The L3 loop and C-terminal phosphorylation jointly define Smad protein trimerization.
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.
Chacko
B M
BM
Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, 55 Lake Ave. North, Worcester, Massachusetts 01655, USA.
Qin
B
B
Correia
J J
JJ
Lam
S S
SS
de Caestecker
M P
MP
Lin
K
K
D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D002850:Chromatography, Gel; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D005786:Gene Expression Regulation; D006801:Humans; D008954:Models, Biological; D008958:Models, Molecular; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011485:Protein Binding; D020836:Protein Structure, Quaternary / Q000187:drug effects; D017434:Protein Structure, Tertiary / Q000187:drug effects; D021122:Protein Subunits; D015398:Signal Transduction / Q000187:drug effects; D051900:Smad3 Protein; D051901:Smad4 Protein; D055672:Static Electricity; D013431:Sulfates / Q000378:metabolism; D015534:Trans-Activators / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D004268:DNA-Binding Proteins; D010446:Peptide Fragments; D021122:Protein Subunits; C497790:SMAD3 protein, human; C497796:SMAD4 protein, human; D051900:Smad3 Protein; D051901:Smad4 Protein; D013431:Sulfates; D015534:Trans-Activators; D016212:Transforming Growth Factor beta