A selective ER ‐phagy exerts procollagen quality control via a Calnexin‐ FAM 134B complex

Abstract

Autophagy is a cytosolic quality control process that recognizes substrates through receptor‐mediated mechanisms. Procollagens, the most abundant gene products in Metazoa, are synthesized in the endoplasmic reticulum (ER), and a fraction that fails to attain the native structure is cleared by autophagy. However, how autophagy selectively recognizes misfolded procollagens in the ER lumen is still unknown. We performed siRNA interference, CRISPR‐Cas9 or knockout‐mediated gene deletion of candidate autophagy and ER proteins in collagen producing cells. We found that the ER‐resident lectin chaperone Calnexin (CANX) and the ER‐phagy receptor FAM134B are required for autophagy‐mediated quality control of endogenous procollagens. Mechanistically, CANX acts as co‐receptor that recognizes ER luminal misfolded procollagens and interacts with the ER‐phagy receptor FAM134B. In turn, FAM134B binds the autophagosome membrane‐associated protein LC3 and delivers a portion of ER containing both CANX and procollagen to the lysosome for degradation. Thus, a crosstalk between the ER quality control machinery and the autophagy pathway selectively disposes of proteasome‐resistant misfolded clients from the ER.

Emerging lysosomal pathways for quality control at the endoplasmic reticulum

“…The RHD of FAM134B interacts with the TM domain of CANX that binds to PC1 molecules most likely through the N-terminal luminal domain. Consistently, the inhibition of CANX function prevents FAM134B interaction with PC, suggesting that CANX bridges ER luminal PC to FAM134B at the ER membrane [24].…”

ER‐phagy: shaping up and destressing the endoplasmic reticulum

“…This reflects a form of hereditary sensory and autonomic neuropathy (HSAN type II) in humans caused by FAM134B nonsense mutations . However, it remains to be determined whether the primary cause of these neuropathies is defective FAM134B‐mediated procollagen (PC) quality control within the ER lumen, as might be suggested by the in vitro study described above . No effect of Fam134b knockout was reported in other organs, albeit in unchallenged mice.…”

The various shades of ER‐phagy

“…Clearance of an overload of misfolded proteins, large amounts of membranes or ER contents could indeed be called bulk ER‐phagy and presumably depends on a receptor with a broad substrate spectrum. In contrast, dependency on co‐receptors, like in the case of FAM134B and misfolded procollagen , might speak to a more targeted and specific ER‐phagy receptor with a narrow substrate spectrum. Of note, RTN3 specifically regulates the elimination of prohormone aggregates, in particular Akita proinsulin aggregates, from the ER lumen .…”

“…In principle, the following possibilities exist and, most probably, a combination of those takes place: Activation via PTMs: PTMs can reveal or mask binding interfaces, which are necessary for the oligomerization, and modulation of receptor binding affinity to its substrates and to autophagic membranes. Co‐receptors, auxiliary and scaffold proteins (i.e. ER chaperones or ER resident proteins): clustering as well as substrate binding could be facilitated via chaperones, this is likely the case with Calnexin in misfolded procollagen elimination . Multiple chaperones could serve the same receptor or recognize the same intraluminal ER protein aggregates.…”

“…2 Co-receptors, auxiliary and scaffold proteins (i.e. ER chaperones or ER resident proteins): clustering as well as substrate binding could be facilitated via chaperones, this is likely the case with Calnexin in misfolded procollagen elimination [37]. Multiple chaperones could serve the same receptor or recognize the same intraluminal ER protein aggregates.…”