Figure 1.

Autophagy is a unique membrane-trafficking pathway.

Vesicle membrane trafficking generally starts with the deformation of a donor membrane into a bud and the nascent transport vesicle gets pinched off. The vesicle is then actively transported to a target membrane where both membranes are tethered and finally fused (A). In contrast, autophagosomes, a unique type of transport vesicle, are formed at or close to the ER. Instead of a continuous outgrowth of one membrane, the elongation of the autophagosomal precursor is thought to require the concerted action of several membrane sources/vesicles. After its closure, the emerging double membrane vesicle is transported to lytic organelles where the membranes are tethered and the outer autophagosomal membrane finally fuses with the target membrane (B).