PMID: 11224571 , Related PDB id: 1G88
Chacko BM, Qin B, Correia JJ, Lam SS, de Caestecker MP, Lin K
The L3 loop and C-terminal phosphorylation jointly define Smad protein trimerization.
Nat Struct Biol. 2001 Mar;8(3):248-53.
Smad proteins mediate the transforming growth factor beta responses. C-terminal phosphorylation of R-Smads leads to the recruitment of Smad4 and the formation of active signaling complexes. We investigated the mechanism of phosphorylation-induced Smad complex formation with an activating pseudo-phosphorylated Smad3. Pseudo-phosphorylated Smad3 has a greater propensity to homotrimerize, and recruits Smad4 to form a heterotrimer containing two Smad3 and one Smad4. The trimeric interaction is mediated through conserved interfaces to which tumorigenic mutations map. Furthermore, a conserved Arg residue within the L3 loop, located near the C-terminal phosphorylation sites of the neighboring subunit, is essential for trimerization. We propose that the phosphorylated C-terminal residues interact with the L3 loop of the neighboring subunit to stabilize the trimer interaction.