Coupling isolation membrane expansion with Atg8 lipidation

Autophagy (Greek for self-eating) is a process in which cells encapsulate and finally degrade damaged and harmful material that accumulates in them. The encapsulation occurs by the formation of a membrane structure around the substances destined for degradation. When the membrane has fully enwrapped the harmful material is closes to form an autophagosome. The autophagosome subsequently fuses with the cells recycling stations, the lysosomes, wherein the material is broken down and the individual building blocks are recycled and reused. The whole process is analogous to a waste bag (the membrane) wrapping around the waste, just that in our cells the bag is made anew locally at the material. The process of autophagy is important for our health and defects in autophagy have been linked to severe disease such as neurodegeneration and cancer. An outstanding question in the field is how autophagosomes are formed, and in particular how the membrane grows around the material destined for degradation. The formation of autophagosomes is mediated by a number of proteins and protein complexes that have various biochemical activities. Among them is the class III phosphatidylinositol 3-phosphate kinase c1 (PI3K) complex, which produces a signaling lipid (called PI3P) in the membrane. In contrast, the Atg2-Atg18 complex recognizes PI3P and has the ability to transfer lipids between membranes. Finally, the Atg12Atg5-Atg16 complex helps to attach a small protein (Atg8) to the membrane, which is required for the membrane to grow. We will employ a combination of biochemical reconstitution approaches, as well as cell biology using yeast and mammalian cells to study how the interplay of these protein complexes mediates the formation of autophagosomes. Our studies promise to reveal fundamental insights into the enigmatic process by which cells form autophagosomes.

Autophagy (Greek for "self-eating") is a process in which cells encapsulate and ultimately degrade damaged or harmful material that accumulates within them. This encapsulation occurs through the formation of a membrane structure around the substances targeted for degradation. Once the membrane fully envelops the harmful material, it closes to form an autophagosome. The autophagosome subsequently fuses with the cell's recycling centers, the lysosomes, where the material is broken down, and its individual building blocks are recycled and reused. This process is analogous to a waste bag (the membrane) wrapping around waste; however, in our cells, the bag is newly formed at the site of the material. Autophagy is essential for our health, and defects in this process have been linked to severe diseases such as neurodegeneration and cancer. An outstanding question in the field is how autophagosomes form, specifically how the membrane grows around the material destined for degradation. Autophagosome formation is mediated by several proteins and protein complexes with distinct biochemical activities. Among these is the class III phosphatidylinositol 3-phosphate kinase complex 1 (PI3K), which produces a signaling lipid, phosphatidylinositol 3-phosphate (PI3P), in the membrane. The Atg2-Atg18 complex, in contrast, recognizes PI3P and can transfer lipids between membranes. Finally, the Atg12-Atg5-Atg16 complex facilitates the attachment of a small protein (Atg8) to the membrane, which is necessary for membrane expansion. We used a combination of biochemical reconstitution approaches and cell biology methods in yeast and mammalian cells to study how these protein complexes collaborate to mediate autophagosome formation. Our research indicates that autophagy proteins co-assemble on the membrane after PI3K produces PI3P. In cells, this co-assembly occurs on vesicles containing the Atg9 protein, which can serve as seeds for autophagosomes. The Atg2 protein, recruited to these seeds, can import lipids from donor membranes, which the autophagy machinery then uses to attach the Atg8 protein to the membrane. In conclusion, we have uncovered a mechanism by which cells form precursors to autophagosomes, allowing us to hypothesize how these seeds eventually mature into fully formed autophagosomes.