You add epidermal growth factor (EGF) to cells, activating the EGF receptor (EGF

Question

You add epidermal growth factor (EGF) to cells, activating the EGF receptor (EGFR). This activation can be seen by appearance of phosphorylated EGFR (phospho-EGFR; the antibody specifically detects this phosphorylated epitope by immunoblot) that was not present in the absence of EGF. The phospho-EGFR band is absent at 0', then rapidly peaks in abundance after 15', declines at 30' and then returns to residual to absent levels within an hour after treatment (control). You decide to assess total EGFR total protein levels, as well. In contrast, total EGFR protein is abundant at 0' and 15', but similarly declines at 30' and is very low at 60'. You blot for actin as a "loading control" for amount of lysate analyzed, which shows a steady uniform abundance of actin protein across all time points. You generate mutant cells that still respond to EGF but fail to downregulate phospho-EGFR over time (mutant). At 0' +EGF, there is abundant total EGFR protein, but no phospho-EGFR (just like wildtype cells). However, you see high levels of phospho-EGFR at 15' that then persists uniformly high from 30' and 60' post EGF treatment. In addition, total EGFR protein levels also uniformly persist at abundant levels similar to those seen at 0'. The loading control shows even levels actin at all timepoints, indicating even loading of lysate across all conditions. Based on the results, what is a possible explanation for persistent phospho-EGFR in the mutant cells?

Explanation / Answer

The correct answer might be the option (C) “EGFR anterograde trafficking is disrupted,” since the reversal of phosphorylated EGFR is not done and the phospho-EGFR persisted even after 60’.

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