INSULIN-LIKE GROWTH FACTOR-I RECEPTORS ON MOUSE NEURO-BLASTOMA CELLS - 2 BETA-SUBUNITS ARE DERIVED FROM DIFFERENCES IN GLYCOSYLATION

We have characterized receptors for the insulin-like growth factor (IGF-I) on the mouse neuroblastoma cell line N18 as well as NG108, the hybrid cell line of N18 and rat glioma (C6). In the cell-free system, IGF-I and insulin stimulated the phosphorylation of 95-kDa and 105-kDa proteins. Using appropriate antibodies we were able to demonstrate that the IGF-I receptor .beta. subunit has two subtypes of 95 kDa and 105 kDa. On the other hand, insulin receptor .beta. subunit is a separate single 95-kDa protein. Enzymatic digestion of IGF-I receptor .beta. subunit subtypes by glycopeptidase F resulted in similar molecular masses (84 kDa and 86 kDa) on SDS-PAGE, which suggests that the difference in molecular masses between two subtypes is attributable to the differences in N-linked complex-type carbohydrate chains on the extracellular domain of .beta. subunits. This conclusion is further supported by peptides of similar molecular mass following staphylococcal V8 protease digestion. Analysis of IGF-I receptor .beta. subunit subtypes in these cells may provide insights into the mechanism of action of IGF-I on neural tissues.