Interphotoreceptor retinoid-binding protein. Characteristics in bovine and monkey retina

Evidence is presented for the presence of a unique retinoid-binding protein in the retinal interphotoreceptor space (IPS). The protein is a glycoprotein with an apparent native Mr of approx. 250,000 and subunit Mr of about 146,000. It is synthesized by the retina and appears to be quickly secreted into the IPS. The protein binds retinol endogenously and there is an apparent differential light-dark binding of the ligand. The protein will bind other retinoids as well as retinol and thus is not as ligand specific as is the cellular retinol-binding protein (CRBP). With the localization of this protein in the IPS and its relatively catholic taste in retinoid binding, it is reasonable to call it the interphotoreceptor retinoid-binding protein (IRBP). The protein may function as a retinoid transport vehicle between retina and pigment epithelium.