Structures of complete extracellular receptor assemblies mediated by IL-12 and IL-23

Cell-surface receptor complexes mediated by pro-inflammatory interleukin (IL)-12 and IL-23, both validated therapeutic targets, are incompletely understood due to the lack of structural insights into their complete extracellular assemblies. Furthermore, there is a paucity of structural details describing the IL-12-receptor interaction interfaces, in contrast to IL-23-receptor complexes. Here we report structures of fully assembled mouse IL-12/human IL-23-receptor complexes comprising the complete extracellular segments of the cognate receptors determined by electron cryo-microscopy. The structures reveal key commonalities but also surprisingly diverse features. Most notably, whereas IL-12 and IL-23 both utilize a conspicuously presented aromatic residue on their α-subunit as a hotspot to interact with the N-terminal Ig domain of their high-affinity receptors, only IL-12 juxtaposes receptor domains proximal to the cell membrane. Collectively, our findings will help to complete our understanding of cytokine-mediated assemblies of tall cytokine receptors and will enable a cytokine-specific interrogation of IL-12/IL-23 signaling in physiology and disease.

Structural basis of activation and antagonism of receptor signaling mediated by interleukin-27

Interleukin-27 (IL-27) uniquely assembles p28 and EBI3 subunits to a heterodimeric cytokine that signals via IL-27Rα and gp130. To provide the structural framework for receptor activation by IL-27 and its emerging therapeutic targeting, we report here crystal structures of mouse IL-27 in complex with IL-27Rα and of human IL-27 in complex with SRF388, a monoclonal antibody undergoing clinical trials with oncology indications. One face of the helical p28 subunit interacts with EBI3, while the opposite face nestles into the interdomain elbow of IL-27Rα to juxtapose IL-27Rα to EBI3. This orients IL-27Rα for paired signaling with gp130, which only uses its immunoglobulin domain to bind to IL-27. Such a signaling complex is distinct from those mediated by IL-12 and IL-23. The SRF388 binding epitope on IL-27 overlaps with the IL-27Rα interaction site explaining its potent antagonistic properties. Collectively, our findings will facilitate the mechanistic interrogation, engineering, and therapeutic targeting of IL-27.

Structure-aided optimization of non-nucleoside M. tuberculosis thymidylate kinase inhibitors

Mycobacterium tuberculosis thymidylate kinase (MtTMPK) has emerged as an attractive target for rational drug design. We recently investigated new families of non-nucleoside MtTMPK inhibitors in an effort to diversify MtTMPK inhibitor chemical space. We here report a new series of MtTMPK inhibitors by combining the Topliss scheme with rational drug design approaches, fueled by two co-crystal structures of MtTMPK in complex with developed inhibitors. These efforts furnished the most potent MtTMPK inhibitors in our assay, with two analogues displaying low micromolar MIC values against H37Rv Mtb. Prepared inhibitors address new sub-sites in the MtTMPK nucleotide binding pocket, thereby offering new insights into its druggability. We studied the role of efflux pumps as well as the impact of cell wall permeabilizers for selected compounds to potentially provide an explanation for the lack of correlation between potent enzyme inhibition and whole-cell activity.

Distinct EH domains of the endocytic TPLATE complex confer lipid and protein binding

Clathrin-mediated endocytosis (CME) is the gatekeeper of the plasma membrane. In contrast to animals and yeasts, CME in plants depends on the TPLATE complex (TPC), an evolutionary ancient adaptor complex. However, the mechanistic contribution of the individual TPC subunits to plant CME remains elusive. In this study, we used a multidisciplinary approach to elucidate the structural and functional roles of the evolutionary conserved N-terminal Eps15 homology (EH) domains of the TPC subunit AtEH1/Pan1. By integrating high-resolution structural information obtained by X-ray crystallography and NMR spectroscopy with all-atom molecular dynamics simulations, we provide structural insight into the function of both EH domains. Both domains bind phosphatidic acid with a different strength, and only the second domain binds phosphatidylinositol 4,5-bisphosphate. Unbiased peptidome profiling by mass-spectrometry revealed that the first EH domain preferentially interacts with the double N-terminal NPF motif of a previously unidentified TPC interactor, the integral membrane protein Secretory Carrier Membrane Protein 5 (SCAMP5). Furthermore, we show that AtEH/Pan1 proteins control the internalization of SCAMP5 via this double NPF peptide interaction motif. Collectively, our structural and functional studies reveal distinct but complementary roles of the EH domains of AtEH/Pan1 in plant CME and connect the internalization of SCAMP5 to the TPLATE complex.

Molecular architecture of the endocytic TPLATE complex

Eukaryotic cells rely on endocytosis to regulate their plasma membrane proteome and lipidome. Most eukaryotic groups, except fungi and animals, have retained the evolutionary ancient TSET complex as an endocytic regulator. Unlike other coatomer complexes, structural insight into TSET is lacking. Here, we reveal the molecular architecture of plant TSET [TPLATE complex (TPC)] using an integrative structural approach. We identify crucial roles for specific TSET subunits in complex assembly and membrane interaction. Our data therefore generate fresh insight into the differences between the hexameric TSET in Dictyostelium and the octameric TPC in plants. Structural elucidation of this ancient adaptor complex represents the missing piece in the coatomer puzzle and vastly advances our functional as well as evolutionary insight into the process of endocytosis.

Endeavors towards transformation of M. tuberculosis thymidylate kinase (MtbTMPK) inhibitors into potential antimycobacterial agents

As the last enzyme in nucleotide synthesis as precursors for DNA replication, thymidylate kinase of M. tuberculosis (MtbTMPK) attracts significant interest as a target in the discovery of new anti-tuberculosis agents. Earlier, we discovered potent MtbTMPK inhibitors, but these generally suffered from poor antimycobacterial activity, which we hypothesize is due to poor bacterial uptake. To address this, we herein describe our efforts to equip previously reported MtbTMPK inhibitors with targeting moieties to increase the whole cell activity of the hybrid analogues. Introduction of a simplified Fe-chelating siderophore motif gave rise to analogue 17 that combined favorable enzyme inhibitory activity with significant activity against M. tuberculosis (MIC of 12.5 μM). Conjugation of MtbTMPK inhibitors with an imidazo[1,2-a]pyridine or 3,5-dinitrobenzamide scaffold afforded analogues 26, 27 and 28, with moderate MtbTMPK enzyme inhibitory potency, but sub-micromolar activity against mycobacteria without significant cytotoxicity. These results indicate that conjugation with structural motifs known to favor mycobacterial uptake may be a valid approach for discovering new antimycobacterial agents.

Structural basis of latent TGF-β1 presentation and activation by GARP on human regulatory T cells

Transforming growth factor–β1 (TGF-β1) is one of very few cytokines produced in a latent form, requiring activation to exert any of its vastly diverse effects on development, immunity, and cancer. Regulatory T cells (Tregs) suppress immune cells within close proximity by activating latent TGF-β1 presented by GARP (glycoprotein A repetitions predominant) to integrin αVβ8 on their surface. We solved the crystal structure of GARP:latent TGF-β1 bound to an antibody that stabilizes the complex and blocks release of active TGF-β1. This finding reveals how GARP exploits an unusual medley of interactions, including fold complementation by the amino terminus of TGF-β1, to chaperone and orient the cytokine for binding and activation by αVβ8. Thus, this work further elucidates the mechanism of antibody-mediated blockade of TGF-β1 activation and immunosuppression by Tregs.

The substrate-free and -bound crystal structures of the duplicated taurocyamine kinase from the human parasite Schistosoma mansoni

The taurocyamine kinase from the blood fluke Schistosoma mansoni (SmTK) belongs to the phosphagen kinase (PK) family and catalyzes the reversible Mg(2+)-dependent transfer of a phosphoryl group between ATP and taurocyamine. SmTK is derived from gene duplication, as are all known trematode TKs. Our crystallographic study of SmTK reveals the first atomic structure of both a TK and a PK with a bilobal structure. The two unliganded lobes present a canonical open conformation and interact via their respective C- and N-terminal domains at a helix-mediated interface. This spatial arrangement differs from that observed in true dimeric PKs, in which both N-terminal domains make contact. Our structures of SmTK complexed with taurocyamine or l-arginine compounds explain the mechanism by which an arginine residue of the phosphagen specificity loop is crucial for substrate specificity. An SmTK crystal was soaked with the dead end transition state analog (TSA) components taurocyamine-NO3 (2-)-MgADP. One SmTK monomer was observed with two bound TSAs and an asymmetric conformation, with the first lobe semiclosed and the second closed. However, isothermal titration calorimetry and enzyme kinetics experiments showed that the two lobes function independently. A small angle x-ray scattering model of SmTK-TSA in solution with two closed active sites was generated.

The molecular mechanism of thermostable α-galactosidases AgaA and AgaB explained by x-ray crystallography and mutational studies

The α-galactosidase AgaA from the thermophilic microorganism Geobacillus stearothermophilus has great industrial potential because it is fully active at 338 K against raffinose and can increase the yield of manufactured sucrose. AgaB has lower affinity for its natural substrates but is a powerful tool for the enzymatic synthesis of disaccharides by transglycosylation. These two enzymes have 97% identity and belong to the glycoside hydrolase (GH) family GH36 for which few structures are available. To understand the structural basis underlying the differences between these two enzymes, we determined the crystal structures of AgaA and AgaB by molecular replacement at 3.2- and 1.8 Å-resolution, respectively. We also solved a 2.8-Å structure of the AgaA(A355E) mutant, which has enzymatic properties similar to those of AgaB. We observe that residue 355 is located 20 Å away from the active site and that the A355E substitution causes structural rearrangements resulting in a significant displacement of the invariant Trp(336) at catalytic subsite -1. Hence, the active cleft of AgaA is narrowed in comparison with AgaB, and AgaA is more efficient than AgaB against its natural substrates. The structure of AgaA(A355E) complexed with 1-deoxygalactonojirimycin reveals an induced fit movement; there is a rupture of the electrostatic interaction between Glu(355) and Asn(335) and a return of Trp(336) to an optimal position for ligand stacking. The structures of two catalytic mutants of AgaA(A355E) complexed with raffinose and stachyose show that the binding interactions are stronger at subsite -1 to enable the binding of various α-galactosides.

A crystal structure of the catalytic core domain of an avian sarcoma and leukemia virus integrase suggests an alternate dimeric assembly

Integrase (IN) is an important therapeutic target in the search for anti-Human Immunodeficiency Virus (HIV) inhibitors. This enzyme is composed of three domains and is hard to crystallize in its full form. First structural results on IN were obtained on the catalytic core domain (CCD) of the avian Rous and Sarcoma Virus strain Schmidt-Ruppin A (RSV-A) and on the CCD of HIV-1 IN. A ribonuclease-H like motif was revealed as well as a dimeric interface stabilized by two pairs of α-helices (α1/α5, α5/α1). These structural features have been validated in other structures of IN CCDs. We have determined the crystal structure of the Rous-associated virus type-1 (RAV-1) IN CCD to 1.8 Å resolution. RAV-1 IN shows a standard activity for integration and its CCD differs in sequence from that of RSV-A by a single accessible residue in position 182 (substitution A182T). Surprisingly, the CCD of RAV-1 IN associates itself with an unexpected dimeric interface characterized by three pairs of α-helices (α3/α5, α1/α1, α5/α3). A182 is not involved in this novel interface, which results from a rigid body rearrangement of the protein at its α1, α3, α5 surface. A new basic groove that is suitable for single-stranded nucleic acid binding is observed at the surface of the dimer. We have subsequently determined the structure of the mutant A182T of RAV-1 IN CCD and obtained a RSV-A IN CCD-like structure with two pairs of buried α-helices at the interface. Our results suggest that the CCD of avian INs can dimerize in more than one state. Such flexibility can further explain the multifunctionality of retroviral INs, which beside integration of dsDNA are implicated in different steps of the retroviral cycle in presence of viral ssRNA.

Effects of calcitonin on basal and thyrotropin-releasing hormone-stimulated prolactin secretion in man

Plasma PRL fell in nine healthy subjects and four patients with hyperprolactinemia after iv administration of salmon calcitonin (CT). The maximum fall was observed 30--60 min after the infusion. There was no change in the plasma concentrations of the other anterior pituitary hormones tested (GH, FSH, LH, and TSH). In five healthy subjects, TRH was injected 60 min after the CT infusion. This protocol was repeated in the same subjects at 3-day intervals, except CT was not administered. Plasma PRL before TRH injection was clearly lower when CT had been administered. Plasma concentrations of the other anterior pituitary hormones did not change. PRL and TSH responses to TRH were markedly inhibited when CT had been previously infused. These observations are in agreement with preceeding studies showing a similar effect of CT on the plasma concentration of various other polypeptide hormones. This general effect of CT could be attributed to a change in intracellular calcium of the secreting cells.