IGF-I receptors in the bovine neural retina: structure, kinase activity and comparison with retinal insulin receptors.
The retina contains specific high-affinity receptors for insulin-like growth factor-I (IGF-I). Although IGF-I binding was observed in photoreceptor outer segments, the level of this binding was only 10% of that found in whole retina or mixed preparations of rod outer (ROS) and inner (RIS) segments. The higher IGF-I binding activity in RIS and non-photoreceptor regions of the retina suggests these sites as candidates for putative IGF-I action. Data from crosslinking experiments with and without neuraminidase treatment indicate that the binding subunits of the retinal IGF-I receptor exist in two subpopulations (Mr = 121- and 131 kDa), and that the larger of the two subunits has either a greater number or more exposed sialic acid residues. In these characteristics, the retinal IGF-I receptor is similar to the retinal insulin receptor. Retinal IGF-I and insulin receptors possess kinase activity towards their own beta-subunits, a tyrosine containing copolymer, and various molecular forms and subunits of transducin (T alpha-GDP, T alpha-GTP, T beta). The transducin forms are phosphorylated with different efficiencies (e.g. T alpha-GDP is 10-15 times more effective than T alpha-GTP as substrate). These differences are also observed in basal conditions and may reflect differences in transducin subunit affinity for the IGF-I and insulin receptor. In all retinal areas examined, tracer IGF-I binding is 10 to 20-fold higher than insulin binding; however, autophosphorylation levels are approximately equal.
Waldbillig
R J
RJ
Laboratory of Retinal Cell and Molecular Biology, National Eye Institute, Bethesda, MD 20892.
Fletcher
R T
RT
Somers
R L
RL
Chader
G J
GJ
D000818:Animals; D001667:Binding, Competitive; D002417:Cattle; D007328:Insulin / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008970:Molecular Weight; D010766:Phosphorylation; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011972:Receptor, Insulin / Q000378:metabolism*; D017451:Receptors, Somatomedin; D012160:Retina / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism; D013002:Somatomedins / Q000378:metabolism*; D015293:Transducin / Q000378:metabolism
D007328:Insulin; D017451:Receptors, Somatomedin; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D011505:Protein-Tyrosine Kinases; D011972:Receptor, Insulin; D015293:Transducin