Filament formation drives activation of NLR-like proteins 2249724

Abstract

Abstract Introduction The nucleotide-binding domain (NBD) and leucine-rich repeat (LRR)-containing protein (NLR) family plays a key role in immune defense across domains of life. Prior studies on inflammasomes and resistosomes revealed that NLRs tend to form ring-like structure for activation. Here, we focused on a unique family of NLR called NLR-SIR2 that comprises SIR2, NBD, and TPR domains, uncovering a filamentous assembly. This discovery has broad implication in the activation of many other NLRs, including NLRC4 and NLRP6. Methods We determined Cryo-EM structures of NLR-SIR2 in three states and combined them with biochemical and cellular assays to link structures with functions. Results NLR-SIR2 adopts a closed monomer in the resting state. At higher concentration it forms a symmetric dimer via SIR2—SIR2 contacts without major domain rearrangement. Incubation at 37 °C promotes concentration-dependent filament formation driven by SIR2-mediated interfaces, in contrast to the NBD-mediated oligomerization observed for other NLRs. ATP hydrolysis accompanies the transition from apo to filament, indicating a regulated assembly step. Filament formation is required for robust NAD+ hydrolysis, which is essential for anti-phage defense in cells. Thus, NLR-SIR2 activation proceeds through SIR2-guided polymerization that licenses NAD+ cleavage. Conclusion NLR-SIR2 reveals an alternative NLR activation pathway in which oligomerization is mediated by SIR2 domain rather than NBD. Coupling of ATP turnover, filament formation, and NAD+ hydrolysis provides a mechanistic framework for NLR immunity and suggests conserved yet diversified assembly logics across NLRs. Funding Source N/A Topic Categories Innate Immune Responses and Host Defense: Molecular Mechanisms (INM)