Cellular retinoic acid-binding proteins (CRABPs) play a role in the metabolism of vitamin A and retinoic acid. They bind all trans retinoic acid, but not retinol. Retinol, the alcohol form of vitamin A, is an essential dietary nutrient. Within the cell, it gets oxidized into its biologically active acid form, retinoic acid, which interacts with the nuclear receptors (RARs and RXRs). The two CRABPs (CRABP1 AND CRABP2) differ in their pattern of expression across cells and developmental stages. Like other lipid binding proteins, CRABPs serve to solubilize and protect their ligand in the aqueous cytosol and transport retinoic acid between cellular compartments. This subgroup includes CRABP2 (also known as CRABP-II, RBP6) which transports retinoic acid to the nucleus, and delivers all-trans-retinoic acid to nuclear retinoic acid receptors. CRABPs belong to the intracellular fatty acid-binding protein (FABP) family, members of which are small proteins that bind hydrophobic ligands in a non-covalent, reversible manner, and besides CRABPS include the cellular retinol-binding protein (CRBPs) and the fatty acid-binding proteins (FABPs).
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