FIGURE 2.

RBP4 membrane receptors for retinol transport. (A) Extrahepatic tissues including the eye take up retinol via binding of circulating holo-RBP4 to STRA6. Uptake is coupled to intracellular binding, storage and metabolism of retinol. STRA6 can also mediate efflux of cellular retinol to apo-RBP4, which was shown to be further stimulated by binding of calmodulin/Ca²⁺ to an intracellular domain of STRA6. (B) Liver, intestine, and several other organs but not the eye express STRA6L, predicted to have a similar transmembrane domain structure like STRA6. STRA6L catalyzes retinol uptake from holo-RBP4. Whether or not it can mediate bidirectional retinol transport, and how it couples to intracellular binding, storage and metabolism of retinol is currently unknown. RAR, retinoic acid receptor; RBP4, retinol binding protein 4; RXR retinoid X receptor; STRA6, stimulated by retinoic acid 6; STRA6L, stimulated by retinoic acid 6-like; TTR, transthyretin.