Cells are known to take up molecules through membrane transport mechanisms such as active transport, channels, and facilitated transport. We report here a new membrane transport mechanism that employs neither cellular energy like active transport nor a preexisting electrochemical gradient of the free substrate like channels or facilitated transport. Through this mechanism, cells take up vitamin A bound with high affinity to retinol binding protein (RBP) in the blood. This mechanism is mediated by the RBP receptor STRA6, which defines a new type of cell-surface receptor. STRA6 is essential for the proper functioning of multiple human organs, but the mechanisms that enable and control its cellular vitamin A uptake activity are unknown. We found that STRA6-mediated vitamin A uptake is tightly coupled to specific intracellular retinoid storage proteins, but no single intracellular protein is absolutely required for its transport activity. By developing sensitive real-time monitoring techniques, we found that STRA6 is not only a membrane receptor but also catalyzes vitamin A release from RBP. However, vitamin A released from RBP by STRA6 inhibits further vitamin A release by STRA6 unless specific intracellular retinoid storage proteins relieve this inhibition. This mechanism is responsible for its coupling to intracellular storage proteins. The coupling of uptake to storage provides high specificity in cellular uptake of vitamin A and prevents the excessive accumulation of free vitamin A. We have also identified a robust small molecule-based technique to specifically stimulate cellular vitamin A uptake. This technique has implications in treating human diseases.

Vitamin A has diverse biological functions and is essential for human survival. STRA6 is the high-affinity membrane receptor for plasma retinol binding protein (RBP), the principle and specific carrier of vitamin A (retinol) in the blood. It was previously shown that STRA6 couples to lecithin retinol acyltransferase (LRAT) and cellular retinol binding protein I (CRBP-I), but poorly to CRBP-II, for retinol uptake from holo-RBP. STRA6 catalyzes both retinol release from holo-RBP, which is responsible for its retinol uptake activity, and the loading of free retinol into apo-RBP, which can cause retinol efflux. Although STRA6-catalyzed retinol efflux into apo-RBP can theoretically deplete cells of retinoid, it is unclear to what extent this efflux happens and in what context. We show here that STRA6 can couple strongly to both CRBP-I and CRBP-II for retinol efflux to apo-RBP. Strikingly, pure apo-RBP can cause almost complete depletion of retinol taken up by CRBP-I in a STRA6-dependent manner. However, if STRA6 encounters both holo-RBP and apo-RBP (as in blood), holo-RBP blocks STRA6-mediated retinol efflux by competing with apo-RBP's binding to STRA6 and by counteracting retinol efflux with influx. We also found that STRA6 catalyzes efficient retinol exchange between intracellular CRBP-I and extracellular RBP, even in the presence of holo-RBP. STRA6's retinol exchange activity may serve to refresh the intracellular retinoid pool. This exchange is also a previously unknown function of CRBP-I and distinguishes CRBP-I from LRAT.

STRA6 is a multi-transmembrane domain protein not homologous to any other proteins with known function. It functions as the high-affinity receptor for plasma retinol binding protein (RBP) and mediates cellular vitamin A uptake from the vitamin A/RBP complex. Consistent with the diverse roles of vitamin A and the wide tissue expression pattern of STRA6, mutations in STRA6 are associated with severe pathological phenotypes in human. The structural basis for STRA6's biochemical function is unknown. Although computer programs predict 11 transmembrane domains for STRA6, its topology has never been studied experimentally. Elucidating the transmembrane topology of STRA6 is critical for understanding its structure and function. By inserting an epitope tag into all possible extracellular and intracellular domains of STRA6, we systematically analyzed the accessibility of each tag on the surface of live cells, the accessibility of each tag in permeabilized cells, the effect of each tag on RBP binding and STRA6-mediated vitamin A uptake from the vitamin A/RBP complex. In addition, we used a new lysine accessibility technique combining cell-surface biotinylation and tandem-affinity purification to study a region of the protein not revealed by the epitope-tagging method. These studies not only revealed STRA6's extracellular, transmembrane and intracellular domains, but also implicated extracellular regions of STRA6 in RBP binding.Vitamin A and its derivatives (retinoids) are essential for diverse aspects of vertebrate physiology (1-3). Due to the hydrophobic nature of retinoids, it has been assumed that random diffusion is the primary if not the only means of transmembrane transport. However, biochemical evidence suggests that retinol uptake from the small intestine is mediated by a membrane transporter (4). There is also strong evidence for the existence of a specific mechanism to transport 11-cis retinal in the retinal pigment epithelium (RPE) that depends on interphotoreceptor retinoid-binding protein (IRBP). Apo-IRBP is much more effective in promoting the release of 11-cis retinal from the RPE than the apo-forms of other retinoid binding proteins (5). In addition, apo-IRBP is only effective when it is present on the apical, but not basal, side of the RPE (6). Another finding that challenges the assumptions about random diffusion is the identification of an ATP-dependent transporter (ABCR or ABCA4) that transports all-trans retinal released from bleached rhodopsin across membranes (7-9). Mutations in ABCR cause a wide spectrum of human vision diseases from retinitis pigmentosa to macular degeneration. Prior to the surprising discovery of ABCR's role in † This project was supported by National Institute of Health grant 1R01EY018144 (H.S.).

Vitamin A has biological functions as diverse as sensing light for vision, regulating stem cell differentiation, maintaining epithelial integrity, promoting immune competency, regulating learning and memory, and acting as a key developmental morphogen. Vitamin A derivatives have also been used in treating human diseases. If vitamin A is considered a drug that everyone needs to take to survive, evolution has come up with a natural drug delivery system that combines sustained release with precise and controlled delivery to the cells or tissues that depend on it. This “drug delivery system” is mediated by plasma retinol binding protein (RBP), the principle and specific vitamin A carrier protein in the blood, and STRA6, the cell-surface receptor for RBP that mediates cellular vitamin A uptake. The mechanism by which the RBP receptor absorbs vitamin A from the blood is distinct from other known cellular uptake mechanisms. This review summarizes recent progress in elucidating the fundamental molecular mechanism mediated by the RBP receptor and multiple newly discovered catalytic activities of this receptor, and compares this transport system with retinoid transport independent of RBP/STRA6. How to target this new type of transmembrane receptor using small molecules in treating diseases is also discussed.