Assembly Mechanism of Mucin and von Willebrand Factor Polymers

Abstract

The respiratory and intestinal tracts are exposed to physical and biological hazards accompanying the intake of air and food. Likewise, the vasculature is threatened by inflammation and trauma. Mucin glycoproteins and the related von Willebrand factor (VWF) guard the vulnerable cell layers in these diverse systems. Colon mucins additionally house and feed the gut microbiome. Here we present an integrated structural analysis of multimerized intestinal mucin MUC2. Our findings reveal the shared mechanism by which complex macromolecules responsible for blood clotting, mucociliary clearance, and the intestinal mucosal barrier form protective polymers and hydrogels. Specifically, cryoelectron microscopy and crystal structures show how disulfide-rich bridges and pH-tunable interfaces control successive assembly steps in the endoplasmic reticulum and Golgi.Remarkably, a densely O-glycosylated mucin domain performs a specific organizational role in MUC2. The mucin assembly mechanism and its adaptation for hemostasis provide the foundation for rational manipulation of barrier function and coagulation.

Effects of environmental factors on severity and mortality of COVID-19

“…While often considered to be a physical barrier, mucus is actually an active biological barrier that crosslinks viruses and bacteria to mucins, a group of highly glycosylated proteins that are secreted to our mucosal barriers where they self-assemble into long polymers(23). Mucin glycans mimic cell surface glycosylation and by acting as a decoy for viral lectins trap viral particles, which are then transported out of airways by mucociliary clearance(24).…”

Effects of Environmental Factors on Severity and Mortality of COVID-19

“…While often considered to be a physical barrier, mucus is actually an active biological barrier that crosslinks viruses and bacteria to mucins, a group of highly glycosylated proteins that are secreted to our mucosal barriers where they selfassemble into long polymers (23). Mucin glycans mimic cell surface glycosylation and, by acting as a decoy for viral lectins, trap viral particles, which are then transported out of airways by mucociliary clearance (24).…”

Orchestrated Restructuring Events During Secretory Granule Maturation Mediate Intragranular Cargo Segregation

“…Biophysical and single particle EM analysis proposed that the MUC2 mucin is packaged as concatenated polygonal rings, with the mucindomains projecting perpendicular from the base of the rings as parallel rods in a calcium and pH-dependent manner within secretory granules 16 . However, an alternative model suggests that MUC2 complexes exist as linear polymers 24 that are also calcium and pH dependent. Previous studies utilizing single particle EM analysis also identified calcium-mediated cross-links in the VWD-3 domain of MUC5B that could mediate stacking and condensation of the mucin into a round disc-like structure around a central node 25,26,27 .…”

“…Within the mammalian digestive tract, the predominant intestinal mucin MUC2, is thought to undergo a regulated unfolding process during secretion that results in the formation of a hydrated layer that expands to line and protect epithelial cells 15,21,22 . In vitro studies using N-terminal portions of MUC2 suggest that pH-dependent interactions regulate the formation of supramolecular MUC2 structures 16,23,24 . Based on these in vitro studies, one model suggests that C-terminal disulfide bonding along with Ca 2+ -dependent charge shielding may result in the compaction of MUC2 complexes into stack rings within secretory granules 16 , while an alternative model suggests that MUC2 complexes exist as linear polymers 24 .…”

“…In vitro studies using N-terminal portions of MUC2 suggest that pH-dependent interactions regulate the formation of supramolecular MUC2 structures 16,23,24 . Based on these in vitro studies, one model suggests that C-terminal disulfide bonding along with Ca 2+ -dependent charge shielding may result in the compaction of MUC2 complexes into stack rings within secretory granules 16 , while an alternative model suggests that MUC2 complexes exist as linear polymers 24 . Similar in vitro folding studies with truncated portions of the pulmonary mucin, MUC5B, have demonstrated calcium and pH dependent N-and C-terminal interactions that are proposed to result in the formation of compacted polymers arranged around a central node 25,26,27 .…”