Coats the epithelia of the intestines, airways, and other mucus membrane-containing organs. Thought to provide a protective, lubricating barrier against particles and infectious agents at mucosal surfaces. Major constituent of both the inner and outer mucus layers of the colon and may play a role in excluding bacteria from the inner mucus layer (By similarity). Homotrimer; disulfide-linked. Dimerizes in the endoplasmic reticulum via its C-terminal region and polymerizes via its N-terminal region by disulfide-linked trimerization. Interacts with FCGBP (By similarity). Interacts with AGR2; disulfide-linked (By similarity). In the intestine, secreted into the inner and outer mucus layers. Expressed in intestine and airway. O-glycosylated. May undergo proteolytic cleavage in the outer mucus layer of the colon, contributing to the expanded volume and loose nature of this layer which allows for bacterial colonization in contrast to the inner mucus layer which is dense and devoid of bacteria. May undergo autocatalytic cleavage in vivo triggered by the low pH of the late secretory pathway.