Filamentation activates bacterial NLR-like antiviral protein

Bacterial antiviral STANDs (Avs) are evolutionarily related to the nucleotide-binding leucine-rich repeat containing receptors (NLRs) widely distributed in immune systems across animals and plants. Ef Avs5, an Avs type 5 protein from Escherichia fergusonii , contains an N-terminal SIR2 effector domain, a nucleotide-binding oligomerization domain (NOD) and a C-terminal sensor domain, conferring protection against diverse phage invasions. Despite the established roles of SIR2 and STAND in prokaryotic and eukaryotic immunity, the mechanism underlying their collaboration remains unclear. Here we present cryo-EM structures of Ef Avs5 filaments, elucidating the mechanisms of dimerization, filamentation, filament clustering, ATP binding and NAD ⁺ hydrolysis, all of which are crucial for anti-phage defense. The SIR2 domains and NODs engage in the intra- and inter-dimer interaction to form an individual filament, while the outward C-terminal domains contribute to bundle formation. Filamentation potentially stabilizes the dimeric SIR2 configuration, thereby activating the NADase activity of Ef Avs5. Ef Avs5 is deficient in the ATPase activity, but elevated ATP concentrations can impede its NADase activity. Together, we uncover the filament assembly of Avs5 as a unique mechanism to switch enzyme activities and perform anti-phage defenses, emphasizing the conserved role of filamentation in immune signaling across diverse life forms.