Receptor Tyrosine Kinases (RTKs)

Concept and Structure of RTKs

Receptor Tyrosine Kinases (RTKs) are a major class of cell surface receptors that play a critical role in cellular signal transduction. They possess intrinsic enzymatic activity, specifically the ability to phosphorylate tyrosine residues on themselves and other proteins.

• Definition: RTKs are single-pass transmembrane receptors with an intracellular tyrosine kinase domain.

• Function: RTKs phosphorylate tyrosine (Tyr) residues on specific target proteins, initiating various signaling cascades.

Domains of RTK Monomers

• Extracellular Ligand-Binding Domain: Binds specific signaling molecules (ligands) outside the cell.

• Transmembrane Domain: Anchors the receptor in the cell membrane.

• Intracellular Tyrosine Kinase Domain: Catalyzes phosphorylation of tyrosine residues.

Diagram:

• Illustrates the three main domains: extracellular ligand-binding, transmembrane, and intracellular tyrosine kinase domains.

Dimerization and Autophosphorylation of RTKs

RTKs are typically monomeric in the absence of ligand but dimerize upon ligand binding, which is essential for their activation.

• Dimerization: Ligand binding induces two RTK monomers to form a dimer.

• Autophosphorylation: The kinase domains phosphorylate each other on specific tyrosine residues (cross-phosphorylation).

• Result: Phosphorylated tyrosines serve as docking sites for downstream signaling proteins.

Example: RTK Dimerization and Autophosphorylation

1. Ligand binds to RTK monomers.

2. RTKs dimerize.

3. Kinase domains cross-phosphorylate tyrosine residues.

4. Phosphorylated RTKs recruit and activate downstream signaling proteins.

SH2 Domains Bind Phosphorylated Tyrosine

SH2 (Src Homology 2) domains are protein modules that specifically recognize and bind to phosphorylated tyrosine residues on activated RTKs.

• Mechanism: Proteins with SH2 domains bind directly to phosphorylated tyrosines on RTKs, facilitating the assembly of signaling complexes.

• Function: This interaction transmits the signal downstream, leading to diverse cellular responses.

Diagram:

• Shows SH2-containing proteins binding to phosphorylated RTKs and mediating signal transduction.

Practice Questions (Concept Review)

• Monomeric receptors are almost always dimerized upon ligand binding.

• RTKs contain intrinsic kinase activity and do not require G proteins for activation.

• SH2 domains are critical for recruiting signaling proteins to activated RTKs.

Key Properties of RTKs

• Contain a cytoplasmic tyrosine kinase domain.

• Undergo ligand-induced dimerization and autophosphorylation.

• Phosphorylated tyrosines serve as binding sites for SH2 domain-containing proteins.

• Initiate multiple signaling pathways, including cell growth, differentiation, and metabolism.

Summary Table: RTK Activation and Signal Transduction

Additional info: RTKs are involved in many physiological processes and are frequently implicated in cancer when mutated or overactive.