This domain represents the N terminus of LIP1, a leucine-rich-repeat protein that interacts with kinase LKB1/STK11, implicated in Peutz-Jeghers syndrome. LIP1 (LKB1 serine/threonine kinase interacting protein 1), also known as STK11IP, interacts with the TGF-beta-regulated transcription factor SMAD4 to form a LKB1-LIP1-SMAD4 ternary complex. Mutations in SMAD4 lead to juvenile polyposis, suggesting a mechanistic link between these two diseases [, ].
This group represents zinc finger FYVE domain-containing proteins SARA and endofin.SARA (also known as zinc finger FYVE domain-containing protein 9) is a component of the TGFbeta pathway that functions to recruit Smad2 to the TGFbeta receptor []. Endofin (zinc finger FYVE domain-containing protein 16) facilitates TGF-beta signaling by bringing Smad4 to the proximity of the receptor complex []. Both are involved in regulating membrane trafficking in the endosomal pathway [, ].
This entry includes Sec8 from fungi and EXOC4 from animals. Sec8 is a component of the exocyst complex essential for targeting exocytic vesicles to specific docking sites on the plasma membrane []. In fission yeasts, the exocyst complex is also required for polarized cell growth and division septum assembly []. EXOC4 has been shown to regulate N-cadherin expression by controlling Smad3 and Smad4 expression through CREB binding protein, thereby mediating the epithelial-mesenchymal transition and early embryonic development [].
Spectrins are involved in the support of general membrane integrity, stabilisation of cell-cell interactions, axonal growth, normal functioning of the Golgi complex and organisation of synaptic vesicles [, , ]. Spectrin is a tetrameric actin cross-linking protein, which contains two alpha and two beta subunits. Two genes for alpha-spectrin [, , ]and five for beta-spectrin have been identified in both mice and humans, each of which is alternatively spliced to produce multiple spectrin isoforms [, ]. Beta-spectrins are more diverse than alpha-spectrin, and include mammalian erythrocytic beta-spectrin, non-erythroid beta-spectrin/Fodrin (beta-G, the general form of beta-spectrin expressed in multiple tissues), a novel beta-G spectrin (ELF1-4) that lacks the C-terminal PH (pleckstrin homology) domain, the brain-specific SPTBN2, and beta-V spectrin.ELF (), a modulator of the Smad adaptor proteins involved in the TGF-beta signalling pathway, was originally identified from endodermal stem/progenitor cells committed to foregut lineage [, ]. ELF is a beta-spectrin that is important for distinct functional membrane generation, protein sorting, cell adhesion and the development of a polarized differentiated epithelial cell [, ]. ELF-deficient mice display disruption of transforming growth factor-beta (TGF-beta) signalling by Smad proteins []. Evidence from null mutants of ELF confirms that ELF is a novel beta-G spectrin and not an isoform of beta-spectrins []. Aberrations in Elf's involvement in Smad4 localization and subsequent activation of Smad4 could result in tumourigenesis.
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 [].
RING finger protein 11 (RNF11) is an E3 ubiquitin-protein ligase that acts both as an adaptor and a modulator of itch-mediated control of ubiquitination events underlying membrane traffic. It is the downstream of an enzymatic cascade for the ubiquitination of specific substrates. It is also a molecular adaptor of homologous to E6-associated protein C terminus (HECT)-type ligases []. RNF11 has been implicated in the regulation of several signaling pathways. It enhances the transforming growth factor receptor (TGFR) signaling by both abrogating Smurf2-mediated receptor ubiquitination and by promoting the Smurf2-mediated degradation of AMSH (associated molecule with the SH3 domain of STAM), a de-ubiquitinating enzyme that enhances transforming growth factor-beta (TGF-beta) signalling and epidermal growth factor receptor (EGFR) endosomal recycling [, ]. It also acts directly on Smad4 to enhance Smad4 function, and plays a role in prolonged TGF-beta signalling []. Moreover, RNF11 functions as a critical component of the A20 ubiquitin-editing protein complex that negatively regulates tumor necrosis factor (TNF)-mediated nuclear factor (NF)-kappaB activation []. It also interacts with Smad anchor for receptor activation (SARA) and the endosomal sorting complex required for transport (ESCRT)-0 complex, thus participating in the regulation of lysosomal degradation of EGFR []. Furthermore, RNF11 acts as a novel GGA cargo actively participating in regulating the ubiquitination of the GGA protein family []. In addition, RNF11 functions together with TAX1BP1 to target TANK-binding kinase 1 (TBK1)/IkappaB kinase IKKi, and further restricts antiviral signaling and type I interferon (IFN)-beta production []. RNF11 contains an N-terminal PPPY motif that binds WW domain-containing proteins such as AIP4/itch, Nedd4 and Smurf1/2 (SMAD-specific E3 ubiquitin-protein ligase 1/2), and a C-terminal C3H2C3-type RING-H2 finger that functions as a scaffold for the coordinated transfer of ubiquitin to substrate proteins together with the E2 enzymes UbcH527 and Ubc13.
Mammalian dwarfins are phosphorylated in response to transforming growth factor beta and are implicated in control of cell growth [, ]. The dwarfin family also includes the Drosophila protein MAD that is required for the function of decapentaplegic (DPP) and may play a role in DPP signalling. Drosophila Mad binds to DNA and directly mediates activation of vestigial by Dpp [, ]. This domain is also found in nuclear factor I (NF-I) or CCAAT box-binding transcription factor (CTF) [].This entry represents the SMAD (Mothers against decapentaplegic (MAD) homologue) (also called MH2 for MAD homology 2) domain found at the carboxy terminus of MAD related proteins such as Smads. This domain is separated from the MH1 domain by a non-conserved linker region. The MH2 domain mediates interaction with a wide variety of proteins and provides specificity and selectivity to Smad function and also is critical for mediating interactions in Smad oligomers. Unlike MH1, MH2 does not bind DNA. The well-studied MH2 domain of Smad4 iscomposed of five alpha helices and three loops enclosing a beta sandwich. Smads are involved in the propagation of TGF-beta signals by direct association with the TGF-beta receptor kinase which phosphorylates the last two Ser of a conserved 'SSXS' motif located at the C terminus of MH2 [, , ].
