Filamins: promiscuous organizers of the cytoskeleton

The role of FilGAP-filamin A interactions in mechanoprotection

Cells in mechanically active environments are subjected to high-amplitude exogenous forces that can lead to cell death. Filamin A (FLNa) may protect cells from mechanically induced death by mechanisms that are not yet defined. We found that mechanical forces applied through integrins enhanced Rac-mediated lamellae formation in FLNa-null but not FLNa-expressing cells. Suppression of force-induced lamella formation was mediated by repeat 23 of FLNa, which also binds FilGAP, a recently discovered Rac GTPase-activating protein (GAP). We found that FilGAP is targeted to sites of force transfer by FLNa. This force-induced redistribution of FilGAP was essential for the suppression of Rac activity and lamellae formation in cells treated with tensile forces. Depletion of FilGAP by small interfering RNA, inhibition of FilGAP activity by dominant-negative mutation or deletion of its FLNa-binding domain, all resulted in a dramatic force-induced increase of the percentage of annexin-V-positive cells. FilGAP therefore plays a role in protecting cells against force-induced apoptosis, and this function is mediated by FLNa.

In-frame deletion in the seventh immunoglobulin-like repeat of filamin C in a family with myofibrillar myopathy

Myofibrillar myopathies (MFMs) are an expanding and increasingly recognized group of neuromuscular disorders caused by mutations in DES, CRYAB, MYOT, and ZASP. The latest gene to be associated with MFM was FLNC; a p.W2710X mutation in the 24th immunoglobulin-like repeat of filamin C was shown to be the cause of a distinct type of MFM in several German families. We studied an International cohort of 46 patients from 39 families with clinically and myopathologically confirmed MFM, in which DES, CRYAB, MYOT, and ZASP mutations have been excluded. In patients from an unrelated family a 12-nucleotide deletion (c.2997_3008del) in FLNC resulting in a predicted in-frame four-residue deletion (p.Val930_Thr933del) in the seventh repeat of filamin C was identified. Both affected family members, mother and daughter, but not unrelated control individuals, carried the p.Val930_Thr933del mutation. The mutation is transcribed and, based on myopathological features and immunoblot analysis, it leads to an accumulation of dysfunctional filamin C in the myocytes. The study results suggest that the novel p.Val930_Thr933del mutation in filamin C is the cause of MFM but also indicate that filamin C mutations are a comparatively rare cause of MFM.

Terminal assembly of sarcomeric filaments by intermolecular beta-sheet formation

The contraction-relaxation cycle of muscle cells translates into large movements of several filament systems in sarcomeres, requiring special molecular mechanisms to maintain their structural integrity. Recent structural and functional data from three filaments harboring extensive arrays of immunoglobulin-like domains - titin, filamin and myomesin--have, for the first time, unraveled a common function of their terminal domains: assembly and anchoring of the respective filaments. In each case, the protein-protein interactions are mediated by antiparallel dimerization modules via intermolecular beta-sheets. These observations on terminal filament assembly indicate an attractive model for several other filament proteins that require structural characterization.

Structural basis of the migfilin-filamin interaction and competition with integrin beta tails

A link between sites of cell adhesion and the cytoskeleton is essential for regulation of cell shape, motility, and signaling. Migfilin is a recently identified adaptor protein that localizes at cell-cell and cell-extracellular matrix adhesion sites, where it is thought to provide a link to the cytoskeleton by interacting with the actin cross-linking protein filamin. Here we have used x-ray crystallography, NMR spectroscopy, and protein-protein interaction studies to investigate the molecular basis of migfilin binding to filamin. We report that the N-terminal portion of migfilin can bind all three human filamins (FLNa, -b, or -c) and that there are multiple migfilin-binding sites in FLNa. Human filamins are composed of an N-terminal actin-binding domain followed by 24 immunoglobulin-like (IgFLN) domains and we find that migfilin binds preferentially to IgFLNa21 and more weakly to IgFLNa19 and -22. The filamin-binding site in migfilin is localized between Pro(5) and Pro(19) and binds to the CD face of the IgFLNa21 beta-sandwich. This interaction is similar to the previously characterized beta 7 integrin-IgFLNa21 interaction and migfilin and integrin beta tails can compete with one another for binding to IgFLNa21. This suggests that competition between filamin ligands for common binding sites on IgFLN domains may provide a general means of modulating filamin interactions and signaling. In this specific case, displacement of integrin tails from filamin by migfilin may provide a mechanism for switching between different integrin-cytoskeleton linkages.

ASB2 targets filamins A and B to proteasomal degradation

The ordered series of proliferation and differentiation from hematopoietic progenitor cells is disrupted in leukemia, resulting in arrest of differentiation at immature proliferative stages. Characterizing the molecular basis of hematopoietic differentiation is therefore important for understanding and treating disease. Retinoic acid induces expression of ankyrin repeat-containing protein with a suppressor of cytokine signaling box 2 (ASB2) in acute promyelocytic leukemia cells, and ASB2 expression inhibits growth and promotes commitment, recapitulating an early step critical for differentiation. ASB2 is the specificity subunit of an E3 ubiquitin ligase complex and is proposed to exert its effects by regulating the turnover of specific proteins; however, no ASB2 substrates had been identified. Here, we report that ASB2 targets the actin-binding proteins filamin A and B for proteasomal degradation. Knockdown of endogenous ASB2 in leukemia cells delays retinoic acid-induced differentiation and filamin degradation; conversely, ASB2 expression in leukemia cells induces filamin degradation. ASB2 expression inhibits cell spreading, and this effect is recapitulated by knocking down both filamin A and filamin B. Thus, we suggest that ASB2 may regulate hematopoietic cell differentiation by modulating cell spreading and actin remodeling through targeting of filamins for degradation.

ECSM2, an endothelial specific filamin a binding protein that mediates chemotaxis

OBJECTIVE

We aimed to characterize the expression and function of a novel transcript that bioinformatics analysis predicted to be endothelial specific, called endothelial-specific molecule-2 (ECSM2).

METHODS AND RESULTS

A full-length cDNA was isolated and predicted ECSM2 to be a putative 205-amino acid transmembrane protein that bears no homology to any known protein. Quantitative polymerase chain reaction analysis in vitro and in situ hybridization analysis in vivo confirmed ECSM2 expression to be exclusively endothelial, and localization to the plasma membrane was shown. Knockdown of ECSM2 expression in human umbilical vein endothelial cells using siRNA resulted in both reduced chemotaxis and impaired tube formation on matrigel, a solubilized basement membrane, both processes involved in angiogenesis. A yeast 2 hybrid analysis using the ECSM2 intracellular domain identified filamin A as an interacting protein. This interaction was confirmed by precipitation of filamin-A from endothelial cell lysates by a GST-tagged intracellular domain of ECSM2.

CONCLUSIONS

This study is the first to characterize a novel cell surface protein ECSM2 that regulates endothelial chemotaxis and tube formation, and interacts with filamin A. These studies implicate a role for ECSM2 in angiogenesis via modulation of the actin cytoskeleton.

A B-Myb complex containing clathrin and filamin is required for mitotic spindle function

B-Myb is one member of the vertebrate Myb family of transcription factors and is ubiquitously expressed. B-Myb activates transcription of a group of genes required for the G2/M cell cycle transition by forming the dREAM/Myb-MuvB-like complex, which was originally identified in Drosophila. Mutants of zebrafish B-myb and Drosophila myb exhibit defects in cell cycle progression and genome instability. Although the genome instability caused by a loss of B-Myb has been speculated to be due to abnormal cell cycle progression, the precise mechanism remains unknown. Here, we have purified a B-Myb complex containing clathrin and filamin (Myb-Clafi complex). This complex is required for normal localization of clathrin at the mitotic spindle, which was previously reported to stabilize kinetochore fibres. The Myb-Clafi complex is not tightly associated with the mitotic spindles, suggesting that this complex ferries clathrin to the mitotic spindles. Thus, identification of the Myb-Clafi complex reveals a previously unrecognized function of B-Myb that may contribute to its role in chromosome stability, possibly, tumour suppression.

Specificity of ADAR-mediated RNA editing in newly identified targets

Adenosine deaminases that act on RNA (ADARs) convert adenosines to inosine in both coding and noncoding double-stranded RNA. Deficiency in either ADAR1 or ADAR2 in mice is incompatible with normal life and development. While the ADAR2 knockout phenotype can be attributed to the lack of editing of the GluR-B receptor, the embryonic lethal phenotype caused by ADAR1 deficiency still awaits clarification. Recently, massive editing was observed in noncoding regions of mRNAs in mice and humans. Moreover, editing was observed in protein-coding regions of four mRNAs encoding FlnA, CyFip2, Blcap, and IGFBP7. Here, we investigate which of the two active mammalian ADAR enzymes is responsible for editing of these RNAs and whether any of them could possibly contribute to the phenotype observed in ADAR knockout mice. Editing of Blcap, FlnA, and some sites within B1 and B2 SINEs clearly depends on ADAR1, while other sites depend on ADAR2. Based on our data, substrate specificities can be further defined for ADAR1 and ADAR2. Future studies on the biological implications associated with a changed editing status of the studied ADAR targets will tell whether one of them turns out to be directly or indirectly responsible for the severe phenotype caused by ADAR1 deficiency.

IKAP localizes to membrane ruffles with filamin A and regulates actin cytoskeleton organization and cell migration

Loss-of-function mutations in the IKBKAP gene, which encodes IKAP (ELP1), cause familial dysautonomia (FD), with defective neuronal development and maintenance. Molecular mechanisms leading to FD are poorly understood. We demonstrate that various RNA-interference-based depletions of IKAP lead to defective adhesion and migration in several cell types, including rat primary neurons. The defects could be rescued by reintroduction of wild-type IKAP but not by FD-IKAP, a truncated form of IKAP constructed according to the mutation found in the majority of FD patients. Cytosolic IKAP co-purified with proteins involved in cell migration, including filamin A, which is also involved in neuronal migration. Immunostaining of IKAP and filamin A revealed a distinct co-localization of these two proteins in membrane ruffles. Depletion of IKAP resulted in a significant decrease in filamin A localization in membrane ruffles and defective actin cytoskeleton organization, which both could be rescued by the expression of wild-type IKAP but not by FD-IKAP. No downregulation in the protein levels of paxillin or beclin 1, which were recently described as specific transcriptional targets of IKAP, was detected. These results provide evidence for the role of the cytosolic interactions of IKAP in cell adhesion and migration, and support the notion that cell-motility deficiencies could contribute to FD.

Cloning and characterization of a cDNA fragment encoding a Schistosoma mansoni actin-binding protein (Smfilamin)

To identify vaccine candidates for Schistosoma mansoni, the IgG fraction of rabbit antiserum raised against immature female worms affinity purified over a NP-40 extract of 3-h schistosomula was used to immunoscreen a cercarial lambdagt11 cDNA library. One clone with a 1.5-kb cDNA insert revealed an encoded peptide of 479 amino acids, which bears homology to human actin-binding protein (ABP-280=filamin). Northern blot analysis revealed a transcript of about 8.6 kb, indicating that the complete gene was not cloned. Overlapping clones, which encode a composite sequence of 983 amino acids (45% identity with filamin), were subsequently isolated from the cDNA library. The 1.5-kb insert was cloned into pGEX, overexpressed, and the 479 amino acid peptide purified. Western blot analysis using polyclonal antisera specific to the peptide identified a 280-kDa molecule in adult worm extracts. RT-PCR demonstrated that Smfilaimin is expressed in various stages. Immunofluorescence studies with specific antisera revealed a tegument-associated fluorescence in adult worms. IgG specific to the Smfilamin fragment showed 36.6% killing of schistosomules in an in vitro killing assay.

Trouble making the first move: interpreting arrested neuronal migration in the cerebral cortex

Postmitotic cortical neurons that fail to initiate migration can remain near their site of origin and form persistent periventricular nodular heterotopia (PH). In human telencephalon, this malformation is most commonly associated with Filamin-A (FLNa) mutations. The lack of genetic animal models that reliably produce PH has delayed our understanding of the underlying molecular mechanisms. This review examines PH pathogenesis using a new mouse model. Although PH have not been observed in Flna-deficient mice generated thus far, the loss of MEKK4, a regulator of Flna, produces striking PH in mice and offers insight into the mechanisms involved in neuronal migration initiation. Elucidating the basic functions of FLNa and associated molecules is crucial for understanding the causes of PH and for developing prevention for at-risk patients.

Palladin is an actin cross-linking protein that uses immunoglobulin-like domains to bind filamentous actin

Palladin is a recently described phosphoprotein that plays an important role in cell adhesion and motility. Previous studies have shown that palladin overexpression results in profound changes in actin organization in cultured cells. Palladin binds to the actin-associated proteins alpha-actinin, vasodilator-stimulated phosphoprotein, profilin, Eps8, and ezrin, suggesting that it may affect actin organization indirectly. To determine its molecular function in generating actin arrays, we purified palladin and asked if it is also capable of binding to F-actin directly. In co-sedimentation and differential sedimentation assays, palladin was found to both bind and cross-link actin filaments. This bundling activity was confirmed by fluorescence and electron microscopy. Palladin fragments were then purified and used to determine the sequences necessary to bind and bundle F-actin. The Ig3 domain of palladin bound to F-actin, and a palladin fragment containing Ig3, Ig4, and the region linking these domains was identified as a fragment that was able to bundle F-actin. Because palladin has multiple Ig domains, and only one of them binds to F-actin, this suggests that different Ig domains may be specialized for distinct biological functions. In addition, our results suggest a potential role for palladin in generating specialized, actin-based cell morphologies via both direct actin cross-linking activity and indirect scaffolding activity.

Structural basis of filamin A functions

Filamin A (FLNa) can effect orthogonal branching of F-actin and bind many cellular constituents. FLNa dimeric subunits have N-terminal spectrin family F-actin binding domains (ABDs) and an elongated flexible segment of 24 immunoglobulin (Ig) repeats. We generated a library of FLNa fragments to examine their F-actin binding to define the structural properties of FLNa that enable its various functions. We find that Ig repeats 9–15 contain an F-actin–binding domain necessary for high avidity F-actin binding. Ig repeats 16–24, where most FLNa-binding partners interact, do not bind F-actin, and thus F-actin does not compete with Ig repeat 23 ligand, FilGAP. Ig repeats 16–24 have a compact structure that suggests their unfolding may accommodate pre-stress–mediated stiffening of F-actin networks, partner binding, mechanosensing, and mechanoprotection properties of FLNa. Our results also establish the orientation of FLNa dimers in F-actin branching. Dimerization, mediated by FLNa Ig repeat 24, accounts for rigid high-angle FLNa/F-actin branching resistant to bending by thermal forces, and high avidity F-actin binding and cross-linking.

Molecular mechanics of filamin's rod domain

Rearrangement of the actin cytoskeleton is integral to cell shape and function. Actin-binding proteins, e.g., filamin, can naturally contribute to the mechanics and function of the actin cytoskeleton. The molecular mechanical bases for filamin's function in actin cytoskeletal reorganization are examined here using molecular dynamics simulations. Simulations are performed by applying forces ranging from 25 pN to 125 pN for 2.5 ns to the rod domain of filamin. Applying small loads ( approximately 25 pN) to filamin's rod domain supplies sufficient energy to alter the conformation of the N-terminal regions of the rod. These forces break local hydrogen bond coordination often enough to allow side chains to find new coordination partners, in turn leading to drastic changes in the conformation of filamin, for example, increasing the hydrophobic character of the N-terminal rod region and, alternatively, activating the C-terminal region to become increasingly stiff. These changes in conformation can lead to changes in the affinity of filamin for its binding partners. Therefore, filamin can function to transduce mechanical signals as well as preserve topology of the actin cytoskeleton throughout the rod domain.

Identification of CAP as a costameric protein that interacts with filamin C

Cbl-associated protein (CAP) is an adaptor protein that interacts with both signaling and cytoskeletal proteins. Here, we characterize the expression, localization and potential function of CAP in striated muscle. CAP is markedly induced during myoblast differentiation, and colocalizes with vinculin during costamerogenesis. In adult mice, CAP is enriched in oxidative muscle fibers, and it is found in membrane anchorage complexes, including intercalated discs, costameres, and myotendinous junctions. Using both yeast two-hybrid and proteomic approaches, we identified the sarcomeric protein filamin C (FLNc) as a binding partner for CAP. When overexpressed, CAP recruits FLNc to cell-extracellular matrix adhesions, where the two proteins cooperatively regulate actin reorganization. Moreover, overexpression of CAP inhibits FLNc-induced cell spreading on fibronectin. In dystrophin-deficient mdx mice, the expression and membrane localization of CAP is increased, concomitant with the elevated plasma membrane content of FLNc, suggesting that CAP may compensate for the reduced membrane linkage of the myofibrils due to the loss of the dystroglycan-sarcoglycan complex in these mice. Thus, through its interaction with FLNc, CAP provides another link between the myofibril cytoskeleton and the plasma membrane of muscle cells, and it may play a dynamic role in the regulation and maintenance of muscle structural integrity.

A novel Amoeba proteus 120 kDa actin-binding protein with only 1 filamin repeat and a coiled-coil region

A novel 120 kDa actin-binding protein (ApABP-F1) was found in Amoeba proteus. It was distributed throughout the cytoplasm, mainly in the subplasma membrane and perinuclear-nuclear areas, enriched in actin. The full-length cDNA of ApABP consisted of 2672 nucleotides with an open reading frame of 878 amino acids, giving a ~95 kDa protein with a theoretical pI value of 5.11. It had a novel domain organization pattern: the N terminus (residues 1-104) contained 1 calponin-homology (CH) domain, followed by only 1 region that was homologous to the filamin repeat (FR, residues 209-324), and a central region (residues 344-577) exhibiting a very high probability of coiled-coil formation, probably engaged in the observed protein dimerization. A phylogenetic tree constructed for CH domains from 25 various proteins revealed that the CH domain of ApABP was most related to that of the hypothetical mouse KIAA0903-like protein, whereas not much relationship to either filamins or the gelation factor (ABP-120) of Dictyostelium discoideum and Entamoeba histolytica was found.

Filamin B represses chondrocyte hypertrophy in a Runx2/Smad3-dependent manner

FILAMIN B, which encodes a cytoplasmic actin binding protein, is mutated in several skeletal dysplasias. To further investigate how an actin binding protein influences skeletogenesis, we generated mice lacking intact Filamin B. As observed in spondylocarpotarsal synostosis syndrome patients, Filamin B mutant mice display ectopic mineralization in many cartilaginous elements. This aberrant mineralization is due to ectopic chondrocyte hypertrophy similar to that seen in mice expressing Runx2 in chondrocytes. Accordingly, removing one copy of Runx2 rescues the Filamin B mutant phenotype, indicating that Filamin B is a regulator of Runx2 function during chondrocyte differentiation. Filamin B binds Smad3, which is known to interact with Runx2. Smad3 phosphorylation is increased in the mutant mice. Thus, Filamin B inhibits Runx2 activity, at least in part, through the Smad3 pathway. Our results uncover the involvement of actin binding proteins during chondrogenesis and provide a molecular basis to a human genetic disease.

Structure of three tandem filamin domains reveals auto-inhibition of ligand binding

Human filamins are large actin-crosslinking proteins composed of an N-terminal actin-binding domain followed by 24 Ig-like domains (IgFLNs), which interact with numerous transmembrane receptors and cytosolic signaling proteins. Here we report the 2.5 A resolution structure of a three-domain fragment of human filamin A (IgFLNa19-21). The structure reveals an unexpected domain arrangement, with IgFLNa20 partially unfolded bringing IgFLNa21 into close proximity to IgFLNa19. Notably the N-terminus of IgFLNa20 forms a beta-strand that associates with the CD face of IgFLNa21 and occupies the binding site for integrin adhesion receptors. Disruption of this IgFLNa20-IgFLNa21 interaction enhances filamin binding to integrin beta-tails. Structural and functional analysis of other IgFLN domains suggests that auto-inhibition by adjacent IgFLN domains may be a general mechanism controlling filamin-ligand interactions. This can explain the increased integrin binding of filamin splice variants and provides a mechanism by which ligand binding might impact filamin structure.

Disruption of the Flnb gene in mice phenocopies the human disease spondylocarpotarsal synostosis syndrome

Spondylocarpotarsal synostosis syndrome (SCT) is an autosomal recessive disease that is characterized by short stature, and fusions of the vertebrae and carpal and tarsal bones. SCT results from homozygosity or compound heterozygosity for nonsense mutations in FLNB. FLNB encodes filamin B, a multifunctional cytoplasmic protein that plays a critical role in skeletal development. Protein extracts derived from cells of SCT patients with nonsense mutations in FLNB did not contain filamin B, demonstrating that SCT results from absence of filamin B. To understand the role of filamin B in skeletal development, an Flnb-/- mouse model was generated. The Flnb-/- mice were phenotypically similar to individuals with SCT as they exhibited short stature and similar skeletal abnormalities. Newborn Flnb-/- mice had fusions between the neural arches of the vertebrae in the cervical and thoracic spine. At postnatal day 60, the vertebral fusions were more widespread and involved the vertebral bodies as well as the neural arches. In addition, fusions were seen in sternum and carpal bones. Analysis of the Flnb-/- mice phenotype showed that an absence of filamin B causes progressive vertebral fusions, which is contrary to the previous hypothesis that SCT results from failure of normal spinal segmentation. These findings suggest that spinal segmentation can occur normally in the absence of filamin B, but the protein is required for maintenance of intervertebral, carpal and sternal joints, and the joint fusion process commences antenatally.

Interactions with filamin A stimulate surface expression of large-conductance Ca2+-activated K+ channels in the absence of direct actin binding

Large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels play an important role in the regulation of cell physiology in a wide variety of excitable and nonexcitable tissues. Filamin A is a conserved and ubiquitous actin-binding protein that forms perpendicular actin cross-links and contributes to changes in cell shape, stiffness, and motility. A variety of membrane proteins bind to filamin A, which regulates their trafficking in and out of the plasma membrane. Filamin A is therefore believed to couple membrane dynamics with those of the underlying cytoskeleton. Filamin A was identified in a yeast two-hybrid screen of a neuronal transcriptome using a subunit of BK(Ca) channels as bait, and the interaction was confirmed by a variety of biochemical assays in native neuronal cells and in human embryonic kidney 293T cells expressing BK(Ca) channels. BK(Ca) channels do not traffic to the plasma membrane in M2 melanoma cells, which lack filamin A, but normal trafficking is seen in A7 cells, which express filamin A, or in M2 cells transiently transfected with filamin A. It is noteworthy that stimulation of plasma membrane expression of BK(Ca) channels also occurs when M2 cells are transfected with filamin A constructs that lack the actin binding domain and that do not bind actin in vivo or in vitro. Filamin A is necessary for normal trafficking of BK(Ca) channels to the plasma membrane, but this effect does not require interactions with actin microfilaments, and it is possible that other actions of the filamin family of scaffolding proteins are independent of effects on actin.

Filamin A is mutated in X-linked chronic idiopathic intestinal pseudo-obstruction with central nervous system involvement

We have previously reported that an X-linked recessive form of chronic idiopathic intestinal pseudo-obstruction (CIIPX) maps to Xq28. To select candidate genes for the disease, we analyzed the expression in murine fetal brain and intestine of 56 genes from the critical region. We selected and sequenced seven genes and found that one affected male from a large CIIPX-affected kindred bears a 2-bp deletion in exon 2 of the FLNA gene that is present at the heterozygous state in the carrier females of the family. The frameshift mutation is located between two close methionines at the filamin N terminus and is predicted to produce a protein truncated shortly after the first predicted methionine. Loss-of-function FLNA mutations have been associated with X-linked dominant nodular ventricular heterotopia (PVNH), a central nervous system (CNS) migration defect that presents with seizures in females and lethality in males. Notably, the affected male bearing the FLNA deletion had signs of CNS involvement and potentially has PVNH. To understand how the severe frameshift mutation we found can explain the CIIPX phenotype and its X-linked recessive inheritance, we transiently expressed both the wild- type and mutant filamin in cell culture and found that filamin translation can start from either of the two initial methionines in these conditions. Therefore, translation of a normal shorter filamin can occur in vitro from the second methionine downstream of the 2-bp insertion we found. We confirmed this, demonstrating that the filamin protein is present in the patient's lymphoblastoid cell line that shows abnormal cytoskeletal actin organization compared with normal lymphoblasts. We conclude that the filamin N terminal region between the initial two methionines is crucial for proper enteric neuron development.

Expression, crystallization and preliminary crystallographic data analysis of filamin A repeats 14-16

Human filamin A is a 280 kDa protein involved in actin-filament cross-linking. It is structurally divided into an actin-binding headpiece (ABD) and a rod domain containing 24 immunoglobulin-like (Ig) repeats. A fragment of human filamin A (Ig repeats 14-16) was cloned and expressed in Escherichia coli and the purified protein was crystallized in 1.6 M ammonium sulfate, 2% PEG 1000 and 100 mM HEPES pH 7.5. The crystals diffracted to 1.95 A and belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 50.63, b = 52.10, c = 98.46 A, alpha = beta = gamma = 90 degrees.

Filamin B deficiency in mice results in skeletal malformations and impaired microvascular development

Mutations in filamin B (FLNB), a gene encoding a cytoplasmic actin-binding protein, have been found in human skeletal disorders, including boomerang dysplasia, spondylocarpotarsal syndrome, Larsen syndrome, and atelosteogenesis phenotypes I and III. To examine the role of FLNB in vivo, we generated mice with a targeted disruption of Flnb. Fewer than 3% of homozygous embryos reached term, indicating that Flnb is important in embryonic development. Heterozygous mutant mice were indistinguishable from their wild-type siblings. Flnb was ubiquitously expressed; strong expression was found in endothelial cells and chondrocytes. Flnb-deficient fibroblasts exhibited more disorganized formation of actin filaments and reduced ability to migrate compared with wild-type controls. Flnb-deficient embryos exhibited impaired development of the microvasculature and skeletal system. The few Flnb-deficient mice that were born were very small and had severe skeletal malformations, including scoliotic and kyphotic spines, lack of intervertebral discs, fusion of vertebral bodies, and reduced hyaline matrix in extremities, thorax, and vertebrae. These mice died or had to be euthanized before 4 weeks of age. Thus, the phenotypes of Flnb-deficient mice closely resemble those of human skeletal disorders with mutations in FLNB.

Crystal structure of human filamin C domain 23 and small angle scattering model for filamin C 23-24 dimer

Filamin C is a dimeric, actin-binding protein involved in organization of cortical cytoskeleton and of the sarcomere. We performed crystallographic, small-angle X-ray scattering and analytical ultracentrifugation experiments on the constructs containing carboxy-terminal domains of the protein (domains 23-24 and 19-21). The crystal structure of domain 23 of filamin C showed that the protein adopts the expected immunoglobulin (Ig)-like fold. Small-angle X-ray scattering experiments performed on filamin C tandem Ig-like domains 23 and 24 reveal a dimer that is formed by domain 24 and that domain 23 has little interactions with itself or with domain 24, while the analytical ultracentrifugation experiments showed that the filamin C domains 19-21 form elongated monomers in diluted solutions.