Apg2p functions in autophagosome formation on the perivacuolar structure

Autophagy is a degradative process in which cytoplasmic components are non- selectively sequestered by double-membrane structures, termed autophagosome s, and transported to the vacuole. We have identified and characterized a n ovel protein Apg2p essential for autophagy in yeast. Biochemical and fluore scence microscopic analyses indicate that Apg2p functions at the step of au tophagosome formation. Apg2p localizes to some membranous structure distinc t from any known organelle. Using fluorescent protein-tagged Apg2p, we show ed that Apg2p localizes to a dot structure close to the vacuole, where Apg8 p also exists, but not on autophagosomes unlike Apg8p. This punctate locali zation of Apg2p depends on the function of Apg1p kinase, phosphatidylinosit ol 3-kinase complex and Apg9p. Apg2p(G83E), encoded by an apg2-2 allele, sh ows a severely reduced activity of autophagy and a dispersed localization i n the cytoplasm. Overexpression of the mutant Apg2p lessens the defect in a utophagy. These results suggest that the dot structure is physiologically i mportant. Apg2p and Apg8p are independently recruited to the structure but coordinately function there to form the autophagosome.