Subunit arrangement of insulin receptors in hepatoma cells

Insulin receptors from rat hepatoma cells were studied by the three following methods. Firstly, the alpha subunit (Mr 130000) was labelled using a 125I-photoreactive insulin analogue and UV irradiation. Secondly, using phosphorylation of partially purified and immunoprecipitated receptors with [gamma-32P]ATP, the beta subunit (Mr 95000) was labelled. Thirdly, both alpha and beta subunits were labelled by surface iodination catalysed by lactoperoxidase followed by cell solubilization and immunoprecipitation of the receptor with anti-receptor antibodies. The results show that the native insulin receptor exists under different forms: free alpha and beta subunits and the following combinations of disulphide-linked oligomers: alpha beta, alpha 2, alpha 2 beta and alpha 2 beta 2. In addition, it appears that there is at least one insulin binding site per alpha subunit, and that the alpha and beta subunits may be in close physical association in the plasma membrane even when they are not linked by disulphide bonds. In intact cells, only the alpha subunit is sensitive to extracellular proteases that cleave preferentially the region of the alpha subunit bearing the sulphydryl groups responsible for the interchain binding.