pH-dependent regulation of SQSTM1/p62 during autophagy

Abstract

During macroautophagy/autophagy, SQSTM1/p62 plays dual roles as a key mediator of cargo selection and as an autophagic substrate. SQSTM1 links N-degrons and/or ubiquitinated cargoes to the autophagosome by forming homo- or hetero-oligomers, although its N-degron recognition and oligomerization mechanisms are not well characterized. We recently found that SQSTM1 is a novel type of N-recognin whose ZZ domain provides a negatively-charged binding pocket for Arg-charged N-degron (Nt-Arg), a prototype type-1 substrate. Although differences in binding affinity exist for each N-degron, SQSTM1 also interacts with type-2 N-degrons, such as Nt-Tyr and Nt-Trp. Intriguingly, interactions between SQSTM1's ZZ domain and various N-degrons are greatly influenced by pH-dependent SQSTM1 oligomerization via its PB1 domain. Because cellular pH conditions vary from neutral to acidic depending on the stage of autophagy, the pH-dependent regulation of SQSTM1's oligomerization must be tightly coupled with the autophagic process.

Keywords: Aggrephagy; HSPA5/BiP/GRP78; N-end rule; UBR box; autophagy adaptor; pH-dependent oligomerization.

Figure 1.

pH-dependent regulation of SQSTM1 during autophagy. The ZZ domain of SQSTM1 recognizes cargo-bound R*-HSPA5 or unknown N-degron (type-1 or type-2) proteins. The PB1, ZZ, LIR, and UBA domains in SQSTM1 are shown as orange, green, purple, and yellow, respectively. The R*-HSPA5 chaperone (gray rectangle) binds to the ubiquitinated aggregate (Ub, beige circle) under certain conditions, such as ER stress. At physiological pH, SQSTM1 forms small oligomers composed of 6 to 8 monomers bearing a relatively weak affinity to the cargo. Phagophore assembly site (PAS) formation, followed by autophagy induction, results in a decreased pH, which promotes further oligomerization of SQSTM1 and facilitates a stronger interaction with the cargo. Autophagosomal pH is lowered even further and generates a higher-order and more flexible filament, which may represent the form of SQSTM1 with the strongest cargo interaction via enhanced avidity. Finally, following fusion of the autophagosome with the lysosome, lysosomal pH facilitates separation of the large, strongly bound complexes (aggregates and SQSTM1) into smaller molecules easily degraded by lysosomal proteases (red crescent). LIR, LC3-interacting region.