Disruption of proteostasis, or protein homeostasis, is often associated with aberrant accumulation of misfolded
proteins or
protein aggregates. Autophagy offers protection to cells by removing toxic
protein aggregates and injured organelles in response to proteotoxic stress. However, the exact mechanism whereby autophagy recognizes and degrades misfolded or aggregated
proteins has yet to be elucidated. Mounting evidence demonstrates the selectivity of autophagy, which is mediated through autophagy receptor
proteins (e.g. p62/SQSTM1) linking autophagy cargos and autophagosomes. Here we report that proteotoxic stress imposed by the
proteasome inhibition or expression of
polyglutamine expanded huntingtin (
polyQ-Htt) induces p62 phosphorylation at its
ubiquitin-association (UBA) domain that regulates its binding to
ubiquitinated proteins. We find that autophagy-related
kinase ULK1 phosphorylates p62 at a novel phosphorylation site S409 in UBA domain. Interestingly, phosphorylation of p62 by ULK1 does not occur upon nutrient
starvation, in spite of its role in canonical autophagy signaling. ULK1 also phosphorylates S405, while S409 phosphorylation critically regulates S405 phosphorylation. We find that S409 phosphorylation destabilizes the UBA dimer interface, and increases binding affinity of p62 to
ubiquitin. Furthermore, lack of S409 phosphorylation causes accumulation of p62, aberrant localization of autophagy
proteins and inhibition of the clearance of
ubiquitinated proteins or
polyQ-Htt. Therefore, our data provide mechanistic insights into the regulation of selective autophagy by ULK1 and p62 upon proteotoxic stress. Our study suggests a potential novel
drug target in developing autophagy-based
therapeutics for the treatment of
proteinopathies including
Huntington's disease.