Catalytic domain of the Serine/Threonine Kinases, Transforming Growth Factor beta family Type I Receptors
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. This subfamily is composed of type I receptors for the TGFbeta family of secreted signaling molecules including 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 are low-affinity receptors that bind ligands only after they are recruited by the ligand/type II high-affinity receptor complex. Following activation through trans-phosphorylation by type II receptors, 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. They are inhibited by the immunophilin FKBP12, which is thought to control leaky signaling caused by receptor oligomerization in the absence of ligand. The TGFbR-I 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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