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,Hui Sun

doi:10.1074/jbc.m801060200

Riki Kawaguchi, Jiamei Yu + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m801060200

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Abstract

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.

Highlights

(11–13), choroid plexus [12, 14], Sertoli cells and peritubular cells of the testis [12, 15,16,17,18], macrophages [19], and skin [12, 20]
By analyzing the individual mutations (Q310H and L315P), we found that L315P is responsible for the loss of retinol-binding protein (RBP) binding activity
STRA6 is not homologous to any protein of known function, its multi-transmembrane domains and its function as the RBP receptor in mediating vitamin A uptake imply that it has extracellular domains that participate in RBP interaction

Summary

Introduction

(11–13), choroid plexus [12, 14], Sertoli cells and peritubular cells of the testis [12, 15,16,17,18], macrophages [19], and skin [12, 20]. We identified the functional defects in these naturally occurring human STRA6 mutants and polymorphisms. Cell Surface Expression and RBP Binding Activity of STRA6 Mutants—␣-Bungarotoxin and BBS provide an efficient tagging system to study membrane proteins [30, 32].

Results

Conclusion