We have previously noted that sequential extraction of an asthmatic mucous exudate with 6 M guanidinium chloride yielded a fraction of the mucins that were most resistant to solubilization and of high Mr [Sheehan, Richardson, Fung, Howard and Thornton (1995) Am. J. Respir. Cell Mol. Biol. 13, 748–756]. Here we show that this mucin fraction is dominated (at least 96% of the total) by the low-charge glycoform of the MUC5B gene product. Seen in the electron microscope the mucins appeared mainly as compact ‘island ’ structures composed of linear threads often emanating from globular ‘nodes ’ rather than the discrete linear threads more typical of mucins that we have previously described. The effect of reducing agents was as expected for other gel-forming mucins, i.e. reduced subunits or monomers of Mr 3×106) were produced within 15 min of treatment. Kinetic experiments on the cleavage of the intact mucins with the proteinase trypsin indicated two clear regimes of fragmentation. An initial rapid cleavage generated mucins ranging from Mr = 4×106 to 30×106 that in the electron microscope appeared as polydisperse threads (500–3000 nm in length), similar to normal and other respiratory mucins that we have previously characterized. A subsequent slower fragmentation over many hours yielded a major fragment of Mr 3×106 and length 200–600 nm, very similar in size and Mr to the subunits obtained by reduction. The results suggest that the MUC5B mucin is assembled, first into polydisperse linear threads, which are then linked together via a protein-mediated process. This might involve part of the mucin polypeptide or an as yet unidentified protein(s). The high proteinase susceptibility of the linkage suggests that it might be a point of control for mucin size and thus mucus rheology.

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