Endoplasmic reticulum lectin XTP3-B inhibits endoplasmic reticulum-associated degradation of a misfolded α1-antitrypsin variant

Authors

  • Tsutomu Fujimori,

    1. Department of Molecular and Cellular Biology, Institute for Frontier Medical Sciences, Kyoto University, Japan
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  • Yukiko Kamiya,

    1. Okazaki Institute for Integrative Bioscience and Institute for Molecular Science, National Institutes of Natural Sciences, Japan
    2. Graduate School of Pharmaceutical Sciences, Nagoya City University, Japan
    Current affiliation:
    1. Graduate School of Engineering, Nagoya University, Japan
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  • Kazuhiro Nagata,

    1. Laboratory of Molecular and Cellular Biology, Faculty of Life Sciences, Kyoto Sangyo University, Japan
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  • Koichi Kato,

    1. Okazaki Institute for Integrative Bioscience and Institute for Molecular Science, National Institutes of Natural Sciences, Japan
    2. Graduate School of Pharmaceutical Sciences, Nagoya City University, Japan
    3. The Glycoscience Institute, Ochanomizu University, Japan
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  • Nobuko Hosokawa

    Corresponding author
    • Department of Molecular and Cellular Biology, Institute for Frontier Medical Sciences, Kyoto University, Japan
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Correspondence

N. Hosokawa, Department of Molecular and Cellular Biology, Institute for Frontier Medical Sciences, Kyoto University, Kyoto 606-8397, Japan

Fax: +81 75 751 4646

Tel: +81 75 751 3849

E-mail: nobukoh@frontier.kyoto-u.ac.jp

Abstract

The endoplasmic reticulum (ER) is an organelle that synthesizes many secretory and membrane proteins. However, proteins often fold incorrectly. Terminally misfolded polypeptides in the ER are retro-translocated to the cytosol, where they are ultimately degraded by the ubiquitin–proteasome system, a process termed ER-associated degradation (ERAD). By recognizing the specific structures of N-linked oligosaccharides attached to polypeptides, lectins play an important role in the quality control of glycoproteins in the ER. Mammalian OS-9 and XTP3-B are ER-resident lectins that contain mannose 6-phosphate receptor homology (MRH) domains, which recognize sugar moieties; OS-9 has one MRH domain and XTP3-B has two. Both are involved in ERAD, but the functional differences between the two are poorly understood. The present study analyzed the function of human XTP3-B, and found, by frontal affinity chromatography analysis, that its C-terminal MRH domain specifically recognized the Man9GlcNAc2 (M9) glycan in vitro and M9 glycans on an ERAD substrate NHK, a terminally misfolded α1-antitrypsin variant, in vivo. Furthermore, endogenous XTP3-B was a component of the HRD1–SEL1L membrane-embedded ubiquitin ligase complex, an association that was stabilized by a direct interaction with SEL1L. The lectin activity of XTP3-B was required for its binding to NHK, but not for its association with SEL1L. Unlike OS-9, XTP3-B did not enhance the degradation of misfolded glycoproteins, but instead inhibited the degradation of NHK bearing M9 oligosaccharides. Therefore, we propose that XTP3-B recognizes M9 glycans on unfolded polypeptides, thereby acting as a negative regulator of ERAD, and also protects newly synthesized immature polypeptides from premature degradation.

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