The role of the Atg2 protein in tethering pre-autophagosomal membranes to the endoplasmic reticulum

October 23, 2018

Postdoctoral Researcher Tetsuya Kotani, Associate Professor Hitoshi Nakatogawa, Honorary Professor Yoshinori Ohsumi and colleagues at Tokyo Institute of Technology have analyzed the Atg protein [1] Atg2, whose function had been completely unknown, and have discovered that Atg2 tethers the pre-autophagosomal membrane to the endoplasmic reticulum during autophagosome formation.

Autophagy is the mechanism which degrades cellular components such as proteins, ribonucleic acids (RNA) and organelles, and is highly conserved from yeast to humans. When autophagy is induced by stresses such as starvation of nutrients, a flattened membrane vesicle called an isolation membrane appears and expands spherically to engulf materials to be degraded, closing to form a double-membrane vesicle called the autophagosome (Figure 1).

The completed autophagosome then fuses with a lysosome or a vacuole [2] and the materials inside the autophagosome are degraded by enzymes inside the lysosome or vacuole. Many Atg proteins playing roles in autophagosome formation have been identified, however, the specific roles of individual Atg proteins in membrane formation are still poorly understood, and the details of the mechanism of autophagosome formation have still not been elucidated.

Overview of Research Achievement

The research group analyzed the function of Atg2, one of the Atg proteins playing a role in autophagosome formation in the budding yeast Saccharomyces cerevisiae. Atg2 is a large protein made of 1,592 amino acids, however, any functional domains are not predicted based on the amino acid sequence. It had been known that Atg2 forms a complex with Atg18, a protein which binds to phosphatidylinositol 3-phosphate (PI3P) [3], and that this was the last among Atg proteins to localize at the site of autophagosome formation. However, its specific function had remained unknown.

The comparison of Atg2's amino acid sequence among different organisms revealed that N- and C-terminal regions [4] have been highly conserved. The group then prepared many Atg2 variants, focusing on these two regions, and evaluated autophagy activity. As a result, they found that regions which are vital to the function of Atg2 exist in N- and C-terminal regions.

Further, by in vitro experiments using Atg2 purified from yeast cells and liposomes (artifical membrane vesicles), the group demonstrated that both the N- and C-terminal regions of Atg2 have the ability to bind to a membrane, and that Atg2 tethers the two membrane structures together by means of these two membrane-binding regions. They also revealed that the C-terminal region is required for Atg18 to bind to membranes containing PI3P, and that it is required for the Atg2-Atg18 complex to localize to the site of autophagosome formation.

Additionally, they demonstrated that the N-terminal region, in contrast, plays a vital role after the Atg2-Atg18 complex has localized to the site of autophagosome formation, and the possibility that it is involved in the association of Atg2 with the endoplasmic reticulum. In view of the above, the group proposes the model that Atg2 tethers pre-autophagosomal membranes (autophagosome precursors) to the endoplasmic reticulum, initiates autophagosome formation, and mediates membrane expansion (Figure 2).

Future Development

Although the formation of the autophagosome with its double-membrane structure is the most unique feature of autophagy, there remain many questions about this process, such as where the membrane comes from and how it is supplied. Previous reports had suggested that the endoplasmic reticulum supplies the membrane for autophagosome formation. Consistent with this model, this research demonstrated the possibility that Atg2 tethers pre-autophagosomal membrane and the endoplasmic reticulum.

However, the mechanism of membrane expansion, such as how lipids are transported from the endoplasmic reticulum to pre-autophagosomal membrane, remains unknown. Analyzing in detail where on the endoplasmic reticulum Atg2 binds and what happens after tethering between pre-autophagosomal membranes and the endoplasmic reticulum is anticipated to lead to unraveling the mechanism of the membrane expansion.

Autophagy is reported to be related to various diseases such as neurodegeneration and cancer. Understanding the mechanism of autophagosome formation is anticipated to provide fundamental information to discovering drugs to treat such autophagy-related diseases.
-end-
[1] Atg protein: Autophagy-related protein. Over 40 Atg proteins have been reported so far in budding yeast, among which nineteen are involved in autophagosome formation. Most of these were discovered by the group of Professor Yoshinori Ohsumi, the winner of the Nobel Prize in Physiology or Medicine 2016.

[2] Lysosome, vacuole: Organelles in the cytoplasm which contain a variety of hydrolases which degrade substances. Lysosomes are present in animal cells, and vacuoles serve this function in plant and yeast cells.

[3] Phosphatidylinositol 3-phosphate: A type of phospholipid that has a phosphate group at the hydroxyl group at position 3 of the inositol ring of phosphatidylinositol (PI). During autophagosome formation, a complex of phosphatidylinositol 3-kinase phosphorylates PI to form PI3P.

[4] N- and C-terminal regions : Proteins are polymers made of amino acids. When the carboxyl group of one amino acid forms a peptide bond with the amino group of another amino acid, and this is repeated, a linear polymer (protein) is formed. The protein terminus which with the amino group is called the N terminus, and the terminus with the carboxyl group is called the C terminus.

Tokyo Institute of Technology

Related Autophagy Articles from Brightsurf:

Surprising insights into the role of autophagy in neuron
Autophagy protects our neurons in the brain, but for entirely different reasons than previously assumed, as researchers from the Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) and Charité in Berlin have shown.

Revealing the identity of the last unknown protein of autophagy
Japanese scientists discovered that Atg9, one of the proteins that function to mediate autophagy, has phospholipid-translocation activity (the lipid scramblase activity) between the two layers of the lipid bilayer?and elucidated that the protein's activity brings about autophagosome membrane expansion.

Lipids, lysosomes, and autophagy: The keys to preventing kidney injury
Lysosomes are cellular waste disposal organelles containing potent enzymes that cause cellular damage if they leak out of ruptured lysosomes.

How zika virus degrades essential protein for neurological development via autophagy
Researchers at the University of Maryland (UMD) shed new light on how Zika virus hijacks our own cellular machinery to break down an essential protein for neurological development, getting it to ''eat itself''.

Autophagy: the beginning of the end
Autophagy, from the Greek for 'self-eating', is an essential process that isolates and recycles cellular components under conditions of stress or when resources are limited.

Cellular cleanup! Atg40 folds the endoplasmic reticulum to facilitate its autophagy
Scientists at Tokyo Institute of Technology (Tokyo Tech) and Institute of Microbial Chemistry investigated 'ER-phagy,' the degradation mechanism of the endoplasmic reticulum (ER), an important organelle with multiple biologically necessary functions like the synthesis of proteins and lipids.

How cells decide the way they want to recycle their content
Researchers from Tokyo Medical and Dental University (TMDU) identified a new phosphorylation site of Ulk1 as a novel regulating mechanism of alternative autophagy.

Autophagy: Scientists discover novel role for self-recycling process in the brain
Proteins classically associated with autophagy regulate the speed of intracellular transport.

Insights into the diagnosis and treatment brain cancer in children
In a recent study published in Autophagy, researchers at Kanazawa University show how abnormalities in a gene called TPR can lead to pediatric brain cancer.

Autophagy degrades liquid droplets, but not aggregates, of proteins
Autophagy is a mechanism through which cellular protein is degraded.

Read More: Autophagy News and Autophagy Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.