1. Academic Validation
  2. A technique for delineating the unfolding requirements for substrate entry into retrotranslocons during endoplasmic reticulum-associated degradation

A technique for delineating the unfolding requirements for substrate entry into retrotranslocons during endoplasmic reticulum-associated degradation

  • J Biol Chem. 2019 Dec 27;294(52):20084-20096. doi: 10.1074/jbc.RA119.010019.
Junfen Shi 1 2 Xianyan Hu 1 2 Yuan Guo 1 2 Linhan Wang 1 2 Jia Ji 1 2 Jiqiang Li 1 2 Zai-Rong Zhang 3 2
Affiliations

Affiliations

  • 1 Interdisciplinary Research Center on Biology and Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 201210, China.
  • 2 University of Chinese Academy of Sciences, Shijingshan District, Beijing 100049, China.
  • 3 Interdisciplinary Research Center on Biology and Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 201210, China zrzhang@sioc.ac.cn.
Abstract

The endoplasmic reticulum-associated degradation (ERAD) pathway mediates the endoplasmic reticulum-to-cytosol retrotranslocation of defective proteins through protein complexes called retrotranslocons. Defective proteins usually have complex conformations and topologies, and it is unclear how ERAD can thread these conformationally diverse protein substrates through the retrotranslocons. Here, we investigated the substrate conformation flexibility necessary for transport via retrotranslocons on the ERAD-L, ERAD-M, and HIV-encoded protein Vpu-hijacked ERAD branches. To this end, we appended various ERAD substrates with specific domains whose conformations were tunable in flexibility or tightness by binding to appropriate ligands. With this technique, we could define the capacity of specific retrotranslocons in disentangling very tight, less tight but well-folded, and unstructured conformations. The Hrd1 complex, the retrotranslocon on the ERAD-L branch, permitted the passage of substrates with a proteinase K-resistant tight conformation, whereas the E3 Ligase gp78-mediated ERAD-M allowed passage only of nearly completely disordered but not well-folded substrates and thus may have the least unfoldase activity. Vpu-mediated ERAD, containing a potential retrotranslocon, could unfold well-folded substrates for successful retrotranslocation. However, substrate retrotranslocation in Vpu-mediated ERAD was blocked by enhanced conformational tightness of the substrate. On the basis of these findings, we propose a mechanism underlying polypeptide movement through the endoplasmic reticulum membrane. We anticipate that our biochemical system paves the way for identifying the factors necessary for the retrotranslocation of membrane proteins.

Keywords

E3 ubiquitin ligase; ER quality control; ER-associated degradation; ER-to-cytosol transport; dihydrofolate reductase; membrane enzyme; protein misfolding; retrotranslocation; tunable folding domain; ubiquitylation (ubiquitination); unfolded substrate.

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