Catalytic domain of Insulin Receptor-like Protein Tyrosine Kinases
PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. The InsR subfamily is composed of InsR, Insulin-like Growth Factor-1 Receptor (IGF-1R), and similar proteins. InsR and IGF-1R are receptor PTKs (RTKs) composed of two alphabeta heterodimers. Binding of the ligand (insulin, IGF-1, or IGF-2) to the extracellular alpha subunit activates the intracellular tyr kinase domain of the transmembrane beta subunit. Receptor activation leads to autophosphorylation, stimulating downstream kinase activities, which initiate signaling cascades and biological function. InsR and IGF-1R, which share 84% sequence identity in their kinase domains, display physiologically distinct yet overlapping functions in cell growth, differentiation, and metabolism. InsR activation leads primarily to metabolic effects while IGF-1R activation stimulates mitogenic pathways. In cells expressing both receptors, InsR/IGF-1R hybrids are found together with classical receptors. Both receptors can interact with common adaptor molecules such as IRS-1 and IRS-2. The InsR-like subfamily is part of a larger superfamily that includes the catalytic domains of serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.