In vitro and in vivo characterization of the multiple isoforms of Schistosoma mansoni hypoxanthine-guanine phosphoribosyltransferases.

Schistosoma mansoni, the parasite responsible for schistosomiasis, lacks the "de novo" purine biosynthetic pathway and depends entirely on the purine salvage pathway for the supply of purines. Numerous reports of praziquantel resistance have been described, as well as stimulated efforts to develop new drugs against schistosomiasis. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is a key enzyme of the purine salvage pathway. Here, we describe a crystallographic structure of the S. mansoni HPGRT-1 (SmHGPRT), complexed with IMP at a resolution of 2.8 Ǻ. Four substitutions were identified in the region of the active site between SmHGPRT-1 and human HGPRT. We also present data from RNA-Seq and WISH, suggesting that some isoforms of HGPRT might be involved in the process related to sexual maturation and reproduction in worms; furthermore, its enzymatic assays show that the isoform SmHGPRT-3 does not present the same catalytic efficiency as other isoforms. Finally, although other studies have previously suggested this enzyme as a potential antischistosomal chemotherapy target, the kinetics parameters reveal the impossibility to use SmHGPRT as an efficient chemotherapeutic target.

Affimer proteins are versatile and renewable affinity reagents.

Molecular recognition reagents are key tools for understanding biological processes and are used universally by scientists to study protein expression, localisation and interactions. Antibodies remain the most widely used of such reagents and many show excellent performance, although some are poorly characterised or have stability or batch variability issues, supporting the use of alternative binding proteins as complementary reagents for many applications. Here we report on the use of Affimer proteins as research reagents. We selected 12 diverse molecular targets for Affimer selection to exemplify their use in common molecular and cellular applications including the (a) selection against various target molecules; (b) modulation of protein function in vitro and in vivo; (c) labelling of tumour antigens in mouse models; and (d) use in affinity fluorescence and super-resolution microscopy. This work shows that Affimer proteins, as is the case for other alternative binding scaffolds, represent complementary affinity reagents to antibodies for various molecular and cell biology applications.

Structural and kinetic analysis of Schistosoma mansoni Adenylosuccinate Lyase (SmADSL).

Schistosoma mansoni is the parasite responsible for schistosomiasis, a disease that affects about 218 million people worldwide. Currently, both direct treatment and disease control initiatives rely on chemotherapy using a single drug, praziquantel. Concerns over the possibility of resistance developing to praziquantel, have stimulated efforts to develop new drugs for the treatment of schistosomiasis. Schistosomes do not have the de novo purine biosynthetic pathway, and instead depend entirely on the purine salvage pathway to supply its need for purines. The purine salvage pathway has been reported as a potential target for developing new drugs against schistosomiasis. Adenylosuccinate lyase (SmADSL) is an enzyme in this pathway, which cleaves adenylosuccinate (ADS) into adenosine 5'-monophosphate (AMP) and fumarate. SmADSL kinetic characterization was performed by isothermal titration calorimetry (ITC) using both ADS and SAICAR as substrates. Structures of SmADSL in Apo form and in complex with AMP were elucidated by x-ray crystallography revealing a highly conserved tetrameric structure required for their function since the active sites are formed from residues of three different subunits. The active sites are also highly conserved between species and it is difficult to identify a potent species-specific inhibitor for the development of new therapeutic agents. In contrast, several mutagenesis studies have demonstrated the importance of dimeric interface residues in the stability of the quaternary structure of the enzyme. The lower conservation of these residues between SmADSL and human ADSL could be used to lead the development of anti-schistosomiasis drugs based on disruption of subunit interfaces. These structures and kinetics data add another layer of information to Schistosoma mansoni purine salvage pathway.

Conformational changes in IgE contribute to its uniquely slow dissociation rate from receptor FcɛRI.

Among antibody classes, IgE has a uniquely slow dissociation rate from, and high affinity for, its cell surface receptor FcɛRI. We show the structural basis for these key determinants of the ability of IgE to mediate allergic hypersensitivity through the 3.4-Å-resolution crystal structure of human IgE-Fc (consisting of the Cɛ2, Cɛ3 and Cɛ4 domains) bound to the extracellular domains of the FcɛRI α chain. Comparison with the structure of free IgE-Fc (reported here at a resolution of 1.9 Å) shows that the antibody, which has a compact, bent structure before receptor engagement, becomes even more acutely bent in the complex. Thermodynamic analysis indicates that the interaction is entropically driven, which explains how the noncontacting Cɛ2 domains, in place of the flexible hinge region of IgG antibodies, contribute together with the conformational changes to the unique binding properties of IgE.

Structure of 5-formyltetrahydrofolate cyclo-ligase from Bacillus anthracis (BA4489).

Bacillus anthracis is a spore-forming bacterium and the causative agent of the disease anthrax. The Oxford Protein Production Facility has been targeting proteins from B. anthracis in order to develop high-throughput technologies within the Structural Proteomics in Europe project. As part of this work, the structure of 5-formyltetrahydrofolate cyclo-ligase (BA4489) has been determined by X-ray crystallography to 1.6 A resolution. The structure, solved in complex with magnesium-ion-bound ADP and phosphate, gives a detailed picture of the proposed catalytic mechanism of the enzyme. Chemical differences from other cyclo-ligase structures close to the active site that could be exploited to design specific inhibitors are also highlighted.

High-resolution structure of the catalytic region of MICAL (molecule interacting with CasL), a multidomain flavoenzyme-signaling molecule.

Semaphorins are extracellular cell guidance cues that govern cytoskeletal dynamics during neuronal and vascular development. MICAL (molecule interacting with CasL) is a multidomain cytosolic protein with a putative flavoprotein monooxygenase (MO) region required for semaphorin-plexin repulsive axon guidance. Here, we report the 1.45-A resolution crystal structure of the FAD-containing MO domain of mouse MICAL-1 (residues 1-489). The topology most closely resembles that of the NADPH-dependent flavoenzyme p-hydroxybenzoate hydroxylase (PHBH). Comparison of structures before and after reaction with NADPH reveals that, as in PHBH, the flavin ring can switch between two discrete positions. In contrast with other MOs, this conformational switch is coupled with the opening of a channel to the active site, suggestive of a protein substrate. In support of this hypothesis, distinctive structural features highlight putative protein-binding sites in suitable proximity to the active site entrance. The unusual juxtaposition of this N-terminal MO (hydroxylase) activity with the characteristics of a multiprotein-binding scaffold exhibited by the C-terminal portion of the MICALs represents a unique combination of functionality to mediate signaling.

A procedure for setting up high-throughput nanolitre crystallization experiments. Crystallization workflow for initial screening, automated storage, imaging and optimization.

Crystallization trials at the Division of Structural Biology in Oxford are now almost exclusively carried out using a high-throughput workflow implemented in the Oxford Protein Production Facility. Initial crystallization screening is based on nanolitre-scale sitting-drop vapour-diffusion experiments (typically 100 nl of protein plus 100 nl of reservoir solution per droplet) which use standard crystallization screening kits and 96-well crystallization plates. For 294 K crystallization trials the barcoded crystallization plates are entered into an automated storage system with a fully integrated imaging system. These plates are imaged in accordance with a pre-programmed schedule and the resulting digital data for each droplet are harvested into a laboratory information-management system (LIMS), scored by crystal recognition software and displayed for user analysis via a web-based interface. Currently, storage for trials at 277 K is not automated and for imaging the crystallization plates are fed by hand into an imaging system from which the data enter the LIMS. The workflow includes two procedures for nanolitre-scale optimization of crystallization conditions: (i) a protocol for variation of pH, reservoir dilution and protein:reservoir ratio and (ii) an additive screen. Experience based on 592 crystallization projects is reported.

Sample preparation and mass-spectrometric characterization of crystal-derived protein samples.

Mass spectrometry is often used to ascertain the accurate mass of purified protein samples prior to crystallization screening. However, in many cases data regarding the form of the protein crystallizing can also be useful, as this may differ from the original sample. Development of a simple method for the preparation and mass spectrometry of crystal-derived protein samples is described. The method is exemplified by the determination of the phosphorylation state of protein in a crystal derived from a mixture of phosphorylated and unphosphorylated protein.

Benefits of automated crystallization plate tracking, imaging, and analysis.

We describe the design of a database and software for managing and organizing protein crystallization data. We also outline the considerations behind the design of a fast web interface linking protein production data, crystallization images, and automated image analysis. The database and associated interfaces underpin the Oxford Protein Production Facility (OPPF) crystallization laboratory, collecting, in a routine and automatic manner, up to 100,000 images per day. Over 17 million separate images are currently held in this database. We discuss the substantial scientific benefits automated tracking, imaging, and analysis of crystallizations offers to the structural biologist: analysis of the time course of the trial and easy analysis of trials with related crystallization conditions. Features of this system address requirements common to many crystallographic laboratories that are currently setting up (semi-)automated crystallization imaging systems.

The crystal structure of IgE Fc reveals an asymmetrically bent conformation.

The distinguishing structural feature of immunoglobulin E (IgE), the antibody responsible for allergic hypersensitivity, is the C epsilon 2 domain pair that replaces the hinge region of IgG. The crystal structure of the IgE Fc (constant fragment) at a 2.6-A resolution has revealed these domains. They display a distinctive, disulfide-linked Ig domain interface and are folded back asymmetrically onto the C epsilon 3 and C epsilon 4 domains, which causes an acute bend in the IgE molecule. The structure implies that a substantial conformational change involving C epsilon 2 must accompany binding to the mast cell receptor Fc epsilon RI. This may be the basis of the exceptionally slow dissociation rate of the IgE-Fc epsilon RI complex and, thus, of the ability of IgE to cause persistent allergic sensitization of mast cells.