TGF-beta Superfamily Receptors
The TGF-beta superfamily of receptors comprises two groups, the type I and the type II serine/threonine kinases. These kinases are type I transmembrane proteins, and the two subgroups are distinguished by the presence of a glycine/serine-rich juxta-membrane domain found in the type I receptors. Whether the type I or the type II receptor binds first is ligand-dependent, and the second type I or type II receptor is then recruited to form a heteromeric signaling complex. A functional receptor complex has one dimeric ligand interacting with two type I and two type II receptors. Type I receptors are referred to as the Activin-like Kinases (ALKs), while the type II receptors are named for the ligands they bind.