Activated type I receptor kinase directly phosphorylates two of the C-terminal serine residues of SMAD1, SMAD5 or SMAD8. Binding of these R-SMADs to the L45 loop of the type I receptor is critical for this event.
Souchelnytskyi, S, ten Dijke, P, Heldin, CH, Rönnstrand, L
transmembrane receptor protein serine/threonine kinase activity of TGFB1:p-TGFBR:ZFYVE9:SMAD2/3 [early endosome membrane]
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