The oligomerization of a family of four genetically clustered human gastrointestinal mucins

Mucins are synthesized and secreted by many epithelia. They are complexglycoproteins that offer cytoprotection. In their functional configuration,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 study oligomerization ofthese related mucins, we performed metabolic labeling experiments with[35S]amino acids in LS174T cells, and isolated mucin precursors by specificimmunoprecipitations that were analyzed on SDS-PAGE. Each of the precursorsof MUC2, MUC5AC, MUC5B, and MUC6 formed a single species ofdisulfide-linked homo-oligomer within 1 h after pulse labeling. Based onapparent molecular masses, these oligomeric precursors were most likelydimers. Inhibition of vesicular RER-to-Golgi transport, with brefeldin Aand CCCP, did not affect the dimerization of MUC2 precursors, localizingdimerization to the RER. O-Glycosylation of MUC2 followed dimerization.Inhibition of N- glycosylation by tunicamycin retarded, but did notinhibit, dimerization, indicating that N-glycans play a role in efficientdimerization of MUC2 precursors. Based on sequence homology, the ability ofMUC2, MUC5AC, MUC5B and MUC6 to dimerize most likely resides in theirC-terminal domains. Thus, the RER-localized dimerization of secretorymucins likely proceeds by similar mechanisms, which is an essential step inthe formation of the human gastrointestinal mucus- gels.