Characterization of a Novel Receptor in Toad Retina with Dual Specificity for Insulin and Insulin‐Like Growth Factor I

The biochemical properties of insulin receptors from toad retinal membranes were examined in an effort to gain insight into the role this receptor plays in the retina. Competition binding assays revealed that toad retinal membranes contained binding sites that displayed an equal affinity for insulin and insulin‐like growth factor I (IGF‐I). Affinity labeling of toad retinal membrane proteins with I25 I‐insulin resulted in the specific labeling of insulin receptor α‐subunits of ∼ 105 kDa. Sodium dodecyl sulfate‐polyacrylamide gel electrophoresis of partially reduced (αβ‐heterodimer) receptors affinity‐labeled with I25 I‐insulin indicated the presence of a disulfide‐linked β‐subunit of ∼95 kDa. Endoglycosidase F digestion of the affinity‐labeled a‐subunits increased their mobility by reducing their apparent mass to ∼83 kDa. This receptor was not detected by immunoblot analysis with a site‐specific antipeptide antibody directed against residues 657–670 of the carboxy terminal of the human insulin receptor α‐subunit, whereas this antibody did label insulin receptor a‐subunits from pig, cow, rabbit, and chick retinas. In in vitro autophosphorylation assays insulin stimulated the tyrosine phosphorylation of toad retina insulin receptor β‐ subunits. These data indicate that toad retinal insulin receptors have a heterotetrameric structure whose a‐subunits are smaller than other previously reported neuronal insulin receptors. They further suggest that a single receptor may account for both the insulin and IGF‐I binding activities associated with toad retinal membranes.