This superfamily represents SMAD (Mothers against decapentaplegic (MAD) homolog) (also called MH2 for MAD homology 2) domains as well as their structural homologues, such as the transactivation domain of interferon regulatory protein 3 (IRF3), both of which have a β-sandwich structural fold.SMAD domains are found at the carboxy terminus of MAD related proteins such as Smads. SMAD domain proteins are found in a range of species from nematodes to humans. These highly conserved proteins contain an N-terminal MH1 domain that contacts DNA, and is separated by a short linker region from the C-terminal MH2 domain, the later showing a striking similarity to FHA domains. SMAD proteins mediate signalling by the TGF-beta/activin/BMP-2/4 cytokines from receptor Ser/Thr protein kinases at the cell surface to the nucleus. SMAD proteins fall into three functional classes: the receptor-regulated SMADs (R-SMADs), including SMAD1, -2, -3, -5, and -8, each of which is involved in a ligand-specific signalling pathway []; the co-mediator SMADs (co-SMADs), including SMAD4, which interact with R-SMADs to participate in signalling []; and the inhibitory SMADs (I-SMADs), including SMAD6 and -7, which block the activation of R-SMADs and Co-SMADs, thereby negatively regulating signalling pathways [].
FHA and SMAD (MH2) domains share a common structure consisting of a sandwich of eleven β-strands in two sheets with Greek key topology. Forkhead-associated (FHA) domains were originally identified as a sequence profile of about 75 amino acids, whereas the full-length domain is closer to about 150 amino acids. FHA domains are found in transcription factors, kinesin motors, and in a variety of other signalling molecules in organisms ranging from eubacteria to humans. FHA domains are protein-protein interaction domains that are specific for phosphoproteins. FHA-containing proteins function in maintaining cell-cycle checkpoints, DNA repair and transcriptional regulation. FHA domain proteins include the Chk2/Rad53/Cds1 family of proteins that contain one or more FHA domains, as well as a Ser/Thr kinase domain [, , ]. SMAD (Mothers against decapentaplegic (MAD) homologue) domain proteins are found in a range of species from nematodes to humans. These highly conserved proteins contain an N-terminal MH1 domain that contacts DNA, and is separated by a short linker region from the C-terminal MH2 domain, the later showing a striking similarity to FHA domains. SMAD proteins mediate signalling by the TGF-beta/activin/BMP-2/4 cytokines from receptor Ser/Thr protein kinases at the cell surface to the nucleus. SMAD proteins fall into three functional classes: the receptor-regulated SMADs (R-SMADs), including SMAD1, -2, -3, -5, and -8, each of which is involved in a ligand-specific signalling pathway []; the comediator SMADs (co-SMADs), including SMAD4, which interact with R-SMADs to participate in signalling []; and the inhibitory SMADs (I-SMADs), including SMAD6 and -7, which block the activation of R-SMADs and Co-SMADs, thereby negatively regulating signalling pathways []. Domains with this fold are also found as the transactivation domain of interferon regulatory factor 3 (IRF3), which has a weak homology to SMAD domains [], and the N-terminal domain of EssC protein in Staphylococcus aureus.
Smad proteins are signal transducers and transcriptional comodulators of the TGF-beta superfamily of ligands, which play a central role in regulating a broad range of cellular responses, including cell growth, differentiation, and specification of developmental fate, in diverse organisms from Caenorhabditis elegans to humans. Ligand binding to specific transmembrane receptor kinases induces receptor oligomerisation and phosphorylation of the receptor specific Smad protein (R-Smad) in the cytoplasm. The R-Smad proteins regulate distinct signalling pathways. Smad1, 5 and 8 mediate the signals of bone morphogenetic proteins (BMPs), while Smad2 and 3 mediate the signals of activins and TGF-betas. Upon ligand stimulation, R-Smad proteins are phosphorylated at the conserved C-terminal tail sequence, SS*xS* (where S* denotes a site of phosphorylation). The phosphorylated states of R-Smad proteins form heteromeric complexes with Smad4 and are translocated into the nucleus. In the nucleus, the heteromeric complexes function as gene-specific transcription activators by binding to promoters and interacting with transcriptional coactivators. Smad6 and Smad7 are inhibitory Smad proteins that inhibit TGF-beta signalling by interfering with either receptor-mediated phosphorylation or hetero-oligomerisation between Smad4 and R-Smad proteins. Smad proteins comprise two conserved MAD homology domains, one in the N terminus (MH1) and one in the C terminus (MH2), separated by a more variable, proline-rich linker region. The MH1 domain has a role in DNA binding and negatively regulates the functions of MH2 domain, whereas the MH2 domain is responsible for transactivation and mediates phosphorylation-triggered heteromeric assembly between Smad4 and R-Smad [, ]. The MH1 domain adopts a compact globular fold, with four alpha helices, six short beta strands, and five loops. The N-terminal half of the sequence consists of three alpha helices, and the C-terminal half contains all six beta strands, which form two small beta sheets and one beta hairpin. The fourth alpha helix is located in the hydrophobic core of the molecule, surrounded by the N-terminal three alpha helices on one side and by the two small beta sheets and the beta hairpin on the other side. These secondary structural elements are connected with five intervening surface loops. The MH1 domain employs a novel DNA-binding motif, an 11-residue β-hairpin formed by strands B2 and B3, to contact DNA in the major groove. Two residues in the L3 loop and immediately preceding strand B2 also contribute significantly to DNA recognition. The beta hairpin appears to protrude outward from the globular MH1 core [].
Smad proteins are signal transducers and transcriptional comodulators of the TGF-beta superfamily of ligands, which play a central role in regulating a broad range of cellular responses, including cell growth, differentiation, and specification of developmental fate, in diverse organisms from Caenorhabditis elegans to humans. Ligand binding to specific transmembrane receptor kinases induces receptor oligomerisation and phosphorylation of the receptor specific Smad protein (R-Smad) in the cytoplasm. The R-Smad proteins regulate distinct signalling pathways. Smad1, 5 and 8 mediate the signals of bone morphogenetic proteins (BMPs), while Smad2 and 3 mediate the signals of activins and TGF-betas. Upon ligand stimulation, R-Smad proteins are phosphorylated at the conserved C-terminal tail sequence, SS*xS* (where S* denotes a site of phosphorylation). The phosphorylated states of R-Smad proteins form heteromeric complexes with Smad4 and are translocated into the nucleus. In the nucleus, the heteromeric complexes function as gene-specific transcription activators by binding to promoters and interacting with transcriptional coactivators. Smad6 and Smad7 are inhibitory Smad proteins that inhibit TGF-beta signalling by interfering with either receptor-mediated phosphorylation or hetero-oligomerisation between Smad4 and R-Smad proteins. Smad proteins comprise two conserved MAD homology domains, one in the N terminus (MH1) and one in the C terminus (MH2), separated by a more variable, proline-rich linker region. The MH1 domain has a role in DNA binding and negatively regulates the functions of MH2 domain, whereas the MH2 domain is responsible for transactivation and mediates phosphorylation-triggered heteromeric assembly between Smad4 and R-Smad [, ]. The MH1 domain adopts a compact globular fold, with four alpha helices, six short beta strands, and five loops. The N-terminal half of the sequence consists of three alpha helices, and the C-terminal half contains all six beta strands, which form two small beta sheets and one beta hairpin. The fourth alpha helix is located in the hydrophobic core of the molecule, surrounded by the N-terminal three alpha helices on one side and by the two small beta sheets and the beta hairpin on the other side. These secondary structural elements are connected with five intervening surface loops. The MH1 domain employs a novel DNA-binding motif, an 11-residue β-hairpin formed by strands B2 and B3, to contact DNA in the major groove. Two residues in the L3 loop and immediately preceding strand B2 also contribute significantly to DNA recognition. The beta hairpin appears to protrude outward from the globular MH1 core [].
