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A New Role for the p24 Family in Antitrypsin Z Clearance from the Endoplasmic Reticulum
Presenter: Prasanna Satpute-Krishnan
Co-authors: Benjamin S. Roberts, Debashree Mitra, Richa Beher, Sudhanshu Abhishek
Affiliation: Uniformed Services University of the Health Sciences, Bethesda, MD
We report a novel role for p24-family proteins as protein quality control (PQC) factors that associate with and facilitate the clearance of the misfolding alpha1-antitrypsin Z mutant (ATZ) from the endoplasmic reticulum (ER). ATZ has been well established to be retained in and cleared from the ER to ER-derived vesicles for direct delivery to lysosomes via one or more ER-to-lysosome-associated degradation (ERLAD) pathways that require the involvement of lipidated LC3 (LC3-II) molecules and are inhibited with autophagy inhibitor, 3-methyladenine [1]. In addition to the ERLAD components calnexin and FAM134B that were previously reported by the Molinari lab [1], we discovered that ATZ co-immunoprepicipated with p24-family members including Tmp21 and TMED9. This contrasts with wild type alpha1-antitrypsin (AAT), which did not co-immunoprecipitate with FAM134B, calnexin or the p24-family members. Live cell imaging experiments revealed that ATZ and the p24-family members trafficked together from the ER to lysosomes but were trapped together at the ER-exit sites marked by Sec24C upon treatment with brefeldin A and nocodazole. Similarly, ATZ and p24-family members were trapped together in ER-derived compartments upon treatment with 3-methyladenine. Importantly, depletion of Tmp21 or TMED9 significantly increased the intracellular levels of ATZ and prevented the trafficking of ATZ to lysosomes. Knocking down Sec24C also impeded trafficking of ATZ to lysosomes. Conversely, overexpression of these p24-family members promoted the degradation of ATZ. Taken together, our results suggest a model in which the p24-family plays a key role in ATZ-clearance by bridging ATZ together with ERLAD machinery and COPII coat proteins for the formation of ER-derived ERLAD vesicles. This reveals a new role for the p24-family in clearing misfolded proteins by-way-of the ERLAD pathway.
[1] Fregno et al. (2018) ER-to-lysosome-associated degradation of proteasome-resistant ATZ polymers occurs via receptor-mediated vesicular transport.