Gel-forming and cell-associated mucins: preparation for structural and functional studies.

Secreted and transmembrane mucins are important components of innate defence at the body's mucosal surfaces. The secreted mucins are large, polymeric glycoproteins, which are largely responsible for the gel-like properties of mucus secretions. The cell-tethered mucins, however, are monomeric but are typically composed of two subunits, a larger extracellular subunit which is heavily glycosylated while the smaller more sparsely glycosylated subunit has a short extracellular region, a single-pass transmembrane domain, and a cytoplasmic tail. These two families of mucins represent high-molecular-weight glycoproteins containing serine and threonine-rich domains that are the attachment sites for large numbers of O-glycans. The high-M ( r ) and high sugar content have been exploited for the separation of mucins from the majority of components in mucus secretions. In this chapter, we describe current and well-established methods (caesium chloride density-gradient centrifugation, gel-filtration and anion-exchange chromatography, and agarose gel electrophoresis) for the extraction and purification of gel-forming and cell-surface mucins which can subsequently be used for a variety of structural and functional studies.