Structural insights into the assembly of large oligomeric signalosomes in the Toll-like receptor-interleukin-1 receptor superfamily

Abstract

The Toll-like receptor (TLR)-interleukin 1 receptor (IL-1R) superfamily plays fundamentally important roles in innate immune and inflammatory responses. Structural studies have begun to show that upon ligand stimulation, TLRs and IL-1Rs assemble large oligomeric intracellular signaling complexes, or "signalosomes," to induce the activation of kinases and E3 ubiquitin ligases, leading eventually to the activation of the transcription factors that are responsible for the expression of genes whose products mediate immune and inflammatory responses. The different scaffolds identified by these structural studies provide a molecular foundation for understanding the formation of microscopically visible signaling clusters that have long been known to cell biologists. Here, we illustrate the potential mechanisms of step-by-step assembly from the membrane-proximal interactions to the more downstream events. Formation of large oligomeric signalosomes may help to establish a digital threshold response in TLR and IL-1R signaling.

Figure 1. Model of TLR/IL-1R:MyD88:IRAK signalosome formation

A. Domain organization of TLR/IL-1R family members (top), MyD88 (middle), and IRAKs (bottom). B. Model of TLR/IL-1R signaling. Top left: IL-1RI (pink) bound to IL-1β (yellow) and co-receptor IL-1RAcP (light green). Top right: TLR1 (Cyan)/TLR2 (Green) dimer bound to lipopeptide (magenta). Cytosolic receptor TIR domains interact with MyD88 TIRs (light blue) to promote formation of the Myddosome DD complex. MyD88 DDs (blue) oligomerize with IRAK4 (red) and IRAK1/2 (orange) to promote phosphorylation and activation of IRAK kinase domains (bottom right).

Figure 2. Model of TRAF6 2-dimensional lattice

A. Domain organization of TRAF6. B. Crystal structure of TRAF6 TRAF-C domain (red) trimerized with TRAF2 CC (light blue) based on TRAF2 crystal structure. C. Model of N-Terminal TRAF6 RING and ZF1-3 bound to Ubc13 (yellow)/Uev1A (green) E2 ubiquitin ligase complex and Ub (orange). D. Model of 2-dimensional TRAF6 lattice assembled through TRAF-C (red) trimerization and N-terminal domain (blue) dimerization.

Figure 3. Model of IKK:NEMO:polyUb signalosome

A. Domain organization of IKKβ and NEMO. B. Crystal structure of IKKβ dimer, showing IKKβ KD (red), ULD (orange), and SDD (yellow). C. Model of IKKβ:NEMO complex. Catalytic dimeric IKKβ KD:ULD:SDD domain interacts with NEMO-KBD (light blue) through a helical C-terminal NBD (green). The KBD of NEMO is followed by a the helical HLX2 domain (violet), the Ub (grey) binding UBAN domain (pink), and a C-terminal ZF (blue).

Figure 4. Overview of TLR/IL-1R Signaling

Ligand binding induces both dimerization and high order oligomerization of TLRs (blue, top). MyD88 (red) is recruited to receptors through homotypic TIR:TIR interactions and facilitates Myddosome assembly with IRAK4 (green) and IRAK1/2 (orange) through homotypic DD:DD interactions, leading to IRAK phosphorylation and activation. Activated IRAK1/2 interacts with TRAF6 and promotes the formation of an infinite 2D lattice, stimulating TRAF6 K63-linked Ub ligase activity. PolyUb (grey) is recognized by TAB2/3 (purple) and NEMO (pink) to allow for the recruitment and activation of TAK1 (purple) and IKK (teal) kinases. Activated IKK phosphorylates IκB, promoting its degradation and allowing for NFκB nuclear translocation.