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Glycobiology Advance Access originally published online on July 22, 2009
Glycobiology 2009 19(10):1127-1135; doi:10.1093/glycob/cwp104
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© The Author 2009. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Sugar-binding activity of the MRH domain in the ER {alpha}-glucosidase II β subunit is important for efficient glucose trimming

Dan Hu2, Yukiko Kamiya3,5, Kiichiro Totani4, Daiki Kamiya5, Norihito Kawasaki2, Daisuke Yamaguchi2, Ichiro Matsuo4, Naoki Matsumoto2, Yukishige Ito4,6, Koichi Kato3,5,6 and Kazuo Yamamoto1,2,6

2 Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, Chiba 277-8562
3 Institute for Molecular Science and Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences, Aichi 444-8787
4 RIKEN, The Institute of Physical and Chemical Research, Saitama 351-0198
5 Graduate School of Pharmaceutical Sciences, Nagoya City University, Aichi 467-8603
6 CREST, JST, Saitama 332-1102, Japan

1 To whom correspondence should be addressed: Tel: +81-4-7136-3614; Fax: +81-4-7136-3619; e-mail: yamamoto{at}k.u-tokyo.ac.jp

Received on April 10, 2009; revised on June 15, 2009; accepted on July 10, 2009

Glucosidase II (GII) is a glycan-processing enzyme that trims two {alpha}1,3-linked glucose residues from N-glycan on newly synthesized glycoproteins. Trimming of the first {alpha}1,3-linked glucose from Glc2Man9GlcNAc2 (G2M9) is important for a glycoprotein to interact with calnexin/calreticulin (CNX/CRT), and cleavage of the innermost glucose from Glc1Man9GlcNAc2 (G1M9) sets glycoproteins free from the CNX/CRT cycle and allows them to proceed to the Golgi apparatus. GII is a heterodimeric complex consisting of a catalytic {alpha} subunit (GII{alpha}) and a tightly associated β subunit (GIIβ) that contains a mannose 6-phosphate receptor homology (MRH) domain. A recent study has suggested a possible involvement of the MRH domain of GIIβ (GIIβ-MRH) in the glucose trimming process via its putative sugar-binding activity. However, it remains unknown whether GIIβ-MRH possesses sugar-binding activity and, if so, what role this activity plays in the function of GII. Here, we demonstrate that human GIIβ-MRH binds to high-mannose-type glycans. Frontal affinity chromatography revealed that GIIβ-MRH binds most strongly to the glycans with the {alpha}1,2-linked mannobiose structure. GII with the mutant GIIβ that lost the sugar-binding activity of GIIβ-MRH hydrolyzes p-nitrophenyl-{alpha}-glucopyranoside, but the capacity to remove glucose residues from G1M9 and G2M9 is significantly decreased. Our results clearly demonstrate the capacity of the GIIβ-MRH to bind high-mannose-type glycans and its importance in efficient glucose trimming of N-glycans.

Key words: glucosidase II / lectin / mannose 6-phosphate receptor / MRH domain / sugar-binding specificity


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