Synthesis and macromolecular organization of gastrointestinal mucin: evidence for the origin of mucin "link protein".
Mucus glycoproteins (mucins), the principal determinants of mucus protective qualities and mucosal defense, have been studied extensively to define pathological aberrations in relation to gastrointestinal disease. Recent work from our laboratory provided evidence as to the initial stages of gastrointestinal mucin synthesis, the molecular size of the apomucin, and its macromolecular organization. Using monoclonal antibodies (MAbs) against apomucin (clone 1H7), O-glycosylated with N-acetylgalactosamine apomucin (clone 2B4), and that against carboxyl terminal of the apomucin (clone 3G12), the mucin synthesizing polysomes were isolated and glycosylated peptides ranging in size from 6 to 60 kDa identified. In vitro synthesis in a cell-free system also afforded 60- to 64-kDa products recognized by 1H7 and 3G12 antimucin MAbs. The results obtained provided evidence that the mucin core consists of 60-kDa peptide that at the cotranslational stage is O-glycosylated with N-acetylgalactosamine. Studies on mucin polymer assembly revealed that mucin preparations prepared by equilibrium density gradient centrifugation and Sepharose 2B chromatography are not completely purified and contain DNA and extraneous proteins. The evidence was obtained that so-called mucin "link protein", a 118-kDa glycopeptide, is a N-glycosylated fragment of fibronectin, whereas the supposedly native undergraded mucin isolated by Carlstedt et al. was found to contain mucin-fibronectin-DNA complexes. The general picture that emerged from the studies is that the pure mucin consists of 60 kDa glycosylated peptides only. The carboxyl terminal (8- to 12-kDa fragment) of these peptides is not glycosylated (naked) and is responsible for mucin interaction with fibronectin and other fibronectin-like extracellular matrix proteins. Although the formation of the mucosal coat depends on many other factors and extracellular components, our findings on mucin structure and interaction with the extracellular matrix proteins provide an explanation as to the possible mechanism of mucin adherence to the epithelial surfaces.
Slomiany
A
A
Research Center, University of Medicine and Dentistry of New Jersey, Newark 07103-2400.
Okazaki
K
K
Slomiany
B L
BL
D000911:Antibodies, Monoclonal / Q000032:analysis; D004591:Electrophoresis, Polyacrylamide Gel; D016326:Extracellular Matrix Proteins / Q000502:physiology; D005353:Fibronectins / Q000737:chemistry / Q000502:physiology; D005752:Gastric Mucins / Q000737:chemistry* / Q000276:immunology / Q000502:physiology; D006023:Glycoproteins / Q000032:analysis* / Q000737:chemistry; D006801:Humans; D010452:Peptide Biosynthesis; D011485:Protein Binding
D000911:Antibodies, Monoclonal; D016326:Extracellular Matrix Proteins; D005353:Fibronectins; D005752:Gastric Mucins; D006023:Glycoproteins; C053366:apomucin