Spinster is an Ambra1 interacting protein required for autophagosome maturation

Autophagy is a self-degradative process involved in the turnover of cellular components in response to nutrient starvation or to organelle damage. During this process, portions of cytoplasm are sequestered by double-membrane vesicles, the autophagosomes, and degraded after fusion with lysosomes. Different protein complexes participate to autophagosome formation including various components of the class III phosphatidylinositol-3-OH kinase complex (Ambra1, Beclin 1, Vps34, Vps15, UVRAG) and most of the Atg genes. Here, I show that Ambra1 also plays a role in the regulation of the late steps of autophagy via the interaction with the lysosomal protein Spinster. Spinster is trans-membrane protein known to regulate the endosomal pathway in Drosophila neurons. Recent studies have shown that Spinster is able to bind the antiapoptotic protein Bcl2 and is involved in the execution of a caspase-independent cell death associated to autophagic vacuole formation. In this study, I demostrate that Spinster is involved in the regulation of autophagosome maturation. Spinster localizes with LC3, a marker of the autophagosomal compartment. Notably, Spinster overexpression induces autophagy, while its down-regulation leads to an alteration of autophagolysosomal acidification causing a block of autophagic degradation. Finally, I showed that Ambra1 regulates Spinster activity by regulating its levels of ubiquitination. Taken together, my data show that Spinster is a positive regulator of the autophagic process and that Ambra1 has a novel role in the regulation of autophagosome maturation.