Receptor Tyrosine Kinases

Receptor tyrosine kinases (RTKs) are cell surface receptors that bind ligands, typically growth factors, and catalyze phosphate transfer from ATP to tyrosine residues on protein substrates. RTKs transmit molecular signals through cells to regulate growth, differentiation, survival, and migration, making them critical for cellular communication and control.

RTKs activate through two main mechanisms. In ligand-induced dimerization, a dimeric ligand like PDGF binds two RTKs simultaneously to bring them closer. In receptor-mediated dimerization, a monomeric ligand like EGF binds each RTK individually and induces conformational changes that crosslink the two receptors together, enabling activation.

Trans-autophosphorylation occurs after RTK dimerization, where the kinase domain of one monomer phosphorylates tyrosine residues on the activation loop of the second RTK and vice versa. This phosphorylation activates the kinase domain by pulling the activation loop away, exposing the substrate protein-binding region and allowing RTKs to phosphorylate downstream signaling proteins.

Phosphorylated tyrosine residues on RTKs serve as binding sites for intracellular signaling proteins containing Src homology 2 (SH2) or phospho-tyrosine binding (PTB) domains. These proteins bind the phosphotyrosines and become phosphorylated and activated by RTKs, allowing them to propagate signals through the pi3k mtor akt signaling pathway and other downstream cascades.

RTKs contain three main structural domains: an extracellular ligand-binding domain that recognizes growth factors, a transmembrane domain that anchors the receptor in the cell membrane, and a cytosolic tail with intrinsic kinase activity that catalyzes phosphorylation of intracellular target proteins.

Phosphotyrosines on activated RTKs serve as binding sites for Cbl, an SH2 domain-containing protein that causes ubiquitin-mediated receptor degradation. This mechanism switches off RTK signaling by removing the activated receptors from the cell surface, providing negative feedback to prevent excessive or prolonged signal transmission.

RTKs remain monomeric and inactive in the absence of ligand binding because their kinase domains are spatially separated and cannot phosphorylate each other. Ligand binding brings two RTK monomers into close proximity through dimerization, positioning their kinase domains so they can perform trans-autophosphorylation and become catalytically active.