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Real-time Analyses of Retinol Transport by the Membrane Receptor of Plasma Retinol Binding Protein

Published: January 28th, 2013



1Department of Physiology, Jules Stein Eye Institute and Howard Hughes Medical Institute, University of California, Los Angeles

Here we describe an optimized technique to produce high-quality vitamin A/RBP complex and two real-time monitoring techniques to study vitamin A transport by STRA6, the RBP receptor.

Vitamin A is essential for vision and the growth/differentiation of almost all human organs. Plasma retinol binding protein (RBP) is the principle and specific carrier of vitamin A in the blood. Here we describe an optimized technique to produce and purify holo-RBP and two real-time monitoring techniques to study the transport of vitamin A by the high-affinity RBP receptor STRA6. The first technique makes it possible to produce a large quantity of high quality holo-RBP (100%-loaded with retinol) for vitamin A transport assays. High quality RBP is essential for functional assays because misfolded RBP releases vitamin A readily and bacterial contamination in RBP preparation can cause artifacts. Real-time monitoring techniques like electrophysiology have made critical contributions to the studies of membrane transport. The RBP receptor-mediated retinol transport has not been analyzed in real time until recently. The second technique described here is the real-time analysis of STRA6-catalyzed retinol release or loading. The third technique is real-time analysis of STRA6-catalyzed retinol transport from holo-RBP to cellular retinol binding protein I (CRBP-I). These techniques provide high sensitivity and resolution in revealing RBP receptor's vitamin A uptake mechanism.

Vitamin A is an organic molecule that is essential for human survival and the proper functioning of almost all human organs. Vitamin A derivatives (retinoids) participate in diverse biochemical and cellular events including the sensing of light for vision 1,2 and the regulation of gene expression and protein translation during embryonic development and in adult tissues 3-6. Although retinol has the ability to diffuse systemically, evolution came up with plasma retinol binding protein, a specific carrier protein for vitamin A transport in the blood to achieve high efficiency and specificity and to avoid toxicity associated with random diffusion

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1. Production, Refolding, and HPLC Purification of Holo-RBP

  1. Transform BL-21cells with the pET3a vector harboring the cDNA for human RBP with 6x His tag on the N-terminus. Grow the transformed BL-21 cells in a shaker at 37 °C in 40 ml LB media with carbenicillin until O.D. at 600 nm reaches 0.5. Induce RBP protein expression by adding IPTG to 1mM. Grow the bacteria at 37 °C another 5 hr.
  2. RBP produced in E.coli is mostly present in insoluble inclusion bodies. To enrich inclusion bodie.......

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We present here representative results for holo-RBP production and purification by HPLC (Figure 1), real-time analysis of STRA6-catalyzed retinol release from holo-RBP and retinol loading into apo-RBP (Figure 2) and real-time analysis of STRA6-catalyzed retinol transport from holo-RBP to EGFP-CRBP-I (Figure 3).

Without refolding, RBP produced in bacteria is almost completely misfolded due to the presence of many incorrect disulfide bonds. Ther.......

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We share here an optimized RBP production protocol because RBP production and purification procedures are critical to generating correctly folded RBP. Given the possibility of misfolded RBP species and the presence of trace amounts of bacterial proteins even in HPLC purified bacteria-produced RBP, it is helpful to use native RBP from serum to confirm a conclusion related to RBP. Urine RBP, which is commercially available, is a complex mixture of many species of RBP including apo-RBP and holo-RBP 48,49.

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Supported by National Institutes of Health grant R01EY018144.


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Name Company Catalog Number Comments
Name of Reagent/Material Company Catalog Number Comments
guanidine hydrochloride EMD 5010
cystine Sigma C8755
cysteine Sigma C7352
EDTA Fisher BP118-500
Tris Fisher 7786-1
DTT EMD 3860
retinol Sigma R7632
carbenicillin Fisher BP2648-5
PBS EMD 6508
NaCl Fisher BP358-10
Ni-NTA Qiagen 1018244
imidazole EMD 5720
heptane EMD HX0295-1
Blocker Casein Pierce 37528
Amicon Ultra 15 concentrator (MWCO 10 K) Millipore UFC901024
Microfluor-2 plate Fisher 14-245-177
Hamilton syringe Gastight #1710 Fisher 14-824-655

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