THE OLIGOMERIZATION OF A FAMILY OF 4 GENETICALLY CLUSTERED HUMAN GASTROINTESTINAL MUCINS

Mucins are synthesized and secreted by many epithelia. They are comple x glycoproteins that offer cytoprotection. In their functional configu ration, mucins form oligomers by a biosynthetic process that is poorly understood. A family of four human gastrointestinal mucin genes (MUC2 , MUC5AC, MUC5B, and MUC6) is clustered to chromosome 11p15.5. To stud y oligomerization of these related mucins, we performed metabolic labe ling experiments with [S-35]amino acids in LS174T cells, and isolated mucin precursors by specific immunoprecipitations that were analyzed o n SDS-PAGE. Each of the precursors of MUC2, MUC5AC, MUC5B, and MUC6 fo rmed a single species of disulfide-linked homooligomer within 1 h afte r pulse labeling. Based on apparent molecular masses, these oligomeric precursors were most likely dimers. Inhibition of vesicular RER-to-Go lgi transport, with brefeldin A and CCCP, did not affect the dimerizat ion of MUC2, precursors, localizing dimerization to the RER. O-Glycosy lation of MUC2 followed dimerization. Inhibition of N-glycosylation by tunicamycin retarded, but did not inhibit, dimerization, indicating t hat N-glycans play a role in efficient dimerization of MUC2 precursors . Based on sequence homology, the ability of MUC2, MUC5AC, MUC5B and M UC6 to dimerize most likely resides in their C-terminal domains. Thus, the RER-localized dimerization of secretory mucins likely proceeds by similar mechanisms, which is an essential step in the formation of th e human gastrointestinal mucus-gels.