The structure of FCGBP is formed as a disulfide‐mediated homodimer between its C‐terminal domains

Mucus in the colon is crucial for intestinal homeostasis by forming a barrier that separates microbes from the epithelium. This is achieved by the structural arrangement of the major mucus proteins, such as MUC2 and FCGBP, both of which are comprised of several von Willebrand D domains (vWD) and assemblies. Numerous disulfide bonds stabilise these domains, and intermolecular bonds generate multimers of MUC2. The oligomeric nature of FCGBP is not known. Human hFCGBP contains 13 vWD domains whereas mouse mFCGBP consists of only 7. We found unpaired cysteines in the vWD1 (human and mouse) and vWD5 (mouse)/vWD11 (human) assemblies which were not involved in disulfide bonds. However, the most C‐terminal vWD domains, vWD7 (mouse)/vWD13 (human), formed disulfide‐linked dimers. The intermolecular bond between C 5284 and C 5403 of human hFCGBP was observed by using mass spectrometry to generate the dimer. Cryo‐EM structure analysis of recombinant mouse mFCGBP revealed a compact dimer with two symmetric intermolecular disulfide bonds between C 2462 and C 2581 , corresponding to the dimerising cysteines in the human hFCGBP. This compact conformation involves interactions between the vWD assemblies, but although the domains involved at the interface are the same, the nature of the interactions differ. Mouse mFCGBP was also found to exist in a semi‐extended conformation. These different interactions offer insights into the dynamic nature of the FCGBP homodimer.