Structural basis for SARM1 inhibition and activation under energetic stress.

Michael Sporny,Ran Zalk,Michail N Isupov,Tami Khazma,Yarden Opatowsky,Moshe Dessau,Avraham Yaron,Michael Hons,Yoel Shkolnisky,Julia Guez-Haddad,Carsten Mim

doi:10.7554/elife.62021

Abstract

SARM1, an executor of axonal degeneration, displays NADase activity that depletes the key cellular metabolite, NAD+, in response to nerve injury. The basis of SARM1 inhibition and its activation under stress conditions are still unknown. Here, we present cryo-EM maps of SARM1 at 2.9 and 2.7 Å resolutions. These indicate that SARM1 homo-octamer avoids premature activation by assuming a packed conformation, with ordered inner and peripheral rings, that prevents dimerization and activation of the catalytic domains. This inactive conformation is stabilized by binding of SARM1's own substrate NAD+ in an allosteric location, away from the catalytic sites. This model was validated by mutagenesis of the allosteric site, which led to constitutively active SARM1. We propose that the reduction of cellular NAD+ concentration contributes to the disassembly of SARM1's peripheral ring, which allows formation of active NADase domain dimers, thereby further depleting NAD+ to cause an energetic catastrophe and cell death.

Highlights

SARM1 was first discovered as a negative regulator of TRIF (TIR domain–containing adaptor inducing interferon-b) in TLR (Toll-like receptor) signaling (Carty et al, 2006)
As it became clear that the compact two-ring structure is inhibited for NADase activity, we considered whether the purified hSARM1, that was not subjected to gradient fixation (GraFix) and predominantly presents just the inner SAM ring in cryo-EM 2D averaging and 3D reconstruction (Figure 1C and D), is active in vitro
Our octamer ring structure of a near-intact hSARM1 reveals an inhibited conformation, in which the catalytic TIR domains are kept apart from each other, unable to form close homodimers, which are required for their NADase activity

Summary

Introduction

SARM1 (sterile a and HEAT/armadillo motif–containing protein; Mink et al, 2001) was first discovered as a negative regulator of TRIF (TIR domain–containing adaptor inducing interferon-b) in TLR (Toll-like receptor) signaling (Carty et al, 2006). SARM1 was later shown to promote neuronal death by oxygen and glucose deprivation (Kim et al, 2007) and viral infections (Hou et al, 2013; Mukherjee et al, 2013; Sundaramoorthy et al, 2020; Uccellini et al, 2020), while having a protective role against bacterial and fungal infections in Caenorhabditis elegans (Couillault et al, 2004; Liberati et al, 2004). The domain composition of SARM1 includes an N-terminal peptide, an ARM-repeats region, two SAM, and one TIR domain (Figure 1A, Figure 1—figure supplement 1), which mediate mitochondria targeting (Panneerselvam et al, 2012), auto-inhibition

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Conclusion