PROTEIN CHEMISTRY AND STRUCTURE
O-Glycosylated MUC2 Monomer and Dimer from LS 174T Cells Are Water-soluble, whereas Larger MUC2 Species Formed Early during Biosynthesis Are Insoluble and Contain Nonreducible Intermolecular Bonds*

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The MUC2 mucin is the major gel-forming mucin in the small and large intestine. Due to its sequence similarities with the von Willebrand factor, it has been suggested to dimerize in the endoplasmic reticulum and polymerize in the trans-Golgi network. Using an O-glycosylation-sensitive MUC2 antiserum, a dimerization has been shown to occur in the endoplasmic reticulum of LS 174T cells (Asker, N., Axelsson, M. A. B., Olofsson, S.-O., and Hansson, G. C. (1998) J. Biol. Chem. 273, 18857–18863). Using an antiserum immunoprecipitating O-glycosylated MUC2 mucin, monomers and dimers were shown to occur in soluble form in the lysate of LS 174T cells. The amount of O-glycosylated dimer was small, and no larger species were found even after long chase periods. However, most of the labeled MUC2 mucin was found in pelleted debris of the cell lysate. This insoluble MUC2 mucin was recovered by immunoprecipitation after reduction of disulfide bonds. Analysis by agarose gel electrophoresis revealed two bands, of which the smaller migrated as the O-glycosylated monomer and the larger migrated as the O-glycosylated dimer of the cell lysis supernatant. Mucins insoluble in 6 m guanidinium chloride could also be obtained from LS 174T cells. Such mucins have earlier been found in the small intestine (Carlstedt, I., Herrmann, A., Karlsson, H., Sheehan, J., Fransson, L.-Å., and Hansson, G. C. (1993) J. Biol. Chem.268, 18771–18781). Reduction of the mucins followed by purification by isopycnic density gradient ultracentrifugation and analysis by agarose gel electrophoresis revealed two bands reacting with an anti-MUC2 tandem repeat antibody after deglycosylation. These bands migrated identically to the bands shown by metabolic labeling, and they could also be separated by rate zonal ultracentrifugation. These results suggest that the MUC2 mucin is forming nonreducible intermolecular bonds early in biosynthesis, but after initial O-glycosylation.

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This work was supported by Swedish Medical Research Council Grants 7461 and 10443, the IngaBritt and Arne Lundbergs Stiftelse, the Göteborg Medical Society, and the Swedish Cystic Fibrosis Foundation.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.