Secretoglobins (SCGBs) are a family of small, alpha-helical, disulfide linked, dimeric proteins found only in mammals. This family was formerly known as the Uteroglobin/Clara cell 10-kDa family, after the two aliases of its founding member Uteroglobin.
The proteins are mostly alpha-helical, and the dimer is formed in an antiparallel way. The dimer interface features a cavity formed across the two monomers, which can accommodate small to medium sized ligands like steroids and phospholipids.[1] The binding and release may be coupled with the redox state of the cystines, i.e. the presence of these disulfide bonds.[2]
Many have regulatory functions.
The family was classified by sequence homology into 6 subfamilies in 2006.[3] The human and mouse genomes only contain the first three families, per an 2011 update. Not every family is monophyletic.[4]
| Subfamily | Group | Member | |||||
|---|---|---|---|---|---|---|---|
| Symbol | Name | Ligand(s) | |||||
| 1 | UGB | A | UGB | SCGB1A1 | Uteroglobin (UGB); blastokinin, CCSP | phosphatidylcholine, phosphatidylinositol, PCB | |
| B | -like | Scgb1b2 (rodent) | in general: steroid pheromones | ||||
| C | SCGB1C1 | ||||||
| SCGB1C2 | |||||||
| D | Lipophilin A/B | ||||||
| 2 | A | Mammaglobin | SCGB2A1 | Mammaglobin-B; lipophilin-C | |||
| SCGB2A2 | Mammaglobin-A | ||||||
| B | -like | Scgb2b1 (rodent) | in general: steroid pheromones | ||||
| 3 | UGB-like | A | UGB-like | SCGB3A1 | |||
| SCGB3A2 | |||||||
Rat prostatein (rat prostatic steroid binding protein) is a three component, tetrameric protein complex built from three lipophilin-like genes in groups 2 and 3. Some sources describe it as the ortholog of lipophilin.[3]
Subfamilies 4 and 6 were originally described as found in human and mice, but they are now treated as duplicates of existing groups. Subfamily 5 is found in neither to begin with.[3]
Subfamily 4 formerly included what is now known as group 1B and 2B as group 4A. Pairs of 1B/2B (ABP) genes are arranged head-to-head on the chromosome into "modules" that have been independently duplicated.[4]
Subfamily 6 was supposed to include human and rat RYD5,[3] now reassigned Scgb1c1.[4]
Fel d 1 was supposed to be put into two subfamilies: chain 1 in subfamily 4, and chain 2 in subfamily 5.[3] Although it is not mentioned in the 2011 update, InterPro matches suggest that CH1 is similar to 1B and CH2 is similar to 2B.[5] Extra computational research confirms the similarity, invalidating subfamilies 4 and 5 fully.[6]