Autophagosome Precursor Maturation Requires Homotypic Fusion

Claudia Puri,David C. Rubinsztein,Maurizio Renna,Brinda Ravikumar,Kevin Moreau

doi:10.1016/j.cell.2011.06.023

Abstract

SummaryAutophagy is a catabolic process in which lysosomes degrade intracytoplasmic contents transported in double-membraned autophagosomes. Autophagosomes are formed by the elongation and fusion of phagophores, which can be derived from preautophagosomal structures coming from the plasma membrane and other sites like the endoplasmic reticulum and mitochondria. The mechanisms by which preautophagosomal structures elongate their membranes and mature toward fully formed autophagosomes still remain unknown. Here, we show that the maturation of the early Atg16L1 precursors requires homotypic fusion, which is essential for subsequent autophagosome formation. Atg16L1 precursor homotypic fusion depends on the SNARE protein VAMP7 together with partner SNAREs. Atg16L1 precursor homotypic fusion is a critical event in the early phases of autophagy that couples membrane acquisition and autophagosome biogenesis, as this step regulates the size of the vesicles, which in turn appears to influence their subsequent maturation into LC3-positive autophagosomes.

Highlights

Macroautophagy, which we will refer to as autophagy, is a catabolic process in which cytoplasmic materials are engulfed by double membrane structures, which form autophagosomes
VAMP7 Colocalizes with Atg16L1 and Atg5 Proteins To understand how Atg16L1 precursors mature toward phagophores and subsequently to a fully formed autophagosome, we hypothesized that fusion events may occur between early precursors
Vesicle fusion depends on multiple classes of proteins, including SNARE proteins, which play a major role by docking vesicles with their target compartments and by catalyzing the fusion of the opposing membranes

Summary

Introduction

Macroautophagy, which we will refer to as autophagy, is a catabolic process in which cytoplasmic materials are engulfed by double membrane structures, which form autophagosomes. The phagophore membranes extend in a manner that is not understood and fuse to create a double-membrane autophagosome (Mizushima et al, 2008; Ravikumar et al, 2009; Xie and Klionsky, 2007). More than 30 autophagy-related proteins (Atg) have been discovered to regulate autophagy Some of these proteins contribute to two conjugation reactions that are required for autophagosome formation. The first involves Atg conjugation to Atg (Xie and Klionsky, 2007) This Atg12-Atg conjugate interacts noncovalently with Atg16L1 to form a complex essential for the phagophore expansion. The complexes formed by Atg16L1-Atg12-Atg are localized to the phagophore and dissociate upon completion of autophagosome formation. A second conjugation reaction involving Atg8/LC3 contributes to the completion of autophagosome formation. LC3-II levels correlate with autophagic vesicle numbers, which can be assessed by western blotting or by scoring LC3-positive vesicle numbers (Rubinsztein et al, 2009)

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