FIP200, a ULK-interacting protein, is required for autophagosome formation in mammalian cells

TLDR

Autophagy is a membrane‑mediated intracellular degradation system in which the serine/threonine kinase Atg1 is essential for autophagosome formation. The study aims to clarify the roles of mammalian ULK1 and ULK2 in autophagy. The authors screened ULK binding partners and identified FIP200, a 200‑kDa protein involved in cell size, proliferation, and migration. ULK1 and ULK2 localize to the isolation membrane during starvation, and loss of their kinase activity blocks autophagosome formation; similarly, FIP200 relocates to the isolation membrane, and its absence abolishes autophagy induction, destabilizes and dephosphorylates ULK1, indicating that FIP200 is a novel autophagy factor that cooperates with ULKs.

Abstract

Autophagy is a membrane-mediated intracellular degradation system. The serine/threonine kinase Atg1 plays an essential role in autophagosome formation. However, the role of the mammalian Atg1 homologues UNC-51–like kinase (ULK) 1 and 2 are not yet well understood. We found that murine ULK1 and 2 localized to autophagic isolation membrane under starvation conditions. Kinase-dead alleles of ULK1 and 2 exerted a dominant-negative effect on autophagosome formation, suggesting that ULK kinase activity is important for autophagy. We next screened for ULK binding proteins and identified the focal adhesion kinase family interacting protein of 200 kD (FIP200), which regulates diverse cellular functions such as cell size, proliferation, and migration. We found that FIP200 was redistributed from the cytoplasm to the isolation membrane under starvation conditions. In FIP200-deficient cells, autophagy induction by various treatments was abolished, and both stability and phosphorylation of ULK1 were impaired. These results suggest that FIP200 is a novel mammalian autophagy factor that functions together with ULKs.