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B. Li, I. L. Pop, J. M. Frederick, P. S. Bernstein; Further Identification and Characterization of the Lutein-Binding Protein in Human and Monkey Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4814.
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We have previously reported that the lutein-binding protein in human retina is a member of the steroidogenic acute regulatory (StAR) protein family with significant cross reactivity with CBP, the StAR protein responsible for lutein uptake in the silkworm gut and silk gland (Biochemistry, 2009, 48 (22), pp 4798-4807). The human genome contains fifteen genes encoding StAR domain (StARD) proteins. Here we identify which StARD protein is responsible for lutein binding in the human retina.
Western blots were performed using fifteen StAR protein antibodies against total protein extracts from human and mouse retina, RPE and liver. Corresponding mRNA expressions of lutein binding protein candidates were tested by RT-PCR using human retina total RNA. Tissue distributions of lutein binding protein candidates and CBP were determined by immunohistochemistry of monkey retina sections.
Immunoblots revealed that StARD3 and StARD8 are the two best lutein-binding protein candidates of the fifteen human StARD proteins. StARD3 labels human macula with a band ~48kD, while retina and RPE labeling is weak. StARD8 labels human macula, human peripheral retinal and human RPE with a band ~118kD. mRNA expressions of StARD3 and StARD8 were demonstrated by RT-PCR with bands of 728 bp and 654 bp, respectively. RT-PCR product identities were confirmed by DNA sequencing. Although broadly distributed in neurons, StARD3 and CBP colocalize prominently in photoreceptors of monkey retina. By contrast, StARD8 distribution was strong in selected cell bodies of the inner nuclear layer (INL) with axons extending into the inner plexiform layer ( IPL). StARD8 did not colocalize with the Müller cell marker, glutamine synthetase.
StARD3 and StARD8 are both found in human retina. Based on the immunolocalization and immunoblot experiments, StARD3 is the best candidate for the human retinal lutein-binding protein, especially since it has significant sequence homology with silkworm CBP. Protein expression and quantitative binding studies are required to confirm its physiological function.
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