Protein Domain : Vitamin D-binding protein IPR000213

Type	Family
Description	A number of serum transport proteins are known to be evolutionarily related, including albumin, alpha-fetoprotein, vitamin D-binding protein and afamin[ , , ]. Albumin is the main protein of plasma; it binds water, cations (suchas Ca 2+, Na +and K +), fatty acids, hormones, bilirubin and drugs - its main function is to regulate the colloidal osmotic pressure of blood. Alphafeto-protein (alpha-fetoglobulin) is a foetal plasma protein that binds various cations, fatty acids and bilirubin. The biological role of afamin (alpha-albumin) has not yet been characterised. Vitamin D-binding protein (DBP) is an abundant serum glycoprotein secretedby the liver; the protein transports vitamin D sterols, binds to actin, and is found on the surface of B-lymphocytes and subpopulations of T-lymphocytes[ ]. The full length DBP contains 476-amino acids, including a 16-aminoacid signal sequence. Sequence analysis reveals 23% similarity to albumin and to alpha-fetoprotein []. DBP contains a characteristic placement ofcysteine residues, identical to that in albumin, suggesting a similar folding structure. Albumin and alpha-fetoprotein contain three internallyrepeated domains [ ]. DBP shows similarity to the first two domains andhas a truncated third domain, supporting the view that DBP is a member of the albumin/alpha-fetoprotein multigene family []. Within the sequence, regularly-spaced disulphide bridges generate a 3-domain folding structure, each domain containing ~170 amino acids, with 5 or 6internal disulphide bonds, as shown schematically below: +---+ +----+ +-----+ \| \| \| \| \| \|xxCxxxxxxxxxxxxxxxxCCxxCxxxxCxxxxxCCxxxCxxxxxxxxxCxxxxxxxxxxxxxxCCxxxxCxxxx \| \| \| \| \| \|+-----------------+ +-----+ +---------------+
Short Name	VitD-bd