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The insect immune deficiency (IMD) pathway resembles the tumour necrosis factor receptor network in mammals and senses diaminopimelic-type peptidoglycans present in Gram-negative bacteria. Whether unidentified chemical moieties activate the IMD signalling cascade remains unknown. Here, we show that infection-derived lipids 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) and 1-palmitoyl-2-oleoyl diacylglycerol (PODAG) stimulate the IMD pathway of ticks. The tick IMD network protects against colonization by three distinct bacteria, that is the Lyme disease spirochete Borrelia burgdorferi and the rickettsial agents Anaplasma phagocytophilum and A. marginale. Cell signalling ensues in the absence of transmembrane peptidoglycan recognition proteins and the adaptor molecules Fas-associated protein with a death domain (FADD) and IMD. Conversely, biochemical interactions occur between x-linked inhibitor of apoptosis protein (XIAP), an E3 ubiquitin ligase, and the E2 conjugating enzyme Bendless. We propose the existence of two functionally distinct IMD networks, one in insects and another in ticks.

Helicobacter pylori (Hp) strains that carry the cag type IV secretion system (cag-T4SS) to inject the cytotoxin-associated antigen A (CagA) into host cells are associated with peptic ulcer disease and gastric adenocarcinoma. CagA translocation by Hp is mediated by β1 integrin interaction of the cag-T4SS. However, other cellular receptors or bacterial outer membrane adhesins essential for this process are unknown. Here, we identify the HopQ protein as a genuine Hp adhesin, exploiting defined members of the carcinoembryonic antigen-related cell adhesion molecule family (CEACAMs) as host cell receptors. HopQ binds the amino-terminal IgV-like domain of human CEACAM1, CEACAM3, CEACAM5 or CEACAM6 proteins, thereby enabling translocation of the major pathogenicity factor CagA into host cells. The HopQ-CEACAM interaction is characterized by a remarkably high affinity (K from 23 to 268 nM), which is independent of CEACAM glycosylation, identifying CEACAMs as bona fide protein receptors for Hp. Our data suggest that the HopQ-CEACAM interaction contributes to gastric colonization or Hp-induced pathologies, although the precise role and functional consequences of this interaction in vivo remain to be determined.

The bacterial flagellar filament has long been studied to understand how a polymer composed of a single protein can switch between different supercoiled states with high cooperativity. Here we present near-atomic resolution cryo-EM structures for flagellar filaments from both Gram-positive Bacillus subtilis and Gram-negative Pseudomonas aeruginosa. Seven mutant flagellar filaments in B. subtilis and two in P. aeruginosa capture two different states of the filament. These reliable atomic models of both states reveal conserved molecular interactions in the interior of the filament among B. subtilis, P. aeruginosa and Salmonella enterica. Using the detailed information about the molecular interactions in two filament states, we successfully predict point mutations that shift the equilibrium between those two states. Further, we observe the dimerization of P. aeruginosa outer domains without any perturbation of the conserved interior of the filament. Our results give new insights into how the flagellin sequence has been “tuned” over evolution.

The mechanisms of HLA-DM catalyzed peptide exchange remain uncertain. We found that all stages of the interaction of DM with HLA-DR were dependent on the occupancy state of the peptide binding groove. High-affinity peptides were protected from removal by DM through two mechanisms: peptide binding induced dissociation of a long-lived complex of empty DR and DM, and high-affinity DR-peptide complexes bound DM only very slowly. Non-binding covalent DR-peptide complexes were converted to efficient DM binders upon truncation of an N-terminal peptide segment that emptied the P1 pocket and disrupted conserved hydrogen bonds to MHC. DM thus only binds to DR conformers in which a critical part of the binding site is vacant, due to spontaneous peptide motion.

Severe acute respiratory syndrome (SARS) emerged in November 2002 as a case of atypical pneumonia in China, and the causative agent of SARS was identified to be a novel coronavirus, severe acute respiratory syndrome coronavirus (SARS-CoV). Bone marrow stromal antigen 2 (BST-2; also known as CD317 or tetherin) was initially identified to be a pre-B-cell growth promoter, but it also inhibits the release of virions of the retrovirus human immunodeficiency virus type 1 (HIV-1) by tethering budding virions to the host cell membrane. Further work has shown that BST-2 restricts the release of many other viruses, including the human coronavirus 229E (hCoV-229E), and the genomes of many of these viruses encode BST-2 antagonists to overcome BST-2 restriction. Given the previous studies on BST-2, we aimed to determine if BST-2 has the ability to restrict SARS-CoV and if the SARS-CoV genome encodes any proteins that modulate BST-2's antiviral function. Through an in vitro screen, we identified four potential BST-2 modulators encoded by the SARS-CoV genome: the papain-like protease (PL Pro ), nonstructural protein 1 (nsp1), ORF6, and ORF7a. As the function of ORF7a in SARS-CoV replication was previously unknown, we focused our study on ORF7a. We found that BST-2 does restrict SARS-CoV, but the loss of ORF7a leads to a much greater restriction, confirming the role of ORF7a as an inhibitor of BST-2. We further characterized the mechanism of BST-2 inhibition by ORF7a and found that ORF7a localization changes when BST-2 is overexpressed and ORF7a binds directly to BST-2. Finally, we also show that SARSCoV ORF7a blocks the restriction activity of BST-2 by blocking the glycosylation of BST-2. IMPORTANCEThe severe acute respiratory syndrome coronavirus (SARS-CoV) emerged from zoonotic sources in 2002 and caused over 8,000 infections and 800 deaths in 37 countries around the world. Identifying host factors that regulate SARS-CoV pathogenesis is critical to understanding how this lethal virus causes disease. We have found that BST-2 is capable of restricting SARS-CoV release from cells; however, we also identified a SARS-CoV protein that inhibits BST-2 function. We show that the SARS-CoV protein ORF7a inhibits BST-2 glycosylation, leading to a loss of BST-2's antiviral function. Severe acute respiratory syndrome coronavirus (SARS-CoV) was identified to be the causative agent of a 2002 to 2004 outbreak of severe respiratory disease that emerged from the Guangdong province of China, resulting in 8,096 cases and 774 deaths across 37 countries (1, 2). SARS-CoV is an enveloped virus with a positive-sense, single-stranded RNA genome of roughly 30,000 nucleotides encoding four structural proteins: the spike (S), envelope (E), membrane (M), and nucleocapsid (N) proteins (3). N protein forms the nucleocapsid, while E and M are minor virion membrane proteins. SARS-CoV entry into the cell is mediated by S-protein binding to angiotensin-converting enzyme 2 (ACE2) on the cell surface (4). In addition to the structural proteins, the SARS-C...

Interleukin-1 (IL-1) family cytokines are key signaling molecules in both the innate and adaptive immune systems, mediating inflammation in response to a wide range of stimuli. The basic mechanism of signal initiation is a stepwise process in which an agonist cytokine binds its cognate receptor. Together, this cytokine-receptor complex recruits an often-common secondary receptor. Intracellularly, the Toll/IL-1 Receptor (TIR) domains of the two receptors are brought into close proximity, initiating an NF-κB signal transduction cascade. Due to the potent inflammatory response invoked by IL-1 family cytokines, several physiological mechanisms exist to inhibit IL-1 family signaling, including antagonist cytokines and decoy receptors. The numerous cytokines and receptors in the IL-1 superfamily are further classified into four subfamilies, dependent on their distinct cognate receptors—the IL-1, IL-33, and IL-36 subfamilies share IL-1RAcP as their secondary receptor, while IL-18 subfamily utilizes a distinct secondary receptor. Here, we describe how structural biology has informed our understanding of IL-1 family cytokine signaling, with a particular focus on molecular mechanisms of signaling complex formation and antagonism at the atomic level, as well as how these findings have advanced therapeutics to treat some chronic inflammatory diseases that are the result of dysregulated IL-1 signaling.

Superantigens (SAGs) crosslink MHC class II and TCR molecules, resulting in an overstimulation of T cells associated with human disease. SAGs interact with several different surfaces on MHC molecules, necessitating the formation of multiple distinct MHC-SAG-TCR ternary signaling complexes. Variability in SAG-TCR binding modes could also contribute to the structural heterogeneity of SAG-dependent signaling complexes. We report crystal structures of the streptococcal SAGs SpeA and SpeC in complex with their corresponding TCR beta chain ligands that reveal distinct TCR binding modes. The SpeC-TCR beta chain complex structure, coupled with the recently determined SpeC-HLA-DR2a complex structure, provides a model for a novel T cell signaling complex that precludes direct TCR-MHC interactions. Thus, highly efficient T cell activation may be achieved through structurally diverse strategies of TCR ligation.