Cyclic nucleotide-induced helical structure activates a TIR immune effector.

Cyclic nucleotide signalling is a key component of antiviral defence in all domains of life. Viral detection activates a nucleotide cyclase to generate a second messenger, resulting in activation of effector proteins. This is exemplified by the metazoan cGAS-STING innate immunity pathway, which originated in bacteria. These defence systems require a sensor domain to bind the cyclic nucleotide and are often coupled with an effector domain that, when activated, causes cell death by destroying essential biomolecules. One example is the Toll/interleukin-1 receptor (TIR) domain, which degrades the essential cofactor NAD when activated in response to infection in plants and bacteria or during programmed nerve cell death. Here we show that a bacterial antiviral defence system generates a cyclic tri-adenylate that binds to a TIR-SAVED effector, acting as the 'glue' to allow assembly of an extended superhelical solenoid structure. Adjacent TIR subunits interact to organize and complete a composite active site, allowing NAD degradation. Activation requires extended filament formation, both in vitro and in vivo. Our study highlights an example of large-scale molecular assembly controlled by cyclic nucleotides and reveals key details of the mechanism of TIR enzyme activation.