Transforming Growth Factor beta (TGF-B) superfamily ligands exert their biological activities via binding to heteromeric receptor complexes consisting of two types (I and II) of serine/threonine kinases. Type II receptors are constitutively active kinases that phosphorylate type I receptors upon ligand binding. In turn, activated type I kinases phosphorylate downstream signaling molecules including the various Smad proteins. Transmembrane proteoglycans, including the type III receptor (betaglycan) and endoglin, can bind and present some of the TGF-B superfamily ligands to type I and II receptor complexes and enhance their cellular responses. Seven type I receptors [also termed activin receptor like kinase (ALK)] and five type II receptors have been isolated from mammals. ALK2, 3, 4, 5, and 6 are also known as Activin R1A, BMPR1A, Activin R1B, TGF-B R1, and BMPR1B, respectively, reflecting their ligand preferences. ALK7 contains a cysteine-rich domain with conserved cysteine spacing in the extracellular region that is shared by other type I receptors, and a glycine-and-serine-rich domain (the GS domain) preceding the kinase domain.
Intended for research use only. Not for use in human, therapeutic, or diagnostic applications.