In this paper, we describe the first prospective application of the shape-comparison program ROCS (Rapid Overlay of Chemical Structures) to find new scaffolds for small molecule inhibitors of the ZipA-FtsZ protein-protein interaction, a proposed antibacterial target. The shape comparisons are made relative to the crystallographically determined, bioactive conformation of a high-throughput screening (HTS) hit. The use of ROCS led to the identification of a set of novel, weakly binding inhibitors with scaffolds presenting synthetic opportunities to further optimize biological affinity and lacking development issues associated with the HTS lead. These ROCS-identified scaffolds would have been missed using other structural similarity approaches such as ISIS 2D fingerprints. X-ray crystallographic analysis of one of the new inhibitors bound to ZipA reveals that the shape comparison approach very accurately predicted the binding mode. These experimental results validate this use of ROCS for chemotype switching or "lead hopping" and suggest that it is of general interest for lead identification in drug discovery endeavors.

Optimal immune responses require both an antigen-specific and a co-stimulatory signal. The shared ligands B7-1 and B7-2 on antigen-presenting cells deliver the co-stimulatory signal through CD28 and CTLA-4 on T cells. Signalling through CD28 augments the T-cell response, whereas CTLA-4 signalling attenuates it. Numerous animal studies and recent clinical trials indicate that manipulating these interactions holds considerable promise for immunotherapy. With the consequences of these signals well established, and details of the downstream signalling events emerging, understanding the molecular nature of these extracellular interactions becomes crucial. Here we report the crystal structure of the human CTLA-4/B7-1 co-stimulatory complex at 3.0 A resolution. In contrast to other interacting cell-surface molecules, the relatively small CTLA-4/B7-1 binding interface exhibits an unusually high degree of shape complementarity. CTLA-4 forms homodimers through a newly defined interface of highly conserved residues. In the crystal lattice, CTLA-4 and B7-1 pack in a strikingly periodic arrangement in which bivalent CTLA-4 homodimers bridge bivalent B7-1 homodimers. This zipper-like oligomerization provides the structural basis for forming unusually stable signalling complexes at the T-cell surface, underscoring the importance of potent inhibitory signalling in human immune responses.

Human calcium-sensing receptor (CaSR) is a G-protein-coupled receptor (GPCR) that maintains extracellular Ca2+ homeostasis through the regulation of parathyroid hormone secretion. It functions as a disulfide-tethered homodimer composed of three main domains, the Venus Flytrap module, cysteine-rich domain, and seven-helix transmembrane region. Here, we present the crystal structures of the entire extracellular domain of CaSR in the resting and active conformations. We provide direct evidence that L-amino acids are agonists of the receptor. In the active structure, L-Trp occupies the orthosteric agonist-binding site at the interdomain cleft and is primarily responsible for inducing extracellular domain closure to initiate receptor activation. Our structures reveal multiple binding sites for Ca2+ and PO43- ions. Both ions are crucial for structural integrity of the receptor. While Ca2+ ions stabilize the active state, PO43- ions reinforce the inactive conformation. The activation mechanism of CaSR involves the formation of a novel dimer interface between subunits.DOI: http://dx.doi.org/10.7554/eLife.13662.001

In Escherichia coli, FtsZ, a homologue of eukaryotic tubulins, and ZipA, a membrane-anchored protein that binds to FtsZ, are two essential components of the septal ring structure that mediates cell division. Recent data indicate that ZipA is involved in the assembly of the ring by linking FtsZ to the cytoplasmic membrane and that the ZipA±FtsZ interaction is mediated by their C-terminal domains. We present the X-ray crystal structures of the C-terminal FtsZbinding domain of ZipA and a complex between this domain and a C-terminal fragment of FtsZ. The ZipA domain is a six-stranded b-sheet packed against three a-helices and contains the split b±a±b motif found in many RNA-binding proteins. The uncovered side of the sheet incorporates a shallow hydrophobic cavity exposed to solvent. In the complex, the 17-residue FtsZ fragment occupies this entire cavity of ZipA and binds as an extended b-strand followed by a-helix. An alanine-scanning mutagenesis analysis of the FtsZ fragment was also performed, which shows that only a small cluster of the buried FtsZ side chains is critical in binding to ZipA. Keywords: bacterial cell division/crystal structure/FtsZ/ single isomorphous replacement/ZipA

Human GABAB receptor is a G-protein coupled receptor central to inhibitory neurotransmission in the brain. It functions as an obligatory heterodimer of GBR1 and GBR2 subunits. Here we present the first crystal structures of a heterodimeric complex between the extracellular domains of GBR1 and GBR2 in the apo, agonist-bound, and antagonist-bound forms. The apo and antagonist-bound structures represent the resting state of the receptor; the agonist-bound complex corresponds to the active state. Both subunits adopt an open conformation at rest, and only GBR1 closes upon agonist-induced receptor activation. The agonists and antagonists are anchored in the interdomain crevice of GBR1 by an overlapping set of residues. An antagonist confines GBR1 to the open conformation of the inactive state, while an agonist induces its domain closure for activation. Our data reveals a unique activation mechanism for GABAB receptor that involves the formation of a novel heterodimer interface between subunits.