Adduct-Induced LIR Mimicry Governs the Hierarchical Selectivity of Mitochondrial Autophagy

Hypothesis

Lipid peroxidation adducts on lysine residues create LC3‑interacting region (LIR) mimics that are only accessible on solvent‑exposed proteins. Matrix‑localized enzymes such as IDH2 acquire MDA/HNE adducts on surface lysines, exposing hydrophobic patches that dock into the LC3 pockets of autophagy receptors (e.g., p62, NBR1) without requiring ubiquitin. In contrast, adducts formed on transmembrane subunits like SDHA are buried within the inner mitochondrial lipid bilayer, sterically hindering receptor binding. Consequently, the autophagy machinery first clears adduct‑modified soluble proteins, leaving membrane‑embedded ETC components to accumulate and drive bioenergetic decline with age.

Mechanistic Reasoning

Adduct formation converts a positively charged lysine ε‑amino group into a Schiff base that adds a hydrophobic aldehyde moiety. This modification mimics the consensus LIR motif [W/F/Y]‑X‑X‑[L/I/V] by presenting a hydrophobic side chain flanked by polar residues. Soluble proteins present these motifs on their surface, allowing direct LC3 binding. Membrane proteins sequester the same adducts inside the hydrophobic core of the bilayer, where the lipid environment prevents LC3 access. The hierarchy thus reflects biophysical accessibility rather than damage severity.

Testable Predictions

1. Site‑specific lysine‑to‑arginine mutants of a matrix protein (IDH2‑K→R) will resist adduct formation yet show unchanged turnover, whereas the same mutant in an inner‑membrane protein (SDHA‑K→R) will restore adduct‑dependent clearance.
2. In vitro LC3‑pull‑down using synthetic peptides containing adduct‑modified lysines from IDH2 will bind LC3 with micromolar affinity, while analogous SDHA peptides will not.
3. Live‑cell mitophagy flux measured with mt‑Keima will increase in cells expressing adduct‑deficient SDHA mutants under oxidative stress, correlating with improved OXPHOS capacity.
4. Pharmacological blockade of Schiff‑base formation (e.g., with aminoguanidine) should abolish the preferential removal of matrix proteins and equalize the half‑life of soluble and mitochondrial membrane adducts.

Falsifiability

If mutating adduct‑prone lysines fails to alter the clearance order of mitochondrial proteins, or if adduct‑modified peptides do not show enhanced LC3 binding in vitro, then the adduct‑induced LIR mimicry mechanism is refuted, implying that other signals (e.g., ubiquitin‑dependent pathways) dictate the observed hierarchy.