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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, which 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 that the mucin domains and their C termini projects 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 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 the combination of intracellular packing of the MUC5B mucin and the submucosal gland morphology creates an efficient machine for producing linear mucin bundles.

To understand the mucociliary clearance system, mucins were visualized by light, confocal and electron microscopy, and mucus was stained by Alcian blue and tracked by video microscopy on tracheal explants of newborn piglets. We observed long linear mucus bundles that appeared at the submucosal gland openings and were transported cephalically. The mucus bundles were shown by mass spectrometry and immunostaining to have a core made of MUC5B mucin and were coated with MUC5AC mucin produced by surface goblet cells. The transport speed of the bundles was slower than the airway surface liquid flow. We suggest that the goblet cell MUC5AC mucin anchors the mucus bundles and thus controls their transport. Normal clearance of the respiratory tree of pigs and humans, both rich in submucosal glands, is performed by thick and long mucus bundles.