Small intestinal mucin: polymerization and the "link glycopeptide".

Extensive polymerization of core glycopeptide subunits appears to be a constant feature of mammalian mucins when care is taken to avoid polymer disruption by high shear forces and proteases. Depolymerization by reducing agents releases a 118 kDa glycopeptide from small intestinal mucins, with features that suggest that one of its roles may be to link highly glycosylated glycopeptide subunits. "Link" peptides are widely distributed, have a similar amino and carbohydrate content, quite distinct from the major mucin glycopeptides and can be liberated by proteases which also depolymerize mucin. In collaboration with Dr. R. Specian, (Shreveport, Louisiana) we have used a monospecific antibody to the 118 kDa glycopeptide to show that it is localized in rat intestine to goblet cells, where it is distributed in a patchy fashion throughout the mucus granule population and is also present in the golgi cysternae. Secreted mucins are distinguished by a very high content of dimerized 118 kDa glycopeptide (200 kDa glycopeptide) which may play a role is selecting or facilitating mucin transport pathways. The 200 kDa glycopeptide may be a integral component of some polymerized mucin molecules since it can be released from intracellular mucins by controlled proteolysis with trypsin, and is subsequently converted to the 118 kDa glycopeptide by thiol reduction.