Characterization of the metabolic turnover of epidermal growth factor receptor protein in A‐431 cells

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Abstract
The metabolism of the receptor for epidermal growth factor (EGF) in A‐431 cells has been measured by labeling the receptor in vivo with radioactive amino acid precursors and then determining, by immunoprecipitation with specific anti‐EGF receptor antisera, the rate of degradation of the receptor when the cells are placed in a nonradioactive medium. The rate of EGF receptor degradation (t½12 = 20 hr) was faster than the rate of degradation of total cell protein (t½12 = 52 hr). When EGF was added at the beginning of the chase, the half‐life of prelabeled receptor decreased to 8.9 hr. This decrease was specific, as the level of total cellular protein and another plasma membrane protein, the transferrin receptor, were relatively unaffected by EGF. The carbohydrate portion of the receptor is degraded, in the presence or absence of EGF, at approximately the same rate as the protein moiety. The amount of EGF receptor protein in A‐431 cells has been quantitated by radiolabeling total cellular protein and quantitating the immunoprecipitable receptor. The EGF receptor constitutes approximately 0.15% of the total cell protein in A‐431 cells. These cells, therefore, have approximately 30 times more EGF receptor protein than fibroblasts. The EGF receptor constitutes an even higher proportion of 3H‐glucosamine‐ or 3H‐mannose‐labeled macromolecules in A‐431 cells, 1.5% or 5.2%, respectively. The EGF receptor from A‐431 cells can easily be identified by submitting carbohydrate‐labeled, solubilized cells to electrophoresis as described by Laemmli (1970).