MINIREVIEW What MAN1 does to the Smads TGFb/BMP signaling and the nuclear envelope Luiza Bengtsson Institute for Chemistry and Biochemistry, Free University Berlin, Germany Introduction Our knowledge about the nuclear membrane has advanced dramatically in the recent years. We now know that protein residents of the nuclear membrane regulate processes as diverse as DNA replication and transcription, control of the shape and stability of the nucleus, cell cycle progression, chromatin organiza- tion, cell development and differentiation, nuclear anchoring and migration, and apoptosis (reviewed in [1,2]). Mutations in several of the integral membrane proteins of the inner nuclear membrane (emerin, MAN1, lamin B receptor) and their common binding partners (lamins) cause distinct diseases, the molecular mechanisms of which are not yet understood [1,3,4]. One of the current hypotheses suggests that the diseases result from altered gene expression in affec- ted tissues and that integral membrane proteins of the inner nuclear membrane (INM) regulate gene expression either directly, or as components of tran- scription regulating protein complexes [3,5,6]. Indeed, both emerin and MAN1 bind the transcriptional repressors germ cell-less (GCL) and Bcl-2-associated transcription factor (Btf) [7,8]. In addition, loss of emerin leads to up-regulation of expression of 28 genes, which can be rescued by reintroducing emerin [9]. LAP2b, another INM protein, can repress trans- cription by recruiting histone deacetylase [10], or Keywords BMP; laminopathy; MAN1; nuclear envelope; phosphatase; signal transduction; Smad; TGFb Correspondence L. Bengtsson, Institute for Chemistry and Biochemistry, Free University Berlin, Thielallee 63 14195 Berlin, Germany Tel: +49 30 838 54789 E-mail: lbengts@chemie.fu-berlin.de Previous address Department of Cell Biology, Johns Hopkins University School of Medicine, 725 N. Wolfe St, Baltimore, MD 21205, USA (Received 8 March 2006, accepted 8 Janu- ary 2007) doi:10.1111/j.1742-4658.2007.05696.x The inner nuclear membrane protein MAN1 has been identified as an important factor in transforming growth factor b⁄ bone morphogenic pro- tein (TGFb ⁄ BMP) signaling. Loss of MAN1 results in three autosomal dominant diseases in humans; all three characterized by increased bone density. Xenopus embryos lacking MAN1 develop severe morphological defects. Both in humans and in Xenopus embryos the defects originate from deregulation of TGFb ⁄ BMP signaling. Several independent studies have shown that MAN1 is antagonizing TGFb ⁄ BMP signaling through binding to regulatory Smads. Here, recent progress in understanding MAN1 func- tions is summarized and a model for MAN1-dependent regulation of TGFb ⁄ BMP signaling is proposed. Abbreviations BAF, barrier-to-autointegration factor; BMP, bone morphogenic protein; Btf, Bcl-2-associated transcription factor; GCL, germ cell-less; INM, inner nuclear membrane; LAP, lamina associated polypeptide; MH-domain, Mad homology domain; pRb, retinoblastoma protein; PP, protein phosphatase; RR-motif, RNA recognition motif; R-Smads, regulatory Smads; SANE, Smad1 antagonistic effector; TGFb, transforming growth factor b; UHM, U2AF homology motif; WH, winged-helix. 1374 FEBS Journal 274 (2007) 1374–1382 ª 2007 The Author Journal compilation ª 2007 FEBS through binding to GCL [11]. Lamin A binds the transcription repressors retinoblastoma protein (pRb) and MOK2 (reviewed in [1,12]). Finally, the nuclear envelope protein MAN1, the subject of this review, has been shown to bind regulatory Smads (R-Smads) and antagonize the transforming growth factor b ⁄ bone morphogenic protein (TGFb ⁄ BMP)-induced signal transduction pathway [13–17]. Who is MAN1? MAN1 was first discovered as one of the autoantigens for the autoantibodies from a patient with collagen vascular disease [18]. MAN1 is an integral membrane protein of the INM and belongs to the LEM (Lap2- emerin-MAN1)-domain family of proteins [18,19]. The LEM domain is a structural motif [20–22] also found in emerin, lamina associated polypeptide (LAP)2, Lem2 [23,24], the Drosophila specific proteins otefin [25] and Bocksbeutel [26], and other as yet uncharac- terized proteins named Lem3–5 [23]. LEM domains bind barrier-to-autointegration factor (BAF [8,27–29]), an essential DNA-binding protein that has been impli- cated in the organization of chromatin structure [30– 32] and recruitment of nuclear envelope proteins to the chromosomes during nuclear assembly [33]. The LEM domain in MAN1, located at the very N-terminus of this 100 kDa protein ([18], Fig. 1), is highly conserved with 82% identity between human and Xenopus MAN1 (xMAN1 [14]). In contrast, the N-terminus outside the LEM domain is only 30% identical between human and Xenopus MAN1 [14]. The functions of a MAN1 homolog, the Lem2 pro- tein, might be representative for the functions of the MAN1 N-terminus. Lem2 is 19% homologous to MAN1, has an N-terminal LEM domain, two trans- membrane domains and a conserved C-terminal nucleo- plasmic domain [24], but is lacking the C-terminal RNA recognition motif (RR-motif) found in MAN1 (Fig. 1). Thus, structurally, Lem2 appears as a shorter version of MAN1. Overexpression of Lem2 in mamma- lian cells does not affect cell viability, but disturbs nuc- lear organization, which is manifested by protein bridges containing lamins and BAF connecting nuclei of cells that have otherwise completed mitosis [24]. In contrast, knockdown of the Caenorhabditis elegans ortholog, the Ce-Lem2 (the gene product of C. elegans lem-2 gene, also known as ‘Ce-MAN1’ [8,24]), is lethal in 15% of embryos [34]. Interestingly, simultaneous down-regulation of Ce-Lem2 and Ce-emerin was lethal in 100% of embryos by the 100-cell stage [34], while reduction of Ce-emerin had no noticeable effect [35], suggesting that Ce-Lem2 and Ce-emerin can substitute for each other to some extent. It is not yet known whe- ther MAN1 and emerin are redundant, however, func- tional overlap is likely, because mammalian MAN1 and mammalian emerin do have many common part- ners (see below). MAN1 needs lamins in order to localize to the INM [34,36,37]. The N-terminus and the first transmem- brane domain of MAN1 are necessary and sufficient for MAN1 INM localization [13,38]. The N-terminus of human MAN1 (up to the first transmembrane domain) binds prelamin A and B1 [8] in vitro, while the LEM domain alone is sufficient to bind BAF (Fig. 1; [8]). Prelamin A and BAF are also binding partners of emerin [39]. Interestingly, the N-terminus of human MAN1 binds the human emerin itself (Fig. 1; [8]). Emerin is an integral membrane protein and localizes to the nuclear envelope [40]. Mutations in emerin cause Emery–Dreifuss muscular dystrophy [41]. Although most disease causing mutations result in loss of emerin, in some cases the mutated emerin is present at normal levels and is also correctly localized (reviewed in [39]). Two of such mutations, the deletion of residues 95–99 and the substitution Q133H, do affect MAN1 N-terminus binding to emerin: the bind- ing was abolished when tested in vitro [8]. Given the possibility that MAN1 overlaps functionally with emerin, one might assume that MAN1 stabilizes ⁄ regu- lates emerin’s functions. Thus, loss of emerin binding to MAN1 N-terminus and ⁄ or loss of the MAN1–emer- in complex functions could directly contribute to the Emery–Dreifuss muscular dystrophy disease mechanism. The C-terminus of MAN1 (human MAN1 residues 649–911; Fig. 1) is 87% identical between human and Fig. 1. Map of binding sites on MAN1. Human and Xenopus MAN1 and C. elegans lem2 sequences were retrieved from NCBI data bank and pairwise aligned to human MAN1 using CLUSTALW [8,13,14,16,17,34,69,70]. Gaps between the boxed areas represent gaps in the alignment that were larger than 10 amino acids. Domains were either predicted using SMART [71] or taken from the NMR structure [44]. Numbers above the sequence mark the first and last amino acid of each functional domain. WH, Winged helix domain; UHM, U2AF homology motif; L, LEM domain; 1, first transmembrane domain; 2, second transmembrane domain; R, RR-motif. Black thick lines depict the smallest part of MAN1 required to bind each partner [8,13,14,16,44,69]. L. Bengtsson What MAN1 does to the Smads FEBS Journal 274 (2007) 1374–1382 ª 2007 The Author Journal compilation ª 2007 FEBS 1375 Xenopus [14] and 55% identical between human and Ciona intestinalis (a simple eukaryote of the chordate lineage from which all vertebrates originate), implying an evolutionarily conserved function. The C-terminus does not localize to the nuclear envelope by itself [13,14,38], suggesting it has roles other than targeting. This part of MAN1 indeed binds several regulators of gene expression, including transcriptional repressors GCL and Btf and, surprisingly, also binds BAF [8,34]. There is no LEM domain in the MAN1 C-terminus, however, a different BAF binding motif common to MAN1 C-terminus, Histone H1 and the transcription factor cone-rod homeobox (Crx) has been proposed [8]. The residues 801–857 in human MAN1 (655–734 in xMAN1) comprise an RR-motif (Fig. 1 [8,13–15]). RR- motifs in other proteins are known to mediate associ- ation with RNA [42], but can also function as protein– protein interaction domains [43]. Several studies have identified the RR-motif in MAN1 as a binding site for transcription regulators, the R-Smads [14]. A detailed NMR analysis of human MAN1 C-terminus revealed the existence of two globular domains: the experiment- ally confirmed winged-helix (WH) domain comprising the residues 655–750 and a putative U2 auxillary factor homology motif (UHM) consisting of residues 782–911 and including the RR-motif [15,44]. Both the WH domain and the UHM domain adopt a stable a ⁄ b-fold found in several DNA-interacting transcription factors [45]. Indeed, a MAN1 fragment consisting of the WH domain binds DNA with nanomolar affinity and the binding is further increased by the presence of the UHM domain [44]. Because the DNA binding site on MAN1 does not overlap with the Smad binding site, it seems possible for MAN1 to bind DNA and Smads simultaneously [44]. MAN1 is essential for early development and later tissue-specific functions MAN1 mRNA is maternally expressed in Xenopus embryos [14]. By the tailbud stage, the expression of xMAN1 is restricted to anterior central nervous sys- tem, eyes, otic vesicles and bronchial arches [14]. Strik- ingly, xMAN1 expression starts to diminish at stage 34 and is completely down-regulated by stage 45 [14,46]. It is not known whether xMAN1 is expressed in adult frogs, however, various human cell lines do contain endogenous MAN1 [13,15], which implies that MAN1 is reactivated in somatic cells. Interestingly, as the expression of xMAN1 is turned off, expression of both Xenopus emerin genes is turned on [46], which suggests yet another link between MAN1 and emerin functions. Xenopus embryos injected with antisense morpholino oligos against xMAN1 gastrulated normally [14]. Like- wise, down-regulation of Drosophila MAN1 by RNAi does not affect the early development of the embryos [37]. At later stages however, the Xenopus embryos showed severe morphological anomalies: their right eyes were absent or poorly formed [14]. The eye defects correlated with several target genes of BMP signaling being up-regulated in the xMAN1 morphants, implica- ting xMAN1 in BMP signaling [14]. It is not clear whether treatment with antisense morpholino oligos against xMAN1 resulted in a true null-phenotype, because, due to partial tetraploidy there might be another xMAN1 gene in Xenopus. In mammalian cells, MAN1 siRNA enhanced TGFb, activin and BMP signaling, because several gene targets of these pathways were up-regulated com- pared to controls [15]. Reduced MAN1 expression also made the cells more sensitive to TGFb-induced growth inhibition [15]. Mutations in human MAN1 result in osteopoikilo- sis, Buschke–Ollendorff syndrome and melorheostosis [17]. All three disorders are autosomal dominant and are characterized by increased bone density [47]. In Buschke–Ollendorff syndrome, the osteopoikilosis is associated with disseminated connective tissue nevi. In melorheostosis, the bone hyperostosis is accompanied by abnormalities of adjacent soft tissues, such as joint contractures, sclerodermatous skin lesions, muscle atrophy, hemangiomas and lymphoedema [17]. The disease causing mutations result in haploinsufficiency with respect to full-length MAN1 [17]. There are two possibilities for how the mutations in MAN1 could cause disease: (a) the mutated protein is specifically interfering with remaining wildtype MAN1 functions, and ⁄ or (b) half the amount of MAN1 in cells is not enough to keep up MAN1 functions. The latter alter- native is more likely, because overexpression of mutated proteins in tissue culture cells expressing nor- mal levels of full-length endogenous MAN1 did not resemble the MAN1 siRNA phenotype, e.g., TGFb signaling was not enhanced [17]. TGFb/BMP signaling: the basics BMP, TGFb and activin belong to a family of pleio- tropic cytokines. Each cytokine has many different iso- forms with highly specific functions. These functions include the context-specific inhibition or stimulation of cell proliferation, control of extracellular matrix synthesis and degradation, and the control of epi- thelial ⁄ mesenchymal interactions during embryogene- sis. Other functions include wound healing and the What MAN1 does to the Smads L. Bengtsson 1376 FEBS Journal 274 (2007) 1374–1382 ª 2007 The Author Journal compilation ª 2007 FEBS modulation of immune functions. Misregulation of these specific pathways results in developmental disor- ders, cancer, fibrosis and autoimmune disorders. Signa- ling is initiated by binding of the cytokine to a homodimeric complex of cytokine receptor type II, which recruits type I receptor and activates it by phos- phorylation. Phosphorylated and thereby activated type I receptor phosphorylates Smads, which then form oligomeric complexes and enter the nucleus to either induce or suppress gene expression by interact- ing with cell type and signal-specific transcription acti- vators or repressors. There are three classes of Smads: regulatory Smads (BMP-responsive R-Smads 1, 5 and 8 and TGFb-responsive R-Smads 2 and 3), the co- Smad Smad4 and the inhibitory Smads 6 and 7. All R-Smads and the co-Smad consist of three domains: the N-terminal MH1 domain, the variable proline-rich linker, and the C-terminal Mad homology (MH)2 domain. The MH2 domain is highly conserved in all Smads and is primarily responsible for binding to different partners in a series of mutually exclusive protein–protein interactions. The specificity of the BMP ⁄ TGFb ⁄ activin signal is conferred by mixing and matching of receptor subtypes in the oligomeric recep- tor complexes as well as by regulation of Smad interac- tions in the cytoplasm and in the nucleus. Smads can be either activated or inhibited by phosphorylation, sumoylation and ubiquitination (reviewed in [48–54]). MAN1 antagonizes TGFb/BMP signaling by binding R-Smads Xenopus MAN1 was identified as a gene involved in neuralization and neural patterning during Xenopus development [14]. The RR-motif in MAN1 was neces- sary but not sufficient for the neuralizing activity, while neither the LEM domain nor the whole N-termi- nus of MAN1 showed any activity [14]. Furthermore, both full-length MAN1 [16] and the C-terminus alone [14,16] could induce a partial secondary axis formation in Xenopus embryos [14]. Both the neuralizing activity and the secondary axis induction indicate inhibited BMP signaling. An independent study also discovered xMAN1 as a negative regulator of the BMP signaling, but named the protein ‘SANE’ (Smad1 antagonistic effector) [13]. The cDNA sequences of SANE and xMAN1 in the NCBI gene database are identical (gi|56849616 and gi|29335751, respectively) and are orthologous to human MAN1. The C-terminus of human MAN1 interacted with Smads 2 and 3 in a yeast two-hybrid skeletal muscle library [15]. Additionally, in an affinity-purification of Smad3 interacting proteins from TGFb-responsive Hep3B (human liver carcinoma) and RIE-1 (rat intest- inal epithelial) cells, MAN1 was among the proteins that bound specifically [15]. Various independent meth- ods ranging from in vivo coimmunoprecipitation to direct in vitro binding assays confirmed the direct inter- action between MAN1 and all regulatory Smads (BMP and TGFb-responsive) but not the co-Smad or the inhibitory Smads [13–17]. The interaction was mapped to the RR-motif in MAN1 and the MH2 domain of R-Smads (Fig. 1; [14,16]). RNAse treatment had no effect on the MAN1 ⁄ Smad binding suggesting that the RR-motif in MAN1 is a protein–protein inter- action domain [13–17]. Several independent experiments suggest that the antagonizing activity of MAN1 in TGFb ⁄ BMP signa- ling depends on its ability to bind R-Smads. When tes- ted using luciferase reporters containing response elements from the BMP-responsive gene Xvent2, both full-length xMAN1 and the C-terminus alone inhibited luciferase gene expression after BMP4 stimulation, while the N-terminus alone had no effect [16]. Although TGFb and activin signaling were unaffected by MAN1 overexpression in Xenopus embryos [13,15,17], in mammalian cell lines both the full-length MAN1 [13] and its C-terminus alone [15,17] were cap- able of antagonizing TGFb-, BMP- and activin-signa- ling. Similarly, human MAN1 with mutated RR-motif was defective in antagonizing both BMP and TGFb signaling in tissue culture cells [15]. MAN1 does not bind inhibitory Smads or the co-Smad [15]. Moreover, MAN1 does not bind R- Smad–co-Smad complexes [15]. The association of MAN1 with R-Smads is not regulated by the signaling pathway, because neither stimulation with TGFb or BMP, nor overexpression of constitutively active type I receptor for TGFb, BMP or activin increases the amount of R-Smad bound to MAN1 [15]. MAN1 binds both phosphorylated and unphosphorylated R-Smads [15]. At the same time, overexpression of MAN1 lowers the cellular pool of phosphorylated R- Smads [15,16] and prevents accumulation of R-Smads in the nucleus after cytokine-induced activation [15]. Importantly, the R-Smads are not being degraded as a result of MAN1 overexpression (shown for Smad3 [13], Smad2 [16] and xSmad1 [16]). The model: MAN1 disrupts the R-Smad–co-Smad complexes and promotes dephosphorylation of R-Smads How can MAN1 attenuate TGFb ⁄ BMP signaling by binding R-Smads? As an INM protein and not a part L. Bengtsson What MAN1 does to the Smads FEBS Journal 274 (2007) 1374–1382 ª 2007 The Author Journal compilation ª 2007 FEBS 1377 of the nuclear pore complexes, MAN1 is unlikely to block Smad entry into the nucleus. It is also unlikely that MAN1 simply sequesters R-Smads at the nuclear envelope and thus prevents transcription from their target genes [15,36,55] – this would result in an accu- mulation of the R-Smads at the nuclear periphery and not in the observed cytoplasmic accumulation [15]. MAN1 is predicted to be able to bind DNA and R-Smads simultaneously [44], thus it may assist in acti- vation or repression of TGFb ⁄ BMP target genes at the nuclear envelope. It is formally possible that such genes code for antagonists of TGFb ⁄ BMP signaling and their expression results in overall signal attenu- ation. However, effects on Smad phosphorylation and Smad nuclear localization were studied after 1 h of TGFb1 stimulation [15] implicating that the antagon- izing mechanism is more direct. Smad-mediated signaling has two important proper- ties: (a) only phosphorylated complexed R-Smads are retained in the nucleus, and (b) only phosphorylated R-Smads in complex with the co-Smad can initiate or inhibit transcription of TGFb ⁄ BMP target genes [48–54,56]. Thus MAN1 has to either disrupt R-Smad– co-Smad complexes and ⁄ or induce dephosphorylation of R-Smads in order to attenuate the Smad-mediated signal. This hypothesis is supported by several experi- mental data: (a) it has been shown that MAN1 bound Smad3 is not associated with the co-Smad, in contrast to ‘free’ Smad3 [15]; (b) overabundance of MAN1 cor- relates with lower cellular pool of phosphorylated Smads [16]; (c) upon overexpression of MAN1 R-Smads do not accumulate in the nucleus, indicating lost retention in the nucleus and accelerated nuclear export [14–16], and (d) full-length MAN1 antagonizes TGFb ⁄ BMP signaling more effectively than the C-ter- minus alone, implying that the correct nuclear envel- ope localization of MAN1 is beneficial, but not necessary for MAN1 functions in TGFb ⁄ BMP signa- ling [14]. Taken together the data suggests a role for MAN1 similar to that of the inhibitory Smad 6. Smad 6 inhibits TGFb ⁄ BMP signaling not only by binding the respective type I receptors and interfering with phosphorylation of Smads, but also by binding R-Smads and preventing them from heterooligomeriz- ing with co-Smad and forming active complexes (reviewed in [57]). Hypothetically, MAN1 may be act- ing as a ‘molecular filter’, catching a portion of the Smad complexes that enter the nucleus and forcing the complexes apart by binding the R-Smad and displacing the co-Smad. Monomeric Smads would become rap- idly dephosphorylated and exported out of the nucleus. MAN1 may also recruit a nuclear phosphatase to dephosphorylate Smads and reinforce Smad complex disassembly. Two nuclear Smad phosphatases have recently been identified: pyruvate dehydrogenase phos- phatase (PDP) for BMP responsive R-Smads [58] and PPM1A for TGFb responsive R-Smads [59]; both are members of the metal-ion-dependent protein phospha- tase family and both are distributed throughout the nucleus. Two further phosphatases, the protein phos- phatase 1 (PP1) and the protein phosphatase 2 A (PP2A) are anchored at the nuclear periphery [60–62]. Overexpression of the catalytic domains of PP1 and PP2A did not have any effect on Smad phosphoryla- tion [58,59]; however, both phosphatases need a regu- latory subunit in order to find their targets [63]. PP1 is responsible for dephosphorylating lamins throughout the interphase, while PP2A dephosphorylates pRb in a cell cycle and lamin dependent manner [60–62]. More- over, inhibition of PP2A increases the phospho-Smad pool in the cells only when lamins are present. Thus, both PP1 and PP2A are potentially in the right place to dephosphorylate MAN1-bound Smads. The pro- posed model is summarized in Fig. 2. Why MAN1? Any inhibition of BMP ⁄ TGFb signaling by MAN1 has to be a strictly local process restricted to the nuc- lear envelope. Why is it important to have a signaling antagonist posted there? MAN1 can potentially bind both DNA and R-Smads and is therefore able to influ- ence gene expression directly [44]. It is not yet known which exact genes are under transcriptional control by MAN1, but the fact that haploinsufficiency of MAN1 causes severe bone disorders [17] suggests that the genes in question are central for cell functions and have to be tightly regulated. Thus MAN1 would hypo- thetically both transduce the Smad-mediated signal and attenuate it at the same time. Alternatively, MAN1 might be safeguarding the nuclear periphery against concentration of active Smad complexes which could potentially interact with other INM proteins and the lamina and negatively influence regulation of gene expression [2]. Could emerin be involved? At least in C. elegans embryos, Ce-emerin seems to provide a backup mechanism for functions of the MAN1 homolog Ce-lem2 [34]. In Xenopus embryos emerin gene expression begins as MAN1 expression diminishes [46]. In somatic human cells, both emerin and MAN1 are expressed [17,18,39]. Human emerin and human MAN1 have many common binding part- ners [8,39], but it is not yet known if emerin also binds What MAN1 does to the Smads L. Bengtsson 1378 FEBS Journal 274 (2007) 1374–1382 ª 2007 The Author Journal compilation ª 2007 FEBS Smads. Emerin binds the N-terminus of MAN1 [8] and has thus the potential to regulate the TGFb ⁄ BMP sign- aling antagonizing activity of MAN1. Emerin is retained at the nuclear membrane by lamins (reviewed in [39]) and Nesprin 2 [64,65]. Interestingly, the expres- sion of synaptic nuclear envelope-2, a short isoform of the giant Nesprin 2 [64–66] also located at the nuclear membrane, is specifically up-regulated in response to TGFb signal [67,68]. If nesprins serve as scaffolds for protein complexes containing MAN1, emerin, lamins, protein phosphatases and other components, then the up-regulation of nesprin expression might function as a feedback mechanism. In such a feedback mechanism, the cytokine signal results in translocation of phos- phorylated Smads into the nucleus, leading to higher expression of nesprins. More nesprins could then hypo- thetically link more emerin ⁄ phosphatases ⁄ MAN1 pro- tein complexes which would eventually lead to enhanced dephosphorylation of Smads and attenu- ated ⁄ terminated signal. The discovery that the INM protein MAN1 binds Smads and antagonizes cytokine signaling also raises the question what roles other nuclear envelope proteins might have in cellular signal transduction. We know that several of them (LAP2b, emerin, lamin A) can regulate gene expression [1,9,11,12]; future studies will have to tell whether they do it on orders coming from the plasma membrane. Acknowledgements The first version of this review was written while I was a postdoctoral fellow in Katherine L. Wilson’s lab (spring 2005). Warmest thanks to Katherine L. Wilson and members of the Wilson lab, especially K. E. Tifft, M. Mansharamani and M. S. Zastrow for comments on the manuscript, to R. Schwappacher for fruitful discussions and to Petra Knaus for her support. LB was funded by a postdoctoral fellowship from the Deutsche Forschungsgemeinschaft. References 1 Gruenbaum Y, Margalit A, Goldman RD, Shumaker DK & Wilson KL (2005) The nuclear lamina comes of age. Nat Rev Mol Cell Biol 6, 21–31. 2 D’Angelo MA & Hetzer MW (2006) The role of the nuclear envelope in cellular organization. Cell Mol Life Sci 63, 316–332. 3 Broers JL, Hutchison CJ & Ramaekers FC (2004) Laminopathies. J Pathol 204, 478–488. Fig. 2. Proposed model for TGFb ⁄ BMP signaling regulation by MAN1. (1) MAN1 binds through its C-terminal RR-motif to the MH2-domain of incoming R-Smads–co-Smad complexes. MAN1 at the nuclear envelope is in a complex with emerin, other proteins and a putative phos- phatase. (2) The recruitment of R-Smads to a MAN1 complex causes disassembly of the R-Smad–co-Smad complex, dephosphorylation of Smads and increased nuclear export. (3) This results in fewer active Smad complexes capable of recruiting coactivators ⁄ corepressors to DNA in the nuclear interior and overall less activation of gene expression. Thus, MAN1 function is to fine tune the TGFb ⁄ BMP signaling. NPC, nuclear pore complex; PP, protein phosphatase. L. Bengtsson What MAN1 does to the Smads FEBS Journal 274 (2007) 1374–1382 ª 2007 The Author Journal compilation ª 2007 FEBS 1379 4 Broers JL, Ramaekers FC, Bonne G, Yaou RB & Hutchison CJ (2006) Nuclear lamins: laminopathies and their role in premature ageing. Physiol Rev 86, 9671008. 5 Wilson KL (2000) The nuclear envelope, muscular dys- trophy and gene expression. Trends Cell Biol 10, 125–129. 6 Somech R, Shaklai S, Amariglio N, Rechavi G & Simon AJ (2005) Nuclear envelopathies – raising the nuclear veil. 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