ALK4 (Activin R1B)

Activin RIB, also known as ALK4, is a glycosylated 58 kDa type I transmembrane receptor that belongs to the superfamily of TGFβ serine/threonine kinase receptors. Activin RIB associates with Activin RIIB to form a receptor complex for activin and inhibin molecules. These ligands bind to Activin RIIB which then associates with, and phosphorylates, the cytoplasmic domain of Activin RIB to initiate signal transduction. Mature mouse Activin RIB consists of a 103 amino acid (aa) extracellular domain (ECD), a 23 aa transmembrane segment, and a 356 aa cytoplasmic region that includes the kinase domain. Within the ECD, mouse Activin RIB shares 93% and 98% aa sequence identity with human and rat Activin RIB, respectively. It shares 23% 38% aa sequence identity with other mouse type I receptors Activin RIA, BMPRIA, BMPRIB, and TGFβ RI. Activin receptor signaling is modulated by the direct interaction of Activin RIB with cripto or inhibin binding protein. Activin RIB is excluded from the signaling complex if Activin RIIB first binds inhibin and betaglycan (8) . Activin RIB functions in a wide variety of growth and differentiation processes, including embryonic cell fate and axis determination, cell proliferation, apoptosis, and tumorigenesis.