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. Running title: Endomembrane remodeling for autophagy

Atg2: A novel phospholipid transfer protein that mediates de novo autophagosome biogenesis

“…Several mechanisms have been proposed for membrane supply during autophagosome formation, including direct connections between the IM and ER, and vesicular transport (Fig. ) . One of the biggest shortcomings of these models is that it would be difficult to supply lipids while excluding membrane proteins that are abundant in membrane source organelles, including the ER.…”

The TRAPP complex mediates secretion arrest induced by stress granule assembly

“…By contrast, conditions that are not permissive for cell proliferation, such as nutrient starvation, stabilize the association of COPII/TRAPP with ERES membranes and prevent their translocation to SGs. Indeed, a profound remodeling of ERES is known to occur during nutrient starvation and coincides with the rerouting of COPII and TRAPP from their role in mediating the ER export of newly synthesized proteins to their role in autophagosome formation (Imai et al , ; Kim et al , ; Lamb et al , ; Ge et al , ; Ramírez‐Peinado et al , ; van Leeuwen et al , ). The kinetics of the COPII cycling at the sites of phagosome formation in the ER have not been specifically explored.…”

Hijacking intracellular membranes to feed autophagosomal growth

“…Furthermore, it has been shown that a pool of FIP200 enriched in the ERES‐ERGIC region tethers adjacent Sec12‐positive ERES to favor membrane expansion independently of ULK1 and ATG13 (Fig. ) . This evidence came from observations in yeast where the FIP200/RB1CC1 homolog Atg17 is able to tether highly curved vesicles by cooperating with ULK1/ATG1.…”

“…The relocation of components of the ERES to the ERGIC has also been shown to contribute to autophagosome biogenesis. In cell-free assays, ERGIC was found to be a source of membranes that then undergo LC3 lipidation [78,79]. Interestingly, these membranes originate by a starvation-induced relocation of COPII machinery from the ERES to the ERGIC to generate ERGIC-derived COPII-dependent membrane templates for autophagosome biogenesis (Fig.…”