Gel-forming mucins. Notions from in vitro studies

Mucus secretions form a protective barrier in the mucosa of the auditory, gastrointestinal, respiratory, and urogenital systems, and the conjunctiva in the eyes. A family of glycoproteins known as gel forming mucins is the major component of the mucus. Gel-forming mucins are among the largest and most complex proteins known. Their polypeptide chains comprise thousands of amino acid residues organized into different domains with diverse post-translational modifications, including O- and N-glycosylation, sulfation, proteolysis, and likely C-mannosylation. Moreover, these glycoproteins form disulfide-linked oligomers/multimers with molecular weights in the millions. Molecular polydispersity in terms of length, carbohydrate content and composition, is an invariable feature of purified mucins. This structural complexity makes it technically very difficult to study mucin biochemical and physical properties. It is not surprising, therefore, that our knowledge on mucin structure, biosynthesis and function still is incomplete. During the last decade, the use of recombinant mucins has allowed researchers to study the biochemical properties of protein domains, peptide motifs and amino acid residues common to all gel-forming mucins, and to propose specific roles for them. We review here the relative impact that these in vitro studies have had for our current understanding of two of the most important features of these macromolecules: formation of disulfide linked oligomers and mucin intragranular packaging.