SQSTM1/p62 UFMylation Enhances Autophagic Clearance of Pathogenic Mutant Huntingtin

Ubiquitin-fold modifier 1 (UFM1) covalently modifies protein substrates (UFMylation) and alters their biological functions. Genetic screening disclosed that enzymes in the UFMylation system play critical roles in regulating autophagy. However, it is still elusive which protein is UFMylated and how this modification modulates autophagy. Here, our quantitative proteomics and biochemical experiments identify SQSTM1/p62 as a UFMylation substrate and discover its two major UFMylation sites, K420 and K435. Mutating them to Arg (p62^2KR) completely abolishes the effect of p62 on autophagic activity. Fusion of UFM1^ΔC4 to p62^2KR (p62^2KR-UFM1^ΔC4) restores the p62-mediated pathogenic autophagic degradation in primary cortical neurons and Huntington's disease mouse striatum. Mechanistically, p62 UFMylation enhances its interaction with LC3, augments autophagic flux, and eliminates pathogenic mutant huntingtin. Collectively, this work discovers a new post-translational modification, UFMylation, on p62 and establishes this modification as a key regulator of autophagy that promotes the clearance of mutant huntingtin, offering a potential target for therapeutic intervention.