Mucin subunits linked by disulphide bridges?

Translation of messenger RNA from canine tracheal epithelial cells: identification of mucin core protein

A high-molecular-weight mucin (Mr approximately 11.0 x 10(6)) was purified from canine tracheal pouch secretions. The mucin was deglycosylated by treatment with trifluoromethane sulfonic acid for 8 h at 8 degrees C and subsequently with alpha-N-acetylgalactosaminidase. These treatments almost completely removed the carbohydrate moieties. The amino acid compositions of the deglycosylated and native mucins were similar, indicating that the deglycosylation procedure used did not cause notable degradation of the protein core. Antiserum specific for deglycosylated canine tracheal mucin was produced by immunization of rabbit with the antigen. RNA was isolated from fresh canine tracheal epithelial cells by extraction with guanidine isothiocyanate/hydrochloride and further fractionated by chromatography on oligo(dT)-cellulose to yield poly(A)+ RNA. The poly(A)+ RNA was translated in a rabbit reticulocyte cell-free translation system using [35S]methionine and [3H]leucine as radiolabels. The translation products were analyzed by gel electrophoresis and fluorography before and after immunoprecipitation with the antiserum to deglycosylated mucin. A labeled product of molecular weight 72,000 was present in the immunoprecipitate. When canine liver poly(A)+ RNA was used as control, no radioactivity above background was detected in the immunoprecipitate. It is concluded that the primary translation product of the canine tracheal epithelial cells is a 72,000-D protein and the monomer subunit of the mucin is about 167,000 D. Thus, in the native state, the canine tracheal mucin consists of several associating subunits.

The complexity of mucins

Mucins represent the main components of gel-like secretions, or mucus, secreted by mucosae or some exocrine glands. These high-molecular-weight glycoproteins are characterized by the large number of carbohydrate chains O-glycosidically linked to the peptide. The determination of mucin molecular weight and conformation has been controversial for several reasons: 1) the methods used to solubilize mucus and to purify mucins are different and 2) the molecules have a strong tendency to aggregate or to bind to other molecules (peptides or lipids). Recently, electron microscopy has shown the filamentous shape of most mucins and their polydisperse character which, in some secretions, might correspond to a polymorphism of the peptide part of these molecules. The recent development of high pressure liquid chromatography and high-resolution proton NMR spectroscopy has allowed major progress in the structural study of mucin carbohydrate chains. These chains may have from 1 to about 20 sugars and bear different antigenic determinants, such as A, B, H, I, i, X, Y or Cad antigens. In some mucins, such as human respiratory mucins, the carbohydrate chain diversity is remarkable, which raises many questions. Mucins are molecules located at the interface between mucosae and the external environment. The carbohydrate chain diversity might allow many interactions between mucins and microorganisms and play a major role in the colonization or the defense of mucosae.

Complex structure of human bronchial mucus glycoprotein

Human bronchial mucus glycoproteins or mucins were isolated from the sputum of two patients by a method avoiding reducing agents and involving water extraction and gel filtration on Sepharose CL-2B in 6 M guanidinium chloride. The chemical analysis indicated approximately 25-40% lipid. The amino acid and carbohydrate analysis differ quantitatively from that of mucins purified after prior reduction of mucus. These fractions also have a higher proportion of aspartic and glutamic acids than that of the mucins from reduced sputum. These mucins are still contaminated by small amounts of peptides but do not seem to contain disulfide-attached cross-linking protein. Human bronchial mucins have a strong tendency to form aggregates except in 6 M guanidinium chloride. Electron microscopy performed with various procedures indicates the presence of both micelles and flexible threads measuring 200-1000 nm. Delipidation removes most of the micellar forms. Thereafter mucins appear mainly as polydisperse flexible extended threads and also as aggregates. These features of bronchial mucins do not fit with the generally accepted idea of mucin subunits linked by disulfide bridges (unless they are linked end to end) and alternatively favour a model where mucin molecules behave like filaments that could easily aggregate according to the solvent system (mucin concentration, absence of dissociating conditions).