Receptor Tyrosine Kinase
Receptor Tyrosine Kinase (RTK) refers to a family of cell surface receptors, which are the most common type of enzyme-linked receptors. RTKs are important for regulation of cell growth and reproduction. Example of ligands for these receptors include insulin, Epidermal growth factor (EGF), Fibroblast growth factor (FGF), Vascular endothelial growth factor (VEGF), cytokines etc. Cancer is often associated with problems in growth factor signalling which involve this family of receptors.
RTKs exists as 2 separate monomers in the inactive state and 1 linked dimer in the activated state. Each individual monomer is composed of 1 alpha-helix transmembrane region, which consists of one extracellular ligand binding site and one intracellular tail with multiple tyrosine residues.
Mechanism of Action
Before the binding of the ligand, RTK is in the inactive monomer form. When the signal molecule binds to the extracellular receptor site of the RTK, it causes the monomers to associate with each other, to form a cross-linked dimer. Dimerization will activate RTK to cross-phosphorylate or autophosphorylate, where each intracellular tyrosine kinase tail will phosphorylate the multiple tyrosine residues on the other monomer. Phosphorylated tyrosine residues will then act as docking sites for other intracellular signalling proteins to bind to RTK. This will cause a conformational change in the proteins and each relay protein will then trigger a signal transduction pathway, eventually leading to a cellular response. Termination of the signal happens due to protein tyrosine phosphatases which will remove the phosphate groups on the tyrosine residues of RTK, or by endocytosis of the ligand.