Structure of chicken plasma retinol-binding protein

The crystal structure of the specific carrier of retinol (retinol-binding protein, RBP) purified from chicken plasma has been determined (space group P2(1)2(1)2(1), with a = 46.06(5) Angstrom, b = 53.56(6) Angstrom, c = 73.41(8) Angstrom, and one protein molecule in the asymmetric unit). Despite being obtained from a species phylogenetically distant from mammals, chicken holoRBP has an overall structure that closely resembles the previously determined structures of mammalian holoRBPs. The lack in chicken RBP of eight carboxy-terminal amino acid residues characteristic of mammalian RBPs does not significantly affect the protein structure. A distinctive feature of the avian protein is a better definition of the loop 63-67, close to the opening of the beta -barrel cavity accommodating the retinol molecule, which is rather disordered in the structures of mammalian RBPs.