A PI3K-WIPI2 positive feedback loop allosterically activates LC3 lipidation in autophagy

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Autophagy is a fundamental cellular process for degrading cytoplasmic components via lysosomes within autophagosomes.
• Key steps include PI(3)P synthesis by PI3KC3-C1 and ATG8/LC3 conjugation to phagophore membranes by the ATG12-ATG5-ATG16L1 (E3) complex.
• WIPI2 acts as a crucial linker between these two essential autophagosome biogenesis pathways.

Purpose of the Study:

• To reconstitute and analyze the molecular mechanisms connecting PI(3)P synthesis and LC3 lipidation.
• To elucidate the role of WIPI2 in the recruitment and activation of the E3 complex.
• To investigate the interplay between PI3KC3-C1 and WIPI2 in autophagosome biogenesis.

Main Methods:

• Reconstitution of autophagic processes on giant unilamellar vesicles (GUVs).
• Assays to measure LC3 lipidation and protein recruitment.
• Investigating the effects of ectopic E3 complex targeting in the absence of WIPI2.

Main Results:

• LC3 lipidation on GUVs strictly requires WIPI2 recruitment, which is dependent on PI(3)P.
• WIPI2 allosterically activates the E3 complex, as ectopic E3 targeting without WIPI2 fails to support LC3 lipidation.
• PI3KC3-C1 and WIPI2 engage in a mutual positive feedback loop, enhancing each other's recruitment.
• Simultaneous PI 3-kinase and LC3 lipidation reactions exhibit rapid kinetics due to positive feedback, mimicking cellular autophagosome formation.

Conclusions:

• WIPI2 is a critical allosteric activator of the E3 complex for LC3 lipidation.
• A positive feedback loop between PI3KC3-C1 and WIPI2 accelerates autophagosome biogenesis.
• This reconstituted system provides insights into the fundamental molecular events driving autophagosome formation.