Advanced Placement (AP) Biology Practice Exam

What characterizes receptor tyrosine kinases?

1. They bind only steroid molecules

2. They dimerize and phosphorylate tyrosines on proteins

3. They are involved in dephosphorylation processes

4. They relay signals through second messengers

The correct answer is: They dimerize and phosphorylate tyrosines on proteins

Receptor tyrosine kinases (RTKs) are characterized by their ability to dimerize and phosphorylate tyrosine residues on themselves as well as on target proteins. When a ligand binds to the extracellular domain of an RTK, it causes the receptors to form dimers—a process known as dimerization. This dimerization is crucial because it activates the kinase activity of the receptors, leading to the phosphorylation of specific tyrosine amino acid residues in the cytoplasmic domain of the receptors and on other proteins. This phosphorylation initiates a cascade of downstream signaling pathways that play significant roles in various cellular processes such as growth, differentiation, metabolism, and apoptosis. These pathways often involve multiple proteins and can lead to various cellular responses. Other options do not accurately describe the key features of receptor tyrosine kinases. For instance, RTKs specifically interact with peptide hormones or growth factors rather than only steroid molecules, which is a characteristic of a different class of receptors. Additionally, while there are proteins involved in dephosphorylation processes, RTKs are primarily associated with phosphorylation, serving as kinases rather than phosphatases. Lastly, although RTKs can initiate signaling cascades that might involve second messengers, the defining