ULK1 forms distinct oligomeric states and nanoscopic structures during autophagy initiation

Area of Science:

• Cellular Biology
• Molecular Biology
• Biochemistry

Background:

• Autophagy is a crucial cellular process involving significant molecular and membrane rearrangements.
• The precise mechanisms governing autophagy initiation, particularly the role of ULK1, are not fully elucidated.

Purpose of the Study:

• To investigate the nanoscopic distribution and behavior of endogenous ULK1 during autophagy initiation.
• To understand the molecular requirements for ULK1 clustering and its role in autophagosome biogenesis.

Main Methods:

• Quantitative photoactivated localization microscopy (qPALM) with single-molecule sensitivity.
• Cross-correlation analysis to determine molecular stoichiometry within ULK1 clusters.

Main Results:

• Under starvation, ULK1 forms large clusters (up to 161 molecules) at the endoplasmic reticulum.
• ULK1 clusters require at least 30 molecules to engage in autophagosome formation.
• ULK1 activity is not required for initial clustering but is essential for subsequent cluster expansion and involves interactions with Atg14, Atg16, LC3B, and Vps34.

Conclusions:

• ULK1 clustering is a critical early step in autophagy initiation.
• A minimum number of ULK1 molecules are necessary for initiating autophagosome formation.
• ULK1 activity and interactions with specific autophagy proteins drive cluster expansion and autophagosome biogenesis.