ABSTRACT
AbstractT ransport o f vitamin A to the target cells is mediated by the lipocalin retinol-binding protein. In plasma, RBP is found in a complex with its carrier protein transthyretin (TTR). The structures of RBP free and in complex with T T R provide the details of the protein-protein interaction.Introduction Vitamin A is unique among the vitamins in having two separable functions. The parent
compound, retinol (also known as vitamin A alcohol) is itself inactive but serves as the precur sor for the production of various active forms that include the aldehyde retinal and retinoic acid. The light-induced cis-trans isomerization of the former is the initial signal required for vision, while the latter regulates gene expression by serving as a ligand for specific nuclear receptors o f the steroid hormone super-family.1 Thus the vitamin is essential not only for vi sion, but also has a hormonal action as retinoic acid in processes such as morphogenesis and proper differentiation and maintenance of various tissues,2 particularly epithelium. All o f the physiologically active forms of the vitamin are hydrophobic molecules that readily partition into lipid-rich sites and consequendy the specific transport of the vitamin is mediated by both intra and extra cellular binding proteins. Only small amounts of the vitamin are required (sev eral mg o f retinol per day are adequate to support good health for humans) and these specific transport proteins allow for efficient mobilization o f the vitamin by minimizing the loss to nonspecific interactions with membranes. The extra cellular transport protein for retinol (ret inol binding protein, RBP) is a member of the lipocalin super-family. RBP circulates in the plasma bound to its carrier protein transthyretin (TTR) and this protein-protein complex is thought to interact with specific cell receptors to deliver retinol to the target cells. Because o f its role in the specific transport of an essential vitamin, this lipocalin has been the focus o f a great deal o f research. This review represents a very brief summary o f the structure and function o f the RBP:TTR complex.
