Properties of an interphotoreceptor retinoid-binding protein from bovine retina

Washes and extracts of frozen and fresh cattle retina contain a water-soluble high-molecular-weight, retinoid-binding protein that is distinct from three other retinoid-binding proteins previously isolated from this tissue. The protein can be purified to apparent homogeneity from retinal homogenates by a combination of gel filtration, lectin, and ion-exchange chromatography. Overestimation of the protein molecular weight was observed in several systems involving migration of the protein through a porous network. The approximate molecular weight obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was 140,000, a value consistent with those reported by other laboratories. However, a more detailed analysis using the method of Ferguson revealed the protein to behave anomalously relative to several proteins used as sodium dodecyl sulfate-polyacrylamide gel electrophoresis standards. The apparent radius of the native protein, estimated from calibrated gel filtration, corresponded to a globular protein with a molecular weight of 240,000-280,000, suggesting that the protein was a dimer. However, when the molecular weight of native interphotoreceptor retinoid-binding protein (IRBP) was determined by a method with no shape dependence, sedimentation equilibrium, a value of 131,700 +/- 3,900 g/mol, was obtained. Sedimentation equilibrium in a dissociating solvent (6 M guanidine HCl) yielded a molecular weight of the smallest component of 120,100 +/- 2,300 g/mol. The similarity of values for the denatured and native molecular weight by sedimentation equilibrium demonstrates that the protein is a monomer. In further support of this, no evidence for a dimer was observed in cross-linking experiments with dimethyl suberimidate. The sedimentation coefficient (S0(20),w = 5.73 +/- 0.15 S) and molecular weight from sedimentation equilibrium were employed to calculate the frictional coefficient and Stokes radius of IRBP (f/f0 = 1.64, Rs = 55 A). The high value of f/f0 of the protein provides a reasonable explanation for the over-estimation of the molecular weight of native IRBP on gel filtration. Approximately 2 mol of exogenous all-trans- or 11-cis-retinol were bound per mol of protein (131,000). Approximately 7% of the binding sites were saturated with endogenous ligand (11-cis-retinol, 88%; all-trans-retinol, 12%) following isolation from partially bleached cattle eyes.(ABSTRACT TRUNCATED AT 400 WORDS)