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J. Biol. Chem., Vol. 283, Issue 22, 15160-15168, May 30, 2008

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An Essential Ligand-binding Domain in the Membrane Receptor for Retinol-binding Protein Revealed by Large-scale Mutagenesis and a Human Polymorphism^*

Riki Kawaguchi, Jiamei Yu, Patrick Wiita, Jane Honda, and Hui Sun^¶1

From the Department of Physiology, the Jules Stein Eye Institute, and the ^¶Brain Research Institute, David Geffen School of Medicine at UCLA, Los Angeles, California 90095

Plasma retinol-binding protein (RBP), the principal carrier of vitamin A in the blood, delivers vitamin A from liver, the site of storage, to distant organs that need vitamin A, such as the eye, brain, placenta, and testis. STRA6 is a high-affinity membrane receptor for RBP and mediates vitamin A uptake in these target organs. STRA6 is a 74-kDa multi-transmembrane domain protein that represents a new class of membrane transport protein. In this study, we used an unbiased strategy by analyzing >900 random mutants of STRA6 to study its structure and function, and we identified an essential RBP-binding domain in STRA6. Mutations in any of the three essential residues in this domain can almost completely abolish binding of STRA6 to RBP and its vitamin A uptake activity from holo-RBP without affecting its cell surface expression. We have also functionally characterized the mutations in human STRA6 that cause severe birth defects as well as several human polymorphisms. All STRA6 mutants associated with severe birth defects have largely abolished vitamin A uptake activity, consistent with the severe clinical phenotypes. In addition, we have identified a human polymorphism that significantly reduces the vitamin A uptake activity of STRA6. Interestingly, the residue affected by this polymorphism is located in the RBP-binding domain we identified, and the polymorphism causes decreased vitamin A uptake by reducing RBP binding. This study identifies an essential functional domain in STRA6 and a human polymorphism in this domain that leads to reduced vitamin A uptake activity.

Received for publication, February 8, 2008 , and in revised form, March 28, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grant 1R01EY018144 (to H. S.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Physiology and Jules Stein Eye Inst., UCLA School of Medicine, 53-231 CHS, 650 Charles E. Young Dr. S., Los Angeles, CA 90095-1751. Fax: 310-206-5661; E-mail: hsun{at}mednet.ucla.edu.

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