| A recently cloned inhibin binding protein , InhBP ( p 120 ) , associates strongly with the type IB activin receptor ( Alk 4 ) in a ligand responsive manner and interacts to a lesser extent with other activin and bone morphogenetic protein ( BMP ) type 1 and activin type 2 receptors . ^^^ Activin stimulates the association of InhBP and Alk 4 , and inhibin B , but not inhibin A , interferes with InhBP Alk 4 complex formation . ^^^ |