Retinol-Binding Protein 4 (RBP4) is a plasma protein that transports retinol (vitamin A) from the liver stores to the peripheral tissues. The RBP4-retinol complex interacts with transthyretin (TTR - transports thyroxine and retinol) which protects it from renal excretion. In addition to retinol, other endogenous and synthetic retinoids bind RBP4, including all-trans and 13-cis retinoic acid, retinyl acetate, N-(ethyl)retinamide, and fenretinide. This group also includes purpurin, a retinol-specific protein that plays a role in neural retina cell adhesion during development of the chicken retina; it also binds retinol and may participate in retinol transporter in the retina. This group belongs to the lipocalin/cytosolic fatty-acid binding protein family which have a large beta-barrel ligand-binding cavity. Lipocalins are mainly low molecular weight extracellular proteins that bind principally small hydrophobic ligands, and form covalent or non-covalent complexes with soluble macromolecules, as well as membrane bound-receptors. They participate in processes such as ligand transport, modulation of cell growth and metabolism, regulation of immune response, smell reception, tissue development and animal behavior. Cytosolic fatty-acid binding proteins, also bind hydrophobic ligands in a non-covalent, reversible manner, and have been implicated in intracellular uptake, transport and storage of hydrophobic ligands, regulation of lipid metabolism and sequestration of excess toxic fatty acids, as well as in signaling, gene expression, inflammation, cell growth and proliferation, and cancer development.
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