ALK6 (BMP R1B)

Cellular responses to bone morphogenetic proteins (BMPs) have been shown to be mediated by the formation of heterooligomeric complexes of the type I and type II serine/threonine kinase receptors. BMP receptor IB (BMPRIB), also known as activin receptor-like kinase (ALK)6, 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 BMPRIA, BMPRIB/ ALK6, and ActRI/ ALK2) can independently bind the various BMP family proteins in the absence of type II receptors. Recombinant soluble BMPRIB binds BMP4 with high-affinity in solution and is a potent BMP4 antagonist in vitro. BMPRIB is expressed in various tissues during embryogenesis. In adult tissues, BMPRIB is only found in the brain. The extracellular domain of BMPRIB shares little amino acid sequence identity with the other mammalian ALK type I receptor kinases, but the cysteine residues are conserved. Human and mouse BMPRIB are highly conserved and share 98% amino acid sequence identity.