Ambra1 regulates the late stages of autophagy by modulating the activity of the lysosomal protein Spinster-1

Autophagy is a highly-regulated multistep process by which cytoplasmic proteins and organelles are enwrapped into double membrane vesicles (autophagosomes) and delivered to the lysosomes for degradation. Different protein complexes are known to regulate specific steps of the autophagic process. Autophagosome nucleation and elongation are under the control of the Ulk1 and Beclin 1/class III PI3-kinase complexes and two ubiquitin-like conjugation systems (Atg12/Atg5 and LC3), while autophagosome maturation and fusion with lysosomes are regulated by the class C Vps/UVRAG complex, the small GTPase Rab7 and the lysosome membrane proteins Lamp1 and Lamp2. An as yet unanswered question is how different steps of autophagy are activated in a coordinated fashion in order to ensure an orchestrated execution of the process. Ambra1 is a WD40 protein involved in autophagy regulation in mammals. We have previously shown that Ambra1 binds to Beclin 1 and regulates Beclin 1/ class III PI3-kinase complex activity. Here, we report that Ambra1 also plays an important role in the late stages of autophagy by interacting with the lysosomal protein Spinster-1. We found that Spinster-1 is involved in autophagosome maturation, since its downregulation led to a defective degradation of autophagosome cargos, accompanied by aberrant acidification of the lysosomal compartment. Moreover, we showed that Ambra1 regulates Spinster-1 activity by modulating its ubiquitination state through the interaction with the ubiquitin E3 ligase TRAF6. Taken together, our results indicate that Ambra1 participates to both early and late stages of autophagosome-mediated degradation, providing evidences that different steps of autophagy could be under the control of common regulatory factors