The insulin receptor functions normally in Chinese hamster ovary cells after truncation of the C terminus

We studied the structure and function of the human insulin receptor (IR) and a mutant which lacked the last 43 amino acids of the β-subunit (IR(Δct)). This deletion removed tyrosine (Tyr¹³²², Tyr¹³¹⁶) and threonine (Thr¹³³⁶) phosphorylation sites. In Chinese hamster ovary (CHO) cells, insulin binding to the mutant receptor was normal, and [³⁵S]methionine labeling indicated that both the IR and IR(Δct) were processed normally; however, the β-subunit of IR(Δct) was 5 kDa smaller than that of the IR. The time course of insulin-stimulated autophosphorylation of the partially purified IR(Δct) was normal, but the maximum autophosphorylation was reduced 20-30%. Tryptic phosphopeptide mapping confirmed the absence of the C-terminal phosphorylation sites and indicated that phosphorylation of the regulatory region (Tyr¹¹⁴⁶, Tyr¹¹⁵⁰, Tyr¹¹⁵¹) occurred normally; kinase activity of the IR and IR(Δct) was activated normally by insulin-stimulated autophosphorylation. In the intact CHO cells, insulin-stimulated serine and threonine phosphorylation of the IR(Δct) was reduced 20%, suggesting that most Ser/Thr phosphorylation sites are located outside of the C terminus. During insulin stimulation, the wild-type and mutant insulin receptor activated the phosphatidylinositol 3-kinase. Moreover, insulin itself or human-specific anti-insulin receptor antibodies stimulated glycogen and DNA synthesis equally in both CHO/IR and CHO/IR(Δct) cells. These data suggest that the C terminus plays a minimal role in IR function and signal transmission in CHO cells.