E3 autophagy complex and LC3/GABARAP proteins in autophagosome generation

Abstract

In macroautophagy, autophagosome (AP) formation in humans requires the concerted action of the LC3/GABARAP and ATG12 ubiquitin-like conjugation systems. In the present study, the conjugation to the membrane of six members of the LC3/GABARAP family has been reconstituted in the presence and absence of the product of the ATG12 system (the E3 complex). In addition, the mechanisms by which the different LC3/GABARAP proteins and E3 could participate in vesicle tethering and fusion during AP expansion have been investigated. In the absence of E3, GABARAP and GABARAPL1 showed the highest activities. E3 increases and speeds up lipidation and LC3/GABARAP-promoted tethering. However, E3 hampers LC3/GABARAP capacity to induce inter-vesicular lipid mixing or subsequent fusion, presumably through formation of a rigid scaffold on the vesicle surface. Our results suggest a model of AP expansion in which the growing regions would be areas where the LC3/GABARAP proteins involved should be susceptible to lipidation in the absence of E3, or else a regulation should exist to inhibit the formationof an E3 immobile scaffold.