Functional protective role for mucin glycosylated repetitive domains

Mucins carry out a number of protective roles, some of which are more easil y studied than others. One mucin function is believed to be the protection of the mucosal epithelium against acidic and proteolytic damage in the stom ach and intestines. In the present work, a portion of stomach mucin tandem repeat sequence (Muc6) was joined to the catalytic domain of a reporter enz yme [human milk cholesterol esterase (CE)] to determine whether the former can protect the latter protein from damage. This Muc6 domain replaced a uni que series of glycosylated C-terminal repeats normally present in CE. The c himeric protein (CE/Muc6) was expressed in two different cell lines and its properties compared to recombinant full-length CE and a truncated version of CE which contained only the catalytic domain (CE/trunc). Results showed that both CE and CE/Muc6 were resistant to denaturation by acid and to prot eolysis by pepsin at low pH values or by pancreatic proteases compared to C E/trunc. Thus, a stomach Muc6 domain is sufficient to confer stability on t he CE catalytic domain, demonstrating a protective effect by a glycosylated mucin sequence.