title: Retinoid-binding proteins: similar protein architectures bind similar ligands via completely different ways.
abstract: BACKGROUND
  Retinoids are a class of compounds that are chemically related to vitamin A, which is an essential nutrient that plays a key role in vision, cell growth and differentiation. In vivo, retinoids must bind with specific proteins to perform their necessary functions. Plasma retinol-binding protein (RBP) and epididymal retinoic acid binding protein (ERABP) carry retinoids in bodily fluids, while cellular retinol-binding proteins (CRBPs) and cellular retinoic acid-binding proteins (CRABPs) carry retinoids within cells. Interestingly, although all of these transport proteins possess similar structures, the modes of binding for the different retinoid ligands with their carrier proteins are different.
  
  
  RESULTS
  In this work, we analyzed the various retinoid transport mechanisms using structure and sequence comparisons, binding site analyses and molecular dynamics simulations. Our results show that in the same family of proteins and subcellular location, the orientation of a retinoid molecule within a binding protein is same, whereas when different families of proteins are considered, the orientation of the bound retinoid is completely different. In addition, none of the amino acid residues involved in ligand binding is conserved between the transport proteins. However, for each specific binding protein, the amino acids involved in the ligand binding are conserved. The results of this study allow us to propose a possible transport model for retinoids.
  
  
  CONCLUSIONS
  Our results reveal the differences in the binding modes between the different retinoid-binding proteins.
authors.affiliation: State Key Laboratory of Genetic Resources and Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan, China.
authors.forename: Yu-Ru
authors.initials: YR
authors.lastname: Zhang
authors.forename: Yu-Qi
authors.initials: YQ
authors.lastname: Zhao
authors.forename: Jing-Fei
authors.initials: JF
authors.lastname: Huang
mesh_terms: D001665:Binding Sites; D001692:Biological Transport; D008024:Ligands; D056004:Molecular Dynamics Simulation; D011485:Protein Binding; D011487:Protein Conformation; D012177:Retinol-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D013379:Substrate Specificity; D014212:Tretinoin / Q000378:metabolism
chemical_list: D008024:Ligands; D012177:Retinol-Binding Proteins; D014212:Tretinoin