CCPG1 Is a Non-canonical Autophagy Cargo Receptor Essential for ER-Phagy and Pancreatic ER Proteostasis

Abstract

SummaryMechanisms of selective autophagy of the ER, known as ER-phagy, require molecular delineation, particularly in vivo. It is unclear how these events control ER proteostasis and cellular health. Here, we identify cell-cycle progression gene 1 (CCPG1), an ER-resident protein with no known physiological role, as a non-canonical cargo receptor that directly binds to core autophagy proteins via an LIR motif to mammalian ATG8 proteins and, independently and via a discrete motif, to FIP200. These interactions facilitate ER-phagy. The CCPG1 gene is inducible by the unfolded protein response and thus directly links ER stress to ER-phagy. In vivo, CCPG1 protects against ER luminal protein aggregation and consequent unfolded protein response hyperactivation and tissue injury of the exocrine pancreas. Thus, via identification of this autophagy protein, we describe an unexpected molecular mechanism of ER-phagy and provide evidence that this may be physiologically relevant in ER luminal proteostasis.

ER ‐to‐lysosome‐associated degradation of proteasome‐resistant ATZ polymers occurs via receptor‐mediated vesicular transport

“…So far, our data on endolysosomal delivery of ATZ polymers echo the findings on nutrient deprivation‐induced, FAM134B‐dependent, and LC3‐dependent ER‐phagy. Yet, nutrient deprivation‐induced ER‐phagy critically depends on FIP200 (Khaminets et al , ), a core component of the ULK kinase complex that is required for the generation of autophagosomes that eventually capture and deliver ER fragments to the lysosomal compartments (Hara et al , ; Hosokawa et al , ; Smith et al , ). Strikingly, our analyses reveal that the FAM134B‐dependent ATZ delivery to endolysosomes is not affected in cells lacking FIP200 (Fig D and I, and EV3B) or other components of the autophagosome biogenesis machinery such as ULK1 and ULK2 (Fig E and I, and EV3A and B), ATG13 (Fig F and I, and EV3A and B) or ATG9 (Fig G and I, and EV3A and B; Saitoh et al , ; Shang et al , ; McAlpine et al , ; Bento et al , ; Kaizuka & Mizushima, ; Hurley & Young, ; Kakuta et al , ).…”

“…Strikingly, our analyses reveal that the FAM134B‐dependent ATZ delivery to endolysosomes is not affected in cells lacking FIP200 (Fig D and I, and EV3B) or other components of the autophagosome biogenesis machinery such as ULK1 and ULK2 (Fig E and I, and EV3A and B), ATG13 (Fig F and I, and EV3A and B) or ATG9 (Fig G and I, and EV3A and B; Saitoh et al , ; Shang et al , ; McAlpine et al , ; Bento et al , ; Kaizuka & Mizushima, ; Hurley & Young, ; Kakuta et al , ). Thus, defective autophagosome biogenesis impairs several types of receptor‐mediated ER‐phagy (Khaminets et al , ; Grumati et al , ; Fregno & Molinari, ; Loi et al , ; Smith et al , ) and conventional macroautophagy (Fig EV3B; Hara et al , ; Hosokawa et al , ; Kakuta et al , ). However, it does not affect delivery of proteasome‐resistant ATZ polymers to endolysosomes for clearance.…”

“…Thus, an intervention of macroautophagy in clearance of luminal aggregates would imply their dislocation across the ER membrane, or the capture of ER portions containing them by autophagosomes (Marciniak et al , ). This latter option is particularly intriguing in view of the recent identification of ER‐resident, LC3‐binding proteins proposed to selectively label ER subdomains for lysosomal clearance via processes collectively defined as ER‐phagy (Khaminets et al , ; Fumagalli et al , ; Grumati et al , ; Fregno & Molinari, ; Loi et al , ; Smith et al , ). Conventional macroautophagy inducers such as rapamycin and starvation enhance clearance of cytosolic inclusions (Lamark & Johansen, ; Bento et al , ; Hurley & Young, ; Menzies et al , ) and of some proteasome‐resistant, large polypeptides misfolding in the ER lumen such as proalpha1(I) chains of type I collagen (Ishida et al , ) and mutant dysferlin (Fujita et al , ).…”

ER‐phagy: shaping up and destressing the endoplasmic reticulum

“…RNAi data from Saos‐2 cells do show that CCPG1 may have minor roles in PC clearance . Indeed, when Ccpg1 function is ablated in mice, exocrine pancreatic acinar and gastric chief cells display ER expansion . In particular, CCPG1‐deficient ER in the pancreas harbours numerous lumenal protein inclusions, resulting in UPR elevation.…”

The various shades of ER‐phagy

“…Much like CCPG1, Atg39 can bind autophagy‐related proteins other than mammalian ATG8 (mATG8s: LC3s and GABARAPs). While Atg39 binds Atg11, CCPG1 interacts specifically with FIP200 .…”