Autophagic tubes: Vacuolar invaginations involved in lateral membrane sorting and inverse vesicle budding

Many intracellular compartments of eukaryotic cells do not adopt a spherica l shape, which would be expected in the absence of mechanisms organizing th eir structure. However, little is known about the principles determining th e shape of organelles. We have observed very defined structural changes of vacuoles, the lysosome equivalents of yeast. The vacuolar membrane can form a large tubular invagination from which vesicles bud off into the lumen of the organelle. Formation of the tube is regulated via the Apg/Aut pathway. Its lumen is continuous with the cytosol, making this inverse budding reac tion equivalent to microautophagocytosis. The tube is highly dynamic, often branched, and defined by a sharp kink of the vacuolar membrane at the site of invagination. The tube is formed by vacuoles in an autonomous fashion. It persists after vacuole isolation and, therefore, is independent of surro unding cytoskeleton. There is a striking lateral heterogeneity along the tu be, with a high density of transmembrane particles at the base and a smooth zone devoid of transmembrane particles at the tip where budding occurs. We postulate a lateral sorting mechanism along the tube that mediates a deple tion of large transmembrane proteins at the tip and results in the inverse budding of Lipid-rich vesicles into the lumen of the organelle.