Recognition by a T-cell antigen receptor (TCR) of peptide complexed with a major histocompatibility complex (MHC) molecule occurs through variable loops in the TCR structure which bury almost all the available peptide and a much larger area of the MHC molecule. The TCR fits diagonally across the MHC peptide-binding site in a surface feature common to all class I and class II MHC molecules, providing evidence that the nature of binding is general. A broadly applicable binding mode has implications for the mechanism of repertoire selection and the magnitude of alloreactions.

Follicle-stimulating hormone (FSH) is central to reproduction in mammals. It acts through a G-protein-coupled receptor on the surface of target cells to stimulate testicular and ovarian functions. We present here the 2.9-A-resolution structure of a partially deglycosylated complex of human FSH bound to the extracellular hormone-binding domain of its receptor (FSHR(HB)). The hormone is bound in a hand-clasp fashion to an elongated, curved receptor. The buried interface of the complex is large (2,600 A2) and has a high charge density. Our analysis suggests that all glycoprotein hormones bind to their receptors in this mode and that binding specificity is mediated by key interaction sites involving both the common alpha- and hormone-specific beta-subunits. On binding, FSH undergoes a concerted conformational change that affects protruding loops implicated in receptor activation. The FSH-FSHR(HB) complexes form dimers in the crystal and at high concentrations in solution. Such dimers may participate in transmembrane signal transduction.

Inhibitory natural killer (NK) cell receptors down-regulate the cytotoxicity of NK cells upon recognition of specific class I major histocompatibility complex (MHC) molecules on target cells. We report here the crystal structure of the inhibitory human killer cell immunoglobulin-like receptor 2DL1 (KIR2DL1) bound to its class I MHC ligand, HLA-Cw4. The KIR2DL1-HLA-Cw4 interface exhibits charge and shape complementarity. Specificity is mediated by a pocket in KIR2DL1 that hosts the Lys80 residue of HLA-Cw4. Many residues conserved in HLA-C and in KIR2DL receptors make different interactions in KIR2DL1-HLA-Cw4 and in a previously reported KIR2DL2-HLA-Cw3 complex. A dimeric aggregate of KIR-HLA-C complexes was observed in one KIR2DL1-HLA-Cw4 crystal. Most of the amino acids that differ between human and chimpanzee KIRs with HLA-C specificities form solvent-accessible clusters outside the KIR-HLA interface, which suggests undiscovered interactions by KIRs.

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

SWI/SNF, an evolutionarily conserved ATP-dependent chromatin-remodeling complex, has an important role in transcriptional regulation 1 . In Saccharomyces cerevisiae, SWI/SNF regulates the expression of ~6% of total genes through activation or repression 2 . Swi1, a subunit of SWI/SNF, contains an N-terminal region rich in glutamine and asparagine, a notable feature shared by all characterized yeast prions-a group of unique proteins capable of self-perpetuating changes in conformation and function 3 . Here we provide evidence that Swi1 can become a prion, [ [PIN + ] are three reported prions in S. cerevisiae 6-9 , and their protein determinants are Sup35, Ure2 and Rnq1, respectively. They all contain an intrinsic domain with high glutamine and asparagine content essential for prion formation and propagation (Fig. 1a). Yeast prions can be formed and lost at a low spontaneous rate 10 . Overproduction of a prion protein considerably increases de novo formation of the corresponding prion 8,11,12 . [PSI + ] induction by Sup35 overproduction requires a [PSI + ] inducible factor (Pin + ), which can be an event such as overproduction of particular glutamine-, asparagine-or glutamine and asparagine-rich proteins, or another prion 13-15 . Notably, several members of the yeast SWI/SNF complex, including Swi1 and Snf5, also contain glutamine and asparagine-rich regions 16 (Fig. 1a), and Swi1 overproduction has been implicated as a Pin + factor 9 . SWI/SNF is an evolutionarily conserved, ATP-dependent chromatin-remodeling complex that has a regulatory role in gene expression 17 . In S. cerevisiae, the SWI/SNF complex is composed of at least 11 subunits, for a total molecular weight of ~1 MDa 18 . Null mutants of SWI/SNF are viable, but show phenotypes of slow growth, reduced mating-type switching, and inability to utilize nonfermentable carbon sources, such as raffinose, galactose, glycerol and sucrose 19 . In mammals, Ini1, a Snf5 homolog, is essential for embryo viability and tumor suppression 20 . Various mutations affecting the human Ini1 or Snf2 homologs, BRM and BRG-1, are (Fig. 1b-d). Although the Nterminal region of Snf5 is also rich in glutamine (Fig. 1a), overexpression of SNF5 did not function as a Pin + (Fig. 1b) Only two were able to grow on raffinose after 5 mM guanidine hydrochloride treatment (Fig. 2a). This guanidine hydrochloride treatment eliminates all known yeast prions, but does not usually cause nucleic acid mutations 23 . Like SWI/SNF mutants, both candidates grew poorly in galactose (Gal -) and glycerol (Gly -) (Fig. 2b). However, they showed only a mild defect in using sucrose (Fig. 2b). Unlike SWI/SNF mutants, they did not show any detectable sensitivity to 0.3 M LiCl or 1 M NaCl ( Fig. 2b and data not shown). These results suggest that the two [SWI + ] candidates show an epigenetically conferred, partial loss-of-function SWI/SNF phenotype.To further investigate whether the observed effect is SWI/SNF specific, we developed a bluewhite visual assay using a previously described lacZ repor...

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.

Heterocysts, formed when filamentous cyanobacteria, such as Anabaena sp. strain PCC 7120, are grown in the absence of combined nitrogen, are cells that are specialized in fixing atmospheric nitrogen (N 2 ) under oxic conditions and that transfer fixed nitrogen to the vegetative cells of the filament. Anabaena sp. mutants whose sepJ gene (open reading frame alr2338 of the Anabaena sp. genome) was affected showed filament fragmentation and arrested heterocyst differentiation at an early stage. In a sepJ insertional mutant, a layer similar to a heterocyst polysaccharide layer was formed, but the heterocyst-specific glycolipids were not synthesized. The sepJ mutant did not exhibit nitrogenase activity even when assayed under anoxic conditions. In contrast to proheterocysts produced in the wild type, those produced in the sepJ mutant still divided. SepJ is a multidomain protein whose N-terminal region is predicted to be periplasmic and whose C-terminal domain resembles an export permease. Using a green fluorescent protein translationally fused to the carboxyl terminus of SepJ, we observed that in mature heterocysts and vegetative cells, the protein is localized at the intercellular septa, and when cell division starts, it is localized in a ring whose position is similar to that of a Z ring. SepJ is a novel composite protein needed for filament integrity, proper heterocyst development, and diazotrophic growth.

SummaryPhotoreduction of dinitrogen by heterocyst-forming cyanobacteria is of great importance ecologically and for subsistence rice agriculture. Their heterocysts must have a glycolipid envelope layer that limits the entry of oxygen if nitrogenase is to remain active to fix dinitrogen in an oxygen-containing milieu (the Fox + + + + phenotype). Genes alr5354 ( hglD ), alr5355 ( hglC ) and alr5357 ( hglB ) of the filamentous cyanobacterium, Anabaena sp. strain PCC 7120, and hglE of Nostoc punctiforme are required for synthesis of heterocyst envelope glycolipids. Newly identified Fox -mutants bear transposons in nearby open reading frames (orfs) all5343 , all5345-asr5349 and alr5351-alr5358 . Complementation and other analysis provide evidence that at least orfs all5343 (or a co-transcribed gene), all5345 , all5347 , alr5348 , asr5350-alr5353 and alr5356 , but not asr5349 , are also required for a Fox + + + + phenotype. Lipid and sequence analyses suggest that alr5351-alr5357 encode the enzymes that biosynthesize the glycolipid aglycones. Electron microscopy indicates a role of all5345 through all5347 in the normal deposition of the envelope glycolipids.