Practice Test 4

A peptide ligand L binds receptor tyrosine kinase (RTK) on the plasma membrane, causing receptor dimerization and autophosphorylation of cytosolic tyrosines. An adaptor protein binds these phosphotyrosines and recruits Ras, which becomes GTP-bound. Active Ras triggers a kinase cascade: kinase 1 phosphorylates kinase 2, which phosphorylates kinase 3. Kinase 3 phosphorylates cytosolic target T, changing its activity within 2 minutes. When a drug prevents RTK dimerization, Ras remains GDP-bound and T is not phosphorylated. Which change would most likely allow T phosphorylation even when RTK dimerization is blocked?

A steroid-like ligand cannot cross the plasma membrane and instead binds a cell-surface receptor R. Binding activates an intracellular enzyme that produces IP$_3$ from a membrane lipid. IP$_3$ diffuses to the endoplasmic reticulum (ER) and opens IP$_3$-gated Ca$^{2+}$ channels, increasing cytosolic Ca$^{2+}$. Elevated Ca$^{2+}$ activates a cytosolic protease Q by binding to an inhibitory subunit that dissociates. When ER Ca$^{2+}$ stores are depleted experimentally, ligand binding still increases IP$_3$ levels but Q activity does not increase. Which step is most directly disrupted by ER Ca$^{2+}$ depletion?