Insulin-like growth factor I receptors in retinal rod outer segments.
We have previously reported that the GDP-bound alpha-subunit of the GTP-binding protein transducin, present in outer segments of retinal rod cells (ROS), serves as a high affinity in vitro substrate (Km = 1 microM) for the insulin receptor kinase. The present study demonstrates that transducin also serves as in vitro substrate for an endogenous IGF-I receptor kinase isolated from ROS membranes. The presence of insulin-like growth factor I (IGF-I) receptors in ROS is evident from the high affinity and specific binding of 125I-IGF-I to ROS membranes (Kd = 3 nM) which contain 110 fmol of IGF-I binding sites/mg of membrane protein. Furthermore, cross-linking of 125I-IGF-I labels the 135-kDa alpha-subunit of this receptor. 125I-Insulin binding capacity to ROS membranes is less than 5% that of IGF-I. The IGF-I-stimulated tyrosine kinase activity in solubilized and partially purified receptors from ROS autophosphorylates its own 95-kDa beta-subunits as well as other substrates like transducin. Insulin, which is 200-fold less potent than IGF-I in competing for 125I-IGF-I binding, is only 5-fold less potent than IGF-I in stimulating the receptor kinase activity. This suggests that insulin is much more potent than IGF-I in coupling ligand binding with kinase activation. The previously reported presence of IGF-I in the vitreous, together with our present studies, strongly suggest that the IGF-I receptor kinase, through phosphorylation of endogenous proteins like transducin, could play a role in mediating transmembrane signal transduction in ROS.
D000818:Animals; D006153:Guanosine Diphosphate / Q000378:metabolism; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D008565:Membrane Proteins / Q000378:metabolism; D008970:Molecular Weight; D010766:Phosphorylation; D010786:Photoreceptor Cells / Q000032:analysis*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D051381:Rats; D011972:Receptor, Insulin / Q000032:analysis*; D017451:Receptors, Somatomedin; D012374:Rod Cell Outer Segment / Q000032:analysis*; D012995:Solubility; D013379:Substrate Specificity; D015293:Transducin
D007527:Isoenzymes; D008565:Membrane Proteins; D017451:Receptors, Somatomedin; D006153:Guanosine Diphosphate; D011505:Protein-Tyrosine Kinases; D011972:Receptor, Insulin; D015293:Transducin