Advanced Placement (AP) Biology Practice Exam

What commonly initiates the action of receptor tyrosine kinases?

1. The binding of a signaling molecule

2. The presence of a calcium ion

3. The dimerization of kinases

4. The activation of phosphatases

The correct answer is: The binding of a signaling molecule

The initiation of receptor tyrosine kinases (RTKs) is primarily triggered by the binding of a signaling molecule, commonly referred to as a ligand. When a ligand, such as a growth factor, attaches to the extracellular domain of an RTK, it leads to a conformational change in the receptor. This change is crucial as it promotes the dimerization of two RTK molecules, which subsequently activates their intrinsic kinase activity. This dimerization allows the associated tyrosine residues on each receptor to be phosphorylated, a process known as autophosphorylation. This phosphorylation serves as a docking site for various signaling proteins, which can then propagate the signal downstream, leading to cellular responses like proliferation, differentiation, or migration. Other options, such as the presence of a calcium ion, the dimerization of kinases, or the activation of phosphatases, do not serve as primary initiators of RTK action. While calcium ions can play a role in other signaling pathways, they are not directly involved in initiating RTK signaling. The dimerization of kinases refers to a state that occurs post-ligand binding and is not the initiating factor, and phosphatases generally function to remove phosphate groups, countering the