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.
Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, 55 Lake Ave. North, Worcester, Massachusetts 01655, USA.
B M
BM
Chacko
B
B
Qin
J J
JJ
Correia
S S
SS
Lam
M P
MP
de Caestecker
K
K
Lin
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