HUMAN MUC5AC MUCIN DIMERIZES IN THE ROUGH ENDOPLASMIC-RETICULUM, SIMILARLY TO THE MUC2 MUCIN

Biosynthetic studies on the human MUC5AC mucin were performed by immun oprecipitations with antisera recognizing only the non-O-glycosylated apomucin in the colon adenocarcinoma cell line LS 174T. Pulse-chase st udies and subcellular fractionations showed that MUC5AC formed dimers in the rough endoplasmic reticulum within 15 min of the initiation of biosynthesis. No non-O-glycosylated species larger than dimers were id entified. The dimerization was N-glycosylation-dependent, because tuni camycin treatment significantly lowered the rate of dimerization. When the biosynthesis of MUC5AC apomucin was compared with that of MUC2 ap omucin, also produced in the LS 174T cell line, both apomucins were as sembled in similar ways with respect to their rates of dimerization wi th and without inhibition of N-glycosylation. No heterodimerization wa s observed between the human MUC5AC and the MUC2 apomucins despite the extensive sequence similarities in the positions of the cysteine resi dues in the C-termini proposed to be involved in mucin dimerization.