Photoaffinity labeling of the insulin receptor in H4 hepatoma cells: Lack of cellular receptor processing

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Abstract
Photoaffinity labeling techniques were used to identify insulin‐binding components of the plasma membrane in insulin‐responsive, monolayer‐cultured hepatoma cells. The activated, photosensitive reagent, an n‐hydroxysuccinimide ester of 4‐azidobenzoic acid, was coupled with highly purifed insulin, and the hormone derivative was subsequently iodinated, bound to cell surface receptors of intact H4 cells, and photoactivatcd. After dissolution of the cells, labeled proteins were analyzed by SDS/polyacrylamide gel electrophoresis under reducing conditions. The main labeled band exhibited an apparent molecular weight of 130,000. Two minor components of apparent mol wt 95,000 and 40,000 were also identified. Specific labeling of all 3 bands was inhibited by simultaneous incubation of the cells with native insulin, but not by the heterologous hormone, glucagon, prior to photoactivation. Binding of azidobenzoyl‐insulin to H4 cells was time‐dependent, as was the correlated labeling of receptor components. Band‐labeling by the photosensitive insulin derivative was totally light‐dependent; spontaneous covalent linking of insulin and receptor was not observed. The labeled receptor‐related proteins were not degraded by the cells under our experimental conditions.