Structure/Function of Cytoplasmic Vitamin A-Binding Proteins

Two cytoplasmic retinol-binding proteins, CRBP and CRBP II, and two cytoplasmic retinoic acid-binding proteins, CRABP-I and CRABP-II, have been well characterized. There has been significant progress in the structural analysis of these four proteins with X-ray crystallography, nuclear magnetic resonance, mutagenesis, and binding studies. In contrast, the cellular functions of these cytoplasmic vitamin A-binding proteins are less well understood. Since these proteins bind their respective ligands with high affinity, they are likely to influence retinoid signaling pathways. Analysis of retinoid metabolism in the presence or absence of these proteins provides support for the hypothesis that these proteins are involved in modulating intracellular retinoid metabolism. Molecular genetic approaches to alteration of the levels of these proteins in tissue culture cells and in whole animals have provided a powerful means toward defining the physiological roles of the cytoplasmic vitamin A-binding proteins in vivo.