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Catalytic domain of the Serine/Threonine Kinase, Bone Morphogenetic Protein Type IA Receptor STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. BMPR1a, also called Activin receptor-Like Kinase 3 (ALK3), functions as a receptor for bone morphogenetic proteins (BMPs), which are involved in the regulation of cell proliferation, survival, differentiation, and apoptosis. BMPs are able to induce bone, cartilage, ligament, and tendon formation, and may play roles in bone diseases and tumors. Germline mutations in BMPR1a are associated with an increased risk to Juvenile Polyposis Syndrome, a hamartomatous disorder that may lead to gastrointestinal cancer. BMPR1a may also play an indirect role in the development of hematopoietic stem cells (HSCs) as osteoblasts are a major component of the HSC niche within the bone marrow. BMPR1a belongs to a group of receptors for the TGFbeta family of secreted signaling molecules that includes TGFbeta, BMPs, 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 BMPR1a, 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 BMPR1a 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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