Remodeling of ER ‐exit sites initiates a membrane supply pathway for autophagosome biogenesis

Abstract

Autophagosomes are double-membrane vesicles generated during autophagy. Biogenesis of the autophagosome requires membrane acquisition from intracellular compartments, the mechanisms of which are unclear. We previously found that a relocation of COPII machinery to the ER-Golgi intermediate compartment (ERGIC) generates ERGIC-derived COPII vesicles which serve as a membrane precursor for the lipidation of LC3, a key membrane component of the autophagosome. Here we employed super-resolution microscopy to show that starvation induces the enlargement of ER-exit sites (ERES) positive for the COPII activator, SEC12, and the remodeled ERES patches along the ERGIC A SEC12 binding protein, CTAGE5, is required for the enlargement of ERES, SEC12 relocation to the ERGIC, and modulates autophagosome biogenesis. Moreover, FIP200, a subunit of the ULK protein kinase complex, facilitates the starvation-induced enlargement of ERES independent of the other subunits of this complex and associates via its C-terminal domain with SEC12. Our data indicate a pathway wherein FIP200 and CTAGE5 facilitate starvation-induced remodeling of the ERES, a prerequisite for the production of COPII vesicles budded from the ERGIC that contribute to autophagosome formation.

COPII gets in shape: Lessons derived from morphological aspects of early secretion

“…Traffic routes from the ER are not exclusively directed to the Golgi complex. It has been known for many years that the traffic machinery of COPII is utilized in the formation of autophagosomes . Functional hub‐like contacts between ERES and membranes other than ERGIC or cis Golgi may be established to support traffic.…”

Crosstalk between Autophagy and Nanomaterials: Internalization, Activation, Termination

“…Many molecular machineries are involved in the process of autophagy (Figure ). First, during the formation of the phagophore assembly sites, uncoordinated 51‐like kinase (ULK) protein kinase complex (including FIP200, ULK1, Atg13, and Atg101), class III phosphatidylinositol 3‐kinase (PI3K) complex, ubiquitin‐like phosphatidylethanolamine conjugation machinery (including Atg3, Atg7, and Atg12–Atg5–Atg16L1 complex), Atg9 proteins and downstream effectors are coordinated to regulate the process . Notably, ULK complex is implicated in whole autophagosomes formation, which regulates the formation of autophagosome from initiation to maturation .…”

“…Atg9, a conserved cross species transmembrane protein, is necessary for the formation of autophagosome by shuttling between phagophore assembly site and nonphagophore assembly site to deliver membrane to the phagophore assembly site . Subsequently, in maturation of autophagosomes and in autophagosomes fusion with lysosomes/endosomes, the major regulation components contain CapZ, Actin, Rab proteins, soluble N ‐ethylmaleimide‐sensitive factor attachment protein receptor (SNARE) proteins, like Syntaxin 17, and tethering complexes including homotypic fusion and protein sorting (HOPS)‐tethering complex . Lastly, during the fission of autolysosome, clathrin and kinesin are involved in this process .…”

Membrane Transport at an Organelle Interface in the Early Secretory Pathway: Take Your Coat Off and Stay a While

“…Ultimately, elongation of the phagophore enables cytoplasmic engulfment of proteins/organelles, which are delivered to lysosomes for breakdown to serve as a nutrient source under conditions of stress. Perhaps somewhat surprisingly, this entire process requires the redistribution of the Sar1 GEF Sec12 to ERGIC membranes, in a manner that requires cTage5, but cTage5 remains tightly associated with the ER under these conditions . The mechanism by which Sec12 is freed from cTage5 and delivered to ERGIC membranes during starvation remains unclear but may involve COPII‐mediated transport and phosphatidylinositol 3‐kinase signaling, which is also required to generate LC3 lipidation‐active membranes from the ERGIC …”