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.
Kretzschmar, M, Liu, F, Hata, A, Doody, J, Massague, J
Hoodless, PA, Haerry, T, Abdollah, S, Stapleton, M, O'Connor, MB, Attisano, L, Wrana, JL
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