Cryo-EM structure of an active bacterial TIR-STING filament complex.

Stimulator of interferon genes (STING) is an antiviral signalling protein that is broadly conserved in both innate immunity in animals and phage defence in prokaryotes1-4. Activation of STING requires its assembly into an oligomeric filament structure through binding of a cyclic dinucleotide4-13, but the molecular basis of STING filament assembly and extension remains unknown. Here we use cryogenic electron microscopy to determine the structure of the active Toll/interleukin-1 receptor (TIR)-STING filament complex from a Sphingobacterium faecium cyclic-oligonucleotide-based antiphage signalling system (CBASS) defence operon. Bacterial TIR-STING filament formation is driven by STING interfaces that become exposed on high-affinity recognition of the cognate cyclic dinucleotide signal c-di-GMP. Repeating dimeric STING units stack laterally head-to-head through surface interfaces, which are also essential for human STING tetramer formation and downstream immune signalling in mammals5. The active bacterial TIR-STING structure reveals further cross-filament contacts that brace the assembly and coordinate packing of the associated TIR NADase effector domains at the base of the filament to drive NAD+ hydrolysis. STING interface and cross-filament contacts are essential for cell growth arrest in vivo and reveal a stepwise mechanism of activation whereby STING filament assembly is required for subsequent effector activation. Our results define the structural basis of STING filament formation in prokaryotic antiviral signalling.

Structural Evolution of TIR-Domain Signalosomes.

TIR (Toll/interleukin-1 receptor/resistance protein) domains are cytoplasmic domains widely found in animals and plants, where they are essential components of the innate immune system. A key feature of TIR-domain function in signaling is weak and transient self-association and association with other TIR domains. An additional new role of TIR domains as catalytic enzymes has been established with the recent discovery of NAD+-nucleosidase activity by several TIR domains, mostly involved in cell-death pathways. Although self-association of TIR domains is necessary in both cases, the functional specificity of TIR domains is related in part to the nature of the TIR : TIR interactions in the respective signalosomes. Here, we review the well-studied TIR domain-containing proteins involved in eukaryotic immunity, focusing on the structures, interactions and their corresponding functional roles. Structurally, the signalosomes fall into two separate groups, the scaffold and enzyme TIR-domain assemblies, both of which feature open-ended complexes with two strands of TIR domains, but differ in the orientation of the two strands. We compare and contrast how TIR domains assemble and signal through distinct scaffolding and enzymatic roles, ultimately leading to distinct cellular innate-immunity and cell-death outcomes.

The interleukin-1 receptor family.

The activity of each member of the IL-1 family of ligands is mediated by its own receptor. Each ligand binds specifically to the extracellular "ligand binding chain" containing three Ig-like regions. With the exception of the IL-1 and IL-36 receptor antagonists, a second chain, termed the "accessory chain", is recruited, forms a heterotrimetic complex with the ligand binding chain and the ligand, and signal transduction is initiated. Each ligand binding or accessory chain shares a common cytosolic segment termed the Toll-IL-1-receptor (TIR) domain. Another family of 13 receptors, termed Toll-like receptors (TLR), have extracellular leucine-rich repeat domains, which bind a broad spectrum of microbial products. All TLR share a nearly identical TIR domain with all members of the IL-1 receptor family. Hence signal transduction and the biological consequences of TLR ligands and IL-1 family ligands are often the same and both receptor families contribute to innate inflammation and host defense. The IL-1 family of receptors is comprised of ten distinct but related gene products. The receptors are indicated by the term IL-1 receptor (IL-1R) followed by a numeral, assigned chronologically by discovery, for example, IL-1R1, IL-1R2, IL-1R3, etc. The ligand binding chain for IL-1α and IL-1ß is IL-1R1 and the accessory chain is IL-1R3. IL-1α, IL-1ß, IL-33 and IL-36 use IL-1R3 as their accessory chain. IL-1R2 is a non-signalling "decoy" receptor that sequesters the IL-1ß and IL-1R3. IL-1R8 exhibits anti-inflammatory properties by reducing IL-1 and TLR signalling. Presently there are two orphan receptors, IL-1R9 and IL-1R10, which have no known function. This review examines the characteristics and functional roles of the IL-1R family in the regulation of innate inflammation, host defense and acquired immunity.