The transforming growth factor- β (TGF-β) superfamily is a family of proteins that is involved in regulating and mediating processes at the cellular level, including cell proliferation, differentiation, motility, adhesion and apoptosis, as well as processes at the tissue and organism level, including development, wound healing, fibrosis and angiogenesis. The TGF-β superfamily consists of ligands and receptors that all signal, at least in part, through well characterized downstream mediators termed the Smads.
The TGF-β1 ligand is the founding member of the TGF-β superfamily of proteins that includes TGF-βs, bone morphogenetic proteins (BMPs), activins, growth differentiation factors (GDFs), and Müllerian inhibiting substance (MIS). TGF-β1 and all other TGF-β superfamily ligands are dimers that are held together by three disulfide bonds that are part of a characteristic structure called a cysteine knot. Secreted TGF-β is a latent complex that is composed of the TGF-β dimer associated with the latency associated peptide (LAP) and the latent TGF-β binding protein (LTBP). The latent complex is proteolytically cleaved in the extracellular space resulting in the release of the TGF-β dimer, allowing TGF-β to bind cell surface receptors. The cell surface TGF-β superfamily receptors are classified as either type I receptors, type II receptors, or co-receptors.
In mammals, there are seven type I TGF-β superfamily receptors called activin receptor like kinases (ALK) 1-7 in the TGF-β superfamily. The ALKs are approximately 55 kD, are serine/threonine kinases, and have relatively short extracellular domains. The cytoplasmic domain of the type I receptors has a 30 amino acid domain named the GS domain, which is characterized by the sequence SGSGSG. Phosphorylation of the GS domain by the TGF-β superfamily type II receptors results in activation of the type I receptors. Downstream of the GS domain is the kinase domain, which phosphorylates its substrates on serines. Within the kinase domains of type I receptors, there is a region called the L45 loop, which is involved in substrate recognition.
There are five type II receptors in the TGF-β superfamily, the TGF-β type II receptor (TβRII), Activin type II receptor (ActRII), Activin type II receptor B (ActRIIB), BMP type II receptor (BMPRII), and anti-Müllerian hormone receptor (AMHR). These receptors are approximately 70 kDa, are serine/threonine kinases, have short extracellular domains, and autophosphorylate and phosphorylate the type I receptors on serines and threonines. The regulatory region of the type II receptors has two serines that when phosphorylated promote or inhibit the activity of the type II receptors.
In the TGF-β superfamily there are also co-receptors, the type III TGF-β receptor (TβRIII), endoglin, RGMa, DRAGON (RGMb), and hemojuvelin/HFE2 (RGMc). TβRIII and endoglin are transmembrane co-receptors, while RGMa, RGMb, and RGMc lack transmembrane domains and are glycosylphosphatidylinositol (GPI)-linked to the cell membrane and all possess a large extracellular domain. The cytoplasmic domains of the transmembrane co-receptors, TβRIII and endoglin, are serine/threonine rich, have a PDZ binding domain, and lack catalytic activity.
Smads mediate signaling downstream of the TGF-β signaling receptors. There are three classes of Smads: (i) the receptor Smads (R-Smads), Smads 1/2/3/5/8, (ii) the coSmad, Smad 4, (iii) the inhibitory Smads (I-Smads), Smads 6/7. There are two major pathways downstream of the TGF-β superfamily receptors: Smads 1/5/8 downstream of ALK 1, 2, 3, and 6 and Smads 2/3 downstream of ALK 4, 5, and 7. The Smads possess two highly conserved domains, the MH1and the MH2 domains, and a variable linker region. The MH1 domain is conserved between the R-Smads and Smad 4. The MH1 domain basally acts as an inhibitor of MH2 activity by preventing association with Smad 4. However, upon ligand stimulation the MH1 domain releases the MH2 domain and is able to bind DNA. The Smad MH2 domain is phosphorylated by the receptors and is responsible for protein-protein interactions with receptors and Smad4. In addition, the MH2 domain of Smad4 activates transcription, while the MH2 domain of Smad 6/7 is required for inhibition.