Up-to-date membrane biogenesis in the autophagosome formation

doi:10.1016/j.ceb.2013.03.004

Current Opinion in Cell Biology

Volume 25, Issue 4, August 2013, Pages 455-460

https://doi.org/10.1016/j.ceb.2013.03.004 Get rights and content

When cells are starved, are invaded by foreign bodies such as bacteria, and contain damaged organelles or aggregated proteins, double-membrane organelles called autophagosomes are formed within the cytoplasm to surround, isolate and deliver these materials to lysosomes for degradation. This pathway, called ‘autophagy’, is conserved from yeast to mammalian cells. Unlike other organelles, the autophagosome forms de novo, thus raising unique questions regarding its membrane biogenesis. Here we highlight a number of recent findings related to autophagosome formation and possible involvement of autophagy-specific vesicles originating from other organelles, but with particular attention on the formation sites and the relationship of the autophagosome to other organelles.

Introduction

Macroautophagy (hereafter referred to as autophagy) is an intracellular degradation system that is conserved among eukaryotes. Unlike the ubiquitin–proteasome system, another major degradation system that specifically targets ubiquitylated proteins, autophagy deals with bulk degradation through dynamic membrane rearrangements (Figure 1a). During autophagy, a cup-shaped double membrane called the isolation membrane (or phagophore) is formed in the cytoplasm. The isolation membrane elongates and closes to become a double membrane vesicle known as an autophagosome. Eventually, the outer membrane of the autophagosome fuses with a lysosome (in mammals) or the vacuole (a lysosome-like structure in yeast), leading to the degradation of the contents inside the autophagosome, including its inner membrane. This way, autophagy can degrade long-lived proteins and other macromolecules, recycling them into new building blocks. In addition, larger structures, such as protein aggregates, damaged or unnecessary organelles and intracellular pathogens can be eliminated by autophagy [1]. Recent findings show that autophagy suppresses a variety of pathological conditions such as neurodegeneration, tumorigenesis, diabetes and heart failure [2].

Even though autophagy has been known since the 1950s, the question of how autophagosomes are formed, or, more specifically, how membrane lipids are supplied for nucleation and elongation of isolation membranes, remains unclear. However, the discovery of autophagy-related (ATG) genes by yeast genetics in the 1990s [3] revolutionized this field by providing molecular mechanism models and experimental tools. In this review, we will discuss some of the recent findings that have advanced our knowledge of membrane biogenesis in autophagosome formation.

Section snippets

Atg proteins in autophagosome formation

Autophagosome formation is regulated by an orderly action of Atg proteins. Core 18 Atg proteins originally identified in yeast (Atg1–10, 12–14, 16–18, 29, and 31) are mostly conserved in mammals [1]. Moreover, epistasis analyses indicate that the hierarchy of the core Atg proteins is generally conserved between yeast and mammals [4, 5]. In yeast, when Atg proteins are visualized by fluorescence microscopy, they accumulate to form a single punctate structure beside the vacuole. This structure,

Atg9 vesicles

Atg9 is the only transmembrane protein among the core Atg proteins. Atg9 is required for the accumulation of most of the core Atg proteins [4], and for autophagosome formation both in yeast and mammals [7, 8, 9, 10]. In yeast, localization of Atg9 to the PAS requires post-Golgi vesicle trafficking machineries [11, 12, 13, 14]. Although how Atg9 regulates Atg protein recruitment is not yet clear, recent studies on Atg9 have provided new insights into its function.

By performing immunoelectron

Autophagosome formation site versus pre-existing organelles

The Rubinsztein group proposed that the plasma membrane is one of the multiple sources of autophagosome membranes. They showed that Atg16L1 (an isolation membrane-specific Atg protein) interacts with the clathrin heavy chain and localizes to clathrin-coated structures. Moreover, inhibition of clathrin-mediated endocytosis decreased the formation of Atg16L1-positive dots and autophagosomes. They suggested that isolation membranes that are positive for Atg16L1 can be generated from the plasma

New SNARE protein involved in autophagosome formation

Starvation induces Atg14L to form puncta structures [36^••], however, the mechanism was not known. The N-terminus of Atg14 is important for binding with ER while C-terminus is involved in binding to the autophagosome via the Barkor autophagosome targeting sequence (BATS) [37]. In the latest paper, we found a new SNARE protein involved in autophagosome formation [36^••]. Syntaxin 17 (Stx17) was reported as an ER localized Qa-SNARE but its function was unknown [38]. Like Atg14L, Stx17 localizes on

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

• of special interest
•• of outstanding interest

Acknowledgements

We thank Dr Alison Hobro for English proofreading.

Work in the authors’ lab is supported by grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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Up-to-date membrane biogenesis in the autophagosome formation

Introduction

Section snippets

Atg proteins in autophagosome formation

Atg9 vesicles

Autophagosome formation site versus pre-existing organelles

New SNARE protein involved in autophagosome formation

References and recommended reading

Acknowledgements

Cell

Dev Cell

Proc Natl Acad Sci U S A

Cell

Genes Cells

J Cell Biol

Science

Trends Cell Biol

Int Rev Cell Mol Biol

Nature

The role of Atg proteins in autophagosome formation

Annu Rev Cell Dev Biol

Hierarchy of Atg proteins in pre-autophagosomal structure organization

Genes Cells

Characterization of autophagosome formation site by a hierarchical analysis of mammalian Atg proteins

Autophagy

The pre-autophagosomal structure organized by concerted functions of APG genes is essential for autophagosome formation

EMBO J

Autophagy and the cvt pathway both depend on AUT9

J Bacteriol

Apg9p/Cvt7p is an integral membrane protein required for transport vesicle formation in the Cvt and autophagy pathways

J Cell Biol

Starvation and ULK1-dependent cycling of mammalian Atg9 between the TGN and endosomes

J Cell Sci

Atg9a controls dsDNA-driven dynamic translocation of STING and the innate immune response

Proc Natl Acad Sci U S A

Post-Golgi Sec proteins are required for autophagy in Saccharomyces cerevisiae

Mol Biol Cell