title: Complete sequence and model for the A2 subunit of the carotenoid pigment complex, crustacyanin.
abstract: The complete sequence has been determined for the A2 subunit of crustacyanin, an astaxanthin-binding protein from the carapace of the lobster Homarus gammarus. The polypeptide chain is 174 residues long and is similar to proteins of the retinol-binding protein superfamily. Some regions of the sequence are most similar to the retinol-binding protein, beta-lactoglobulin subgroup, while the disulphide bonding pattern is more akin to that seen in the porphyrin binding proteins insecticyanin and bilin-binding protein. It is beginning to appear as though this superfamily of proteins, characterized by a similar gross structural framework, may be further subdivided into interrelated subclasses. Model building based on the coordinates of the known structure of human plasma retinol-binding protein and on empirical prediction algorithms has allowed the putative identification of side-chains which line the binding cavity. This pocket is larger than in retinol binding protein and beta-lactoglobulin but does not allow the carotenoid to adopt a folded conformation. The amino acid composition of the pocket does not support a 'charge-shift'-type hypothesis to support the bathochromic shift phenomenon which takes place on interaction of the chromophore with the protein. Instead aromatic side-chains may play a prominent role.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Leeds, England.
authors.forename: J N
authors.initials: JN
authors.lastname: Keen
authors.forename: I
authors.initials: I
authors.lastname: Caceres
authors.forename: E E
authors.initials: EE
authors.lastname: Eliopoulos
authors.forename: P F
authors.initials: PF
authors.lastname: Zagalsky
authors.forename: J B
authors.initials: JB
authors.lastname: Findlay
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002338:Carotenoids / Q000235:genetics*; D002352:Carrier Proteins; D002850:Chromatography, Gel; D003445:Crustacea; D004591:Electrophoresis, Polyacrylamide Gel; D007782:Lactoglobulins / Q000235:genetics; D008969:Molecular Sequence Data; D011506:Proteins / Q000235:genetics*; D012177:Retinol-Binding Proteins / Q000235:genetics; D054839:Retinol-Binding Proteins, Plasma; D016415:Sequence Alignment
chemical_list: D002352:Carrier Proteins; D007782:Lactoglobulins; D011506:Proteins; D012177:Retinol-Binding Proteins; D054839:Retinol-Binding Proteins, Plasma; C018494:crustacyanins; D002338:Carotenoids