Sensing Membrane Curvature in Macroautophagy

Abstract

In response to intracellular stress events ranging from starvation to pathogen invasion, the cell activates one or more forms of macroautophagy. The key event in these related pathways is the de novo formation of a new organelle called the autophagosome, which surrounds and sequesters either random portions of the cytoplasm or selectively targets individual intracellular challenges. Thus the autophagosome is a flexible membrane platform with dimensions that ultimately depend upon the target cargo. The intermediate membrane, termed the phagophore or isolation membrane, is a cup-like structure with a clear concave face and a highly curved rim. The phagophore is largely devoid of integral membrane proteins, thus its shape and size are governed by peripherally-associated membrane proteins and possibly by the lipid composition of the membrane itself. Growth along the phagophore rim marks the progress of both organelle expansion and ultimately of organelle closure around a particular cargo. These two properties, a reliance on peripheral membrane proteins and a structurally-distinct membrane architecture, suggest that the ability to target or manipulate membrane curvature might be an essential activity of proteins functioning in this pathway. In this review, we discuss the extent to which membranes are naturally curved at each of the cellular sites believed to engage in autophagosome formation, review basic mechanisms used to sense this curvature and then summarize the existing literature concerning which autophagy proteins are capable of curvature recognition.

GABARAP Like-1 enrichment on membranes: Direct observation of trans-homo-oligomerization between membranes and curvature-dependent partitioning into membrane tubules

“…The lipidation of Atg8/LC3 on liposomes is strongly sensitive to membrane curvature 28 , and several other autophagy complexes recognize or depend on local curvature (reviewed here 37 ), suggesting proteins in this pathway may have natural affinities for strongly deformed membranes. Indeed, Atg8-PE has been shown to prefer highly curved surfaces 31 .…”

Curvature-sensitive trans-assembly of human Atg8-family proteins in autophagy-related membrane tethering

“…Notably, when tested at the protein-to-lipid ratio of 1:2,500 (0.2 M LC3B/GATE-16 to 0.5 mM lipids; Figure 2B) that is very close to the ratio calculated above, both LC3B and GATE-16 were still able to exhibit the significant intrinsic tethering activities ( Figure 2B). In addition, as membrane-anchored Atg8 proteins can be selectively sorted into a highly curved membrane segment by several folds [45,46], it is conceivable that the protein-to-lipid ratio of 1:500, which was used in the kinetic turbidity assays and fluorescence microscopy assays with 100-nm-diameter liposomes ( Figure 2C-E), can provide the physiologically-relevant Atg8 density on membrane surfaces. It should be also noted that, assuming that Atg8 proteins are a typically spherical 15-kDa protein with an average radius of 1.6 nm [47], membrane-anchored LC3B and GATE-16 proteins only occupy approximately 4.5% of the surface areas of liposomes in the current reconstitution systems, when tested at the protein-to-lipid ratio of 1:500 ( Figure 2).…”

“…Additional fusogenic proteins acting together with Atg8 proteins may include autophagy-related SNARE-family proteins that have been reported to be targeted to autophagic compartments and involved in autophagosome formation and maturation [22,[48][49][50]. However, it should be noted that, since membrane attachment of cytosolic Atg8 proteins via the conjugation to a nonlamellar-prone PE lipid is proposed to take place at locally-curved unstable lipid bilayers [45,46,51], we will not exclude the possibility that the intrinsic fusogenic potency of membrane-anchored Atg8 proteins directly facilitate membrane fusion events specifically at the highly-curved, fusion-prone membrane segments of autophagic compartments, including the rim of the cup-shaped phagophore membrane.…”

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

“…Second, to promote lipid-mixing in vitro , Atg8 requires somewhat unstable membranes. Interestingly, the highly curved rim of the constricting pore is likely to be naturally unstable [30]. If Atg8 proteins were to concentrate at this site, they may support the local deformation of the membrane.…”