Fasana, E. | bioRxiv Science

bioRxiv2025

The involvement of the ER-phagy receptor FAM134B in membrane contact sites between ER and endolysosomes promotes ERLAD

Membrane contact sites (MCS) between organelles maintain the proximity required for controlled exchange of small molecules and ions yet preventing fusion events that would compromise organelles identity and integrity. Here, by investigating the intracellular fate of the disease-causing Z-variant of alpha1 antitrypsin (ATZ), we report on a novel function of MCS between the endoplasmic reticulum (ER) and RAB7/LAMP1-positive endolysosomes in ER-to-lysosome-associated degradation (ERLAD). For this function, the VAPA:ORP1L:RAB7 multi-protein complex forming MCS between the ER and endolysosomes engages, in an ERLAD client-driven manner, the misfolded protein segregation complex formed by the lectin chaperone Calnexin (CNX), the ER-phagy receptor FAM134B and the ubiquitin-like protein LC3. Generation of this supramolecular complex facilitates the membrane fusion events regulated by the SNARE proteins STX17 and VAMP8 that ensure efficient delivery of ATZ polymers from their site of generation, the ER, to the site of their intracellular clearance, the degradative RAB7/LAMP1-positive endolysosomes.

cell biology↗

bioRxiv2023

FAM134B regulates ER-to-lysosome-associated degradation of misfolded proteins upon pharmacologic or genetic inactivation of ER-associated degradation

About 40% of the eukaryotic cells proteome is synthesized and assembled in the endoplasmic reticulum (ER). Native proteins are transported to their intra- or extra-cellular site of activity. Folding-defective polypeptides are dislocated across the ER membrane into the cytoplasm, poly-ubiquitylated and degraded by 26S proteasomes (ER-associated degradation, ERAD). Large misfolded proteins like mutant forms of collagen or aggregation-prone mutant forms of alpha1 antitrypsin cannot be dislocated across the ER membrane for ERAD. Rather, they are segregated in ER subdomains that vesiculate and deliver their cargo to endolysosomal compartments for clearance by ER-to-lysosome-associated degradation (ERLAD). Here, we show the lysosomal delivery of a canonical ERAD substrate upon pharmacologic and genetic inhibition of the ERAD pathways. This highlights the surrogate intervention of ERLAD to remove defective gene products upon dysfunctional ERAD.

cell biology↗