Activin RIB/ALK-4 Proteins and Enzymes

Receptors for the TGF-beta superfamily comprise 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. Either a type I or type II receptor may initially bind ligand, followed by recruitment of its alternate-type receptor counterpart to generate a heteromeric signaling complex. Type I receptors are referred to as the Activin Receptor-like Kinases (ALKs), while the type II receptors are named for the ligands they bind.