Conserved catalytic activity of immune TIR domains in animals

The Toll/interleukin-1 receptor (TIR) domain is important for immune signaling across bacteria, plants, and animals. In human innate immunity, TIR domains are known to function as adaptors mediating protein-protein interactions, yet studies in bacteria and plants revealed that TIR domains often act as enzymes that produce immune signaling molecules. Here, we show that TIR domains from evolutionarily diverse animals have conserved active sites, implying that they can function as enzymes. In vitro experiments with animal TIRs show that the TIR domain of several Toll-like receptors (TLRs), including that of human TLR4, can produce cyclic ADP-ribose (cADPR), revealing an enzymatic activity previously unknown for TLR TIRs. We show that production of cADPR is a conserved feature of TIR domains across the animal tree of life, implying a role for this molecule in animal TIR signaling. Finally, we report a TIR domain from green algae that synthesizes 3ʹcADPR, suggesting conservation of 3ʹcADPR signaling between bacteria and eukaryotes. Our results reveal that the catalytic activity of TIR domains is widespread in animals and conserved across the tree of life.