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Granule-stored MUC5B mucins are packed by the non-covalent formation of N-terminal head-to-head tetramers

Journal article
Authors Sergio Trillo-Muyo
Harriet E. Nilsson
Christian V Recktenwald
Anna Ermund
C. Ridley
Lauren N Meiss
A. Bahr
N. Klymiuk
Jeffrey J. Wine
P. J. B. Koeck
D. J. Thornton
H. Hebert
Gunnar C. Hansson
Published in Journal of Biological Chemistry
Volume 293
Issue 15
Pages 5746-5754
ISSN 0021-9258
Publication year 2018
Published at Institute of Biomedicine, Department of Medical Biochemistry and Cell Biology
Pages 5746-5754
Language en
Links doi.org/10.1074/jbc.RA117.001014
Keywords von-willebrand-factor, gel-forming-mucin, cystic-fibrosis, secretion, release, mucus, bicarbonate, packing, tubules, domain, Biochemistry & Molecular Biology
Subject categories Biochemistry and Molecular Biology

Abstract

Most MUC5B mucin polymers in the upper airways of humans and pigs are produced by submucosal glands. MUC5B forms N-terminal covalent dimers that are further packed into larger assemblies because of low pH and high Ca2+ in the secretory granule of the mucin-producing cell. We purified the recombinant MUC5B N-terminal covalent dimer and used single-particle electron microscopy to study its structure under intracellular conditions. We found that, at intragranular pH, the dimeric MUC5B organized into head-to-head noncovalent tetramers where the von Willebrand D1-D2 domains hooked into each other. These N-terminal tetramers further formed long linear complexes from which, we suggest, the mucin domains and their C termini project radially outwards. Using conventional and video microscopy, we observed that, upon secretion into the submucosal gland ducts, a flow of bicarbonate-rich fluid passes the mucin-secreting cells. We suggest that this unfolds and pulls out the MUC5B assemblies into long linear threads. These further assemble into thicker mucin bundles in the glandular ducts before emerging at the gland duct opening. We conclude that the combination of intracellular packing of the MUC5B mucin and the submucosal gland morphology creates an efficient machine for producing linear mucin bundles.

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