The coordination of membrane fission and fusion at the end of autophagosome maturation

Abstract

The two major objectives of macroautophagy are to sequester cargo away from the cytoplasm and deliver this material for breakdown in the lysosome. Sequestration is complete when the autophagosome membrane undergoes fission to produce separate inner and outer membranes, while delivery into the lysosome requires fusion of the outer autophagosome membrane with the lysosome membrane. Thus, the merging of membranes through fission and fusion underlies each of the pivotal events in macroautophagic clearance. How these merging events are controlled in the cell is poorly understood. Several recent studies however suggest that the two events may be temporally coordinated and rely upon members of the classic membrane fusion SNARE family as well as the autophagy-specific family of Atg8 proteins.

Figure 1

Membrane fission/fusion events in autophagosome maturation. Two essential membrane fission/fusion events occur during autophagosome maturation. First, membrane fission of a constricting pore closes the autophagosome and separates the outer and inner membranes into distinct structures. The machineries driving this event are still uncertain. The membranes are topologically similar to the endosomal sorting complexes required for transport (ESCRT) targets, but a number of studies including two recent works from the Mizushima and Lazarou labs suggest that the Atg8 family of proteins is actively involved in autophagosome sealing. Second, closed autophagosomes fuse with lysosomes in a classic soluble NSF attachment protein receptor (SNARE)-dependent event. STX17 is recruited to mature autophagsomes and then complexes with SNAP-29. Fusion is driven by the interaction of this complex with lysosomal VAMP8. Atg8 proteins may also regulate this event, through protein-protein interactions including the recruitment of SNARE effector proteins.

Figure 2

Possible mechanisms for Atg8-mediated membrane tethering and fission/fusion. LC3B soluble structure (PDB: 1v49) is shown which adopts the “closed” conformation of Atg8 proteins. The bulk of the protein is the ubiquitin-like fold (colored in blue). At its amino terminus are two alpha-helices (∂1 in orange, ∂2 in red), which in the closed conformation lie near the ubiquitin core. However, other structures of Atg8 family proteins suggest they may also adopt an open conformation, perhaps in a protein isoform specific manner or perhaps as a result of other post-translational modifications including the attachment of a phospholipid at the carboxy-terminus. Lipidated Atg8 family proteins can tether and fuse membranes in vitro which might explain their role in membrane fission in vivo. Tethering and fusion rely upon direct membrane engagement of ∂1 with the apposing membrane (in either the closed (1) or open (2) conformation) and the homo-oligomerization of Atg8 proteins in trans, spanning the two membrane surfaces (3).