Chaperone-Autophagy Coupling Prevents Aggregate Persistence by Directing Seeds…

The chaperone-autophagy coupling hypothesis centers on the critical interaction between CHIP (C-terminus of HSC70-interacting protein, encoded by STUB1) and the heat shock protein 70 (HSP70) chaperone system to prevent pathological protein aggregation through enhanced autophagic clearance rather than proteasomal degradation. CHIP functions as a dual-specificity adaptor that serves as a molecular bridge between the protein folding machinery and selective autophagy pathways, particularly chaperone-mediated autophagy (CMA) and aggrephagy. The mechanism involves CHIP’s interaction with autophagy receptors such as p62/SQSTM1 and NBR1 through its U-box domain, which contains a conserved LIR (LC3-interacting region) motif that directly binds LC3/GABARAP family proteins on autophagosomal membranes. When misfolded proteins such as tau or α-synuclein bind to HSP70, CHIP is recruited to form a tripartite complex. However, instead of ubiquitinating substrates for proteasomal degradation, CHIP facilitates the recruitment of autophagy machinery by promoting the formation of K63-linked polyubiquitin chains on substrate proteins. These K63-linked chains serve as recognition signals for autophagy receptors like p62, which contain both ubiquitin-binding domains (UBA) and LC3-binding domains (LIR). The HSP70-CHIP-substrate complex is then sequestered into autophagosomes through p62-mediated clustering and LC3 interaction. This pathway is particularly activated under conditions of proteasomal stress or when large protein aggregates exceed the capacity of the 26S proteasome. Phosphorylation of CHIP at Ser20 by ULK1 kinase enhances its interaction with autophagy receptors and promotes the switch from proteasomal to autophagic clearance. The chaperone-autophagy coupling mechanism provides a backup clearance system that prevents the accumulation of aggregation seeds when proteasomal capacity is overwhelmed, particularly relevant in aging and neurodegenerative diseases where autophagic flux becomes the primary protein quality control mechanism.