Ceramide and the membrane-fusion activity of LC3/GABARAP autophagy proteins

Macroautophagy is a cellular degradation process characterized by the formation of the double-membrane structure termed autophagosome (AP). The process of AP formation is not fully understood, but it is thought to happen through the combined action of direct lipid transfer and incorporation of new vesicles to the edges of the growing structure. Human LC3/GABARAP autophagy-related proteins are known to induce vesicle tethering and lipid mixing in vitro, which makes them suitable for the latter expansion mechanism. Ceramide (Cer) is a sphingolipid previously described to facilitate membrane fusion. Cer has also been related to macroautophagy modulation previously, although its specific role remains unclear. Moreover, the presence of sphingolipids in the AP has been suggested by recent experiments, increasing the relevance of Cer in macroautophagy. The present work has investigated the potential role that Cer could have on the proposed fusion of new vesicles to the nascent AP membrane. Interaction of purified ATG proteins with lipid vesicles of defined composition has been quantified using fluorescence spectroscopic techniques. Our results suggest that, if present, Cer could promote the vesicle tethering and leakage-free intervesicular lipid mixing induced by GABARAP and GABARAPL1, which would in turn mediate AP membrane expansion.