Glycobiology and Extracellular Matrices
Granule-stored MUC5B mucins are packed by the non-covalent formation of N-terminal head-to-head tetramersStructure of stored MUC5B mucin

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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.

mucin
mucus
lung
secretion
EM
mucin bundle
regulated secretory pathway
submucosal gland

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This work was supported by the Swedish Research Council; the Swedish Cancer Foundation; the Knut and Alice Wallenberg Foundation; the Inga Britt and Arne Lundberg Foundation; Sahlgrenska University Hospital (ALF); the Wilhelm and Martina Lundgren Foundation; NIAID, National Institutes of Health Grant U01AI095473; the Erica Lederhausen Foundation; the Magnus Bergvall Foundation; the Swedish Heart-Lung Foundation; the Center for Innovative Medicine (Karolinska Institutet); the Lederhausen Center for Cystic Fibrosis Research at the University of Gothenburg; the Swedish Cystic Fibrosis Foundation; and the Cystic Fibrosis Foundation (CFFT, Hansso14X0, and Thornt077X0). The authors declare that they have no conflicts of interest with the contents of this article. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

This article contains Figs. S1–S3 and Movies S1 and S2.

The 3D map has been deposited at Electron Microscopy Data Bank (EMDB) with id. EMD-4296.

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Both authors contributed equally to this work.