Signaling by TGF-beta Receptor Complex

TGF beta (TGFB1) is secreted as a homodimer, and as such it binds to TGF beta receptor II (TGFBR2), inducing its dimerization. Binding of TGF beta enables TGFBR2 to form a stable hetero-tetrameric complex with TGF beta receptor I homodimer (TGFBR1). TGFBR2 acts as a serine/threonine kinase and phosphorylates serine and threonine residues within the short GS domain (glycine-serine rich domain) of TGFBR1.

The phosphorylated heterotetrameric TGF beta receptor complex (TGFBR) internalizes into clathrin coated endocytic vesicles where it associates with the endosomal membrane protein SARA. SARA facilitates the recruitment of cytosolic SMAD2 and SMAD3, which act as R-SMADs for TGF beta receptor complex. TGFBR1 phosphorylates recruited SMAD2 and SMAD3, inducing a conformational change that promotes formation of R-SMAD trimers and dissociation of R-SMADs from the TGF beta receptor complex.

In the cytosol, phosphorylated SMAD2 and SMAD3 associate with SMAD4 (known as Co-SMAD), forming a heterotrimer which is more stable than the R-SMAD homotrimers. R-SMAD:Co-SMAD heterotrimer translocates to the nucleus where it directly binds DNA and, in cooperation with other transcription factors, regulates expression of genes involved in cell differentiation, in a context-dependent manner.

The intracellular level of SMAD2 and SMAD3 is regulated by SMURF ubiquitin ligases, which target R-SMADs for degradation. In addition, nuclear R-SMAD:Co-SMAD heterotrimer stimulates transcription of inhibitory SMADs (I-SMADs), forming a negative feedback loop. I-SMADs bind the phosphorylated TGF beta receptor complexes on caveolin coated vesicles, derived from the lipid rafts, and recruit SMURF ubiquitin ligases to TGF beta receptors, leading to ubiquitination and degradation of TGFBR1. Nuclear R-SMAD:Co-SMAD heterotrimers are targets of nuclear ubiquitin ligases which ubiquitinate SMAD2/3 and SMAD4, causing heterotrimer dissociation, translocation of ubiquitinated SMADs to the cytosol and their proteasome-mediated degradation. For a recent review of TGF-beta receptor signaling, please refer to Kang et al. 2009.