ATG16 mediates the autophagic degradation of the 19S proteasomal subunits PSMD1 and PSMD2

2018
Abstract Autophagy and the ubiquitin proteasomesystem are the two major cellular processes for protein and organelle recycling and clearance in eukaryotic cells. Evidence is accumulating that these two pathways are interrelated through adaptor proteins. Here, we found that PSMD1and PSMD2, both components of the 19S regulatory particle of the proteasome, directly interact with Dictyostelium discoideumautophagy 16 (ATG16), a core autophagosomalprotein. ATG16 is composed of an N-terminal domain, which is responsible for homo-dimerization and binding to ATG5and a C-terminal β-propeller structure. Deletion analysis of ATG16 showed that the N-terminal half of ATG16 interacted directly only with PSMD1, while the C-terminal half interacted with both, PSMD1and PSMD2. RFP-tagged PSMD1as well as PSMD2were enriched in large puncta, reminiscent of autophagosomes, in wild-type cells. These puncta were absent in atg16‾ and atg9‾/ 16cellsand weaker and less frequent in atg9‾ cells, showing that ATG16 was crucial and the autophagic process important for their formation. Co-expression of ATG16-GFP or GFP-ATG8a(LC3) with RFP- PSMD1or RFP- PSMD2, respectively, in atg16‾ or wild-type cells revealed many instances of co-localization, suggesting that RFP- PSMD1or RFP- PSMD2positive puncta constitute autophagosomes. LysoTracker ® labeling and a proteolytic cleavage assay confirmed that PSMD1and PSMD2were present in lysosomes in wild-type cells. In vivo , ATG16 is required for their enrichment in ATG8a positive puncta, which mature into autolysosomes. We propose that ATG16 links autophagy and the ubiquitin proteasomesystem.
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