Mammalian Media related proteins

The Smad4/DPC4 tumour suppressor is inactivated in nearly half of pancreatic carcinomas, and to a lesser extent in a variety of other cancers. Smad4 is important because it is the dimerization partner of the other Smads. Smad4/DPC4, and the related tumour suppressor Smad2, belong to the SMAD family of proteins which mediate cell surface to nucleus signaling by the TGF-beta/activin/BMP-2/4 cytokine superfamily; signals from receptor Ser/Thr protein kinases at the cell surface are sent to the nucleus. SMAD proteins, which are phosphorylated by the activated receptor, propagate the signal, in part, through homo- and hetero-oligomeric interactions. Smad4/DPC4 is critical because it is the shared hetero-oligomerization partner for the other SMADs. The conserved carboxy-terminal domains of SMADs are sufficient for inducing most of the ligand-specific effects, and are the primary targets of tumorigenic inactivation. The crystal structure of the C-terminal domain (CTD) of the Smad4/DPC4 tumour suppressor was determined at 2.5 A resolution. The structure reveals that the Smad4/DPC4 CTD forms a crystallographic trimer through a conserved protein-protein interface, to which the majority of the tumour-derived missense mutations map. These mutations disrupt homo-oligomerization in vitro and in vivo, indicating that the trimeric assembly of the Smad4/DPC4 CTD is critical for signaling and is disrupted by tumorigenic mutations. A heterohexamer model is suitable from a structural perspective for Mad protein hetero-oligomerization. The heterohexamer would be formed by a dimerization of Smad4 and Smad2 homotrimers (Shi, 1997).

Smad2 and Smad3 are structurally highly similar and mediate TGF-beta signals. Smad4 is distantly related to Smads 2 and 3, and forms a heteromeric complex with Smad2 after TGF-beta or activin stimulation. Smad4/DPC4 is the shared hetero-oligomerization partner for the other SMADs. Smad2 and Smad3 interact with the kinase-deficient TGF-beta type I receptor (TbetaR)-I after it is phosphorylated by TbetaR-II kinase. TGF-beta1 induces phosphorylation of Smad2 and Smad3 in cultured Mv1Lu mink lung epithelial cells. Smad4 is found to be constitutively phosphorylated in Mv1Lu cells, the phosphorylation level remaining unchanged upon TGF-beta1 stimulation. Similar results are obtained using HSC4 cells, which are also growth-inhibited by TGF-beta. Smads 2 and 3 interact with Smad4 after TbetaR activation in transfected COS cells. In addition, TbetaR-activation-dependent interaction is observed between Smad2 and Smad3. Smads 2, 3 and 4 accumulate in the nucleus upon TGF-beta1 treatment in Mv1Lu cells, and show a synergistic effect in a transcriptional reporter assay using the TGF-beta-inducible plasminogen activator inhibitor-1 promoter. Dominant-negative Smad3 inhibits the transcriptional synergistic response by Smad2 and Smad4. These data suggest that TGF-beta induces heteromeric complexes of Smads 2, 3 and 4, and their concomitant translocation to the nucleus, which is required for efficient TGF-beta signal transduction (Nakao 1997).

About 90 percent of human pancreatic carcinomas show allelic loss at chromosome 18q. Twenty-five of 84 tumors have homozygous deletions at 18q21.1, a site that includes DPC4, a gene similar in sequence to Mad. Potentially inactivating mutations in DPC4 have been identified in six of 27 pancreatic carcinomas that did not have homozygous deletions at 18q21.1. DPC4 is thus a MAD related candidate tumor suppressor gene whose inactivation may play a role in pancreatic and possibly other human cancers (Hahn, 1996).

Familial juvenile polyposis is an autosomal dominant disease characterized by a predisposition to hamartomatous polyps and gastrointestinal cancer. A subset of juvenile polyposis families carries germ line mutations in the gene SMAD4 (also known as DPC4, located on chromosome 18q21.1), which encode a critical cytoplasmic mediator in the transforming growth factor-beta signaling pathway. The mutant SMAD4 proteins are predicted to be truncated at the carboxyl-terminus and lack sequences required for normal function. These results confirm an important role for SMAD4 in the development of gastrointestinal tumors (Howe, 1998).

Smad2 and Smad4 are related tumour-suppressor proteins, which, when stimulated by the growth factor TGF-beta, form a complex to inhibit growth. The effector function of Smad2 and Smad4 is located in the conserved carboxy-terminal domain (C domain) of these proteins and is inhibited by the presence of their amino-terminal domains (N domain). This inhibitory function of the N domain involves an interaction with the C domain that prevents the association of Smad2 with Smad4. This inhibitory function is increased in tumour-derived forms of Smad2 and 4 that carry a missense mutation in a conserved N domain arginine residue. The mutant N domains have an increased affinity for their respective C domains, inhibit the Smad2-Smad4 interaction, and prevent TGF beta-induced Smad2-Smad4 association and signaling. Whereas mutations in the C domain disrupt the effector function of the Smad proteins, N-domain arginine mutations inhibit SMAD signaling through a gain of autoinhibitory function. Gain of autoinhibitory function is a new mechanism for inactivating tumour suppressors (Hata, 1997).

Homologs of Drosophila Mad function as downstream mediators of the receptors for transforming growth factor beta (TGF-beta)-related factors. Two homologs, the receptor-associated Smad3 and the tumor suppressor Smad4/DPC4, synergize to induce ligand-independent TGF-beta activities and are essential mediators of the natural TGF-beta response. Smad3 and Smad4 associate in homomeric and heteromeric interactions, as assessed by yeast two-hybrid and coimmunoprecipitation analyses. Heteromeric interactions are mediated through the conserved C-terminal domains of Smad3 and Smad4. Smad4/DPC4 is critical because it is the shared hetero-oligomerization partner for the other SMADs. In Smad3, the homomeric interaction is mediated by the same domain. In contrast, the homomeric association of Smad4 requires both the N-terminal domain and the C-terminal domain, which by itself does not homomerize. Mutations that have been associated with impaired Mad activity in Drosophila or decreased tumor suppressor activity of Smad4/DPC4 in pancreas cancer, including a short C-terminal truncation and two point mutations in the conserved C-terminal domains, impair the ability of Smad3 and Smad4 to undergo homo- and heteromeric associations. Analyses of the biological activity of Smad3 and Smad4 and their mutants show that full signaling activity correlates with the ability to undergo efficient homo- and heteromeric interactions. Mutations that interfere with these interactions result in decreased signaling activity. The ability of Smad3 or Smad4 to induce transcriptional activation in yeast has also been evaluated. These results correlate the ability of individual Smads to homomerize with transcriptional activation and additionally with their biological activity in mammalian cells (Wu, 1997).

The Ski family of nuclear oncoproteins (see Drosophila snoN) represses TGF-beta signaling through interactions with the Smad proteins. The crystal structure of the Smad4 binding domain of human c-Ski in complex with the MH2 domain of Smad4 reveals specific recognition of the Smad4 L3 loop region by a highly conserved interaction loop (I loop) from Ski. The Ski binding surface on Smad4 significantly overlaps with that required for binding of the R-Smads. Indeed, Ski disrupts the formation of a functional complex between the Co- and R-Smads, explaining how it could lead to repression of TGF-beta, activin, and BMP responses. Intriguingly, the structure of the Ski fragment, stabilized by a bound zinc atom, resembles the SAND domain, in which the corresponding I loop is responsible for DNA binding (Wu, 2002).

c-Ski is a transcriptional corepressor that interacts strongly with Smad2, Smad3, and Smad4 but only weakly with Smad1 and Smad5. Through binding to Smad proteins, c-Ski suppresses signaling of transforming growth factor-β as well as bone morphogenetic proteins (BMPs). A mutant of c-Ski, termed c-Ski (ARPG) inhibits TGF-β/activin signaling but not BMP signaling. Selectivity was confirmed in luciferase reporter assays and by determination of cellular responses in mammalian cells (BMP-induced osteoblastic differentiation of C2C12 cells and TGF-β-induced epithelial-to-mesenchymal transdifferentiation of NMuMG cells) and Xenopus embryos. The ARPG mutant recruited histone deacetylases 1 (HDAC1) to the Smad3-Smad4 complex but not to the Smad1/5-Smad4 complex. c-Ski (ARPG) was unable to interact with Smad4, and the selective loss of suppression of BMP signaling by c-Ski (ARPG) was attributed to the lack of Smad4 binding. It was also found that c-Ski interacts with Smad3 or Smad4 without disrupting Smad3-Smad4 heteromer formation. c-Ski (ARPG) would be useful for selectively suppressing TGF-β/activin signaling (Takeda, 2004; full text of article).

Mammalian Media related proteins and development

Mutations in the SMAD4/DPC4 tumor suppressor gene, a key signal transducer in most TGFbeta-related pathways, are involved in 50% of pancreatic cancers. Homozygous Smad4 mutant mice die before day 7.5 of embryogenesis. Mutant embryos have reduced size, fail to gastrulate or express a mesodermal marker, and show abnormal visceral endoderm development. Growth retardation of the Smad4-deficient embryos results from reduced cell proliferation rather than increased apoptosis. The aggregation of mutant Smad4 ES cells with wild-type tetraploid morulae rescues the gastrulation defect. These results indicate that Smad4 is initially required for the differentiation of the visceral endoderm and that the gastrulation defect in the epiblast is secondary and non-cell autonomous. Rescued embryos show severe anterior truncations, indicating a second important role for Smad4 in anterior patterning during embryogenesis (Sirard, 1998).

The defective visceral endoderm of Smad4 mutant embryos could lead to growth and gastrulation defects in the epiblast by its failure to provide either general nutritional requirements for the embryo or specific inductive signals for gastrulation and patterning. Genetic analysis in the mouse shows that at least two TGF-related proteins (Bmp4 and nodal) are required for gastrulation. While the expression of Bmp4 appears to be restricted to the ectoderm, the expression of nodal is restricted to the visceral endoderm prior to gastrulation. Interestingly, both factors are reduced or absent during early in vitro differentiation of Smad4-deficient embryoid bodies, as compared with wild type. Concurrent with the reduction of these factors, is the absence of the mesoderm marker, Brachyury, suggesting that the deficiency of these factors (and possibly others) could be responsible for the absence of gastrulation in the Smad4 mutant mice. It is not yet clear whether Bmp4 and nodal are direct targets of Smad4, or whether the reduced level of serum proteins, Hnf4, Bmp4, and nodal expression in the visceral endoderm is a consequence of the inability of the Smad4-deficient embryo to develop to the stage at which these genes are normally expressed (Sirard, 1998).

The tumor suppressor SMAD4, also known as DPC4, is believed to be an essential factor that mediates TGF-beta signals. Smad4 is expressed ubiquitously during murine embryogenesis. To explore the functions of SMAD4 in development, it was mutated by truncating its functional C-domain. Mice heterozygous for the Smad4ex8/+ mutation are developmentally normal, whereas homozygotes die between embryonic day 6.5 (E6.5) and 8.5. All Smad4ex8/ex8 mutants are developmentally delayed at E6 and show little or no elongation in the extraembryonic portion of late egg cylinder stage embryos. At E6.5, the mutant embryos are only about one-quarter to one-half the size of their littermates, revealing an essential function for SMAD4 at stages around E6 of embryogenesis. It has been shown that mouse embryos undergo rapid cell proliferation before gastrulation with the epiblast cells increasing from 120 cells per E5.5 embryo to 660 cells per E6.5 embryo. The cell number increases further to 14,290 per embryo at E7.5. Thus, it is possible that SMAD4 functions as a mediator of mitogenic stimuli for epiblast proliferation. Consistent with this view, Smad4ex8/ex8 embryos exhibit considerably reduced staining with an antibody to PCNA when compared to the control embryos. Consistent with this, cultured Smad4ex8/ex8 blastocyst outgrowths suffer cellular proliferation defects and fail to undergo endoderm differentiation. Although a portion of mutant embryos at E8.5 show an increase in the embryonic ectoderm and endoderm, morphological and molecular analyses indicate that they do not form mesoderm. Altogether, these data demonstrate that SMAD4-mediated signals are required for epiblast proliferation, egg cylinder formation, and mesoderm induction (Yang, 1998).

Since the bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta (TGF-beta) superfamily that induce the differentiation of mesenchymal precursor cells into the osteogenic cells, the relevant signaling molecules responsible for mediating BMP-2 effects on mesenchymal precursor cells have been identified. BMP-2 induces osteoblastic differentiation of the pluripotent mesenchymal cell line C2C12 by increasing alkaline phosphatase activity and osteocalcin production. Because recent studies have demonstrated that cytoplasmic Smad proteins are involved in TGF-beta superfamily signaling, the relevant Smad family members involved in osteoblastic differentiation have been identifed. Human Smad5 is highly homologous to Smad1. BMP-2 causes serine phosphorylation of Smad5 as well as Smad1. In contrast, TGF-beta fails to cause serine phosphorylation of Smad1 and Smad5. Smad5 is directly activated by BMP type Ia or Ib receptors through physical association with these receptors. Following phosphorylation, Smad5 binds to DPC4, another Smad family member, and the complex is translocated to the nucleus. Overexpression of point-mutated Smad5 (G419S) or a C-terminal deletion mutant DPC4 (DPC4 delta C) blocks the induction of alkaline phosphatase activity, osteocalcin production, and Smad5-DPC4 signaling cascades upon BMP-2 treatment in C2C12 cells. These data suggest that activation of Smad5 and subsequent Smad5-DPC4 complex formation are key steps in the BMP signaling pathway, which mediates BMP-2-induced osteoblastic differentiation of the C2C12 mesenchymal cells (Nishimura, 1998).

Little is known about how neural stem cells are initially formed during development. Could a default mechanism of neural specification regulate acquisition of neural stem cell identity directly from embryonic stem (ES) cells? ES cells cultured in defined, low-density conditions readily acquire a neural identity. A novel primitive neural stem cell was characterized as a component of neural lineage specification that is negatively regulated by TGFß-related signaling. Primitive neural stem cells have distinct growth factor requirements, express neural precursor markers, generate neurons and glia in vitro, and have neural and non-neural lineage potential in vivo. These results are consistent with a default mechanism for neural fate specification and support a model whereby definitive neural stem cell formation is preceded by a primitive neural stem cell stage during neural lineage commitment (Tropepe, 2001).

Given that very few of the cultured ES cells generated sphere colonies (0.2%), attempts were made to determine if the release of endogenous BMP from the ES cells inhibits neural sphere colony formation, as would be predicted from the neural default model. Given that BMP4 and BMP-receptor-1 are expressed by undifferentiated ES cells, tests were made to see whether BMP could inhibit ES sphere colony formation by adding BMP4 (5 ng/ml) to ES cell cultures containing LIF and FGF2. A >50% decrease in the number of colonies generated was observed, and this effect appeared to be maximal since a 5-fold increase in BMP4 concentration did not further significantly attenuate the number of sphere colonies generated. Addition of the BMP4 protein antagonist Noggin (100 µg/ml) to the primary ES cell cultures caused a 50% increase in the number of sphere colonies generated. This increase appeared to be maximal since an increase in Noggin concentration from 10 µg/ml to 100 µg/ml resulted in no additional increase in the numbers of colonies generated (Tropepe, 2001).

It is evident that although Noggin can enhance the numbers of ES cells that differentiate into neural colony-forming stem cells, the effect is moderate. Noggin is known to be less effective than Chordin in neural induction assays in Xenopus, and targeted null mutations in both Noggin and Chordin in mouse are required to reveal anterior neural deficits in vivo. Thus, the effects of Noggin alone may underestimate the role for BMP-mediated inhibition of neural stem cell colony formation. To determine more directly the effect of blocking BMP signaling, an ES cell line with a targeted null mutation in the Smad4 gene, a critical intracellular transducer of multiple TGFß-related signaling pathways, was used. Smad4-/- ES cells cultured in the presence of LIF generate a 3- to 4-fold increase in the numbers of colonies, compared to the wild-type E14K cell line used to generate the targeted mutation. Interestingly, the rate of proliferation between wild-type and Smad4-/- cells in high or low serum concentration is similar, indicating that the increase in the number of colonies from mutant ES cells is likely not a result of a general increase in proliferation. Taken together, these results indicate that BMP4 signaling has a specific effect in limiting the numbers of single ES cells that differentiate into colony-forming neural stem cells and that inhibition of this pathway is sufficient to enhance neural stem cell colony formation (Tropepe, 2001).

Members of the TGF-ß superfamily are critical regulators for epithelial growth and can alter the differentiation of keratinocytes. Transduction of TGF-ß signaling depends on the phosphorylation and activation of Smad proteins by heteromeric complexes of ligand-specific type I and II receptors. To understand the function of TGF-ß and activin-specific Smad, transgenic mice were generated that overexpress Smad2 in epidermis under the control of keratin 14 promoter. Overexpression of Smad2 increases endogenous Smad4 and TGF-ß1 expression while heterozygous loss of Smad2 reduces their expression levels, suggesting a concerted action of Smad2 and Smad4 in regulating TGF-ß signaling during skin development. These transgenic mice have delayed hair growth, underdeveloped ears, and shorter tails. In their skin, there is severe thickening of the epidermis with disorganized epidermal architecture, indistinguishable basement membrane, and dermal fibrosis. These abnormal phenotypes are due to increased proliferation of the basal epidermal cells and abnormalities in the program of keratinocyte differentiation. The ectodermally derived enamel structure is also abnormal. Collectively, this study presents the first in vivo evidence that, by providing an auto-feedback in TGF-ß signaling, Smad2 plays a pivotal role in regulating TGF-ß-mediated epidermal homeostasis (Ito, 2001).

Somatosensory information from the face is transmitted to the brain by trigeminal sensory neurons. Whether neurons innervating distinct areas of the face possess molecular differences has been an open question. This study identified a set of genes differentially expressed along the dorsoventral axis of the embryonic mouse trigeminal ganglion and thus can be considered trigeminal positional identity markers. Interestingly, establishing some of the spatial patterns requires signals from the developing face. Bone morphogenetic protein 4 (BMP4) was identified as one of these target-derived factors; spatially defined retrograde BMP signaling controls the differential gene expressions in trigeminal neurons through both Smad4-independent and Smad4-dependent pathways. Mice lacking one of the BMP4-regulated transcription factors, Onecut2 (OC2), have defects in the trigeminal central projections representing the whiskers. These results provide molecular evidence for both spatial patterning and retrograde regulation of gene expression in sensory neurons during the development of the somatosensory map (Hodge, 2007).

Induction of the sympathetic nervous system (SNS) from its neural crest (NC) precursors is dependent on BMP signaling from the dorsal aorta. To determine the roles of BMP signaling and the pathways involved in SNS development, components of the BMP pathways were conditionally knocked out. To determine if BMP signaling is a cell-autonomous requirement of SNS development, the Alk3 (BMP receptor IA) was deleted in the NC lineage. The loss of Alk3 does not prevent NC cell migration, but the cells die immediately after reaching the dorsal aorta. The paired homeodomain factor Phox2b, known to be essential for survival of SNS precursors, is downregulated, suggesting that Phox2b is a target of BMP signaling. To determine if Alk3 signals through the canonical BMP pathway, Smad4 was deleted in the NC lineage. Loss of Smad4 does not affect neurogenesis and ganglia formation; however, proliferation and noradrenergic differentiation are reduced. Analysis of transcription factors regulating SNS development shows that the basic helix-loop-helix factor Ascl1 is downregulated by loss of Smad4 and that Ascl1 regulates SNS proliferation but not noradrenergic differentiation. To determine if the BMP-activated Tak1 (Map3k7) pathway plays a role in SNS development, Tak1 was deleted in the NC lineage. Tak1 was shown not to be involved in SNS development. Taken together, these results suggest multiple roles for BMP signaling during SNS development. The Smad4-independent pathway acts through the activation of Phox2b to regulate survival of SNS precursors, whereas the Smad4-dependent pathway controls noradrenergic differentiation and regulates proliferation by maintaining Ascl1 expression (Morikawa, 2009).

Interaction of Smads with the cytoskeleton

Smad proteins are intracellular signaling effectors of the TGFbeta superfamily. Endogenous Smad2, 3, and 4 bind microtubules (MTs) in several cell lines. Binding of Smads to MTs does not require TGFbeta stimulation. TGFbeta triggers dissociation from MTs, phosphorylation, and nuclear translocation of Smad2 and 3, with consequent activation of transcription in CCL64 cells. Destabilization of the MT network by nocodazole, colchicine, or a tubulin mutant disrupts the complex between Smads and MTs and increases TGFbeta-induced Smad2 phosphorylation and transcriptional response in CCL64 cells. These data demonstrate that MTs may serve as a cytoplasmic sequestering network for Smads, controlling Smad2 association with and phosphorylation by activated TGFbeta receptor I, and suggest a novel mechanism for the MT network to negatively regulate TGFbeta function (Dong, 2000).

BMP/SMAD1 signaling sets a threshold for the left/right pathway in lateral plate mesoderm and limits availability of SMAD4

Bistability in developmental pathways refers to the generation of binary outputs from graded or noisy inputs. Signaling thresholds are critical for bistability. Specification of the left/right (LR) axis in vertebrate embryos involves bistable expression of transforming growth factor beta (TGFbeta) member NODAL in the left lateral plate mesoderm (LPM) controlled by feed-forward and feedback loops. This study provides evidence that bone morphogenetic protein (BMP)/SMAD1 signaling sets a repressive threshold in the LPM essential for the integrity of LR signaling. Conditional deletion of Smad1 in the LPM led to precocious and bilateral pathway activation. NODAL expression from both the left and right sides of the node contributed to bilateral activation, indicating sensitivity of mutant LPM to noisy input from the LR system. In vitro, BMP signaling inhibited NODAL pathway activation and formation of its downstream SMAD2/4-FOXH1 transcriptional complex. Activity was restored by overexpression of SMAD4 and in embryos, elevated SMAD4 in the right LPM robustly activated LR gene expression, an effect reversed by superactivated BMP signaling. It is concluded that BMP/SMAD1 signaling sets a bilateral, repressive threshold for NODAL-dependent Nodal activation in LPM, limiting availability of SMAD4. This repressive threshold is essential for bistable output of the LR system (Furtado, 2008).

Human ortholog of Drosophila Melted impedes SMAD2 release from TGF-beta receptor I to inhibit TGF-beta signaling

Drosophila melted encodes a pleckstrin homology (PH) domain-containing protein that enables normal tissue growth, metabolism, and photoreceptor differentiation by modulating Forkhead box O (FOXO), target of rapamycin, and Hippo signaling pathways. Ventricular zone expressed PH domain-containing 1 (VEPH1) is the mammalian ortholog of melted, and although it exhibits tissue-restricted expression during mouse development and is potentially amplified in several human cancers, little is known of its function. This study explored the impact of VEPH1 expression in ovarian cancer cells by gene-expression profiling. In cells with elevated VEPH1 expression, transcriptional programs associated with metabolism and FOXO and Hippo signaling were affected, analogous to what has been reported for Melted. Altered regulation was observed of multiple transforming growth factor-beta (TGF-beta) target genes. Global profiling revealed that elevated VEPH1 expression suppressed TGF-beta-induced transcriptional responses. This inhibitory effect was verified on selected TGF-beta target genes and by reporter gene assays in multiple cell lines. It was further demonstrated that VEPH1 interacts with TGF-beta receptor I (TbetaRI) and inhibits nuclear accumulation of activated Sma- and Mad-related protein 2 (SMAD2). Two TbetaRI-interacting regions (TIRs) were identified with opposing effects on TGF-beta signaling. TIR1, located at the N terminus, inhibits canonical TGF-beta signaling and promotes SMAD2 retention at TbetaRI, similar to full-length VEPH1. In contrast, TIR2, located at the C-terminal region encompassing the PH domain, decreases SMAD2 retention at TbetaRI and enhances TGF-beta signaling. These studies indicate that VEPH1 inhibits TGF-beta signaling by impeding the release of activated SMAD2 from TbetaRI and may modulate TGF-beta signaling during development and cancer initiation or progression (Shathasivam, 2015).

Nuclear import and export of Smads

TGFß stimulation leads to phosphorylation and activation of Smad2 and Smad3, which form complexes with Smad4 that accumulate in the nucleus and regulate transcription of target genes. Following TGF-ß stimulation of epithelial cells, receptors remain active for at least 3-4 hr, and continuous receptor activity is required to maintain active Smads in the nucleus and for TGF-ß-induced transcription. Continuous nucleocytoplasmic shuttling of the Smads during active TGF-ß signaling provides the mechanism whereby the intracellular transducers of the signal continuously monitor receptor activity. These data therefore explain how, at all times, the concentration of active Smads in the nucleus is directly dictated by the levels of activated receptors in the cytoplasm (Inman, 2002).

Rather than existing as a static pool of activated Smads in the nucleus, the R-Smads are being constantly dephosphorylated, which results in dissociation of the R-Smad/co-Smad complexes and export of the inactive Smads to the cytoplasm. If the receptors are still active, the Smads will be reactivated and return to the nucleus. If the receptors are no longer active, the Smads accumulate back in the cytoplasm. It is concluded that the complexes dissociate in the nucleus because the export of Smad2 and 3 from the nucleus occurs independently of Smad4 export. Smad4 export depends on the nuclear transport receptor, CRM1, while Smad2 and 3 are actively exported via a CRM1-independent mechanism. Since recent work has demonstrated that the complex formation is mediated by the phosphoserine residues on the R-Smads, it is concluded that dephosphorylation of the R-Smads by an as yet unidentified nuclear phosphatase is responsible for triggering complex dissociation (Inman, 2002).

Smads and integration of signaling pathways in the nucleus

continued: Medea Evolutionary homologs part 3/3 | back to part 1/3 |

Medea: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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