PROSITE documentation PDOC00338

Secretoglobin (SCGB) family profile

Proteins of the secretoglobin (SCGB) family have the following common characteristics: secretion, small size, α-helical, and dimeric structure [1]. The secretoglobin monomere structure is composed of four α-helices, H1 to H4, that do not form a canonical four helix-bundle motif, but a 'boomerang'-shaped structure in which helices H1, H3, and H4 form the interface of a dimeric structure displaying a large oblong internal cavity (see <PDB:1UTG>). The large cavity, at the interface of the two monomers, joined together in an antiparallel manner, is able to accommodate small- to medium-sized hydrophobic ligands but its access is sealed by two intermolecular disulfide bridges involving conserved cysteines [2,3,4,5]. A schematic representation of the location of the two disulfide bonds in the antiparallel dimer is shown below:

 NH2-xxCxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxCx-COOH
       |                                                              |
 COOH-xCxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxCxx-NH2

Proteins known to be belong to the secretoglobin family are listed below:

• Uteroglobin (UTG), a steroid-inducible, cytokine-like protein with anti- inflammatory and immunomodulatory properties. It seems specific to lagomorphes (rabbit, hare, and pica) [3].
• Clara cell phospholipid-binding protein (CCPBP) (also known as CC10). CCPBP is a protein expressed by non-ciliated cells of pulmonary airway surface epithelium. Its exact function is not known. Like uteroglobin, it is an inhibitor of phospholipase A2. It binds to phosphatidylcholine, phosphatidylinositol and to polychlorinated biphenyls (PCB) [4].
• Domestic cat major allergen chain 1 (fel d I-A) and chain 2 (fel d I-B), which form a heterodimeric structure. Fel DI contains an additional disulfide bridge [5].
• Human mammaglobin, a mammary-specific protein [6].
• Rat prostatic steroid-binding protein C3 [7].

The profile we developed covers the four α-helices of the SCGB monomere.