Structural basis of selective beta-carotene binding by a soluble protein.

β-carotene (BCR) is the most abundant carotenoid, a colorant, antioxidant, and provitamin A. The extreme hydrophobicity of this hydrocarbon requires special mechanisms for distribution in aqueous media, including water-soluble carotenoproteins. However, all known carotenoproteins prefer oxygenated carotenoids and bind BCR inefficiently. Here, we present the crystal structure of the BCR-binding protein (BBP) from gregarious male locusts, which is responsible for their vivid yellow body coloration, in complex with its natural ligand, BCR. BBP forms an antiparallel tubular homodimer with α/β-wrap folded monomers, each forming a hydrophobic 47 Å long, coaxial tunnel that opens outward and is occupied by one s-cisC6-C7, all-trans BCR molecule. In the BCR absence, BBP accepts a range of xanthophylls, with reduced efficiency depending on the position and number of oxygen atoms, but rejects lycopene. The structure captures a pigment complex with a Takeout 1 protein and inspires potential applications of BBP as a BCR solubilizer. [Display omitted] • Crystal structure of a soluble protein in complex with β-carotene (BBP) determined • Extended BBP dimer has tubular subunits each binding a single β-carotene molecule • BBP is selective for β-carotene, rejects lycopene, and tolerates some xanthophylls • BBP apoform shows efficient ligand-assisted refolding when dissolved in 8 M urea Among soluble carotenoproteins, those binding carotenes are rare. Egorkin et al. determined the crystal structure of genuine β-carotene-binding protein (BBP) from gregarious locusts responsible for their yellow body coloration. Reconstitution studies revealed a selectivity for β-carotene, inability to bind lycopene, and unusual blue spectral shifts for carotenoids upon binding to BBP. [ABSTRACT FROM AUTHOR]