Feedback Practice Test

A growth factor activates a receptor tyrosine kinase (RTK), causing autophosphorylation and recruitment of a MAPK cascade that activates ERK. Active ERK phosphorylates a docking site on the RTK’s cytosolic tail, reducing adaptor binding and decreasing further MAPK activation while ligand remains present. Cells expressing an RTK mutant lacking the ERK phosphorylation site show prolonged ERK activity after a brief ligand pulse. Which outcome best explains the effect of this feedback on pathway behavior?

In a sensory cell, a ligand-gated Na$^+$ channel opens when odorant binds, depolarizing the membrane. Depolarization opens voltage-gated Ca$^{2+}$ channels, raising cytosolic Ca$^{2+}$. Ca$^{2+}$ activates a kinase that phosphorylates the odorant-gated Na$^+$ channel, decreasing its open probability even if odorant remains bound. When kinase activity is blocked, depolarization lasts longer for the same odorant pulse. Which outcome best illustrates the feedback’s effect on pathway activity?