B7-1 and B7-2 are generally thought to have comparable structures and affinities for their receptors, CD28 and CTLA-4, each of which is assumed to be bivalent. We show instead (1) that B7-2 binds the two receptors more weakly than B7-1, (2) that, relative to its CTLA-4 binding affinity, B7-2 binds CD28 2- to 3-fold more effectively than B7-1, (3) that, unlike B7-1, B7-2 does not self-associate, and (4) that, in contrast to CTLA-4 homodimers, which are bivalent, CD28 homodimers are monovalent. Our results indicate that B7-1 markedly favors CTLA-4 over CD28 engagement, whereas B7-2 exhibits much less bias. We propose that the distinct structures and binding properties of B7-1 and B7-2 account for their overlapping but distinct effects on T cell responses.

The coagulation system is characterized by the sequential and highly localized activation of a series of serine proteases, culminating in the conversion of fibrinogen into fibrin, and formation of a fibrin clot. Here we show that C-terminal peptides of thrombin, a key enzyme in the coagulation cascade, constitute a novel class of host defense peptides, released upon proteolysis of thrombin in vitro, and detected in human wounds in vivo. Under physiological conditions, these peptides exert antimicrobial effects against Gram-positive and Gram-negative bacteria, mediated by membrane lysis, as well as immunomodulatory functions, by inhibiting macrophage responses to bacterial lipopolysaccharide. In mice, they are protective against P. aeruginosa sepsis, as well as lipopolysaccharide-induced shock. Moreover, the thrombin-derived peptides exhibit helical structures upon binding to lipopolysaccharide and can also permeabilize liposomes, features typical of “classical” helical antimicrobial peptides. These findings provide a novel link between the coagulation system and host-defense peptides, two fundamental biological systems activated in response to injury and microbial invasion.

Naive T cell activation requires signaling by the T cell receptor and by nonclonotypic cell surface receptors. The most important costimulatory protein is the monovalent homodimer CD28, which interacts with CD80 and CD86 expressed on antigen-presenting cells. Here we present the crystal structure of a soluble form of CD28 in complex with the Fab fragment of a mitogenic antibody. Structural comparisons redefine the evolutionary relationships of CD28-related proteins, antigen receptors and adhesion molecules and account for the distinct ligand-binding and stoichiometric properties of CD28 and the related, inhibitory homodimer CTLA-4. Cryo-electron microscopy-based comparisons of complexes of CD28 with mitogenic and nonmitogenic antibodies place new constraints on models of antibody-induced receptor triggering. This work completes the initial structural characterization of the CD28-CTLA-4-CD80-CD86 signaling system.

The bacterial ω‐transaminase from Chromobacterium violaceum (Cv‐ωTA, http://www.chem.qmul.ac.uk/iubmb/enzyme/EC2/6/1/18.html) catalyses industrially important transamination reactions by use of the coenzyme pyridoxal 5′‐phosphate (PLP). Here, we present four crystal structures of Cv‐ωTA: two in the apo form, one in the holo form and one in an intermediate state, at resolutions between 1.35 and 2.4 Å. The enzyme is a homodimer with a molecular mass of ∼ 100 kDa. Each monomer has an active site at the dimeric interface that involves amino acid residues from both subunits. The apo‐Cv‐ωTA structure reveals unique ‘relaxed’ conformations of three critical loops involved in structuring the active site that have not previously been seen in a transaminase. Analysis of the four crystal structures reveals major structural rearrangements involving elements of the large and small domains of both monomers that reorganize the active site in the presence of PLP. The conformational change appears to be triggered by binding of the phosphate group of PLP. Furthermore, one of the apo structures shows a disordered ‘roof ’ over the PLP‐binding site, whereas in the other apo form and the holo form the ‘roof’ is ordered. Comparison with other known transaminase crystal structures suggests that ordering of the ‘roof’ structure may be associated with substrate binding in Cv‐ωTA and some other transaminases. Database The atomic coordinates and structure factors for the Chromobacterium violaceumω‐transaminase crystal structures can be found in the RCSB Protein Data Bank (http://www.rcsb.org) under the accession codes http://www.rcsb.org/pdb/search/structidSearch.do?structureId=4A6U for the holoenzyme, http://www.rcsb.org/pdb/search/structidSearch.do?structureId=4A6R for the apo1 form, http://www.rcsb.org/pdb/search/structidSearch.do?structureId=4A6T for the apo2 form and http://www.rcsb.org/pdb/search/structidSearch.do?structureId=4A72 for the mixed form Structured digital abstract  http://www.uniprot.org/uniprot/Q7NWG4 and http://www.uniprot.org/uniprot/Q7NWG4 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0407 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0038 (http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-8300874)  http://www.uniprot.org/uniprot/Q7NWG4 and http://www.uniprot.org/uniprot/Q7NWG4 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0407 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0114 (http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-8300763)  http://www.uniprot.org/uniprot/Q7NWG4 and http://www.uniprot.org/uniprot/Q7NWG4 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0407 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0114 (http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-8300950)

The development of non-genotoxic therapies that activate wild-type p53 in tumors is of great interest since the discovery of p53 as a tumor suppressor. Here we report the identification of over 100 small-molecules activating p53 in cells. We elucidate the mechanism of action of a chiral tetrahydroindazole (HZ00), and through target deconvolution, we deduce that its active enantiomer (R)-HZ00, inhibits dihydroorotate dehydrogenase (DHODH). The chiral specificity of HZ05, a more potent analog, is revealed by the crystal structure of the (R)-HZ05/DHODH complex. Twelve other DHODH inhibitor chemotypes are detailed among the p53 activators, which identifies DHODH as a frequent target for structurally diverse compounds. We observe that HZ compounds accumulate cancer cells in S-phase, increase p53 synthesis, and synergize with an inhibitor of p53 degradation to reduce tumor growth in vivo. We, therefore, propose a strategy to promote cancer cell killing by p53 instead of its reversible cell cycle arresting effect.