PP1 dephosphorylates TGF-beta receptor-1 (TGFBR1), thereby inhibiting TGF-beta signaling. It has not been precisely examined whether PP1 dephosphorylates all TGFBR1 serine and threonine residues phosphorylated by TGFBR2 (Shi et al. 2004). This was inferred from experiments that used a recombinant mouse Smad7 and recombinant human TGFBR1, TGFBR2 and PP1.
phosphoprotein phosphatase activity of TGFB1:p-TGFBR:I-SMAD7:GADD34:PP1:ZFYVE9 [early endosome membrane]