Signalling by Cooperative Assembly Formation (SCAF) by TIR Domains in Innate Immunity and Cell Death Pathways

TIR (Toll/interleukin-1 receptor) domains are widely distributed in animals, plants and bacteria. They function through self-association and interactions with other TIR domains [1]. They feature in proteins with immune functions – e.g. Toll-like receptors, plant NLRs and bacterial antiphage defence proteins. Many TIR domains have enzymatic activities - cleavage of nucleotides such as NAD+ [2,3]. We used an integrated structural biology approach, combining X-ray crystallography, NMR (nuclear magnetic resonance), cryoEM (cryogenic electron microscopy), microED (microcrystal electron diffraction) and SFX (serial femtosecond crystallography) to characterize the signalosomes formed by different TIR domains [3-6]. We found that all TIR assemblies feature a head-to-tail arrangement of TIR molecules, with the enzyme active site located in the interface between two molecules, explaining the requirement for self-association in enzyme activity. The products of enzymatic reactions have downstream signalling functions in immune pathways. Our studies will form the foundation of applications ranging from the treatment of inflammatory disorders and bacterial infections in humans to the prevention of plant diseases.