Here's why:

* Tyrosine Kinase Activity: RTKs are characterized by their intrinsic tyrosine kinase activity. This means they can directly add phosphate groups (phosphorylate) to tyrosine residues within proteins.

* Membrane Localization: As the name suggests, RTKs are embedded within the cell membrane. They possess an extracellular domain that binds to specific ligands (signaling molecules) and an intracellular domain with the tyrosine kinase activity.

* Signal Transduction: When a ligand binds to the extracellular domain, it triggers a conformational change in the RTK, leading to the activation of its kinase activity. This phosphorylation event initiates a signaling cascade within the cell, ultimately leading to various cellular responses.

Examples of RTKs:

* Insulin receptor

* Epidermal growth factor receptor (EGFR)

* Platelet-derived growth factor receptor (PDGFR)

Important Note: While RTKs are the most common type of membrane receptors with tyrosine kinase activity, there are other types of receptor kinases that phosphorylate different amino acids. For example, serine/threonine kinases phosphorylate serine or threonine residues.