II topoisomerase fragments, the DNA- and C-gates delimit a cavity for DNA and can be open or closed. However, the conformational space accessible has not yet been mapped. Here, we describe the crystal structure of the Bacillus subtilis DNA gyrase A subunit lacking the C-terminal DNA-wrapping domains. Five dimeric states of the GyrA N-terminal domain are observed, with their DNA- and C-gates either closed, or open to different extents. All of these conformations can in principle accommodate double-stranded DNA in the central cavity but only one conformation has its DNA-gate open wide enough for DNA to enter. The structure thus reflects the lower limit of DNA-gate opening that must occur during gyrase catalysis. The DNA-gate is formed by two flat surfaces, with few interactions. In contrast, the C-gate exhibits a highly undulated surface and forms a large number of interactions. None of the dimers in the crystal structures display an open C-gate that would allow DNA passage, in agreement with a transient opening of this gate during the catalytic cycle of DNA supercoiling.

Structure-guided lead optimization of recently described benzimidazolyl acetamides addressed the key liabilities of the previous lead compound 1. These efforts culminated in the discovery of 4-{(S)-2-[2-(4-chloro-phenyl)-5,6-difluoro-benzoimidazol-1-yl]-2-cyclohexyl-acetylamino}-3-fluoro-benzoic acid 7g, a highly potent and selective FXR agonist with excellent physicochemical and ADME properties and potent lipid lowering activity after oral administration to LDL receptor deficient mice.

The catalytic mechanism of Propionibacterium acnes polyunsaturated fatty acid isomerase (PAI) is explored by kinetic, spectroscopic, and thermodynamic studies. The PAI-catalyzed double bond isomerization takes place by selective removal of the pro-R hydrogen from C-11 followed by suprafacial transfer of this hydrogen to C-9 as shown by conversion of C-9-deuterated substrate isotopologs. Data on the midpoint potential, photoreduction, and cofactor replacement suggest PAI to operate via an ionic mechanism with the formation of FADH2 and linoleic acid carbocation as intermediates. In line with this proposal, no radical intermediates were detected neither by stopped flow absorption nor by EPR spectroscopy. The substrate preference toward free fatty acids is determined by the interaction between Arg-88 and Phe-193, and the reaction rate is strongly affected by replacement of these amino acids, indicating that the efficiency of the hydrogen transfer relies on a fixed distance between the free carboxyl group and the N-5 atom of FAD. Combining data obtained for PAI from the structural studies and experiments described here suggests that at least two different prototypical active site geometries exist among polyunsaturated fatty acid double bond isomerases.

DEAD box RNA helicases use the energy of ATP hydrolysis to unwind double-stranded RNA regions or to disrupt RNA/protein complexes. A minimal RNA helicase comprises nine conserved motifs distributed over two RecA-like domains. The N-terminal domain contains all motifs involved in nucleotide binding, namely the Q-motif, the DEAD box, and the P-loop, as well as the SAT motif, which has been implicated in the coordination of ATP hydrolysis and RNA unwinding. We present here the crystal structure of the N-terminal domain of the Thermus thermophilus RNA helicase Hera in complex with adenosine monophosphate (AMP). Upon binding of AMP the P-loop adopts a partially collapsed or half-open conformation that is still connected to the DEAD box motif, and the DEAD box in turn is linked to the SAT motif via hydrogen bonds. This network of interactions communicates changes in the P-loop conformation to distant parts of the helicase. The affinity of AMP is comparable to that of ADP and ATP, substantiating that the binding energy from additional phosphate moieties is directly converted into conformational changes of the entire helicase. Importantly, the N-terminal Hera domain forms a dimer in the crystal similar to that seen in another thermophilic prokaryote. It is possible that this mode of dimerization represents the prototypic architecture in RNA helicases of thermophilic origin.

The T-cell receptor (TCR) is a heterodimeric cell-surface protein consisting of two chains, alpha and beta, each of which is composed of a variable (V) and a constant (C) domain. Crystals of the isolated V(alpha) domain of the murine TCR 2C were grown by serendipity from a solution containing the extracellular domains of the intact TCR 2C and CD3 gamma epsilon-chains. The V(alpha) crystal structure shows how crystal packing can substitute for another V(alpha) domain in a different fashion from that observed in V(alpha)/V(alpha) homodimer and V(alpha)/V(beta) heterodimer structures. Significant conformational changes occur in the CDR3 and beta(3)beta(4) loops that normally form part of the dimer interface. The monomeric V(alpha) domain provides the unique opportunity to study the effect of dimerization on the conformation of the unliganded complementarity-determining regions (CDR) of a TCR. This structure of an individual V(alpha) module has implications for stability and bioengineering of isolated antibody and immunoglobulin domains.

Proliferation, differentiation, and morphology of eucaryotic cells is regulated by a large network of signaling molecules. Among the major players are members of the Ras and Rho/Rac subfamilies of small GTPases that bind to different sets of effector proteins. Recognition of multiple effectors is important for communicating signals into different pathways, leading to the question of how an individual GTPase achieves tight binding to diverse targets. To understand the observed specificity, detailed information about binding energetics is expected to complement the information gained from the three-dimensional structures of GTPase/effector protein complexes. Here, the thermodynamics of the interaction of four closely related members of the Ras subfamily with four different effectors and, additionally, the more distantly related Cdc42/WASP couple were quantified by means of isothermal titration calorimetry. The heat capacity changes upon complex formation were rationalized in light of the GTPase/effector complex structures. Changes in enthalpy, entropy, and heat capacity of association with various Ras proteins are similar for the same effector. In contrast, although the structures of the Ras-binding domains are similar, the thermodynamics of the Ras/Raf and Ras/Ral guanine nucleotide dissociation stimulator interactions are quite different. The energy profile of the Cdc42/WASP interaction is similar to Ras/Ral guanine nucleotide dissociation stimulator, despite largely different structures and interface areas of the complexes. Water molecules in the interface cannot fully account for the observed discrepancy but may explain the large range of Ras/effector binding specificity. The differences in the thermodynamic parameters, particularly the entropy changes, could help in the design of effector-specific inhibitors that selectively block a single pathway.

The K bm1 and K bm8 natural mutants of the murine MHC class I molecule H-2K b were originally identified by allograft rejection. They also bind viral peptides VSV8 and SEV9 with high affinity, but their peptide complexes have substantially decreased thermostability, and the K bm1 complexes do not elicit alloreactive T cell responses. Crystal structures of the four mutant complexes at 1.7–1.9 A resolution are similar to the corresponding wild-type K b structures, except in the vicinity of the mutated residues, which alter the electrostatic potential, topology, hydrogen bonding, and local water structure of the peptide binding groove. Thus, these natural K b mutations define the minimal perturbations in the peptide environment that alter antigen presentation to T cells and abolish alloreactivity.

Wiskott Aldrich syndrome is a rare hereditary disease that affects cell morphology and signal transduction in hematopoietic cells. Different size fragments of the Wiskott Aldrich syndrome protein, W4, W7 and W13, were expressed in Escherichia coli or obtained from proteolysis. All contain the GTPase binding domain (GBD), also called Cdc42/Rac interactive binding region (CRIB), found in many putative downstream effectors of Rac and Cdc42. We have developed assays to measure the binding interaction between these fragments and Cdc42 employing fluorescent N-methylanthraniloyl-guanine nucleotide analogues. The fragments bind with submicromolar affinities in a GTP-dependent manner, with the largest fragment having the highest affinity, showing that the GBD/CRIB motif is necessary but not sufficient for tight binding. Rate constants for the interaction with W13 have been determined via surface plasmon resonance, and the equilibrium dissociation constant obtained from their ratio agrees with the value obtained by fluorescence measurements. Far UV circular dichroism spectra show significant secondary structure only for W13, supported by fluorescence studies using intrinsic protein fluorescence and quenching by acrylamide. Proton and 15N NMR measurements show that the GBD/CRIB motif has no apparent secondary structure and that the region C-terminal to the GBD/CRIB region is alpha-helical. The binding of Cdc42 induces a structural rearrangement of residues in the GBD/CRIB motif, or alternatively, the Wiskott Aldrich syndrome protein fragments have an ensemble of conformations, one of which is stabilized by Cdc42 binding. Thus, in contrast to Ras effectors, which have no conserved sequence elements but a defined domain structure with ubiquitin topology, Rac/Cdc42 effectors have a highly conserved binding region but no defined domain structure in the absence of the GTP-binding protein. Deviating from common belief GBD/CRIB is neither a structural domain nor sufficient for tight binding as regions outside this motif are necessary for structure formation and tight interaction with Rho/Rac proteins.

DEAD box helicases couple ATP hydrolysis to RNA structural rearrangements. T. thermophilus Hera (heat resistant RNA-dependent ATPase) consists of a helicase core and a C-terminal extension. In single molecule FRET experiments we identified fragments of the 23S rRNA comprising hairpin 92 and RNase P RNA as substrates for Hera. RNA binding requires the C-terminal extension. Both substrates switch the helicase core to the closed conformation and stimulate the intrinsic ATPase activity of Hera. ATP-dependent unwinding of a short helix adjacent to hairpin 92 of 23S rRNA suggests a specific role for Hera in ribosome assembly, in analogy to the E. coliand B. subtilis helicases DbpA and YxiN. In addition, the specificity of Hera for RNase P RNA may be required for RNase P RNA folding or RNase P assembly.Hera forms a stable dimer in solution, setting it apart form other helicases. Crystal structures show that the C-terminal extension is bipartite, forming a highly flexible dimerization motif with a novel fold and an additional RNA-binding module that adopts the fold of a degenerated RNA recognition motif (RRM). Comparison with RRM/RNA complexes suggests an RNA binding mode similar to that of the spliceosomal protein U1A. The structure-based model for the complete Hera dimer bound to RNA reveals a likely binding surface for large RNA substrates that spans both RecA-like domains and the RBD. The RNA-binding sites of the helicase cores face each other, possibly enabling subunit communication. The plasticity of the dimerization motif allows for drastic changes in the juxtaposition of the helicase cores within the dimer. Simultaneous action of the Hera subunits in the dimer on the same large RNA molecule may be important for efficient remodeling of in vivo RNA substrates.