Production of recombinant FCGBP protein using FectoCHO® expression system (transient expression). Suspension CHO-S cells were cultured in FectoCHO® CD (chemically defined) medium and successfully transfected with FectoPRO®.

Mucus forms an important protective barrier that minimizes bacterial contact with the colonic epithelium. Intestinal mucus is organized in a complex network with several specific proteins, including the mucin MUC2 and the as abundant IgG Fc-binding protein, FCGBP. FCGBP is expressed in all intestinal goblet cells and is secreted into the mucus. It is comprised of repeated von Willebrand Factor D (vWD) domain assemblies, most of which have a GDPH amino acid sequence that can be autocatalytically cleaved, as previously observed in the mucins MUC2 and MUC5AC. However, the functions of FCGBP in mucus are not understood. We show that all vWD domains of FCGBP with a GDPH sequence are cleaved, and that these cleavages occur early during biosynthesis in the endoplasmic reticulum. All cleaved fragments, however, remain connected via a disulfide bond within each vWD domain. This cleavage generates a C-terminal reactive Asp-anhydride that could react with other molecules, such as MUC2, but this was not observed. Quantitative analyses by mass spectrometry showed that FCGBP was mainly soluble in chaotropic solutions, whereas MUC2 was insoluble, and most of the secreted FCGBP was not covalently bound to MUC2. Although FCGBP has been suggested to bind IgG, we were unable to reproduce this binding in vitro using purified proteins. In conclusion, while the function of FCGBP is still unknown, our results suggest that it does not contribute to covalent crosslinking in the mucus, nor incorporate IgG into mucus, instead the single disulfide bond linking each fragment could mediate controlled dissociation.