Table 2.
Mucin peptide domains.
| Peptide Domain Type | Mucin | Mucin Type | Peptide Domain Function |
|---|---|---|---|
| Cysteine rich CYS domains | MUC2, MUC5AC, MUC5B, MUC19 | Secreted | Non-glycosylated multiple copy domains adjacent or interrupting tandem repeat domains. Important for various mucin–mucin interactions. |
| Cysteine Knot | MUC2, MUC5AC, MUC5B, MUC6, MUC19 | Secreted | Involved in dimerization. |
| Von Willebrand Factor D (D1, D2, D’, D3) |
MUC2, MUC5AC, MUC5B, MUC6, MUC19 | Secreted | Mediate oligomerisation located at N- & C-terminus D3 is directly active in polymerization. |
| Von Willebrand Factor D (D4) |
MUC2, MUC5AC, MUC5B, MUC6 MUC4 |
Secreted & Membrane-associated | Located N-terminally to the D4 is located C-terminally to the VNTR domains, contains the GDPH autocatalytic cleavage site. |
| Cytoplasmic Tail | MUC1, MUC3A/B, MUC12, MUC13, MUC16, MUC17, MUC21 | Membrane-associated | Located on the cytoplasmic side of the cell surface membrane. Contains phosphorylation sites involved in signaling. MUC3, MUC12, and MUC17 have PDZ binding motifs |
| SEA (Sperm protein, Enterokinase & Agrin) |
MUC3A/B, MUC4, MUC12, MUC13, MUC17, MUC21 | Membrane-associated | Protein binding properties. Contains autocatalytic proteolytic cleavage site. |
| EGF (Epidermal Growth Factor) |
MUC1, MUC3A/B, MUC12, MUC13, MUC17 | Membrane-associated | Mediate interactions between mucin subunits and ERBB receptors. |
| Transmembrane | MUC1, MUC3A/B, MUC4, MUC12, MUC13, MUC16, MUC17, MUC20, MUC21 | Membrane-associated | Membrane-spanning sequence typical for membrane proteins |
| GDPH autocatalytic proteolytic site | MUC2, MUC4, MUC5AC | Secreted & Membrane-associated | Autocatalytic site cleaving between GD and PH residues |
| Proteolytic cleavage site | MUC1, MUC3A/B, MUC4, MUC12, MUC13, MUC16, MUC17 | Membrane-associated | Found in MUCs with the SEA domain and in MUC16 |
The major mucin peptide domains are indicated for each of the secreted and membrane-associated mucin genes. An indication of their function is summarized. In addition to the conventional mucin forms, there are similar molecules that have been given names such as mucin-like, see previous papers [54,55,56]. These molecules are different to the mucin family shown in Table 1 and are not considered further in this review.