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

Abstract

Maintenance of cellular proteostasis relies on efficient clearance of defective gene products. For misfolded secretory proteins, this involves dislocation from the endoplasmic reticulum (ER) into the cytosol followed by proteasomal degradation. However, polypeptide aggregation prevents cytosolic dislocation and instead activates ill-defined lysosomal catabolic pathways. Here, we describe an ER-to-lysosome-associated degradation pathway (ERLAD) for proteasome-resistant polymers of alpha1-antitrypsin Z (ATZ). ERLAD involves the ER-chaperone calnexin (CNX) and the engagement of the LC3 lipidation machinery by the ER-resident ER-phagy receptor FAM134B, echoing the initiation of starvation-induced, receptor-mediated ER-phagy. However, in striking contrast to ER-phagy, ATZ polymer delivery from the ER lumen to LAMP1/RAB7-positive endolysosomes for clearance does not require ER capture within autophagosomes. Rather, it relies on vesicular transport where single-membrane, ER-derived, ATZ-containing vesicles release their luminal content within endolysosomes upon membrane:membrane fusion events mediated by the ER-resident SNARE STX17 and the endolysosomal SNARE VAMP8. These results may help explain the lack of benefits of pharmacologic macroautophagy enhancement that has been reported for some luminal aggregopathies.

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

“…We have recently reported that CANX delivers proteasome‐resistant polymers of alpha 1‐antitrypsin Z (ATZ) to ER subdomains en route for FAM134B‐mediated vesicular transport to the lysosomes for degradation (Fregno et al , ). ATZ clearance, however, shows substantial differences compared to the quality control autophagy of endogenous PC that we studied in collagen‐producing cells.…”

“…However, not all misfolded ER proteins are eligible for ERAD and thus must be cleared from the ER through other processes. Autophagy‐dependent and autophagy‐independent lysosomal degradation of proteins from the ER has also been reported (Ishida et al , ; Hidvegi et al , ; Houck et al , ; Fregno et al , ). However, the mechanism by which misfolded ER luminal proteins are recognized by the cytosolic autophagic machinery and delivered to the lysosomes remains to be understood.…”

“…Canx −/− MEF cell lines were previously described (Kraus et al , ). Fam134b CRISPR‐Cas9 MEF cell line was described in Fregno et al (). Fam134b −/− MEF cell line was described in Khaminets et al ().…”

Rhomboid protease RHBDL4 promotes retrotranslocation of aggregation-prone proteins for degradation

“…All together, these results suggest that RHBDL4, in cooperation with the erlin complex, cleaves and thereby induces the degradation of aggregation-prone ERAD-L substrates. In contrast, given the limited dimension of any putative ERAD dislocation channel, macroscopic protein aggregates can only be removed by autophagy-or vesiclebased, lysosomal degradation routes (Fregno et al, 2018;Fu and Sztul, 2009). In addition to controlling the integrity of the membrane proteome as previously described (Fleig et al, 2012), RHBDL4 serves as an important fail-safe mechanism for ER luminal protein homeostasis by lowering the concentration of aggregation-prone luminal ERAD-L substrates.…”

“…Protein aggregates cause cellular toxicity and are a hallmark of several diseases including neurodegenerative disorders, like Alzheimer's or Parkinson's disease (Chiti and Dobson, 2017). Clearance of large misfolded protein species in the ER is accomplished by selective autophagy (Grumati et al, 2018;Kruse et al, 2006a) or a recently described vesicular ER-to-lysosome trafficking pathway (Fregno et al, 2018). Yet, if not terminally aggregated, the best characterized mechanism for turnover of aberrant proteins is ERAD.…”