Vesicle tethering and fusion promoted by LC3/GABARAP proteins is modulated by the ATG12-ATG5-ATG16L1 complex

Abstract

Recently, we have examined the membrane anchoring and subsequent lipidation of six members of the LC3/GABARAP protein family, together with their ability to promote membrane tethering and fusion. GABARAP and GABARAPL1 showed the highest activities. Differences found within LC3/GABARAP proteins suggested the existence of a lipidation threshold as a requisite for tethering and inter-vesicular lipid mixing. The presence of ATG12-ATG5-ATG16L1 (E3 in short) increased and accelerated LC3/GABARAP lipidation and subsequent vesicle tethering. However, E3 hampered LC3/GABARAP capacity to induce inter-vesicular lipid mixing and/or fusion. Our results suggest a model in which, together with the recently described inter-membrane lipid transfer mechanism, LC3/GABARAP could help in the phagophore expansion process through their ability to tether and fuse vesicles. The growing regions would be areas where the LC3/GABARAP proteins could be lipidated in the absence of E3, or else an independent regulatory mechanism would allow lipid/vesicle incorporation and phagophore growth when E3 was present.Abbreviations: Atg/ATG: autophagy-related protein (in yeast/human); E3: ATG12-ATG5-ATG16L1 complex; GABARAP: gamma-aminobutyric acid receptor associated protein; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3.

Keywords: ATG12–ATG5-ATG16L1; ATG8; Autophagy proteins; E3 complex; LC3/GABARAP; autophagosome expansion.

Figure 1.

LC3/GABARAP proteins and E3 in the phagophore expansion. (A) Schematic representation of the LC3/GABARAP protein family. 3D structures of each protein in solution, presented using PyMOL. PDB: LC3A (5C×3), LC3B (2ZJD), LC3C (2NCN), GABARAP (1GNU), GABARAPL1 (5 L×I) and GABARAPL2 (4CO7). (B) Hypothetical model based on the results in Iriondo et al. GABARAP and GABARAPL1 are the main candidates to promote phagophore expansion, particularly on the highly curved edges, as these proteins reach faster the necessary lipidation levels to trigger vesicle tethering and inter-vesicular lipid mixing. In this model, E3 is not forming an immobile scaffold with lipidated LC3/GABARAP proteins on the edges or growing zones of the phagophore.