Running Title: Conformational transitions of MUC5B

Gel forming mucins are the largest complex glycoprotein macromolecules in the body. They form the matrix of gels protecting all the surface epithelia and are secreted as disulphide bonded polymeric structures. The mechanisms by which they are formed and organized within cells and thereafter released to form mucus gels are not understood. In particular, the initial rate of expansion of the mucins after release from their secretory granules is very rapid (seconds) but no clear mechanism for how it is achieved has emerged. Our major interest is in lung mucins but most particularly in MUC5B which is the major gel-forming mucin in mucus and which provides its major protective matrix. In this study, using OptiPrep density gradient ultracentrifugation, we have isolated a small amount of a stable form of the recently secreted and expanding MUC5B mucin which account for less than 2 % of the total mucin present. It has an average mass of around 150 x 106 Da and size Rg of 150 nm in radius of gyration. In transmission electron microscopy (EM), this compact mucin has maintained a circular structure that is characterized by flexible chains connected around protein rich nodes as determined by their ability to bind colloidal gold. The appearance indicates that the assembled mucins in a single granule are organized around a number of nodes each attached to 4 to 8 subunits. The organization of the mucins in this manner is consistent with efficient packing of a number of large heavil