Preparation and Structural Studies of the Silkworm Carotenoid-Binding Protein Complexed with a New Pigment

— Water-soluble carotenoid-binding proteins (CBPs) are found in many organisms and represent an example of convergent evolution, in which the needs for CBPs were fulfilled differently in different organisms. Carotenoid-binding proteins exhibit many biological functions, e.g., they have light-harvesting and photoprotective properties, color organs and tissues of animals, plants, and birds, and are involved in quenching of reactive oxygen species. Carotenoids are very potent antioxidants, which highlights their significance for the potential application in medicine; however, they have poor solubility in aqueous solutions. In recent years, it was suggested to use CBPs for the targeted delivery of carotenoids to prevent oxidative stress. Meanwhile, this application is limited because the structural basis underlying the mechanisms by which carotenoids are bound and transported, as well as the substrate specificity, remain unknown for most CBPs. The structure of recombinant CBP from silkworms (BmCBP) in complex with a new pigment was determined and investigated. The in vitro reconstitution of this complex, its spectroscopic characterization, crystallization, and the X-ray diffraction study are described. Since the crystals of BmCBP rapidly grow under different conditions, we plan to use this protein for the preparation and characterization of its complexes with different natural and synthetic ligands.