Reconstitution of autophagic-like membrane tethering reveals that Atg11 can bind and cluster vesicles on cargo mimetics.

Macroautophagy (hereafter, autophagy) is essential for the degradation of mitochondria from yeast to humans. Mitochondrial autophagy in yeast is initiated when the selective autophagy scaffolding protein Atg11 is recruited to mitochondria through its interaction with the selective autophagy receptor Atg32. This also results in the recruitment of small 30-nm vesicles that fuse to generate the initial phagophore membrane. We demonstrate that Atg11 can bind to autophagic-like membranes in vitro in a curvature-dependent manner in part via a predicted amphipathic helix. Deletion of the amphipathic helix from Atg11 results in a delay in the formation of mitophagy initiation sites in yeast. Furthermore, using a novel biochemical approach, we demonstrate that the interaction between Atg11 and Atg32 results in the tethering of autophagic-like vesicles in clusters to giant unilamellar vesicles containing a lipid composition designed to mimic the outer mitochondrial membrane. We also demonstrate that the N-terminal region of Atg11 is an important mediator of vesicle tethering to cargo mimetics and that clustering of autophagic-like vesicles requires the C-terminal region of Atg11. Taken together, our results reveal that Atg11 clusters into high-order oligomers, can tether autophagic-like membranes and due to its ability to oligomerize can cluster vesicles on the surface of cargo mimetics. This work provides new insight into the mechanisms of protein and membrane clustering by Atg11. Given the increasing importance of protein oligomerization and clustering in autophagy, these results have important implications in the initiation of mitochondrial autophagy. Atg11: autophagy related 11; Atg11-Cterm: C-terminal region of Atg11; Atg11-Nterm: N-terminal region of Atg11; Atg32: autophagy related 32; COV: coefficient of variance; DOPC: 1,2-dioleoyl-sn-glycero-3-phosphocholine; DOPE: 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine; DOPS: 1,2-dioleoyl-sn-glycero-3-phospho-L-serine; FRAP: fluorescence recovery after photobleaching; GLT: GUV and liposome tethering; GUV: giant unilamellar vesicle; MKO: multiple knockout; OMM: outer mitochondrial membrane; PC: phosphatidylcholine; PE: phosphatidylethanolamine; PtdIns: phosphatidylinositol; PtdIns3P: phosphatidylinositol-3-phosphate; RhPE: rhodamine phosphatidylethanolamine; SAR: selective autophagy receptor; SEC: size-exclusion chromatography; SLB: supported lipid bilayers; SMrT: supported membrane templates; YPL: yeast polar lipids.