Autophagy, responsible for the delivery of cytoplasmic components to the lysosome/vacuole for degradation, is the major degradative pathway in eukaryotic cells. This process requires a ubiquitin-like protein conjugation system, in which Apg12 is covalently bound to Apg5. In the yeast Saccharomyces cerevisiae, the Apg12-Apg5 conjugate further interacts with a small coiled-coil protein, Apg16. The Apg12-Apg5 and Apg16 are localized in the cytosol and pre-autophagosomal structures and play an essential role in autophagosome formation. Here we show that the Apg12-Apg5 conjugate and Apg16 form a approximately 350-kDa complex in the cytosol. Because Apg16 was suggested to form a homo-oligomer, we generated an in vivo system that allowed us to control the oligomerization state of Apg16. With this system, we demonstrated that formation of the approximately 350-kDa complex and autophagic activity depended on the oligomerization state of Apg16. These results suggest that the Apg12-Apg5 conjugate and Apg16 form a multimeric complex mediated by the Apg16 homo-oligomer, and formation of the approximately 350-kDa complex is required for autophagy in yeast.
