Characterization of the Function of Interphotoreceptor Retinoid-Binding Proteins at the Molecular Level

The structural studies to elucidate the roles interphotoreceptor retinoid-binding protein (IRBP) in vision and eye diseases pose a challenge similar in magnitude presented by aromatase. This research is being conducted in collaboration with ophthalmologist and vision scientist Dr. Federico Gonzalez-Fernandez, Professor, SUNY Buffalo. IRBP is a usually complex retinoid-binding protein composed of 300 amino acid modules and only one module has ever been crystallized. Structural domains within the modules (and perhaps between them) allow the protein to target and protect retinoids in the rod and cone visual cycles. Understanding how this is accomplished requires combining physiological approaches along with X-ray crystallographic and spectroscopic studies. As interactions between the modules are probably important to IRBPs physiological functions, we have focused our structural studies on the full-length intact IRBPs with bound physiological ligands. Human and Xenopus represent 4-module IRBPs; zebrafish expresses two forms of IRBP composed of modules 1+2+3 (zIRBP1), and modules 1+4 (zIRBP2). We anticipate that these zIRBP orthologs will provide insight into how the more complex 4-module IRBP functions in the rod and cone visual cycles. We have now established conditions to prepare diffraction quality crystals of full-length IRBPs from a variety of species ranging from human to zebrafish. One of our latest results (Ghosh et al., Biochemistry, in revision) is the discovery that IRBP binds and is influenced by fatty acids, and the structural and functional evidences thereof. Our plan is to determine the structures of a variety of IRBPs of different sizes and the mechanisms of interaction with various fatty acids and retinoids.