Achleitner, S. | Search bioRxiv

bioRxiv2026

Activation of the bridge-like lipid transfer protein Atg2 by Atg1-mediated phosphorylation

Bridge-like lipid transfer proteins (BLTPs) have emerged as major contributors to lipid flux and thus cellular organization. BLTPs harbor a hydrophobic channel connecting two membranes to enable the bulk flow of lipids between them. The regulation of BLTPs is incompletely understood. Employing in vitro reconstitution, molecular dynamics simulations, and cell biology, we discovered that the BLTP Atg2, which mediates lipid transfer during autophagosome biogenesis, is activated by the Atg1 kinase. Atg1 phosphorylates two serines in the N-terminal region of Atg2. This triggers membrane binding and opening of the channel to enable lipid transfer. The Atg1 kinase complex is localized to the endoplasmic reticulum through binding of its Atg13 subunit to the VAP protein Scs2. Phosphorylated Atg2 can subsequently establish contacts with Atg9 vesicles. Our study delineates a pathway for the Atg1 kinase mediated activation of the BLTP Atg2 to establish membrane contact site formation and lipid flux.

cell biology↗

bioRxiv2023

Faa1 membrane binding drives positive feedback in autophagosome biogenesis via fatty acid activation

Autophagy serves as a stress response pathway by mediating the degradation of cellular material within lysosomes. In autophagy this material is encapsulated in double membrane vesicles termed autophagosomes, which form from precursors referred to as phagophores. Phagophores grow by lipid influx from the endoplasmic reticulum into Atg9-positive compartments and local lipid synthesis provides lipids for their expansion. How phagophore nucleation and expansion are coordinated with lipid synthesis is unclear. Here, we show that Faa1, an enzyme activating fatty acids, is directly recruited to Atg9 vesicles. We further show that Faa1 binds to negatively charged membranes. We define the membrane binding surface in Faa1 and show that membrane binding is required for its enzymatic activity. In cells, membrane binding by Faa1 is required for its recruitment to phagophores and promotes autophagosome biogenesis. Our results suggest a positive feedback loop coupling phagophore nucleation and expansion to lipid synthesis. SummaryBaumann, Achleitner, Tulli et al. dissect Faa1 function and recruitment during autophagy. They discover that Faa1 directly binds membranes via a positively charged surface. This is a prerequisite for Faa1s enzymatic activity sustaining autophagosome biogenesis.

biochemistry↗