ABSTRACT
Retinoids play important roles in diverse cellular processes including growth, cell differentiation and vision. Many natural and synthetic retinoids are used as drugs in dermatology and oncology. A large amount of data has been accumulated on the cellular activity of different synthetic retinoids. They are stabilized and transported inside the cell cytoplasm by binding and transport proteins, such as cellular retinol-binding proteins and cellular retinoic acid binding proteins (CRABPs). The structures of human CRABP II in complex with two different synthetic retinoids, Ro13-6307 and Ro12--7310 (at 2.1 and 2.0 A resolution, respectively) and of bovine CRABP I in complex with a retinobenzoic acid, Am80 (at 2.8 A resolution) are described. The binding affinities of human CRABP I and II for the retinoids studied here have been determined. All these compounds have comparable binding affinities (nanomolar range) for both CRABPs. Apart from the particular interactions of the carboxylate group of the retinoids with specific protein groups, each structure reveals characteristic interactions. Studying the atomic details of the interaction of retinoids with retinoid-binding proteins facilitates the understanding of the kinetics of retinoid trafficking inside the cytoplasm.
Subject(s)
Receptors, Retinoic Acid/chemistry , Retinoids/chemistry , Animals , Benzoates/chemistry , Cattle , Crystallography, X-Ray , Etretinate/analogs & derivatives , Etretinate/chemistry , Fatty Acids, Unsaturated/chemistry , Humans , Hydrogen Bonding , Models, Molecular , Molecular Sequence Data , Molecular Structure , Protein Binding , Recombinant Proteins/chemistry , Tetrahydronaphthalenes/chemistryABSTRACT
BACKGROUND: Retinoic acid (RA) plays a fundamental role in diverse cellular activities. Cellular RA binding proteins (CRABPs) are thought to act by modulating the amount of RA available to nuclear RA receptors. CRABPs and cellular retinol-binding proteins (CRBPs) share a unique fold of two orthogonal beta-sheets that encapsulate their ligands. It has been suggested that a trio of residues are the prime determinants defining the high specificity of CRBPs and CRABPs for their physiological ligands. RESULTS: Bovine/murine CRABP I and human CRABP II have been crystallized in complex with their natural ligand, all-trans-RA. Human CRABP II has also been crystallized in complex with a synthetic retinoid, 'compound 19'. Their structures have been determined and refined at resolutions of 2.9 A, 1.8 A and 2.2 A, respectively. CONCLUSIONS: The retinoid-binding site in CRABPs differs significantly from that observed in CRBP. Structural changes in three juxtaposed areas of the protein create a new, displaced binding site for RA. The carboxylate of the ligand interacts with the expected trio of residues (Arg132, Tyr134 and Arg111; CRABP II numbering). The RA ligand is almost flat with the beta-ionone ring showing a significant deviation (-33 degrees) from a cis conformation relative to the isoprene tail. The edge atoms of the beta-ionone ring are accessible to solvent in a suitable orientation for presentation to metabolizing enzymes. The bulkier synthetic retinoid causes small conformational changes in the protein structure.
