Figure 7.

Activation segments of three classes of RD (arginine aspartic acid) kinases. Insulin receptor kinase (IRK), fibroblast growth factor receptor 1 (FGFR1), platelet-derived growth factor receptor-β (PDGFRβ), and colony-stimulating factor 1 receptor (CSF1R) represent the receptor tyrosine kinases. Janus kinases (Jaks) and cyclic-AMP-dependent protein kinase (cAPK) represent non-receptor tyrosine kinases and serine/threonine kinase respectively. These kinases are activated by autophosphorylation of conserved tyrosine/threonine residues (underlined) (11). This segment is located between the highly conserved triplets DFG of kinase subdomain VII and APE of kinase subdomain VIII. The activation segment of IRK which causes autoinhibition has five extra amino acids and adopts a different conformation of the segment in cAPK (11, 58).