Receptor Tyrosine Kinases Explained

Receptor tyrosine kinases (RTKs) are a crucial class of cell surface receptors involved in various cellular processes like growth, differentiation, and metabolism. They bind to signaling molecules like growth factors and hormones.

Analyzing the Statements

Let's examine each statement about RTKs to determine which one is incorrect:

• Statement 1: All RTKs are transmembrane proteins with extracellular ligand-binding site and a cytosolic domain.

  This statement is correct. RTKs typically span the cell membrane. They have a part outside the cell that binds the signal molecule (ligand), a part that crosses the membrane, and a part inside the cell (cytosolic domain) which contains the enzymatic tyrosine kinase activity.

• Statement 2: Binding of ligand causes most RTKs to dimerize.

  This statement is correct. A common mechanism for RTK activation is ligand-induced dimerization. When the ligand binds, it often causes two receptor molecules to come together and form a dimer. This dimerization is essential for activating the kinase activity.

• Statement 3: Cytosolic tyrosine kinase phosphorylates a distinct set of tyrosine residues in the cytosolic domain of the dimer.

  This statement is incorrect. The tyrosine kinase activity is not provided by a separate cytosolic enzyme. Instead, the RTK receptor molecule itself contains the tyrosine kinase domain within its cytosolic part. Upon ligand binding and dimerization, this intrinsic kinase activity is activated. The activated receptor then phosphorylates tyrosine residues, often on the partner receptor in the dimer (this is called autophosphorylation) and on other downstream signaling proteins.

• Statement 4: Adapter proteins and enzymes involved in signalling pathways bind to different phosphotyrosine residues via a conserved polypeptide domain called SH2 domain.

  This statement is correct. Once tyrosine residues on the activated RTK (or other substrates) are phosphorylated, these phosphotyrosines serve as binding sites for various intracellular signaling proteins. Many of these proteins contain specific domains, such as the SH2 (Src homology 2) domain, which recognizes and binds to phosphotyrosine residues within a specific sequence context. This binding recruits downstream signaling molecules to the activated receptor complex.

Identifying the Incorrect Statement

Based on the analysis, Statement 3 incorrectly describes the source of tyrosine kinase activity. The activity is inherent to the RTK receptor itself, not a separate cytosolic kinase enzyme phosphorylating the receptor dimer.

Therefore, the incorrect statement is: Cytosolic tyrosine kinase phosphorylates a distinct set of tyrosine residues in the cytosolic domain of the dimer.