Insulin‐like growth factor I receptors on mouse neuroblastoma cells: Two β 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 this 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 β subunit has two subtypes of 95 kDa and 105 kDa. On the other hand, insulin receptor β subunit is a separate single 95‐kDa protein. Enzymatic digestion of IGF‐I receptor β 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 β subunits. This conclusion is further supported by peptides of similar molecular mass following staphylococcal V8 protease digestion. Analysis of IGF‐I receptor β subunit subtypes in these cells may provide insights into the mechanism of action of IGF‐I on neural tissues.