Complete sequence and model for the C1 subunit of the carotenoprotein crustacyanin, and model for the dimer, β‐crustacyanin, formed from the C1 and A2 subunits with astaxanthin

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Abstract
The complete sequence has been determined for the C1 subunit of crustacyanin, an astaxanthin‐binding protein from the carapace of the lobster Homarus gammarus (L.). The polypeptide, 181 residues long, is similar (38% identity) to the other main subunit, A2 and to plasma retinol‐binding protein. The tertiary structure of the C1 subunit has been modelled on that derived for the A2 subunit from the coordinates of retinol‐binding protein. Residues lining the putative binding cavities and at the putative carotenoid binding sites of the two subunits are highly conserved. The carotenoid environments are characterized by a preponderance of aromatic and polar residues and the absence of charged side‐chains. A tentative model for the dimer, β‐crustacyanin, formed between the two subunits with their associated carotenoid ligands, is discussed. The model is based on the crystal structure of the dimer of bilin‐binding protein, a member of the same superfamily. This structure has enabled us to examine mechanisms for the bathochromic spectral shift of the protein‐bound carotenoid and to identify likely contact regions between dimers in octameric α‐crustacyanin.