Design of protein-binding proteins from the target structure alone.

The design of proteins that bind to a specific site on the surface of a target protein using no information other than the three-dimensional structure of the target remains a challenge1-5. Here we describe a general solution to this problem that starts with a broad exploration of the vast space of possible binding modes to a selected region of a protein surface, and then intensifies the search in the vicinity of the most promising binding modes. We demonstrate the broad applicability of this approach through the de novo design of binding proteins to 12 diverse protein targets with different shapes and surface properties. Biophysical characterization shows that the binders, which are all smaller than 65 amino acids, are hyperstable and, following experimental optimization, bind their targets with nanomolar to picomolar affinities. We succeeded in solving crystal structures of five of the binder-target complexes, and all five closely match the corresponding computational design models. Experimental data on nearly half a million computational designs and hundreds of thousands of point mutants provide detailed feedback on the strengths and limitations of the method and of our current understanding of protein-protein interactions, and should guide improvements of both. Our approach enables the targeted design of binders to sites of interest on a wide variety of proteins for therapeutic and diagnostic applications.

Structure of ATP citrate lyase and the origin of citrate synthase in the Krebs cycle.

Across different kingdoms of life, ATP citrate lyase (ACLY, also known as ACL) catalyses the ATP-dependent and coenzyme A (CoA)-dependent conversion of citrate, a metabolic product of the Krebs cycle, to oxaloacetate and the high-energy biosynthetic precursor acetyl-CoA1. The latter fuels pivotal biochemical reactions such as the synthesis of fatty acids, cholesterol and acetylcholine2, and the acetylation of histones and proteins3,4. In autotrophic prokaryotes, ACLY is a hallmark enzyme of the reverse Krebs cycle (also known as the reductive tricarboxylic acid cycle), which fixates two molecules of carbon dioxide in acetyl-CoA5,6. In humans, ACLY links carbohydrate and lipid metabolism and is strongly expressed in liver and adipose tissue1 and in cholinergic neurons2,7. The structural basis of the function of ACLY remains unknown. Here we report high-resolution crystal structures of bacterial, archaeal and human ACLY, and use distinct substrate-bound states to link the conformational plasticity of ACLY to its multistep catalytic itinerary. Such detailed insights will provide the framework for targeting human ACLY in cancer8-11 and hyperlipidaemia12,13. Our structural studies also unmask a fundamental evolutionary relationship that links citrate synthase, the first enzyme of the oxidative Krebs cycle, to an ancestral tetrameric citryl-CoA lyase module that operates in the reverse Krebs cycle. This molecular transition marked a key step in the evolution of metabolism on Earth.

The Structure of the LysR-type Transcriptional Regulator, CysB, Bound to the Inducer, N-acetylserine.

In Escherichia coli and Salmonella typhimurium, cysteine biosynthesis requires the products of 20 or more cys genes co-ordinately regulated by CysB. Under conditions of sulphur limitation and in the presence of the inducer, N-acetylserine, CysB binds to cys promoters and activates the transcription of the downstream coding sequences. CysB is a homotetramer, comprising an N-terminal DNA binding domain (DBD) and a C-terminal effector binding domain (EBD). The crystal structure of a dimeric EBD fragment of CysB from Klebsiella aerogenes revealed a protein fold similar to that seen in Lac repressor but with a different symmetry in the dimer so that the mode of DNA binding was not apparent. To elucidate the subunit arrangement in the tetramer, we determined the crystal structure of intact CysB in complex with N-acetylserine. The tetramer has two subunit types that differ in the juxtaposition of their winged helix-turn-helix DNA binding domains with respect to the effector binding domain. In the assembly, the four EBDs form a core with the DNA binding domains arranged in pairs on the surface. N-acetylserine makes extensive polar interactions in an enclosed binding site, and its binding is accompanied by substantial conformational rearrangements of surrounding residues that are propagated to the protein surface where they appear to alter the arrangement of the DNA binding domains. The results are (i) discussed in relation to the extensive mutational data available for CysB and (ii) used to propose a structural mechanism of N-acetylserine induced CysB activation.

Ym1 protein crystals promote type 2 immunity.

Spontaneous protein crystallization is a rare event, yet protein crystals are frequently found in eosinophil-rich inflammation. In humans, Charcot-Leyden crystals (CLCs) are made from galectin-10 (Gal10) protein, an abundant protein in eosinophils. Although mice do not encode Gal10 in their genome, they do form pseudo-CLCs, made from the chitinase-like proteins Ym1 and/or Ym2, encoded by Chil3 and Chil4 and made by myeloid and epithelial cells respectively. Here, we investigated the biological effects of pseudo-CLCs since their function is currently unknown. We produced recombinant Ym1 crystals which were shown to have identical crystal packing and structure by X-ray crystallography as in vivo native crystals derived from murine lung. When administered to the airways of mice, crystalline but not soluble Ym1 stimulated innate and adaptive immunity and acted as a type 2 immune adjuvant for eosinophilic inflammation via triggering of dendritic cells (DCs). Murine Ym1 protein crystals found at sites of eosinophilic inflammation reinforce type 2 immunity and could serve as a surrogate model for studying the biology of human CLCs.

Mechanism of receptor assembly via the pleiotropic adipokine Leptin.

The adipokine Leptin activates its receptor LEP-R in the hypothalamus to regulate body weight and exerts additional pleiotropic functions in immunity, fertility and cancer. However, the structure and mechanism of Leptin-mediated LEP-R assemblies has remained unclear. Intriguingly, the signaling-competent isoform of LEP-R is only lowly abundant amid several inactive short LEP-R isoforms contributing to a mechanistic conundrum. Here we show by X-ray crystallography and cryo-EM that, in contrast to long-standing paradigms, Leptin induces type I cytokine receptor assemblies featuring 3:3 stoichiometry and demonstrate such Leptin-induced trimerization of LEP-R on living cells via single-molecule microscopy. In mediating these assemblies, Leptin undergoes drastic restructuring that activates its site III for binding to the Ig domain of an adjacent LEP-R. These interactions are abolished by mutations linked to obesity. Collectively, our study provides the structural and mechanistic framework for how evolutionarily conserved Leptin:LEP-R assemblies with 3:3 stoichiometry can engage distinct LEP-R isoforms to achieve signaling.

A structural view of the inactivation of the SARS coronavirus main proteinase by benzotriazole esters.

The main proteinase (M(pro)) of the severe acute respiratory syndrome (SARS) coronavirus is a principal target for the design of anticoronaviral compounds. Benzotriazole esters have been reported as potent nonpeptidic inhibitors of the enzyme, but their exact mechanism of action remains unclear. Here we present crystal structures of SARS-CoV M(pro), the active-site cysteine of which has been acylated by benzotriazole esters that act as suicide inhibitors. In one of the structures, the thioester product has been hydrolyzed and benzoic acid is observed to bind to the hydrophobic S2 pocket. This structure also features the enzyme with a shortened N-terminal segment ("amputated N finger"). The results further the understanding of the important role of the N finger for catalysis as well as the design of benzotriazole inhibitors with improved specificity.

Protein crystallization promotes type 2 immunity and is reversible by antibody treatment.

Although spontaneous protein crystallization is a rare event in vivo, Charcot-Leyden crystals (CLCs) consisting of galectin-10 (Gal10) protein are frequently observed in eosinophilic diseases, such as asthma. We found that CLCs derived from patients showed crystal packing and Gal10 structure identical to those of Gal10 crystals grown in vitro. When administered to the airways, crystalline Gal10 stimulated innate and adaptive immunity and acted as a type 2 adjuvant. By contrast, a soluble Gal10 mutein was inert. Antibodies directed against key epitopes of the CLC crystallization interface dissolved preexisting CLCs in patient-derived mucus within hours and reversed crystal-driven inflammation, goblet-cell metaplasia, immunoglobulin E (IgE) synthesis, and bronchial hyperreactivity (BHR) in a humanized mouse model of asthma. Thus, protein crystals may promote hallmark features of asthma and are targetable by crystal-dissolving antibodies.

pH-dependent conformational flexibility of the SARS-CoV main proteinase (M(pro)) dimer: molecular dynamics simulations and multiple X-ray structure analyses.

The SARS coronavirus main proteinase (M(pro)) is a key enzyme in the processing of the viral polyproteins and thus an attractive target for the discovery of drugs directed against SARS. The enzyme has been shown by X-ray crystallography to undergo significant pH-dependent conformational changes. Here, we assess the conformational flexibility of the M(pro) by analysis of multiple crystal structures (including two new crystal forms) and by molecular dynamics (MD) calculations. The MD simulations take into account the different protonation states of two histidine residues in the substrate-binding site and explain the pH-activity profile of the enzyme. The low enzymatic activity of the M(pro) monomer and the need for dimerization are also discussed.

Multifunctional xylooligosaccharide/cephalosporin C deacetylase revealed by the hexameric structure of the Bacillus subtilis enzyme at 1.9A resolution.

Esterases and deacetylases active on carbohydrate ligands have been classified into 14 families based upon amino acid sequence similarities. Enzymes from carbohydrate esterase family seven (CE-7) are unusual in that they display activity towards both acetylated xylooligosaccharides and the antibiotic, cephalosporin C. The 1.9A structure of the multifunctional CE-7 esterase (hereinafter CAH) from Bacillus subtilis 168 reveals a classical alpha/beta hydrolase fold encased within a 32 hexamer. This is the first example of a hexameric alpha/beta hydrolase and is further evidence of the versatility of this particular fold, which is used in a wide variety of biological contexts. A narrow entrance tunnel leads to the centre of the molecule, where the six active-centre catalytic triads point towards the tunnel interior and thus are sequestered away from cytoplasmic contents. By analogy to self-compartmentalising proteases, the tunnel entrance may function to hinder access of large substrates to the poly-specific active centre. This would explain the observation that the enzyme is active on a variety of small, acetylated molecules. The structure of an active site mutant in complex with the reaction product, acetate, reveals details of the putative oxyanion binding site, and suggests that substrates bind predominantly through non-specific contacts with protein hydrophobic residues. Protein residues involved in catalysis are tethered by interactions with protein excursions from the canonical alpha/beta hydrolase fold. These excursions also mediate quaternary structure maintenance, so it would appear that catalytic competence is only achieved on protein multimerisation. We suggest that the acetyl xylan esterase (EC 3.1.1.72) and cephalosporin C deacetylase (EC 3.1.1.41) enzymes of the CE-7 family represent a single class of proteins with a multifunctional deacetylase activity against a range of small substrates.

Structure and properties of a complex of α-synuclein and a single-domain camelid antibody.

The aggregation of the intrinsically disordered protein α-synuclein to form fibrillar amyloid structures is intimately associated with a variety of neurological disorders, most notably Parkinson's disease. The molecular mechanism of α-synuclein aggregation and toxicity is not yet understood in any detail, not least because of the paucity of structural probes through which to study the behavior of such a disordered system. Here, we describe an investigation involving a single-domain camelid antibody, NbSyn2, selected by phage display techniques to bind to α-synuclein, including the exploration of its effects on the in vitro aggregation of the protein under a variety of conditions. We show using isothermal calorimetric methods that NbSyn2 binds specifically to monomeric α-synuclein with nanomolar affinity and by means of NMR spectroscopy that it interacts with the four C-terminal residues of the protein. This latter finding is confirmed by the determination of a crystal structure of NbSyn2 bound to a peptide encompassing the nine C-terminal residues of α-synuclein. The NbSyn2:α-synuclein interaction is mediated mainly by side-chain interactions while water molecules cross-link the main-chain atoms of α-synuclein to atoms of NbSyn2, a feature we believe could be important in intrinsically disordered protein interactions more generally. The aggregation behavior of α-synuclein at physiological pH, including the morphology of the resulting fibrillar structures, is remarkably unaffected by the presence of NbSyn2 and indeed we show that NbSyn2 binds strongly to the aggregated as well as to the soluble forms of α-synuclein. These results give strong support to the conjecture that the C-terminal region of the protein is not directly involved in the mechanism of aggregation and suggest that binding of NbSyn2 could be a useful probe for the identification of α-synuclein aggregation in vitro and possibly in vivo.

Structural insights into the beta-mannosidase from T. reesei obtained by synchrotron small-angle X-ray solution scattering enhanced by X-ray crystallography.

A molecular envelope of the beta-mannosidase from Trichoderma reesei has been obtained by combined use of solution small-angle X-ray scattering (SAXS) and protein crystallography. Crystallographic data at 4 A resolution have been used to enhance informational content of the SAXS data and to obtain an independent, more detailed protein shape. The phased molecular replacement technique using a low resolution SAXS model, building, and refinement of a free atom model has been employed successfully. The SAXS and crystallographic free atom models exhibit a similar globular form and were used to assess available crystallographic models of glycosyl hydrolases. The structure of the beta-galactosidase, a member of a family 2, clan GHA glycosyl hydrolases, shows an excellent fit to the experimental molecular envelope and distance distribution function of the beta-mannosidase, indicating gross similarities in their three-dimensional structures. The secondary structure of beta-mannosidase quantified by circular dichroism measurements is in a good agreement with that of beta-galactosidase. We show that a comparison of distance distribution functions in combination with 1D and 2D sequence alignment techniques was able to restrict the number of possible structurally homologous proteins. The method could be applied as a general method in structural genomics and related fields once protein solution scattering data are available.