Pathways & interactions
Short name: Blactoglobulin
Overlapping homologous superfamilies
- Calycin (IPR012674)
- Lipocalin (IPR002345)
- Beta-lactoglobulin (IPR002447)
Beta-lactoglobulin (Blg) is the major protein component of milk from a wide range of species but not human. Glycodelin or PP14 protein is the human equivalent of Blg and is secreted into the endometrium. Blg binds a wide variety of hydrophobic ligands but its function remains unknown. The crystal structure of Blg has been solved [PMID: 9115437], confirming membership of the lipocalin protein family.
This entry also includes human glycodelin. It has four glycoforms, namely glycodelin-S, -A, -F and -C have been identified in reproductive tissues that differ in glycosylation and biological activity. Glycodelin-A has contraceptive and immunosuppressive activities [PMID: 9918684, PMID: 7531163]. Glycodelin-C stimulates binding of spermatozoa to the zona pellucida [PMID: 17192260]. Glycodelin-F inhibits spermatozoa-zona pellucida binding and significantly suppresses progesterone-induced acrosome reaction of spermatozoa [PMID: 12672671]. Glycodelin-S in seminal plasma maintains the uncapacitated state of human spermatozoa [PMID: 15883155].
The lipocalins are a diverse, interesting, yet poorly understood family of proteins composed, in the main, of extracellular ligand-binding proteins displaying high specificity for small hydrophobic molecules [PMID: 2580349, PMID: 8761444]. Functions of these proteins include transport of nutrients, control of cell regulation, pheromone transport, cryptic colouration and the enzymatic synthesis of prostaglandins.
The crystal structures of several lipocalins have been solved and show a novel 8-stranded anti-parallel beta-barrel fold well conserved within the family. Sequence similarity within the family is at a much lower level and would seem to be restricted to conserved disulphides and 3 motifs, which form a juxtaposed cluster that may act as a common cell surface receptor site [PMID: 8761444]. By contrast, at the more variable end of the fold are found an internal ligand binding site and a putative surface for the formation of macromolecular complexes [PMID: 8573354]. The anti-parallel beta-barrel fold is also exploited by the fatty acid-binding proteins (which function similarly by binding small hydrophobic molecules), by avidin and the closely related metalloprotease inhibitors, and by triabin. Similarity at the sequence level, however, is less obvious, being confined to a single short N-terminal motif. The lipocalin family can be subdivided into kernal and outlier sets. The kernal lipocalins form the largest self consistent group, comprising the subfamily of beta-lactoglobulins. The outlier lipocalins form several smaller distinct subgroups: the OBPs, the von Ebner's gland proteins, alpha-1-acid glycoproteins, tick histamine binding proteins and the nitrophorins.
- PR01172 (BLCTOGLOBULN)