Insulin-like growth factor I receptor beta-subunit heterogeneity. Evidence for hybrid tetramers composed of insulin-like growth factor I and insulin receptor heterodimers.
In both NIH3T3 cells and HepG2 cells, insulin-like growth factor I (IGF-I) receptors possess two beta-subunits that display different electrophoretic mobilities. Increasing concentrations of IGF-I stimulated the phosphorylation of both beta-subunits to a similar extent, whereas insulin stimulated the phosphorylation of both subunits only at elevated concentrations. Both beta-subunits were immunoprecipitated with p5, an insulin receptor-specific anti-peptide antibody, or with A410, a polyclonal anti-insulin receptor antisera. However, if the tetrameric IGF-I receptor was first dissociated into alpha-beta heterodimers with 1 mM dithiothreitol, only the lower molecular weight beta-subunit was immunoprecipitated. These results suggested that p5 and A410 specifically recognized the lower molecular weight beta-subunit but immunoprecipitated the higher molecular weight beta-subunit because it was present in the same disulfide linked tetramer. Similarly, alpha-IR-3, an antibody specific for the alpha-subunit of the IGF-I receptor, immunoprecipitated both types of beta-subunit from the intact tetramer but only the higher molecular weight beta-subunit from the dissociated heterodimers, suggesting that there are two types of alpha-subunits in the same tetramer and that the alpha-subunit recognized by alpha-IR-3 is only associated with the higher molecular weight beta-subunit. Tryptic phosphopeptide maps of the lower molecular weight beta-subunit of IGF-I receptor were different from the higher molecular weight beta-subunit, but were similar to those of the insulin receptor beta-subunit. Thus, by immunochemical cross-reactivity and structural criteria, the lower molecular weight beta-subunit of the IGF-I receptor was similar to the beta-subunit of insulin receptor. These data suggest that there exists a species of IGF-I receptor that is a hybrid composed of an insulin receptor alpha-beta heterodimer and an IGF-I receptor alpha-beta heterodimer. The existence of such a hybrid receptor could have important functional consequences.
Moxham
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CP
Division of Cell Biology, Burroughs Wellcome Co., Research Triangle Park, North Carolina 27709.
Duronio
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V
Jacobs
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D000818:Animals; D006528:Carcinoma, Hepatocellular / Q000378:metabolism; D002460:Cell Line; D004220:Disulfides; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008113:Liver Neoplasms; D051379:Mice; D008970:Molecular Weight; D010449:Peptide Mapping; D010748:Phosphopeptides / Q000302:isolation & purification; D010766:Phosphorylation; D011233:Precipitin Tests; D011487:Protein Conformation; D011972:Receptor, Insulin / Q000302:isolation & purification*; D011956:Receptors, Cell Surface / Q000302:isolation & purification* / Q000378:metabolism; D017451:Receptors, Somatomedin; D013002:Somatomedins / Q000378:metabolism*; D014357:Trypsin
D004220:Disulfides; D010748:Phosphopeptides; D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D011972:Receptor, Insulin; D014357:Trypsin