IL-17-induced dimerization of IL-17RA drives the formation of the IL-17 signalosome to potentiate signaling

Signaling through innate immune receptors such as the Toll-like receptor (TLR)/interleukin-1 receptor (IL-1R) superfamily proceeds via the assembly of large membrane-proximal complexes or “signalosomes.” Although structurally distinct, the IL-17 receptor family triggers cellular responses that are typical of innate immune receptors. The IL-17RA receptor subunit is shared by several members of the IL-17 family. Using a combination of crystallographic, biophysical, and mutational studies, we show that IL-17A, IL-17F, and IL-17A/F induce IL-17RA dimerization. X-ray analysis of the heteromeric IL-17A complex with the extracellular domains of the IL-17RA and IL-17RC receptors reveals that cytokine-induced IL-17RA dimerization leads to the formation of a 2:2:2 hexameric signaling assembly. Furthermore, we demonstrate that the formation of the IL-17 signalosome potentiates IL-17-induced IL-36γ and CXCL1 mRNA expression in human keratinocytes, compared with a dimerization-defective IL-17RA variant.

IL-17RA is the shared co-receptor for several IL-17 family members. Goepfert et al. show that IL-17 induces IL-17RA dimerization, which then drives the formation of a 2:2:2 hexameric signaling assembly with IL-17RC. Furthermore, IL-17RA dimerization potentiates IL-17 signaling in immortalized primary human keratinocytes, compared with cells expressing a dimerization-defective IL-17RA variant.

Structural Analysis Reveals that the Cytokine IL-17F Forms a Homodimeric Complex with Receptor IL-17RC to Drive IL-17RA-Independent Signaling

Interleukin-17A (IL-17A), IL-17F, and IL-17A/F heterodimers are key cytokines of the innate and adaptive immune response. Dysregulation of the IL-17 pathway contributes to immune pathology, and it is therefore important to elucidate the molecular mechanisms that govern IL-17 recognition and signaling. The receptor IL-17RC is thought to act in concert with IL-17RA to transduce IL-17A-, IL-17F-, and IL-17A/F-mediated signals. We report the crystal structure of the extracellular domain of human IL-17RC in complex with IL-17F. In contrast to the expected model, we found that IL-17RC formed a symmetrical 2:1 complex with IL-17F, thus competing with IL-17RA for cytokine binding. Using biophysical techniques, we showed that IL-17A and IL-17A/F also form 2:1 complexes with IL-17RC, suggesting the possibility of IL-17RA-independent IL-17 signaling pathways. The crystal structure of the IL-17RC:IL-17F complex provides a structural basis for IL-17F signaling through IL-17RC, with potential therapeutic applications for respiratory allergy and inflammatory bowel diseases.

The human IL-17A/F heterodimer: a two-faced cytokine with unique receptor recognition properties

IL-17A and IL-17F are prominent members of the IL-17 family of cytokines that regulates both innate and adaptive immunity. IL-17A has been implicated in chronic inflammatory and autoimmune diseases, and anti-IL-17A antibodies have shown remarkable clinical efficacy in psoriasis and psoriatic arthritis patients. IL-17A and IL-17F are homodimeric cytokines that can also form the IL-17A/F heterodimer whose precise role in health and disease remains elusive. All three cytokines signal through the assembly of a ternary complex with the IL-17RA and IL-17RC receptors. Here we report the X-ray analysis of the human IL-17A/F heterodimer that reveals a two-faced cytokine closely mimicking IL-17A as well as IL-17F. We also present the crystal structure of its complex with the IL-17RA receptor. Unexpectedly in view of the much higher affinity of this receptor toward IL-17A, we find that IL-17RA is bound to the “F-face” of the heterodimer in the crystal. Using site-directed mutagenesis, we then demonstrate that IL-17RA can also bind to the “A-face” of IL-17A/F with similar affinity. Further, we show that IL-17RC does not discriminate between the two faces of the cytokine heterodimer either, thus enabling the formation of two topologically-distinct heterotrimeric complexes with potentially different signaling properties.

Conserved inhibitory mechanism and competent ATP binding mode for adenylyltransferases with Fic fold

The ubiquitous FIC domain is evolutionarily conserved from bacteria to human and has been shown to catalyze AMP transfer onto protein side-chain hydroxyl groups. Recently, it was predicted that most catalytically competent Fic proteins are inhibited by the presence of an inhibitory helix α_inh that is provided by a cognate anti-toxin (class I), or is part of the N- or C-terminal part of the Fic protein itself (classes II and III). In vitro, inhibition is relieved by mutation of a conserved glutamate of α_inh to glycine. For the class III bacterial Fic protein NmFic from Neisseria meningitidis, the inhibitory mechanism has been elucidated. Here, we extend above study by including bacterial class I and II Fic proteins VbhT from Bartonella schoenbuchensis and SoFic from Shewanella oneidensis, respectively, and the respective E->G mutants. Comparative enzymatic and crystallographic analyses show that, in all three classes, the ATP substrate binds to the wild-type FIC domains, but with the α-phosphate in disparate and non-competent orientations. In the E->G mutants, however, the tri-phosphate moiety is found reorganized to the same tightly bound structure through a unique set of hydrogen bonds with Fic signature motif residues. The γ-phosphate adopts the location that is taken by the inhibitory glutamate in wild-type resulting in an α-phosphate orientation that can be attacked in-line by a target side-chain hydroxyl group. The latter is properly registered to the Fic active center by main-chain β-interactions with the β-hairpin flap. These data indicate that the active site motif and the exposed edge of the flap are both required to form an adenylylation-competent Fic protein.

Fic domain-catalyzed adenylylation: insight provided by the structural analysis of the type IV secretion system effector BepA

Numerous bacterial pathogens subvert cellular functions of eukaryotic host cells by the injection of effector proteins via dedicated secretion systems. The type IV secretion system (T4SS) effector protein BepA from Bartonella henselae is composed of an N-terminal Fic domain and a C-terminal Bartonella intracellular delivery domain, the latter being responsible for T4SS-mediated translocation into host cells. A proteolysis resistant fragment (residues 10-302) that includes the Fic domain shows autoadenylylation activity and adenylyl transfer onto Hela cell extract proteins as demonstrated by autoradiography on incubation with α-[(32)P]-ATP. Its crystal structure, determined to 2.9-Å resolution by the SeMet-SAD method, exhibits the canonical Fic fold including the HPFxxGNGRxxR signature motif with several elaborations in loop regions and an additional β-rich domain at the C-terminus. On crystal soaking with ATP/Mg(2+), additional electron density indicated the presence of a PP(i) /Mg(2+) moiety, the side product of the adenylylation reaction, in the anion binding nest of the signature motif. On the basis of this information and that of the recent structure of IbpA(Fic2) in complex with the eukaryotic target protein Cdc42, we present a detailed model for the ternary complex of Fic with the two substrates, ATP/Mg(2+) and target tyrosine. The model is consistent with an in-line nucleophilic attack of the deprotonated side-chain hydroxyl group onto the α-phosphorus of the nucleotide to accomplish AMP transfer. Furthermore, a general, sequence-independent mechanism of target positioning through antiparallel β-strand interactions between enzyme and target is suggested.

Crystal Structures of Human MdmX (HdmX) in Complex with p53 Peptide Analogues Reveal Surprising Conformational Changes

p53 tumor suppressor activity is negatively regulated through binding to the oncogenic proteins Hdm2 and HdmX. The p53 residues Leu(26), Trp(23), and Phe(19) are crucial to mediate these interactions. Inhibiting p53 binding to both Hdm2 and HdmX should be a promising clinical approach to reactivate p53 in the cancer setting, but previous studies have suggested that the discovery of dual Hdm2/HdmX inhibitors will be difficult. We have determined the crystal structures at 1.3 A of the N-terminal domain of HdmX bound to two p53 peptidomimetics without and with a 6-chlorine substituent on the indole (which binds in the same subpocket as Trp(23) of p53). The latter compound is the most potent peptide-based antagonist of the p53-Hdm2 interaction yet to be described. The x-ray structures revealed surprising conformational changes of the binding cleft of HdmX, including an "open conformation" of Tyr(99) and unexpected "cross-talk" between the Trp and Leu pockets. Notably, the 6-chloro p53 peptidomimetic bound with high affinity to both HdmX and Hdm2 (K(d) values of 36 and 7 nm, respectively). Our results suggest that the development of potent dual inhibitors for HdmX and Hdm2 should be feasible. They also reveal possible conformational states of HdmX, which should lead to a better prediction of its interactions with potential biological partners.

Evidence for peptide synthesis in the course of in vitro proteolysis

The present paper reports on studies concerned with furnishing evidence for peptide synthesis in the course of in vitro proteolysis. To this end, the oxidized chain B from insulin (INS) (S = 5% in demineralized water) was subjected to tryptic proteolysis (E/S = 1/50; pH 5; 37 degrees C; 24 h). HPLC-as well as amino acid-and sequence analytical studies have shown that the heptapeptide INS 23-29 (Gly-Phe-Phe-Tyr-Thr-Pro-Lys) liberated by way of hydrolysis is linked by tryptic synthesis via transpeptidation or condensation to form a dimer which accounts for 15% of the amount of monomer. The results of the model trials show clearly that during in vitro proteolysis chemical reactions beyond hydrolytic cleavage of peptide bonds take place. In principle, plastein-like reactions (transpeptidation, condensation) can occur during each in vitro proteolysis.

Semi-preparative isolation of phosphopeptides derived from bovine casein and dephosphorylation of casein phosphopeptides

Semi-preparative isolation of casein phosphopeptides (CPP) from a tryptic hydrolysate of bovine casein was performed applying a three-step procedure consisting of solid phase extraction, reversed phase HPLC and ion exchange chromatography. Dephosphorylation of CPP was achieved using immobilized alkaline phosphatase. The purified phosphopeptides and their dephosphorylated forms obtained by these methods are suitable for comparative studies on biological activities, especially mineral binding and immunmodulation.

[Intranasal administration of midazolam (Dormicum) in young children in emergency medicine]

A method for sedation of preschool children in emergency medicine is presented. Midazolam (Dormicum) is frequently used in premedication in children. Various routes of administration have been described for this drug. After use of nasal administration of midazolam for premedication we also used this method for sedation of children in emergencies. This technique provides rapid onset and cardiovascular stability while maintaining spontaneous ventilation. In two case reports the first results are demonstrated; the method is evaluated and discussed.