Co-chaperone BAG3 directly target autophagic degradation via its LC3-interacting regions

Abstract

The co-chaperone BAG3 is a hub for a variety of cellular pathways via its multiple domains and its interaction with HSP70 and HSPB8. Under aging and cellular stress conditions in particular, together with molecular chaperones, BAG3 ensures the sequestration of aggregated or aggregation prone ubiquitinated proteins to the autophagic-lysosomal system via ubiquitin receptors. There are emerging indications that BAG3-mediated selective macroautophagy also copes with non-ubiquitinated cargo. Phylogenetically, BAG3 comprises several highly conserved predicted LIRs, LC3-interacting regions, which might directly target BAG3 including its cargo to ATG8 proteins and directly drive their autophagic degradation. Based on pull-down experiments, peptide arrays and proximity ligation assays, our results provide evidence of an interaction of BAG3 with ATG8 proteins. In addition, we could demonstrate that mutations within the LIRs impair co-localization with ATG8 proteins in immunofluorescence. A BAG3 variant mutated in all LIRs results in a substantial decrease of BAG3 levels within purified native autophagic vesicles compared to wild-type BAG3. These results strongly suggest LC3-mediated sequestration of BAG3. Therefore, we conclude that in addition of being a key co-chaperone to HSP70, BAG3 may also act as cargo receptor for client proteins, which would significantly extend the role of BAG3 in selective macroautophagy and protein quality control.