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PRINCIPAL PUBLICATION AND AUTHORS

The assembly of the Mitochondrial Complex I Assembly complex uncovers a redox pathway coordination, L. McGregor (a), S. Acajjaoui (a), A. Desfosses (b), M. Saïdi (a), M. Bacia-Verloop (b), J.J. Schwarz (c), P. Juyoux (d), J. von Velsen (d), M.W. Bowler (d), A.A. McCarthy (d), E. Kandiah (a), I. Gutsche (b,e), M. Soler-Lopez (a), Nat. Commun. 14, 8248 (2023); https:/doi.org/10.1038/s41467-023-43865-0 (a) ESRF (b) Institut de Biologie Structurale, Univ. Grenoble Alpes, CEA, CNRS (IBS), Grenoble (France) (c) European Molecular Biology Laboratory (EMBL), Heidelberg (Germany) (d) European Molecular Biology Laboratory (EMBL), Grenoble (France) (e) Department of Chemistry, Umeå University (Sweden)

REFERENCES

[1] L. McGregor & M. Soler-López, Curr. Op. Struct. Bio. 80, 102573 (2023). [2] G. Giachin et al., Front. Mol. Biosci. 3, 43 (2016). [3] G. Giachin et al., Angew. Chem. Int. Ed. 60, 4689 (2021).

This work employed an integrative biology approach to determine the structures of ACAD9 alone and in complex with the C-terminal domain of ECSIT at high resolution. Data collected at the cryo-electron microscope CM01 have revealed the interaction site between ACAD9 and ECSIT, highlighting a site of functional interest (Figure 27a,b). Upon comparing the bound and unbound ACAD9 structures, a striking conformational change is observed at a site bridging the ACAD9 cofactor pocket and the ACAD9-ECSIT binding site. A stretch of 20 amino acid residues adopt a downward-facing conformation in ACAD9 alone, acting as a gatekeeper barrier between external solvent and its cofactor (Figure 27a, bottom). However, upon ACAD9-ECSIT complex formation, a short helix of ECSIT residues recognise ACAD9 and induce the opening of the gatekeeper loop. This loop-flipping mechanism results in deflavination and reassigns ACAD9 from an FAO to an OXPHOS enzyme (Figure 27b, bottom) [3]. In addition, cell biology and biophysical analyses have revealed that ECSIT can undergo phosphorylation, a reversible post-translational modification of specific amino acids crucial for signalling processes. Interestingly, an ECSIT threonine residue located at the ACAD9-ECSIT binding site has been identified as a phosphorylation site.

Biophysical analyses suggest that the dephosphorylation of ECSIT may be a prior condition to successful MCIA formation (Figure 27c, left). Furthermore, the exposure of ECSIT to soluble Aβ oligomers decreases the level of ECSIT phosphorylation (Figure 27c, middle), meanwhile, the activity of CI increased (Figure 27c, right). Taken together, these studies suggest that under early amyloidogenic conditions, the MCIA complex would reassign ACAD9 to its OXPHOS functionalities and, in turn, contribute to a functional CI (Figure 28). However, this may result in a negative cycle over time, where the overactivity of CI leads to oxidative stress, promoting the accumulation of Aβ peptides and resulting in mitochondrial dysfunction, compromising neuronal integrity.

Combining molecular biology, biophysics and structural analysis, this work reveals the interactions within the MCIA subcomplex and how its assembly and activity may be regulated in the presence of amyloid toxicity. With information on the structure and behaviour of the core proteins, how the MCIA contributes to bioenergetics can begin to be understood. These findings will be instrumental in determining whether the MCIA proteins can be used as biomarkers for the early stages of AD.

Fig. 28: Proposed mechanism of ACAD9-ECSIT assembly and its functional implications in FAO and OXPHOS pathways. In the absence of ECSIT, ACAD9 acts as an FAO enzyme. There is concomitant binding of ECSIT and deflavination, shutting down the enzymatic activity of ACAD9 and activating its role in CI assembly. ECSIT dephosphorylation enables ACAD9 binding and hence, acts as a potential trigger to facilitate the assembly of the MCIA complex.