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Autophagy is an evolutionally conserved "self-eating" process. Although the genes essential for autophagy (termed Atg) have been identified in yeast, the molecular mechanism of how these Atg proteins control autophagosome formation in mammalian cells remains to be elucidated. Here, we demonstrate that Bif-1 (also known as Endophilin B1) interacts with Beclin 1 through UVRAG and acts as a positive mediator of the class III PI3-kinase (PI3KC3). In response to nutrition deprivation, Bif-1 localizes to autophagosomes where it colocalizes with Atg5, as well as LC3. Furthermore, loss of Bif-1 suppresses autophagosome formation. While the SH3 domain of Bif-1 is sufficient for binding to UVRAG, both the BAR and SH3 domains are required for Bif-1 to activate PI3KC3 and induce autophagosome formation. We also found that Bif-1 ablation prolongs cell survival under nutrient starvation. Moreover, knockout of Bif-1 significantly enhances the development of spontaneous tumors in mice. These findings suggest that Bif-1 joins the UVRAG-Beclin 1 complex as a potential activator of autophagy and tumor suppressor.Autophagy is a tightly orchestrated intracellular process for bulk degradation of cytoplasmic proteins or organelles that appears to be essential for many physiological processes such as cellular homeostasis, development, differentiation, tissue remodeling, cell survival and death, innate immunity, and pathogenesis in various organisms 1-4 . The process of autophagic degradation is initiated when a portion of the cytosolic components are sequestered in cupshaped membrane structures called isolation membranes 1, 2, 5, 6 . The isolation membranes are elongated and eventually sealed to become double-membrane vesicles called autophagosomes, which are then fused with lysosomes resulting in degradation of the enclosed components. Eighteen autophagy-related (Atg) genes have been characterized in S. cerevisiae and can be categorized into four functional groups: (1) the Atg1 protein kinase complex regulating the induction of autophagy, (2) the class III PI3-kinase (PI3KC3) lipid kinase complex controlling vesicle nucleation, (3) the Atg12-Atg5 and Atg8-phosphatidylethanolamine conjugation pathways for vesicle expansion and completion, and (4) the Atg protein retrieval system 2, 7 . Beclin 1, the mammalian homologue of yeast Atg6, is a key component of the PI3KC3 complex, which plays an essential role in autophagosome formation 8-11 . Although the phosphatidylinositol 3-phosphate (PtdIns-3-P) generated by PI3KC3 has been proposed to control membrane dynamics during autophagosome formation 3 , the molecular mechanism underlying this process remains unknown. Results Loss of Bif-1 suppresses caspase-independent cell deathWe have previously reported that Bif-1 localizes to mitochondria and regulates the activation of Bax and Bak during apoptosis induced by intrinsic death stimuli 21 . To examine Bif-1 localization in mouse embryonic fibroblast (MEF) cells during serum deprivation, we added a pancaspase inhibitor, z-VAD-fmk, to ...

Autophagy is a highly orchestrated intracellular bulk degradation process that is activated by various environmental stresses. The serine/threonine kinase ULK1, like its yeast homologue Atg1, is a key initiator of autophagy that is negatively regulated by the mTOR kinase. However, the molecular mechanism that controls the inhibitory effect of mTOR on ULK1-mediated autophagy is not fully understood. Here we identified AMPK, a central energy sensor, as a new ULK1-binding partner. We found that AMPK binds to the PS domain of ULK1 and this interaction is required for ULK1-mediated autophagy. Interestingly, activation of AMPK by AICAR induces 14-3-3 binding to the AMPK-ULK1-mTORC1 complex, which coincides with raptor Ser792 phosphorylation and mTOR inactivation. Consistently, AICAR induces autophagy in TSC2-deficient cells expressing wild-type raptor but not the mutant raptor that lacks the AMPK phosphorylation sites (Ser722 and Ser792). Taken together, these results suggest that AMPK association with ULK1 plays an important role in autophagy induction, at least in part, by phosphorylation of raptor to lift the inhibitory effect of mTOR on the ULK1 autophagic complex.