The Mucins

The mucins are a family of large glycoproteins characterized by long and often repeated sequences rich in proline, threonine, and serine (PTS domain). These proteins are extensively modified with O-glycans, forming mucin domains that extend as stiff rods with glycans pointing in all directions. This dense cluster of glycans makes the central protein core inaccessible to proteases and generates a glycan surface with clustered antigens. Mucins can be categorized into two main groups: the classical gel-forming polymeric mucins (e.g., MUC2, MUC5AC, MUC5B, MUC6) and a more heterogeneous group of monomeric transmembrane mucins (e.g., MUC1, MUC3A, MUC3B, MUC4, MUC12, MUC13, MUC15, MUC16, MUC17, MUC21, MUC22). Most mucins are located at mucosal surfaces, where they contribute to the protective mucus and the epithelial cell glycocalyx. However, some mucins, such as MUC1, are also widely distributed across various cell types, including classical immune cells. Here, we provide an update on the encyclopedia of mucins.