Which of the following is an example of a receptor tyrosine kinase (RTK) ligand?
Receptor Tyrosine Kinase ligands are typically soluble growth factors like EGF, FGF, PDGF, VEGF, Insulin, HGF forming dimers bridging receptors. Fibroblast Growth Factor binds FGFR1-4 extracellular IgII-III domains causing dimerization, trans-autophosphorylation of activation loop tyrosines Y653/654 activating Ras-MAPK, PI3K-AKT, PLCγ signaling driving proliferation, survival, chemotaxis. This mitogenic RTK signaling contrasts Hedgehog which signals via Patched-Smoothened GPCR-like seven-pass mechanism not involving tyrosine kinase, Wnt via Frizzled-LRP beta-catenin, and Delta via Notch proteolytic S2 S3 cleavage producing NICD. Therefore FGF exemplifies classic RTK ligand paradigm used in developmental induction including limb AER maintenance.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: FGF as prototypic receptor tyrosine kinase ligand for induction.