Structure and function of mucus.

A glycoprotein building block is common to mammalian mucins. This structure is composed of several protein chains having the same sequence. The carbohydrate side chains, which constitute over three-quarters of the weight, coat only some two-thirds of the backbone chain. The bare protein chains are linked by disulphide bridges and can be digested away with trypsin. Either procedure rapidly solubilizes mucus and results in a structural unit of about 500 000 molecular weight. Mucus solubilizes spontaneously. The first size unit which reaches solution is about 15 X 10(6) molecular weight but continues to break down further. Mechanical agitation considerably speeds up this process. The gel-like character which is an essential feature of mucus--which cannot otherwise act as transport coupler--is thus a transient phenomenon. The problem of how such a structure can arise from the building blocks known to be available is discussed.