ALK3 (BMP R1A)

Cellular responses to bone morphogenetic proteins (BMPs) have been shown to be mediated by the formation of hetero-oligomeric complexes of the type I and type II serine/threonine kinase receptors. BMP receptor 1A (BMPR1A), also known as activin receptor-like kinase (ALK3), is one of seven known type I serine/threonine kinases that are required for the signal transduction of TGFβ family cytokines. In contrast to the TGFβ receptor system in which the type I receptor does not bind TGFβ in the absence of the type II receptor, type I receptors involved in BMP signaling (including BMPR1A, BMPR1B/ ALK6, and ActR1/ ALK2) can independently bind the various BMP family proteins in the absence of type II receptors. Recombinant soluble BMPR1A binds BMP4 with high affinity in solution and is a potent BMP4 antagonist in vitro. BMPR1A is ubiquitously expressed during embryogenesis. In adult tissues, BMPR1A mRNA is also widely distributed with the highest expression levels found in skeletal muscle. The extracellular domain of BMPR1A shares little amino acid sequence identity with the other mammalian ALK type I receptor kinases, but the cysteine residues are conserved. Human and mouse BMPR1A are highly conserved and share 98% sequence identity.