MUC5B IS A MAJOR GEL-FORMING, OLIGOMERIC MUCIN FROM HUMAN SALIVARY-GLAND, RESPIRATORY-TRACT AND ENDOCERVIX - IDENTIFICATION OF GLYCOFORMS AND C-TERMINAL CLEAVAGE

Mucins from human whole saliva, as well as from respiratory- and cervi cal-tract secretions. were subjected to density-gradient centrifugatio n in CsC1/0.5 M guanidinium chloride. A polydisperse population of MUC 5B mucins was demonstrated in all samples using anti-peptide antisera (LUM5B-2, LUM5B-3 and LUM5B-4) raised against sequences within the MUC 5B mucin. The sequences recognized by the LUM5B-2 and LUM5B-3 antisera are located within the domains flanking the highly glycosylated regio ns of MUC5B, and reduction increased the reactivity with these antibod ies, suggesting that the epitopes are partially shielded and that thes e regions are folded and stabilized by disulphide bonds, Rate-zonal ce ntrifugation before and after reduction showed MUC5B to be a large oli gomeric mucin composed of disulphide-linked subunits. In saliva and re spiratory-tract secretions, populations of MUC5B mucins with different charge densities were identified by ion-exchange HPLC, suggesting the presence of MUC5B 'glycoforms'. In trachea, the F2 monoclonal antibod y against the sulpho-Lewis C structure reacted preferentially with the later-to-be-eluted populations, An antibody (LUM5B-4) recognizing a s equence in the C-terminal domain of MUC5B identified, after reduction, the mucin subunits as well as smaller fragments, suggesting that some of the MUC5B mucins are cleaved within the C-terminal domain. Immunoh istochemistry revealed that MUC5B is produced by cells dispersed throu ghout the human submandibular and sublingual glands, in the airway sub mucosal glands as well as the goblet cells, and in the epithelium and glands of the endocervix. The F2 antibody stained a subpopulation of t he MUC5B-producing cells in the airway submucosal glands, suggesting t hat different cells may produce different glycoforms of MUC5B in this tissue.