The purpose of this research project is to investigate the role of specific retinoid-binding proteins in mediating the action of retinoids in ocular tissues. A recombinant peptide of about 35 kDa from the first domain of human interphotoreceptor retinoid-binding protein (IRBP) was expressed in yeast (Pichia pastoris). This peptide bound to concanavalin A Sepharose, indicating glycosylation, and was recognized by antibodies to IRBP. Specific binding of retinol was also observed. This IRBP peptide will be useful in elucidating the retinoid-binding site of IRBP. A new gene encoding a retinoid- and fatty acid-binding glycoprotein (RFABG) from Drosophila has been characterized. The gene, localized to chromosome 4 (102F region), encodes a 3351- amino acid protein with high sequence similarity to insect lipophorins that could serve as a precursor for the about 70 kDa and >200 kDa polypeptides associated with RFABG. It is expressed in the Drosophila embryo during development in cells that make up the amnioserosa and fat bodies. The RFABG precursor contains a signal peptide and exhibits a multidomain mosaic protein structure, which is typical of extracellular proteins. It has structural domains similar to vitellogenins and apolipoprotein B and also contains a domain similar to the D domain of von Willebrand factor and mucin. Immunocytochemistry showed specific localization of RFABG in the cone (Semper) cells of the fly compound eye. Western blotting using antibodies raised against Drosophila RFABG demonstrated specific immunoreactivity with >200 kDa and 70 kDa protein bands in adult mouse retinal pigment epithelium (RPE) and a 70 kDa protein band in both monkey and human RPE cells in culture, raising the possibility of the existence of a mammalian counterpart to this new Drosophila gene.
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