Figure 1.

Schematic of Smad protein structure. A general schematic of the structure of a human Smad protein is shown. Smad proteins consist of two domains (based on sequence conservation among identified Smad proteins) termed MH₁ and MH₂ (for MAD homology domain) located in their amino- and carboxyl-terminal halves, respectively. These regions are separated by a more variable, proline-rich domain known as the linker region. Certain signaling or receptor-associated Smads, such as Smad1 and Smad2, have a series of serine residues (termed the SSXS motif) present in their carboxyl termini, which are the sites of type 1 receptor-mediated phosphorylation. Human Smad4 and Smad7 do not possess the SSXS sequence.