The Drosophila melanogaster LEM-domain protein MAN1

Developmental changes in nuclear lamina components during germ cell differentiation

The nuclear lamina (NL) changes composition for regulation of nuclear events. We investigated changes that occur in Drosophila oogenesis, revealing switches in NL composition during germ cell differentiation. Germline stem cells (GSCs) express only LamB and predominantly emerin, whereas differentiating nurse cells predominantly express LamC and emerin2. A change in LamC-specific localization also occurs, wherein phosphorylated LamC redistributes to the nuclear interior only in the oocyte, prior to transcriptional reactivation of the meiotic genome. These changes support existing concepts that LamC promotes differentiation, a premise that was tested. Remarkably ectopic LamC production in GSCs did not promote premature differentiation. Increased LamC levels in differentiating germ cells altered internal nuclear structure, increased RNA production, and reduced female fertility due to defects in eggshell formation. These studies suggest differences between Drosophila lamins are regulatory, not functional, and reveal an unexpected robustness to level changes of a major scaffolding component of the NL.

PIGB maintains nuclear lamina organization in skeletal muscle of Drosophila

The nuclear lamina (NL) plays various roles and participates in nuclear integrity, chromatin organization, and transcriptional regulation. Lamin proteins, the main components of the NL, form a homogeneous meshwork structure under the nuclear envelope. Lamins are essential, but it is unknown whether their homogeneous distribution is important for nuclear function. Here, we found that PIGB, an enzyme involved in glycosylphosphatidylinositol (GPI) synthesis, is responsible for the homogeneous lamin meshwork in Drosophila. Loss of PIGB resulted in heterogeneous distributions of B-type lamin and lamin-binding proteins in larval muscles. These phenotypes were rescued by expression of PIGB lacking GPI synthesis activity. The PIGB mutant exhibited changes in lamina-associated domains that are large heterochromatic genomic regions in the NL, reduction of nuclear stiffness, and deformation of muscle fibers. These results suggest that PIGB maintains the homogeneous meshwork of the NL, which may be essential for chromatin distribution and nuclear mechanical properties.

Checkpoint activation drives global gene expression changes in Drosophila nuclear lamina mutants

The nuclear lamina (NL) lines the inner nuclear membrane. This extensive protein network organizes chromatin and contributes to the regulation of transcription, DNA replication, and repair. Lap2-emerin-MAN1 domain (LEM-D) proteins are key members of the NL, representing proteins that connect the NL to the genome through shared interactions with the chromatin-binding protein Barrier-to-Autointegration Factor (BAF). Functions of the LEM-D protein emerin and BAF are essential during Drosophila melanogaster oogenesis. Indeed, loss of either emerin or BAF blocks germ cell development and causes loss of germline stem cells, defects linked to the deformation of NL structure, and non-canonical activation of Checkpoint kinase 2 (Chk2). Here, we investigate the contributions of emerin and BAF to gene expression in the ovary. Profiling RNAs from emerin and baf mutant ovaries revealed that nearly all baf misregulated genes were shared with emerin mutants, defining a set of NL-regulated genes. Strikingly, loss of Chk2 restored the expression of most NL-regulated genes, identifying a large class of Chk2-dependent genes (CDGs). Nonetheless, some genes remained misexpressed upon Chk2 loss, identifying a smaller class of emerin-dependent genes (EDGs). Properties of EDGs suggest a shared role for emerin and BAF in the repression of developmental genes. Properties of CDGs demonstrate that Chk2 activation drives global misexpression of genes in the emerin and baf mutant backgrounds. Notably, CDGs were found upregulated in lamin-B mutant backgrounds. These observations predict that Chk2 activation might have a general role in gene expression changes found in NL-associated diseases, such as laminopathies.

Survival of Drosophila germline stem cells requires the chromatin-binding protein Barrier-to-autointegration factor

The nuclear lamina (NL) is an extensive protein network that underlies the inner nuclear envelope. This network includes LAP2-emerin-MAN1 domain (LEM-D) proteins that associate with the chromatin and DNA-binding protein Barrier-to-autointegration factor (BAF). Here, we investigate the partnership between three NL Drosophila LEM-D proteins and BAF. In most tissues, only Emerin/Otefin is required for NL enrichment of BAF, revealing an unexpected dependence on a single LEM-D protein. Prompted by these observations, we studied BAF contributions in the ovary, a tissue where Emerin/Otefin function is essential. We show that germ cell-specific BAF knockdown causes phenotypes that mirror emerin/otefin mutants. Loss of BAF disrupts NL structure, blocks differentiation and promotes germ cell loss, phenotypes that are partially rescued by inactivation of the ATR and Chk2 kinases. These data suggest that, similar to emerin/otefin mutants, BAF depletion activates the NL checkpoint that causes germ cell loss. Taken together, our findings provide evidence for a prominent NL partnership between the LEM-D protein Emerin/Otefin and BAF, revealing that BAF functions with this partner in the maintenance of an adult stem cell population.

Drosophila emerins control LINC complex localization and transcription to regulate myonuclear position

Mispositioned nuclei are a hallmark of skeletal muscle disease. Many of the genes that are linked to Emery-Dreifuss muscular dystrophy (EDMD) encode proteins that are critical for nuclear movement in various cells, suggesting that disruptions in nuclear movement and position may contribute to disease progression. However, how these genes are coordinated to move nuclei is not known. Here, we focussed on two different emerin proteins in Drosophila, Bocksbeutel and Otefin, and their effects on nuclear movement. Although nuclear position was dependent on both, elimination of either Bocksbeutel or Otefin produced distinct phenotypes that were based in differential effects on the KASH-domain protein Klarsicht. Specifically, loss of Bocksbeutel reduced Klarsicht localization to the nucleus and resulted in a disruption in nuclear separation. Loss of Otefin increased the transcription of Klarsicht and led to premature separation of nuclei and their positioning closer to the edge of the muscle. Consistent with opposing functions, nuclear position is normal in otefin; bocksbeutel double mutants. These data indicate emerin-dependent regulation of Klarsicht levels in the nuclear envelope is a critical determinant of nuclear position.

Nuclear lamina dysfunction triggers a germline stem cell checkpoint

LEM domain (LEM-D) proteins are conserved components of the nuclear lamina (NL) that contribute to stem cell maintenance through poorly understood mechanisms. The Drosophila emerin homolog Otefin (Ote) is required for maintenance of germline stem cells (GSCs) and gametogenesis. Here, we show that ote mutants carry germ cell-specific changes in nuclear architecture that are linked to GSC loss. Strikingly, we found that both GSC death and gametogenesis are rescued by inactivation of the DNA damage response (DDR) kinases, ATR and Chk2. Whereas the germline checkpoint draws from components of the DDR pathway, genetic and cytological features of the GSC checkpoint differ from the canonical pathway. Instead, structural deformation of the NL correlates with checkpoint activation. Despite remarkably normal oogenesis, rescued oocytes do not support embryogenesis. Taken together, these data suggest that NL dysfunction caused by Otefin loss triggers a GSC-specific checkpoint that contributes to maintenance of gamete quality.

Otefin is a nuclear lamina protein required for survival of Drosophila germ stem cells. Here the authors show that nuclear lamina dysfunction resulting from loss of Otefin activates a DNA damage-independent germ stem cell-specific checkpoint, mediated by the ATR and Chk2 kinases, which ensures that healthy gametes are passed on to the next generation.

LEM domain-containing protein 3 antagonizes TGFβ-SMAD2/3 signaling in a stiffness-dependent manner in both the nucleus and cytosol

Transforming growth factor-β (TGFβ) signaling through SMAD2/3 is an important driver of pathological fibrosis in multiple organ systems. TGFβ signaling and extracellular matrix (ECM) stiffness form an unvirtuous pathological circuit in which matrix stiffness drives activation of latent TGFβ, and TGFβ signaling then drives cellular stress and ECM synthesis. Moreover, ECM stiffness also appears to sensitize cells to exogenously activated TGFβ through unknown mechanisms. Here, using human fibroblasts, we explored the effect of ECM stiffness on a putative inner nuclear membrane protein, LEM domain-containing protein 3 (LEMD3), which is physically connected to the cell's actin cytoskeleton and inhibits TGFβ signaling. We showed that LEMD3-SMAD2/3 interactions are inversely correlated with ECM stiffness and TGFβ-driven luciferase activity and that LEMD3 expression is correlated with the mechanical response of the TGFβ-driven luciferase reporter. We found that actin polymerization but not cellular stress or LEMD3-nuclear-cytoplasmic couplings were necessary for LEMD3-SMAD2/3 interactions. Intriguingly, LEMD3 and SMAD2/3 frequently interacted in the cytosol, and we discovered LEMD3 was proteolytically cleaved into protein fragments. We confirmed that a consensus C-terminal LEMD3 fragment binds SMAD2/3 in a stiffness-dependent manner throughout the cell and is sufficient for antagonizing SMAD2/3 signaling. Using human lung biopsies, we observed that these nuclear and cytosolic interactions are also present in tissue and found that fibrotic tissues exhibit locally diminished and cytoplasmically shifted LEMD3-SMAD2/3 interactions, as noted in vitro Our work reveals novel LEMD3 biology and stiffness-dependent regulation of TGFβ by LEMD3, providing a novel target to antagonize pathological TGFβ signaling.

Laminopathies: what can humans learn from fruit flies

Lamin proteins are type V intermediate filament proteins (IFs) located inside the cell nucleus. They are evolutionarily conserved and have similar domain organization and properties to cytoplasmic IFs. Lamins provide a skeletal network for chromatin, the nuclear envelope, nuclear pore complexes and the entire nucleus. They are also responsible for proper connections between the karyoskeleton and structural elements in the cytoplasm: actin and the microtubule and cytoplasmic IF networks. Lamins affect transcription and splicing either directly or indirectly. Translocation of active genes into the close proximity of nuclear lamina is thought to result in their transcriptional silencing. Mutations in genes coding for lamins and interacting proteins in humans result in various genetic disorders, called laminopathies. Human genes coding for A-type lamin (LMNA) are the most frequently mutated. The resulting phenotypes include muscle, cardiac, neuronal, lipodystrophic and metabolic pathologies, early aging phenotypes, and combined complex phenotypes. The Drosophila melanogaster genome codes for lamin B-type (lamin Dm), lamin A-type (lamin C), and for LEM-domain proteins, BAF, LINC-complex proteins and all typical nuclear proteins. The fruit fly system is simpler than the vertebrate one since in flies there is only single lamin B-type and single lamin A-type protein, as opposed to the complex system of B- and A-type lamins in Danio, Xenopus and Mus musculus. This offers a unique opportunity to study laminopathies. Applying genetic tools based on Gal4 and in vitro nuclear assembly system to the fruit fly model may successfully advance knowledge of laminopathies. Here, we review studies of the laminopathies in the fly model system.

Influence of non-steroidal anti-inflammatory drugs on Drosophila melanogaster longevity

Most age-related diseases and aging itself are associated with chronic inflammation. Thus pharmacological inhibition of inflammatory processes may be effective antiaging strategy. In this study we demonstrated that treatment of Drosophila melanogaster with 10 non-steroidal anti-inflammatory drugs (NSAIDs: CAY10404, aspirin, APHS, SC-560, NS-398, SC-58125, valeroyl salicylate, trans-resveratrol, valdecoxib, licofelone) leads to extension of lifespan, delays age-dependent decline of locomotor activity and increases stress resistance. The effect of the lifespan increase was associated with decrease of fecundity. Depending on the concentration, NSAIDs demonstrated both anti- and pro-oxidant properties in Drosophila tissues. However, we failed to identify clear correlation between antioxidant properties of NSAIDs and their pro-longevity effects. The lifespan extending effects of APHS, SC-58125, valeroyl salicylate, trans-resveratrol, valdecoxib, and licofelone were more pronounced in males, valdecoxib and aspirin - in females. We demonstrated that lifespan extension effect of NSAIDs was abolished in flies with defective genes involved in Pkh2-ypk1-lem3-tat2 pathway.

Unique and shared functions of nuclear lamina LEM domain proteins in Drosophila

The nuclear lamina is an extensive protein network that contributes to nuclear structure and function. LEM domain (LAP2, emerin, MAN1 domain, LEM-D) proteins are components of the nuclear lamina, identified by a shared ∼45-amino-acid motif that binds Barrier-to-autointegration factor (BAF), a chromatin-interacting protein. Drosophila melanogaster has three nuclear lamina LEM-D proteins, named Otefin (Ote), Bocksbeutel (Bocks), and dMAN1. Although these LEM-D proteins are globally expressed, loss of either Ote or dMAN1 causes tissue-specific defects in adult flies that differ from each other. The reason for such distinct tissue-restricted defects is unknown. Here, we generated null alleles of bocks, finding that loss of Bocks causes no overt adult phenotypes. Next, we defined phenotypes associated with lem-d double mutants. Although the absence of individual LEM-D proteins does not affect viability, loss of any two proteins causes lethality. Mutant phenotypes displayed by lem-d double mutants differ from baf mutants, suggesting that BAF function is retained in animals with a single nuclear lamina LEM-D protein. Interestingly, lem-d double mutants displayed distinct developmental and cellular mutant phenotypes, suggesting that Drosophila LEM-D proteins have developmental functions that are differentially shared with other LEM-D family members. This conclusion is supported by studies showing that ectopically produced LEM-D proteins have distinct capacities to rescue the tissue-specific phenotypes found in single lem-d mutants. Our findings predict that cell-specific mutant phenotypes caused by loss of LEM-D proteins reflect both the constellation of LEM-D proteins within the nuclear lamina and the capacity of functional compensation of the remaining LEM-D proteins.

Destination: inner nuclear membrane

The inner nuclear membrane (INM) of eukaryotic cells is enriched in proteins that are required for nuclear structure, chromosome organization, DNA repair, and transcriptional control. Mislocalization of INM proteins is observed in a wide spectrum of human diseases; however, the mechanism by which INM proteins reach their final destination is poorly understood. In this review we discuss how investigating INM composition, dissecting targeting pathways of conserved INM proteins in multiple systems and analyzing the nuclear transport of viruses and signaling complexes have broadened our knowledge of INM transport to include both nuclear pore complex-dependent and -independent pathways. The study of these INM targeting pathways is important to understanding nuclear organization and in both normal and diseased cells.

Essential roles of LEM-domain protein MAN1 during organogenesis in Xenopus laevis and overlapping functions of emerin

Mutations in nuclear envelope proteins are linked to an increasing number of human diseases, called envelopathies. Mutations in the inner nuclear membrane protein emerin lead to X-linked Emery-Dreifuss muscular dystrophy, characterized by muscle weakness or wasting. Conversely, mutations in nuclear envelope protein MAN1 are linked to bone and skin disorders. Both proteins share a highly conserved domain, called LEM-domain. LEM proteins are known to interact with Barrier-to-autointegration factor and several transcription factors. Most envelopathies are tissue-specific, but knowledge on the physiological roles of related LEM proteins is still unclear. For this reason, we investigated the roles of MAN1 and emerin during Xenopus laevis organogenesis. Morpholino-mediated knockdown of MAN1 revealed that MAN1 is essential for the formation of eye, skeletal and cardiac muscle tissues. The MAN1 knockdown could be compensated by ectopic expression of emerin, leading to a proper organ development. Further investigations revealed that MAN1 is involved in regulation of genes essential for organ development and tissue homeostasis. Thereby our work supports that LEM proteins might be involved in signalling essential for organ development during early embryogenesis and suggests that loss of MAN1 may cause muscle and retina specific diseases.

The cell biology of open and closed mitosis

Mitosis is the process of one cell dividing into two daughters, such that each inherits a single and complete copy of the genome of their mother. This is achieved through the equal segregation of the sister chromatids between the daughter cells. However, beyond this simple principle, the partitioning of other cellular components between daughter cells appears to follow a large variety of patterns. We discuss here how the organization of the nuclear envelope during mitosis influences cell division and, subsequently, cellular identity.

Farnesylated nuclear proteins Kugelkern and lamin Dm0 affect nuclear morphology by directly interacting with the nuclear membrane

Nuclear shape changes are observed during a variety of developmental processes, pathological conditions and ageing. Here, the molecular mechanism is analyzed how the farnesylated nuclear proteins interact with the nuclear envelope and deform the phospholipid bilayer.

Nuclear shape changes are observed during a variety of developmental processes, pathological conditions, and ageing. The mechanisms underlying nuclear shape changes in the above-mentioned situations have mostly remained unclear. To address the molecular mechanism behind nuclear shape changes, we analyzed how the farnesylated nuclear envelope proteins Kugelkern and lamin Dm0 affect the structure of the nuclear membrane. We found that Kugelkern and lamin Dm0 affect nuclear shape without requiring filament formation or the presence of a classical nuclear lamina. We also could show that the two proteins do not depend on a group of selected inner nuclear membrane proteins for their localization to the nuclear envelope. Surprisingly, we found that farnesylated Kugelkern and lamin Dm0 protein constructs change the morphology of protein-free liposomes. Based on these findings, we propose that farnesylated proteins of the nuclear membrane induce nuclear shape changes by being asymmetrically inserted into the phospholipid bilayer via their farnesylated C-terminal part.

Lamin-binding Proteins

A- and B-type lamins are the major intermediate filaments of the nucleus. Lamins engage in a plethora of stable and transient interactions, near the inner nuclear membrane and throughout the nucleus. Lamin-binding proteins serve an amazingly diverse range of functions. Numerous inner-membrane proteins help anchor lamin filaments to the nuclear envelope, serving as part of the nuclear "lamina" network that is essential for nuclear architecture and integrity. Certain lamin-binding proteins of the inner membrane bind partners in the outer membrane and mechanically link lamins to the cytoskeleton. Inside the nucleus, lamin-binding proteins appear to serve as the "adaptors" by which the lamina organizes chromatin, influences gene expression and epigenetic regulation, and modulates signaling pathways. Transient interactions of lamins with key components of the transcription and replication machinery may provide an additional level of regulation or support to these essential events.

The Drosophila LEM-domain protein MAN1 antagonizes BMP signaling at the neuromuscular junction and the wing crossveins

BMP signaling responses are refined by distinct secreted and intracellular antagonists in different cellular and temporal contexts. Here, we show that the nuclear LEM-domain protein MAN1 is a tissue-specific antagonist of BMP signaling in Drosophila. MAN1 contains two potential Mad-binding sites. We generated MAN1DeltaC mutants, harbouring a MAN1 protein that lacks part of the C-terminus including the RNA recognition motif, a putative Mad-binding domain. MAN1DeltaC mutants show wing crossvein (CV) patterning defects but no detectable alterations in nuclear morphology. MAN1(DeltaC) pupal wings display expanded phospho-Mad (pMad) accumulation and ectopic expression of the BMP-responsive gene crossveinless-2 (cv-2) indicating that MAN1 restricts BMP signaling. Conversely, MAN1 overexpression in wing imaginal discs inhibited crossvein development and BMP signaling responses. MAN1 is expressed at high levels in pupal wing veins and can be activated in intervein regions by ectopic BMP signaling. The specific upregulation of MAN1 in pupal wing veins may thus represent a negative feedback circuit that limits BMP signaling during CV formation. MAN1DeltaC flies also show reduced locomotor activity, and electrophysiology recordings in MAN1DeltaC larvae uncover a new presynaptic role of MAN1 at the neuromuscular junction (NMJ). Genetic interaction experiments suggest that MAN1 is a BMP signaling antagonist both at the NMJ and during CV formation.

Tissue-specific defects are caused by loss of the Drosophila MAN1 LEM domain protein

The nuclear lamina represents a protein network required for nuclear structure and function. One family of lamina proteins is defined by an approximately 40-aa LAP2, Emerin, and MAN1 (LEM) domain (LEM-D) that binds the nonspecific DNA-binding protein, barrier-to-autointegration factor (BAF). Through interactions with BAF, LEM-D proteins serve as a bridge between chromosomes and the nuclear envelope. Mutations in genes encoding LEM-D proteins cause human laminopathies that are associated with tissue-restricted pathologies. Drosophila has five genes that encode proteins with LEM homology. Using yeast two-hybrid analyses, we demonstrate that four encode proteins that bind Drosophila (d)BAF. In addition to dBAF, dMAN1 associates with lamins, the LEM-D protein Bocksbeutel, and the receptor-regulated Smads, demonstrating parallel protein interactions with vertebrate homologs. P-element mobilization was used to generate null dMAN1 alleles. These mutants showed decreased viability, with surviving adults displaying male sterility, decreased female fertility, wing patterning and positioning defects, flightlessness, and locomotion difficulties that became more severe with age. Increased phospho-Smad staining in dMAN1 mutant wing discs is consistent with a role in transforming growth factor (TGF)-beta/bone morphogenic protein (BMP) signaling. The tissue-specific, age-enhanced dMAN1 mutant phenotypes are reminiscent of human laminopathies, suggesting that studies in Drosophila will provide insights into lamina dysfunction associated with disease.

In vivo dynamics of Drosophila nuclear envelope components

Nuclear pore complexes (NPCs) are multisubunit protein entities embedded into the nuclear envelope (NE). Here, we examine the in vivo dynamics of the essential Drosophila nucleoporin Nup107 and several other NE-associated proteins during NE and NPCs disassembly and reassembly that take place within each mitosis. During both the rapid mitosis of syncytial embryos and the more conventional mitosis of larval neuroblasts, Nup107 is gradually released from the NE, but it remains partially confined to the nuclear (spindle) region up to late prometaphase, in contrast to nucleoporins detected by wheat germ agglutinin and lamins. We provide evidence that in all Drosophila cells, a structure derived from the NE persists throughout metaphase and early anaphase. Finally, we examined the dynamics of the spindle checkpoint proteins Mad2 and Mad1. During mitotic exit, Mad2 and Mad1 are actively imported back from the cytoplasm into the nucleus after the NE and NPCs have reformed, but they reassociate with the NE only later in G1, concomitantly with the recruitment of the basket nucleoporin Mtor (the Drosophila orthologue of vertebrate Tpr). Surprisingly, Drosophila Nup107 shows no evidence of localization to kinetochores, despite the demonstrated importance of this association in mammalian cells.

Otefin, a nuclear membrane protein, determines the fate of germline stem cells in Drosophila via interaction with Smad complexes

Nuclear envelope proteins play important roles in chromatin organization, gene regulation, and signal transduction; however, the physiological role of these proteins remains elusive. We found that otefin (ote), which encodes a nuclear lamin-binding protein [corrected], is essential for germline stem cell (GSC) maintenance. We show that Ote, as an intrinsic factor, is both necessary and sufficient to regulate GSC fate. Furthermore, we demonstrate that ote is required for the Dpp/BMP signaling pathway to silence bam transcription. By structure-function analysis, we demonstrate that the nuclear membrane localization of Ote is essential for its role in GSC maintenance. Finally, we show that Ote physically interacts with Medea/Smad4 at the bam silencer element to regulate GSC fate. Thus, we demonstrate that specific nuclear membrane components mediate signal-dependent transcriptional effects to control stem cell behavior.

Invertebrate lamins

Lamins are the main component of the nuclear lamina and considered to be the ancestors of all intermediate filament proteins. They are localized mainly at the nuclear periphery where they form protein complexes with integral proteins of the nuclear inner membrane, transcriptional regulators, histones and chromatin modifiers. Studying lamins in invertebrate species has unique advantages including the smaller number of lamin genes in the invertebrate genomes and powerful genetic analyses in Caenorhabditis elegans and Drosophila melanogaster. These simpler nuclear lamina systems allow direct analyses of their structure and functions. Here we give an overview of recent advances in the field of invertebrate nuclear lamins with special emphasis on their evolution, assembly and functions.

Early sorting of inner nuclear membrane proteins is conserved

Spodoptera frugiperda (Sf9) importin-alpha-16 is a translocon-associated protein that participates in the early sorting pathway of baculovirus integral membrane proteins destined for the inner nuclear membrane (INM). To discern whether sorting intermediate protein complexes like those observed in insect cells are also formed with mammalian INM proteins, cross-linked complexes of importin-alpha-16 with human lamin B receptor (LBR) and nurim were examined. Both LBR and nurim cross-link with Sf9 importin-alpha-16 during cotranslational membrane integration and remain proximal with importin-alpha-16 after integration into the endoplasmic reticulum membrane and release from the translocon. Human cells encode several isoforms of importin-alpha; to determine whether any of these isoforms may recognize INM-directed proteins, they were tested for their ability to cross-link with the viral-derived INM sorting motif sequence. One cross-linked adduct was detected with a 16-kDa isoform encoded from KPNA4 (KPNA-4-16). KPNA-4-16 was easily detected in microsomal membranes prepared from KPNA4-16 recombinant virus-infected cells and was also detected in microsomes prepared from HeLa cells. Together these observations suggest that elements of the early sorting pathway of INM-directed proteins mediated by importin-alpha-16 are highly conserved, and mammalian KPNA-4-16 is a candidate partner in sorting integral membrane proteins to the INM.

Barrier-to-autointegration factor – a BAFfling little protein

Barrier-to-autointegration factor (BAF) is an abundant, highly conserved, small and essential protein that binds to dsDNA, chromatin, nuclear lamina proteins, histones and various transcription factors. It was discovered as a cellular component of retrovirus pre-integration complex that inhibits their autointegration in vitro. BAF is also required for many cellular functions, including the higher-order organization of chromatin and the transcription of specific genes. Recent findings suggest further roles for BAF, including nuclear envelope assembly, regulating specific developmental processes and regulating retrovirus infectivity. At least some of these roles are controlled by phosphorylation of the BAF N-terminus by the vaccinia-related kinase. Here, we give an overview of recent advances in the field of BAF with special emphasis on evolution, interacting partners and functions.

What MAN1 does to the Smads

The inner nuclear membrane protein MAN1 has been identified as an important factor in transforming growth factor beta/bone morphogenic protein (TGFbeta/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 TGFbeta/BMP signaling. Several independent studies have shown that MAN1 is antagonizing TGFbeta/BMP signaling through binding to regulatory Smads. Here, recent progress in understanding MAN1 functions is summarized and a model for MAN1-dependent regulation of TGFbeta/BMP signaling is proposed.

LEM‐Domain Proteins: New Insights into Lamin‐Interacting Proteins

LEM-domain proteins present a growing family of nonrelated inner nuclear membrane and intranuclear proteins, including emerin, MAN1, LEM2, several alternatively spliced isoforms of LAP2, and various uncharacterized proteins in higher eukaryotes as well as the Drosophila-specific proteins otefin and Bocksbeutel. LEM-domain proteins are involved in diverse cellular processes including replication and cell cycle control, chromatin organization and nuclear assembly, the regulation of gene expression and signaling pathways, as well as retroviral infection. Genetic analyses in different model organisms reveal new insights into the various functions of LEM-domain proteins, lamins, and their involvement in laminopathic diseases. All these findings as well as previously proposed ideas and models have been summarized to broaden our view of this exciting protein family.