Catalytic domain of the Serine/Threonine Kinases, Activin Type I Receptor and Activin receptor-Like Kinase 1
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. ACVR1, also called Activin receptor-Like Kinase 2 (ALK2), and ALK1 act as receptors for bone morphogenetic proteins (BMPs) and they activate SMAD1/5/8. ACVR1 is widely expressed while ALK1 is limited mainly to endothelial cells. The specificity of BMP binding to type I receptors is affected by type II receptors. ACVR1 binds BMP6/7/9/10 and can also bind anti-Mullerian hormone (AMH) in the presence of AMHR2. ALK1 binds BMP9/10 as well as TGFbeta in endothelial cells. A missense mutation in the GS domain of ACVR1 causes fibrodysplasia ossificans progressiva, a complex and disabling disease characterized by congenital skeletal malformations and extraskeletal bone formation. ACVR1 belongs to a group of receptors for the TGFbeta family of secreted signaling molecules that includes TGFbeta, BMPs, activins, growth and differentiation factors, and AMH, among others. These receptors contain an extracellular domain that binds ligands, a single transmembrane (TM) region, and a cytoplasmic catalytic kinase domain. Type I receptors, like ACVR1 and ALK1, are low-affinity receptors that bind ligands only after they are recruited by the ligand/type II high-affinity receptor complex. Following activation, they start intracellular signaling to the nucleus by phosphorylating SMAD proteins. Type I receptors contain an additional domain located between the TM and kinase domains called the GS domain, which contains the activating phosphorylation site and confers preference for specific SMAD proteins. The ACVR1 subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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