Dynamics of the SS Loop Regulates SARM1's Catalysis

Axonal degeneration (AxD) is a defining feature of many neurodegenerative disorders, with SARM1 functioning as a key pro-degenerative NADase that becomes activated through conformational change. Here, we identify a critical regulatory mechanism mediated by the SARM1-specific (SS) loop within its TIR domain. Using mutagenesis, activity assays, and molecular dynamics simulations, we demonstrate that dynamic conformational transitions of the SS loop, particularly the flipping of His640, are essential for NAD recruitment and catalytic site assembly. Zinc ions inhibit SARM1 activity by coordinating with a conserved C₂H₂ motif in the SS loop, thereby restricting His640 flipping and preventing NAD engagement. Notably, chemical tethering of adjacent cysteine residues within the SS loop similarly suppresses its conformational dynamics, resulting in potent inhibition of catalysis. The essential role of SS loop dynamics uncovered here offers new mechanistic insight and establishes a foundation for developing therapeutic strategies targeting SARM1.