Porcine submaxillary mucin forms disulfide-bonded dimers between its carboxyl-terminal domains

COS-7 cells transfected with three different expression vectors encoding the 240-amino acid residue, disulfide-rich domain at the carboxyl terminus of porcine submaxillary mucin have been used to determine the possible function of the domain in forming higher oligomers of the mucin polypeptide chain. The domain is expressed as a disulfide-bonded dimer, as shown by SDS-gel electrophoretic analysis of the immunoprecipitated domain in the presence and absence of reducing agent and the cross-linking agent bis(sulfosuccinimidyl) suberate. Molecular weight determination by gel filtration on agarose columns in 6 M guanidine HCl confirmed dimer formation. However, the domain expressed is heterogeneous as the result of different extents of glycosylation. Pulse-chase studies with the 35S-labeled domain show that dimer formation and secretion from cells occur very rapidly. Moreover, dimer formation is not dependent on the N-linked oligosaccharides on the domain. Evidence is presented that dimer formation most likely occurs in the endoplasmic reticulum before complex-type oligosaccharide synthesis is completed. Neither brefeldin A nor tunicamycin interferes with the rate of dimer formation. These studies suggest that the disulfide-rich domain acts to form dimers of the polypeptide chain of mucin. This role of the domain is consistent with its amino acid sequence similarity to the disulfide-rich domain of human prepro-von Willebrand factor, which also serves to form dimers of this blood coagulation factor.