Molecular and biochemical characterization of a novel oxysterol-binding protein (OSBP2) highly expressed in retina

We are interested in understanding the possible function(s) of the oxystero l-binding proteins in mediating oxysterol cytotoxicity in the retina. In th is study we describe the cloning, localization, and biological activity of a novel oxysterol-binding protein (OSBP2), and complete the molecular chara cterization of the previously known OSBP1. Both OSBP genes contain 14 exons and have similar exon sizes and splice sites suggesting they may have aris en from a gene duplication event. OSBP1 is located in chromosome 11q12.1, a nd OSBP2 is located in 22q12. At the protein level they share 63% overall s imilarity and although they have unique N termini, both have similar plecks trin homology domains within the N terminus region. Northern blot analyses indicate that OSBP1 is broadly expressed in human and monkey tissues. OSBP2 is detected mainly in retina, testis, and fetal liver. Western blot analys is using peptide antibodies specific to OSBP1 and OSBP2 detected the protei ns in different subcellular fractions in the retinal monkey tissue. OSBP1 i s detected mainly in the soluble or cytosolic fraction and nuclei whereas O SBP2 is detected exclusively in the detergent soluble fraction suggesting a ssociation with membranes. Immunohistochemical localization of OSBP1 and OS BP2 in the monkey retina placed these two proteins in similar but distinct areas of the inner retina. OSBP2 was found to bind 7-ketocholesterol but to have very little affinity for cholesterol or 25-hydroxycholesterol.