The Atg16L Complex Specifies the Site of LC3 Lipidation for Membrane Biogenesis in Autophagy

Abstract

Two ubiquitin-like molecules, Atg12 and LC3/Atg8, are involved in autophagosome biogenesis. Atg12 is conjugated to Atg5 and forms an ϳ800-kDa protein complex with Atg16L (referred to as Atg16L complex). LC3/Atg8 is conjugated to phosphatidylethanolamine and is associated with autophagosome formation, perhaps by enabling membrane elongation. Although the Atg16L complex is required for efficient LC3 lipidation, its role is unknown. Here, we show that overexpression of Atg12 or Atg16L inhibits autophagosome formation. Mechanistically, the site of LC3 lipidation is determined by the membrane localization of the Atg16L complex as well as the interaction of Atg12 with Atg3, the E2 enzyme for the LC3 lipidation process. Forced localization of Atg16L to the plasma membrane enabled ectopic LC3 lipidation at that site. We propose that the Atg16L complex is a new type of E3-like enzyme that functions as a scaffold for LC3 lipidation by dynamically localizing to the putative source membranes for autophagosome formation.

Extracellular matrix regulation of autophagy

“…The same E1-like enzyme, ATG7 and a distinct E2-like enzyme, ATG3, mediates this lipidation. Recent biochemical evidence indicates that the ATG12:5 complex possesses a E3-like activity required for the efficient PE-lipidation of ATG8 [46,47]. The lipid modification of ATG8/LC3 causes it to relocate to autophagosome membrane, serving as useful autophagy marker; indeed, monitoring ATG8/LC3 relocation to puncta or measuring the levels of PE-modified LC3 (LC3-II) using immunoblotting are two methods commonly used to measure autophagosome formation [21].…”

A current perspective of autophagosome biogenesis

“…Therefore, Atg12, Atg5, and Atg16(L1) are frequently used as markers for the early steps of autophagosome formation. The localization of the Atg12-Atg5-Atg16(L1) complex appears to be directed by Atg16(L1) [19,45], and it has been shown that mammalian Atg16L1 directly binds to the ULK1 complex subunit FIP200 [46][47][48]. In addition, the membrane-binding ability of yeast Atg5, which can be inhibited by Atg12 and activated by Atg16, has also been reported [49].…”

Starvation Compromises Paneth Cells

“…Herein, we show that Paneth cells of starved mice display aberrant granules, with changes in their composition. Recently, mice deficient in ATG16L1 or ATG5 protein, important in the formation and proper subcellular localization of the autophagosome, 27 showed substantial structural abnormalities in Paneth cells and their granules. More specifically, the Paneth cell granule number was decreased, and Paneth cells showed distorted packaging and exocytosis of their granules.…”