STRA6-Catalyzed Vitamin A Influx, Efflux, and Exchange

STRA6-Catalyzed Vitamin A Influx, Efflux, and Exchange 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Journal of Membrane Biology Springer Journals

STRA6-Catalyzed Vitamin A Influx, Efflux, and Exchange

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Publisher
Springer-Verlag
Copyright
Copyright © 2012 by Springer Science+Business Media, LLC
Subject
Life Sciences; Human Physiology; Biochemistry, general
ISSN
0022-2631
eISSN
1432-1424
D.O.I.
10.1007/s00232-012-9463-1
Publisher site
See Article on Publisher Site

Abstract

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.

Journal

The Journal of Membrane BiologySpringer Journals

Published: Jul 20, 2012

References

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