Transforming growth factor-β (TGF-β) and activin exert their effects by binding to heteromeric complexes of type I and type II receptors. The type II receptors for TGF-β and activin are transmembrane serine-threonine kinases; a series of related receptors, denoted activin receptor-like kinase (ALK) 1 to 5, have recently been identified, and ALK-6 is described here. ALK-5 has been shown to be a functional TGF-β type I receptor. A systematic analysis revealed that most ALKs formed heteromeric complexes with the type II receptors for TGF-β and activin after overexpression in COS cells; however, among the six ALKs, only ALK-5 was a functional TGF-β type I receptor for activation of plasminogen activator inhibitor-1, and only ALK-2 and ALK-4 bound activin with high affinity.
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