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

The Atg8/LC3/GABARAP protein family has been implicated in membrane remodeling events on the growing autophagosome. In particular, each of these proteins can form a protein-lipid conjugate that has been shown in vitro to drive liposome aggregation and in some cases membrane fusion. Furthermore, yeast Atg8 has been described as a curvature sensing protein, through its natural capacity to concentrate on highly curved membranes. A key advance with yeast Atg8, was the introduction of Giant Unilamellar Vesicles (GUVs) as an in vitro support that could allow membrane deformation and tethering to be observed by simple microscopy. Further, micromanipulation of an individual GUV could be used to create local areas of curvature to follow Atg8 partitioning. Here, we use a recently developed method to decorate GUVs with the mammalian Atg8 protein GABARAPL1 and establish the generality of the observations made on yeast Atg8. Then we apply double micromanipulation, the capture and positioning of two independently prepared GUVs, to test elements of the mechanism, speed and reversibility of mammalian Atg8 protein-mediated tethering. We find that the membranes adhere through GABARAPL1/GABARAPL1 homotypic trans-interactions. On a single membrane with two regions with significantly different curvatures we observed that the regions of higher curvature can be enriched up to 10 times in GABARAPL1 compared to the planar regions. This approach has the potential to allow the formation and study of specific topographically-controlled interfaces involving Atg8-proteins and their targets on apposing membranes.