The Molecular Basis of the Coloration Mechanism in Lobster Shell: β-Crustacyanin at 3.2-Å Resolution

• Published in: Proceedings of the National Academy of Sciences - PNAS 2002-07, Vol.99 (15), p.9795-9800
• Main Authors: Cianci, Michele, Rizkallah, Pierre J., Olczak, Andrzej, Raftery, James, Chayen, Naomi E., Zagalsky, Peter F., Helliwell, John R.
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Binding sites; Biological Sciences; Carotenoids; Carrier Proteins; Colors; Crystallography, X-Ray; Dimers; Hydrogen bonds; Lobsters; Models, Molecular; Molecular Sequence Data; Molecular structure; Molecules; Nephropidae - chemistry; Pigments, Biological - chemistry; Polyenes; Protein Conformation; Proteins - chemistry; Proteins - isolation & purification; Stress, Mechanical
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.152088999

The binding of the carotenoid astaxanthin (AXT) in the protein multimacromolecular complex crustacyanin (CR) is responsible for the blue coloration of lobster shell. The structural basis of the bathochromic shift mechanism has long been elusive. A change in color occurs from the orange red of the unbound dilute AXT (λmax472 nm in hexane), the well-known color of cooked lobster, to slate blue in the protein-bound live lobster state (λmax632 nm in CR). Intriguingly, extracted CR becomes red on dehydration and on rehydration goes back to blue. Recently, the innovative use of softer x-rays and xenon derivatization yielded the three-dimensional structure of the A1apoprotein subunit of CR, confirming it as a member of the lipocalin superfamily. That work provided the molecular replacement search model for a crystal form of the β-CR holo complex, that is an A1with A3subunit assembly including two bound AXT molecules. We have thereby determined the structure of the A3molecule de novo. Lobster has clearly evolved an intricate structural mechanism for the coloration of its shell using AXT and a bathochromic shift. Blue/purple AXT proteins are ubiquitous among invertebrate marine animals, particularly the Crustacea. The three-dimensional structure of β-CR has identified the protein contacts and structural alterations needed for the AXT color regulation mechanism.