Scaffolding the cup-shaped double membrane in autophagy

Area of Science:

• Cell Biology
• Molecular Biology
• Biophysics

Background:

• Autophagy is a fundamental cellular process for degrading and recycling cellular components.
• Phagophore formation, the initial step in autophagosome biogenesis, involves creating a cup-shaped membrane structure.
• The precise mechanisms driving phagophore shape determination remain largely unknown.

Purpose of the Study:

• To investigate the biophysical mechanisms underlying phagophore cup-shape formation during autophagy.
• To elucidate the role of Atg1 complexes, specifically the S-shaped Atg17-Atg31-Atg29 subcomplex, in phagophore nucleation.
• To understand the contribution of Atg17-membrane interactions to the transition towards a cup-shaped phagophore.

Main Methods:

• Computational simulations of membrane remodeling processes, with and without membrane-associated Atg17.
• Analysis of vesicle fusion dynamics and membrane shape transitions.
• Experimental validation using yeast mutants with altered Atg17 membrane interaction sites.

Main Results:

• At least three vesicle fusions are necessary to initiate the phagophore shape.
• S-shaped Atg17 complexes are essential for overcoming kinetic barriers and inducing the cup shape, with non-S-shaped or weakly binding Atg17 failing to induce the transition.
• Experimental mutations in Atg17's putative membrane interaction sites significantly reduce or abolish autophagic activity in yeast.

Conclusions:

• Phagophore formation involves a two-step process: initial vesicle fusion followed by Atg17-guided membrane shape remodeling.
• The S-shape and membrane interaction capabilities of Atg17 complexes are critical for efficient phagophore nucleation.
• These findings provide a mechanistic explanation for the recruitment and function of Atg17 at the yeast preautophagosomal structure.