Structure-function study on FIP200 reveals mechanistic insights into selective autophagy

Abstract

Selective autophagy serves a role in maintaining cellular homeostasis by degradation of aggregated proteins, damaged organelles or pathogens. A double membrane engulfs cargo destined for degradation and thereby forms an enclosed compartment called the autophagosome. Fusion with lysosomes leads to cargo digestion and recycling. In this process, cargo is specifically recognized by dedicated cargo receptors. An unresolved question is how cargo recognition and initiation of autophagy are connected. In yeast, the scaffold protein Atg11 mediates the interaction of cargo receptors with the Atg1 complex required for autophagy initiation. In mammals however, this link has not been established. Interestingly, FIP200, a key player in mammalian autophagy initiation, harbors a C-terminal Atg11 homology domain and has recently been shown to bind to cargo receptor p62. To gain insights into the mechanistic details of this interaction, the crystal structure of the dimeric FIP200 C-terminal region (CTR) was determined in this work. Both the extended N-terminal helix and the compact C-terminal ‘Claw’ domain of the CTR contribute to dimer formation. Structure-based mutagenesis allowed the identification of the p62 binding site as a positively charged pocket in the Claw. A point mutant in this pocket was defective for p62-condensate processing, demonstrating the significance of the FIP200 CTR – p62 interaction for selective autophagy. Surface plasmon resonance experiments showed that p62 phosphorylation enhances the weak interaction with FIP200 CTR, therefore identifying receptor phosphorylation as a key regulatory principle. Strikingly, FIP200 CTR also bound to liposomes, suggesting a direct function in membrane recruitment. Thus, this work describes a missing link between cargo recognition and the initiation of selective autophagy. Further insights into this fundamental process will ultimately help to design therapies against diseases caused by malfunctioning of cellular homeostasis.