Catalytic domain of the Serine/Threonine Kinases, Transforming Growth Factor beta Type I Receptor and Activin Type IB/IC Receptors
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. TGFbR1, also called Activin receptor-Like Kinase 5 (ALK5), functions as a receptor for TGFbeta and phoshorylates SMAD2/3. TGFbeta proteins are cytokines that regulate cell growth, differentiation, and survival, and are critical in the development and progression of many human cancers. Mutations in TGFbR1 (and TGFbR2) can cause aortic aneurysm disorders such as Loeys-Dietz and Marfan syndromes. ACVR1b (also called ALK4) and ACVR1c (also called ALK7) act as receptors for activin A and B, respectively. TGFbR1, ACVR1b, and ACVR1c belong to a group of receptors for the TGFbeta family of secreted signaling molecules that includes TGFbeta, bone morphogenetic proteins, activins, growth and differentiation factors, and anti-Mullerian hormone, 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 TGFbR1, ACVR1b, and ACVR1c, 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 TGFbR1 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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