Structural Biology and Electron Microscopy of the Autophagy Molecular Machinery.

Louis Tung Faat Lai,Liwen Jiang,Wilson Chun Yu Lau,Wenxin Zhang,Hao Ye

doi:10.3390/cells8121627

Louis Tung Faat Lai, Liwen Jiang + Show 3 more

Open Access

https://doi.org/10.3390/cells8121627

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Abstract

Autophagy is a highly regulated bulk degradation process that plays a key role in the maintenance of cellular homeostasis. During autophagy, a double membrane-bound compartment termed the autophagosome is formed through de novo nucleation and assembly of membrane sources to engulf unwanted cytoplasmic components and targets them to the lysosome or vacuole for degradation. Central to this process are the autophagy-related (ATG) proteins, which play a critical role in plant fitness, immunity, and environmental stress response. Over the past few years, cryo-electron microscopy (cryo-EM) and single-particle analysis has matured into a powerful and versatile technique for the structural determination of protein complexes at high resolution and has contributed greatly to our current understanding of the molecular mechanisms underlying autophagosome biogenesis. Here we describe the plant-specific ATG proteins and summarize recent structural and mechanistic studies on the protein machinery involved in autophagy initiation with an emphasis on those by single-particle analysis.

Highlights

Macroautophagy is an evolutionarily conserved eukaryotic “self-eating” process for the degradation of damaged proteins and organelles, protein aggregates, and invading pathogens [1,2,3]
ATG2-ATG18/WD-repeat protein interacting with phosphoinositides (WIPI) axis is the downstream effector of phosphatidylinositol 3-phosphate (PI3P) produced by the PI3KC3 complex [110,111,112,113] (Figure 1)
We speculate that the cytoplasmic regions of ATG9 protomers from opposing vesicles/compartments interact during autophagy, pulling them toward each other, and eventually result in membrane fusion, through a process which likely requires the help of additional regulators

Summary

Introduction

Macroautophagy ( known as autophagy) is an evolutionarily conserved eukaryotic “self-eating” process for the degradation of damaged proteins and organelles, protein aggregates, and invading pathogens [1,2,3]. Being a highly dynamic process, autophagosome biogenesis can be divided into four stages including initiation, nucleation, expansion, and maturation [13,14], all of which are highly dependent on and tightly regulated by a dedicated set of protein machineries known as the autophagy-related (Atg/ATG) proteins (denoted by the letters Atg and ATG in yeast and mammals/plants, respectively). There exists several technologies dedicated to producing proteins in plants, including the Agrobacterium-mediated transient gene expression [24], chloroplast transformation [25], and stable transformation that integrates foreign genes into the plant nuclear genome [26] Both the transient expression and the chloroplast transformation are of only limited use at present owing to the inability to either express proteins of large size or in sufficient quantity, which is important for structural studies. Proteins with different compositional and conformational heterogeneity can be studied by cryo-EM [75], which is suitable for studying protein complexes with dynamic and intrinsically disordered properties such as ATG proteins

The PI3KC3 Complex

The Transmembrane Protein ATG9

Findings

Concluding Perspectives