Biology subjects

Schuschnig, M.

Publications and source records attributed to Schuschnig, M..

8 recordsLinked to original sources

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↗

bioRxiv2026

TAX1BP1 recruitment reactivates autophagy of Tau aggregates through ULK1 and TBK1

The accumulation of protein aggregates is a hallmark of numerous neurodegenerative diseases. These aggregates frequently accumulate ubiquitin and the early autophagy factor p62 yet fail to undergo degradation. Whether such aggregates are principally susceptible to autophagic clearance is unknown. Here, we show that Tau aggregates associated with Alzheimers disease (AD) and other tauopathies persist due to defective autophagy initiation. This autophagy resistance correlates with a block in the recruitment of the autophagy receptor TAX1BP1 and the downstream machinery to the aggregates. Artificial targeting of TAX1BP1 to Tau aggregates restores their autophagic turnover. Mechanistically, TAX1BP1 co-activates ULK1- and TBK1-dependent signaling, thereby enabling aggregate sequestration by autophagosomes. We identify exclusion of the TAX1BP1 receptor as a breaking point at which pathological Tau aggregates evade autophagy. Furthermore, we show that Tau aggregates can be rendered susceptible to autophagy, revealing therapeutic strategies for their clearance in disease.

cell biology↗

bioRxiv2025

A lipid transfer-dependent feedback loop activates ATG9A compartments in autophagy initiation

Autophagy degrades cellular material by sequestering it within autophagosomes, which form de novo from precursors called phagophores. Phagophore assembly and expansion require ATG9A-positive seed compartments, the lipid transfer protein ATG2A, and the class III phosphatidylinositol 3-phosphate kinase complex I (PI3KC3-C1). PI3KC3-C1 synthesizes phosphatidylinositol 3-phosphate (PI3P), a key lipid that drives downstream processes for phagophore expansion, including ATG8 lipidation. We find that ATG9A compartments contain only traces of phosphatidylinositol (PI), likely insufficient for efficient PI3P production or recruitment of PI3P-binding effectors. Nevertheless, ATG2A is recruited to these compartments and mediates lipid transfer, including PI, into them. Remarkably, even without detectable PI3P, ATG9A compartments are direct substrates for ATG8 lipidation, and ATG8 proteins themselves enhance ATG2A-mediated lipid transfer. In cells, ATG2A is essential for the appearance of PI3P on ATG9A compartments. Our findings support a model in which a lipid transfer-driven feedback loop activates ATG9A compartments for phagophore expansion.

cell biology↗

bioRxiv2024

Reconstitution of BNIP3/NIX-mediated autophagy reveals two pathways and hierarchical flexibility of the initiation machinery

Selective autophagy is a lysosomal degradation pathway that is critical for maintaining cellular homeostasis by disposing of harmful cellular material. While the mechanisms by which soluble cargo receptors recruit the autophagy machinery are becoming increasingly clear, the principles governing how organelle-localized transmembrane cargo receptors initiate selective autophagy remain poorly understood. Here, we demonstrate that transmembrane cargo receptors can initiate autophagosome biogenesis not only by recruiting the upstream FIP200/ULK1 complex but also via a WIPI-ATG13 complex. This latter pathway is employed by the BNIP3/NIX receptors to trigger mitophagy. Additionally, other transmembrane mitophagy receptors, including FUNDC1 and BCL2L13, exclusively use the FIP200/ULK1 complex, while FKBP8 and the ER-phagy receptor TEX264 are capable of utilizing both pathways to initiate autophagy. Our study defines the molecular rules for initiation by transmembrane cargo receptors, revealing remarkable flexibility in the assembly and activation of the autophagy machinery, with significant implications for therapeutic interventions.

cell biology↗

bioRxiv2024

Mechanism of TAX1BP1 recruitment in aggrephagy to switch from cargo collection to sequestration

Autophagy mediates the degradation of harmful material within lysosomes. In aggrephagy, the pathway mediating the degradation of aggregated, ubiquitinated proteins, this cargo material is collected in larger condensates prior to its sequestration by autophagosomes. In this process, SQSTM1/p62 and NBR1 drive cargo condensation, while TAX1BP1, which binds to NBR1 recruits the autophagy machinery to facilitate autophagosome biogenesis at the condensates. The mechanistic basis for the TAX1BP1 mediated switch from cargo collection to its sequestration is unclear. Here we show that TAX1BP1 is not a constitutive component of the condensates. Its recruitment correlates with the induction of autophagosome biogenesis. TAX1BP1 is sufficient to recruit the TBK1 kinase via the SINTBAD adapter. We define the NBR1 - TAX1BP1 binding site, which is adjacent to the GABARAP/LC3 interaction site and demonstrate that the recruitment of TAX1BP1 to cargo mimetics can be enhanced by an increased ubiquitin load. Our study suggests that autophagosome biogenesis is initiated once sufficient cargo is collected in the condensates.

cell biology↗

bioRxiv2023

Control of mitophagy initiation and progression by the TBK1 adaptors NAP1 and SINTBAD

Mitophagy preserves overall mitochondrial fitness by selectively targeting damaged mitochondria for degradation. The regulatory mechanisms that prevent PINK1/Parkin-dependent mitophagy and other selective autophagy pathways from overreacting while ensuring swift progression once initiated are largely elusive. Here, we demonstrate how the TBK1 adaptors NAP1 and SINTBAD restrict the initiation of OPTN-driven mitophagy by competing with OPTN for TBK1. Conversely, they promote the progression of NDP52-driven mitophagy by recruiting TBK1 to NDP52 and stabilizing its interaction with FIP200. Notably, OPTN emerges as the primary recruiter of TBK1 during mitophagy initiation, which in return boosts NDP52-mediated mitophagy. Our results thus define NAP1 and SINTBAD as cargo receptor rheostats, elevating the threshold for mitophagy initiation by OPTN while promoting the progression of the pathway once set in motion by supporting NDP52. These findings shed light on the cellular strategy to prevent pathway hyperactivity while still ensuring efficient progression.

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↗

bioRxiv2022

Membrane curvature sensing and stabilization by the autophagic LC3 lipidation machinery

How the highly curved phagophore membrane is stabilized during autophagy initiation is a major open question in autophagosome biogenesis. Here, we use in vitro reconstitution on membrane nanotubes and molecular dynamics simulations to investigate how core autophagy proteins in the LC3 lipidation cascade interact with curved membranes, providing insight into possible roles in regulating membrane shape during autophagosome biogenesis. ATG12-5-16L1 was up to 100-fold enriched on highly curved nanotubes relative to flat membranes. At high surface density, ATG12-5-16L1 binding increased the curvature of the nanotubes. While WIPI2 binding directs membrane recruitment, the amphipathic helix 2 of ATG16L1 is responsible for curvature sensitivity. Molecular dynamics simulations revealed that helix 2 of ATG16L1 inserts shallowly into the membrane, explaining its curvature-sensitive binding to the membrane. These observations show how the binding of the ATG12-5-16L1 complex to the early phagophore rim could stabilize membrane curvature and facilitate autophagosome growth.

biophysics↗