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Fig. 4. Schematic model for formation of the autophagosome or Cvt vesicle. The origin of the membrane that forms the Cvt vesicle or autophagosome is not known, nor is the mechanism of vesicle formation. In one model (shown on the left), a membrane sheet from a pre-existing organelle such as the endoplasmic reticulum (ER) is induced to separate and undergo a deformation to form a spherical shape that eventually seals; no additional membrane is needed for expansion. It is not known when or how autophagy proteins such as Atg8 would be targeted to this membrane. In a second model (shown on the right), a portion of a membrane forms the nucleus of the autophagosome or Cvt vesicle; in yeast, this nucleus is the pre-autophagosomal structure (PAS). Additional membrane is added to allow expansion of the forming vesicle, but the origin of this membrane is not known. Most data support the second type of model. In either case, the original membrane is presumably equivalent on all surfaces in terms of protein content and phospholipid composition. During vesicle formation, the membrane may differentiate so that, upon vesicle completion, the two separate membranes have different properties; for example, Atg8-PE is located on both sides of the forming vesicle, but is removed from the outer membrane following cleavage by Atg4 (see Fig. 5). The outer vesicle membrane might contain SNARE elements needed for recognition and fusion with the vacuole (see Fig. 7), whereas the inner vesicle membrane might be more susceptible to degradation within the vacuole lumen. The double-membrane nature of the autophagosome or Cvt vesicle results in a transfer of the cargo from the cytosol into the lumenal space of the cell.
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