Characterization of the actin binding site on smooth muscle filamin

Comparative expression of the extracellular calcium-sensing receptor in the mouse, rat, and human kidney

The calcium-sensing receptor (CaSR) was cloned over 20 years ago and functionally demonstrated to regulate circulating levels of parathyroid hormone by maintaining physiological serum ionized calcium concentration ([Ca(2+)]). The receptor is highly expressed in the kidney; however, intrarenal and intraspecies distribution remains controversial. Recently, additional functions of the CaSR receptor in the kidney have emerged, including parathyroid hormone-independent effects. It is therefore critical to establish unequivocally the localization of the CaSR in the kidney to relate this to its proposed physiological roles. In this study, we determined CaSR expression in mouse, rat, and human kidneys using in situ hybridization, immunohistochemistry (using 8 different commercially available and custom-made antibodies), and proximity ligation assays. Negative results in mice with kidney-specific CaSR ablation confirmed the specificity of the immunohistochemistry signal. Both in situ hybridization and immunohistochemistry showed CaSR expression in the thick ascending limb, distal tubule, and collecting duct of all species, with the thick ascending limb showing the highest levels. Within the collecting ducts, there was significant heterogeneity of expression between cell types. In the proximal tubule, lower levels of immunoreactivity were detected by immunohistochemistry and proximity ligation assays. Proximity ligation assays were the only technique to demonstrate expression within glomeruli. This study demonstrated CaSR expression throughout the kidney with minimal discrepancy between species but with significant variation in the levels of expression between cell and tubule types. These findings clarify the intrarenal distribution of the CaSR and enable elucidation of the full physiological roles of the receptor within this organ.

Expression of FLNa in human melanoma cells regulates the function of integrin α1β1 and phosphorylation and localisation of PKB/AKT/ERK1/2 kinases

FLNa is a ubiquitous cytoskeletal protein that links transmembrane receptors, including integrins, to F-actin and functions as a signalling intermediate. We investigated FLNa's role in the function of integrin-type collagen receptors, EGF-EGFR signalling and regulation of PKB/Akt and ERK1/2. Using FLNa-deficient M2 human melanoma cells, and same cells expressing EGFP-FLNa (M2F) or its Ig-like repeats 1-8+24, 8-15+24 and 16-24, we found that in M2F and M2 8-15+24 cells, EGF induced the increased phosphorylation of PKB/Akt and ERK1/2. In M2F cells EGF induced the localisation of these kinases to cell nucleus and lamellipodia, respectively, and the ERK1/2 phosphorylation-dependent co-immunoprecipitation of FLNa with ERK1/2. Only M2F and M2 8-15+24 cells adhered to and spread on type I collagen whereas on fibronectin all cells behaved similarly. α1β1 and α2β1 were the integrin-type collagen receptors expressed on these cells with primarily α1β1 localising to focal contacts and affecting cell adhesion and migration in a manner dependent on FLNa or its Ig-like repeats 8-15. Our results suggest a role for FLNa repeats 8-15 in the α1-subunit-dependent regulation of integrin α1β1 function, EGF-EGFR signalling to PKB/Akt and ERK1/2, identify ERK1/2 in EGF-induced FLNa-associated protein complexes, and show that the function of different integrins is subjected to differential regulation by FLNa.

Molecular mechanisms of thoracic aortic dissection

Thoracic aortic dissection (TAD) is a highly lethal vascular disease. In many patients with TAD, the aorta progressively dilates and ultimately ruptures. Dissection formation, progression, and rupture cannot be reliably prevented pharmacologically because the molecular mechanisms of aortic wall degeneration are poorly understood. The key histopathologic feature of TAD is medial degeneration, a process characterized by smooth muscle cell depletion and extracellular matrix degradation. These structural changes have a profound impact on the functional properties of the aortic wall and can result from excessive protease-mediated destruction of the extracellular matrix, altered signaling pathways, and altered gene expression. Review of the literature reveals differences in the processes that lead to ascending versus descending and sporadic versus hereditary TAD. These differences add to the complexity of this disease. Although tremendous progress has been made in diagnosing and treating TAD, a better understanding of the molecular, cellular, and genetic mechanisms that cause this disease is necessary to developing more effective preventative and therapeutic treatment strategies.

Crystal structure of the filamin N-terminal region reveals a hinge between the actin binding and first repeat domains

The filamin proteins cross-link F-actin and interact with protein partners to integrate both extracellular and intracellular signalling events with the cytoskeleton and to provide mechanoprotection and sensing to cells. The filamins are large, flexible, multi-domain homodimers with the interactions between domains important for protein function. The crystal structure of the N-terminal region of filamin B, containing the actin binding domain (ABD) and the first filamin repeat (FR1) domain, reveals an extended two-domain conformation with no interaction between the ABD and FR1 other than the connecting linker region. The two FLNB347 structures in the crystallographic asymmetric unit exhibit differing relative domain orientations providing the first high-resolution structural characterisation of a filamin inter-domain conformational change. The structure reveals a new hinge in the linker region between ABD and FR1 that is ideally positioned to orient the ABD for actin binding and adds to the previously described hinge regions, hinge 1 (between repeats 15 and 16) and hinge 2 (repeats 23 and 24), providing an additional mechanism by which filamin can exhibit inter-domain flexibility. The extended structure, with the absence of interactions between the domains, implies that any conformational rearrangements required for actin binding by the ABD, as observed for homologous proteins, can freely occur without being influenced by FR1. The ABD retains its previously observed compact conformation. FR1 exhibits a filamin immunoglobulin-like domain fold with a closed C-D β-strand groove, in contrast to filamin repeats that bind protein partners with this region of the domain surface.

Disease-associated substitutions in the filamin B actin binding domain confer enhanced actin binding affinity in the absence of major structural disturbance: Insights from the crystal structures of filamin B actin binding domains

Missense mutations in filamin B (FLNB) are associated with the autosomal dominant atelosteogenesis (AO) and the Larsen group of skeletal malformation disorders. These mutations cluster in particular FLNB protein domains and act in a presumptive gain-of-function mechanism. In contrast the loss-of-function disorder, spondylocarpotarsal synostosis syndrome, is characterised by the complete absence of FLNB. One cluster of AO missense mutations is found within the second of two calponin homology (CH) domains that create a functional actin-binding domain (ABD). This N-terminal ABD is required for filamin F-actin crosslinking activity, a crucial aspect of filamin's role of integrating cell-signalling events with cellular scaffolding and mechanoprotection. This study characterises the wild type FLNB ABD and investigates the effects of two disease-associated mutations on the structure and function of the FLNB ABD that could explain a gain-of-function mechanism for the AO diseases. We have determined high-resolution X-ray crystal structures of the human filamin B wild type ABD, plus W148R and M202V mutants. All three structures display the classic compact monomeric conformation for the ABD with the CH1 and CH2 domains in close contact. The conservation of tertiary structure in the presence of these mutations shows that the compact ABD conformation is stable to the sequence substitutions. In solution the mutant ABDs display reduced melting temperatures (by 6-7 degrees C) as determined by differential scanning fluorimetry. Characterisation of the wild type and mutant ABD F-actin binding activities via co-sedimentation assays shows that the mutant FLNB ABDs have increased F-actin binding affinities, with dissociation constants of 2.0 microM (W148R) and 0.56 microM (M202V), compared to the wild type ABD K(d) of 7.0 microM. The increased F-actin binding affinity of the mutants presents a biochemical mechanism that differentiates the autosomal dominant gain-of-function FLNB disorders from those that arise through the complete loss of FLNB protein.

Differential localization of autolyzed calpains 1 and 2 in slow and fast skeletal muscles in the early phase of atrophy

Calpains have been proposed to be involved in the cytoskeletal remodeling and wasting of skeletal muscle. However, limited data are available about the specific involvement of each calpain in the early stages of muscle atrophy. The aims of this study were to determine whether calpains 1 and 2 are autolyzed after a short period of muscle disuse, and, if so, where in the myofibers the autolyzed products are localized. In the rat soleus muscle, 5 days of immobilization increased autolyzed calpain 1 in the particulate and not the soluble fraction. Conversely, autolyzed calpain 2 was not found in the particulate fraction, whereas it was increased in the soluble fraction after immobilization. In the less atrophied plantaris muscle, no difference was noted between the control and immobilized groups whatever the fraction or calpain. Other proteolytic pathways were also investigated. The ubiquitin-proteasome pathway was activated in both skeletal muscles, and caspase 3 was activated only in the soleus muscle. Taken together, our data suggest that calpains 1 and 2 are involved in atrophy development in slow type muscle exclusively and that they have different regulation and protein targets. Moreover, the activation of proteolytic pathways appears to differ in slow and fast muscles, and the proteolytic mechanisms involved in fast-type muscle atrophy remain unclear.

Calpain 1-gamma filamin interaction in muscle cells: a possible in situ regulation by PKC-alpha

Calpains are a family of calcium-dependent cysteine-proteases involved in cytoskeleton remodelling and muscle differentiation. In a recent study, we observed the presence of calpain 1 in the muscle contractile apparatus and specifically in the N1- and N2-lines. This calpain isoform was found to be involved in the degradation of muscle fibres via proteolysis of key proteins in Z-disk and costameric junctions. The goal of this study was to determine whether gamma-filamin--a specific muscle isoform of the filamin family--is a calpain 1 substrate and to characterise this interaction. Gamma-filamin is a major muscle architectural protein located in the Z-line and under the sarcolemmal membrane. This protein is a component of the chain binding the sarcolemma to the sarcomeric structure. In this study, we found that gamma-filamin formed a stable complex in vitro and in cells with calpain 1 in the absence of calcium stimulation. We also located the binding domains in the C-terminus of gamma-filamin with a cleavage site between serine 2626 and serine 2627 in the hinge 2 region. The catalytic (80 kDa) and regulatory (28 kDa) subunits of calpain 1 are both involved in high affinity binding at gamma-filamin. Moreover, we showed that phosphorylation of the filamin C-terminus domain by PKC alpha protected gamma-filamin against proteolysis by calpain 1 in COS cells. Stimulation of PKC activity in myotubes, prevented gamma-filamin proteolysis by calpain and resulted in an increase in myotube adhesion.

The Crystal Structure of the Actin Binding Domain from α-Actinin in its Closed Conformation: Structural Insight into Phospholipid Regulation of α-Actinin

Alpha-actinin is the major F-actin crosslinking protein in both muscle and non-muscle cells. We report the crystal structure of the actin binding domain of human muscle alpha-actinin-3, which is formed by two consecutive calponin homology domains arranged in a "closed" conformation. Structural studies and available biochemical data on actin binding domains suggest that two calponin homology domains come in a closed conformation in the native apo-form, and that conformational changes involving the relative orientation of the two calponin homology domains are required for efficient binding to actin filaments. The actin binding activity of muscle isoforms is supposed to be regulated by phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), which binds to the second calponin homology domain. On the basis of structural analysis we propose a distinct binding site for PtdIns(4,5)P2, where the fatty acid moiety would be oriented in a direction that allows it to interact with the linker sequence between the actin binding domain and the first spectrin-like repeat, regulating thereby the binding of the C-terminal calmodulin-like domain to this linker.

Molecular Structure of the Rod Domain of Dictyostelium Filamin

Dictyostelium discoideum filamin (ddFLN) is a two-chain F-actin crosslinking protein with an N-terminal actin-binding domain and a rod domain constructed from six tandem repeats of a 100 residue motif that has an immunoglobulin (Ig) fold. We report the 2.8 A resolution crystal structure of a homodimer of rod repeats 4, 5 and 6. The two chains are arranged in an antiparallel fashion and form an elongated element, which is shortened, however, compared to a fully extended, linear configuration because the long axis of each Ig domain is arranged at an angle to the long axis of the rod. Same arrangement of repeats should also be present in the rod domain of human FLNa, much longer than Dictyostelium FLN, which forms an extended structure able to crosslink F-actin chains over distances of more than 1000 A.

Calpain 3 is expressed in astrocytes of rat and Microcebus brain

The calcium-dependent protease calpain is involved in numerous functions, including the control of cell survival, plasticity and motility. Whereas the isoforms calpain 1 and 2 have been described as ubiquitously expressed enzymes, calpain 3 has been called "muscle-specific", although trace amounts of calpain 3 mRNA have been detected by Northern blot in brain homogenates. In this study, we validated antibodies raised either against the peptides that were specific for a given isoform or the peptides present in all the three isoforms. We then used the anti-calpain 3 antibodies together with antibodies directed against cell-type-specific proteins to determine by double- and triple-labelling immunocytochemistry if the protease is expressed in specific cell populations of rat as well as lesser mouse lemur (Microcebus murinus) brain. Calpain 3 was almost exclusively found in cells displaying astrocyte morphology. These cells, most of which co-expressed glial fibrillary acidic protein, were particularly numerous close to the striatal subventricular zone (where numerous neurones forming the rostral migratory stream (RMS) towards the olfactory bulbs are generated) and the RMS itself. Other immunoreactive cells were found close to the pial surface of the forebrain, in the corpus callosum and in the dentate gyrus. Calpain 3 may be involved in astrocyte plasticity and/or motility.

Transient expression of the t-isoform of plastins/fimbrin in the stereocilia of developing auditory hair cells

The transduction of auditory signals by cochlear hair cells depends upon the integrity of hair cell stereociliary bundles. Stereocilia contain a central core of actin filaments, cross-linked by actin bundling proteins. In the cochlea, the two proteins described to date as responsible for the spatial arrangement of actin filaments in sterocilia are fimbrin and the recently discovered espin. Fimbrin (the chick homolog of human I-plastin) belongs to the plastins/fimbrin family that includes two additional isoforms of plastins, T- and L-plastin. In the present study, we used isoform specific antibodies to investigate the presence of the T- and L-isoforms of plastin/fimbrin in the adult and developing rat cochlea. We found that T-plastin, but not L-plastin, is expressed in the rat cochlea. During postnatal development of the rat organ of Corti, T-plastin can be detected in the core of stereocilia from early stages of hair cell differentiation, and its expression gradually increases in stereocilia as hair cells mature. However, as opposed to other actin-binding proteins expressed in stereocilia, T-plastin is absent from the stereocilia of mature hair cells. Such temporally restricted expression strengthens the idea of functional differences between plastins isoforms, and suggests that T-plastin could have a specific role in stereocilia formation.

Structural and functional aspects of filamins

Filamins are a family of high molecular mass cytoskeletal proteins that organize filamentous actin in networks and stress fibers. Over the past few years it has become clear that filamins anchor various transmembrane proteins to the actin cytoskeleton and provide a scaffold for a wide range of cytoplasmic signaling proteins. The recent cloning of three human filamins and studies on filamin orthologues from chicken and Drosophila revealed unexpected complexity of the filamin family, the biological implications of which have just started to be addressed. Expression of dysfunctional filamin-A leads to the genetic disorder of ventricular heterotopia and gives reason to expect that abnormalities in the other isogenes may also be connected with human disease. In this review aspects of filamin structure, its splice variants, binding partners and biological function will be discussed.

Filamins as integrators of cell mechanics and signalling

Filamins are large actin-binding proteins that stabilize delicate three-dimensional actin webs and link them to cellular membranes. They integrate cellular architectural and signalling functions and are essential for fetal development and cell locomotion. Here, we describe the history, structure and function of this group of proteins.

Mutual effects of alpha-actinin, calponin and filamin on actin binding

The mutual effect of three actin-binding proteins (alpha-actinin, calponin and filamin) on the binding to actin was analyzed by means of differential centrifugation and electron microscopy. In the absence of actin alpha-actinin, calponin and filamin do not interact with each other. Calponin and filamin do not interfere with each other in the binding to actin bundles. Slight interference was observed in the binding of alpha-actinin and calponin to actin bundles. Higher ability of calponin to depress alpha-actinin binding can be due to the higher stoichiometry calponin/actin in the complexes formed. The largest interference was observed in the pair filamin-alpha-actinin. These proteins interfere with each other in the binding to the bundled actin filaments; however, neither of them completely displaced another protein from its complexes with actin. The structure of actin bundles formed in the presence of any one actin-binding protein was different from that observed in the presence of binary mixtures of two actin-binding proteins. In the case of calponin or its binary mixtures with alpha-actinin or filamin the total stoichiometry actin-binding protein/actin was larger than 0.5. This means that alpha-actinin, calponin and filamin may coexist on actin filaments and more than mol of any actin-binding protein is bound per two actin monomers. This may be important for formation of different elements of cytoskeleton.

Crystal structure of the actin-binding region of utrophin reveals a head-to-tail dimer

BACKGROUND

Utrophin is a large multidomain protein that belongs to a superfamily of actin-binding proteins, which includes dystrophin, alpha-actinin, beta-spectrin, fimbrin, filamin and plectin. All the members of this family contain a common actin-binding region at their N termini and perform a wide variety of roles associated with the actin cytoskeleton. Utrophin is the autosomal homologue of dystrophin, the protein defective in the X-linked Duchenne and Becker muscular dystrophies, and upregulation of utrophin has been suggested as a potential therapy for muscular dystrophy patients.

RESULTS

The structure of the actin-binding region of utrophin, consisting of two calponin-homology (CH) domains, has been solved at 3.0 A resolution. It is composed of an antiparallel dimer with each of the monomers being present in an extended dumbell shape and the two CH domains being separated by a long central helix. This extended conformation is in sharp contrast to the compact monomer structure of the N-terminal actin-binding region of fimbrin.

CONCLUSIONS

The crystal structure of the actin-binding region of utrophin suggests that these actin-binding domains may be more flexible than was previously thought and that this flexibility may allow domain reorganisation and play a role in the actin-binding mechanism. Thus utrophin could possibly bind to actin in an extended conformation so that the sites previously identified as being important for actin binding may be directly involved in this interaction.

Calponin interaction with alpha-actinin-actin: evidence for a structural role for calponin

The purpose of this study was to address the paradox of calponin localization with alpha-actinin and filamin, two proteins with tandem calponin homology (CH) domains, by determining the effect of these proteins on the binding of calponin to actin. The results show that actin can accommodate near-saturating concentrations of either calponin and alpha-actinin or calponin and filamin with little change or no change in ligand affinity. Little direct interaction occurred between alpha-actinin and calponin in the absence of actin, so this effect is not likely to explain the co-distribution of these proteins. Calponin, like alpha-actinin, induced elastic gel formation when added to actin. When alpha-actinin was added to newly formed calponin/actin gels, no change was seen in the mechanical properties of the gel compared to calponin and actin alone. However, when calponin was added to newly formed alpha-actinin/actin gels, the resulting gel was much stronger than the gels formed by either ligand alone. Furthermore, gels formed by the addition of calponin to alpha-actinin/actin exhibited a phenomenon known as strain hardening, a characteristic of mechanically resilient gels. These results add weight to the concept that one of the functions of calponin is to stabilize the actin cytoskeleton.

Structural comparisons of calponin homology domains: implications for actin binding

BACKGROUND

The actin-binding site of several cytoskeletal proteins is comprised of two calponin homology (CH) domains in a tandem arrangement. As a single copy, the CH domain is also found in regulatory proteins in muscle and in signal-transduction proteins. The three-dimensional structures of three CH domains are known, but they have not yet clarified the molecular details of the interaction between actin filaments and proteins harbouring CH domains.

RESULTS

We have compared the crystal structure of a CH domain from beta-spectrin, which has been refined to 1.1 A resolution, with the two CH domains that constitute the actin-binding region of fimbrin. This analysis has allowed the construction of a structure-based sequence alignment of CH domains that can be used in further comparisons of members of the CH domain family. The study has also improved our understanding of the factors that determine domain architecture, and has led to discussion on the functional differences that seem to exist between subfamilies of CH domains, as regards binding to F-actin.

CONCLUSIONS

Our analysis supports biochemical data that implicate a surface centered at the last helix of the N-terminal CH domain as the most probable actin-binding site in cytoskeletal proteins. It is not clear whether the C-terminal domains of the tandem arrangement or the single CH domains have this function alone. This may imply that although the CH domains are homologous and have a conserved structure, they may have evolved to perform different functions.

Coevolution of actin and associated proteins: an alpha-actinin-like protein in a cyanobacterium (Spirulina platensis)

Actin, together with associated proteins, such as myosin, cross-linking or capping proteins, has been observed in all eukaryotic cells. Presence of actin or actin-like proteins has also been reported in prokaryotic organisms belonging to the cyanobacteria. Our aim was first to extend the characterization of an actin-like protein to another prokaryotic cell, i.e. Spirulina, then to compare the antigenic reactivity of this new protein with that of Synechocystis and skeletal actins. We observed that some of the conserved antigenic epitopes corresponded to actin regions known to interact with cross-linking proteins. We also report for the first time that alpha-actinin and filamin purified from chicken gizzard both interact with a prokaryotic actin-like protein. Finally, we searched for the occurrence of a cross-linking protein in these cyanobacteria and identified a 105-kDa protein as an alpha-actinin-like protein using specific antibodies.

Iqg1p, a yeast homologue of the mammalian IQGAPs, mediates cdc42p effects on the actin cytoskeleton

The Rho-type GTPase Cdc42p has been implicated in diverse cellular functions including cell shape, cell motility, and cytokinesis, all of which involve the reorganization of the actin cytoskeleton. Targets of Cdc42p that interface the actin cytoskeleton are likely candidates for mediating cellular activities. In this report, we identify and characterize a yeast homologue for the mammalian IQGAP, a cytoskeletal target for Cdc42p. The yeast IQGAP homologue, designated Iqg1p, displays a two-hybrid interaction with activated Cdc42p and coimmunoprecipitates with actin filaments. Deletion of IQG1 results in a temperature-sensitive lethality and causes aberrant morphologies including elongated and round multinucleated cells. This together with its localization at the mother-bud neck, suggest that Iqg1p promotes budding and cytokinesis. At restrictive temperatures, the vacuoles of the mutant cells enlarge and vesicles accumulate in the bud. Interestingly, Iqg1p shows two-hybrid interactions with the ankyrin repeat-containing protein, Akr1p (Kao, L.-R., J. Peterson, J. Ruiru, L. Bender, and A. Bender. 1996. Mol. Cell. Biol. 16:168-178), which inhibits pheromone signaling and appears to promote cytokinesis and/or trafficking. We also show two-hybrid interactions between Iqg1p and Afr1p, a septin-binding protein involved in projection formation (Konopka, J.B., C. DeMattei, and C. Davis. 1995. Mol. Cell. Biol. 15:723-730). We propose that Iqg1p acts as a scaffold to recruit and localize a protein complex involved in actin-based cellular functions and thus mediates the regulatory effects of Cdc42p on the actin cytoskeleton.

Identification of an actin cytoskeletal complex that includes IQGAP and the Cdc42 GTPase

The Rho subfamily of low molecular weight GTPases have been implicated in a variety of cellular functions that include reorganization of the actin cytoskeleton and stress-induced activation of the c-Jun kinase. The downstream targets that mediate the effects of Cdc42 on the actin cytoskeleton have yet to be fully identified. We have used the transient transfection of COS-7 cells with epitope-tagged Cdc42 to identify candidate signaling partners for this GTPase and identified the IQGAP protein as a major in vivo target for activated Cdc42. Epidermal growth factor stimulation of serum-starved COS-7 cells promoted the formation of a Cdc42-IQGAP complex, indicating that growth factors can increase the pool of activated Cdc42. Activated HA-Cdc42 co-localized with IQGAP or F-actin in vivo, whereas cells transfected with dominant-negative forms of Cdc42 (Cdc42(T17N)) showed predominantly dispersed distributions for both HA-Cdc42 and endogenous IQGAP. In detergent lysates from COS-7 cells transiently transfected with different forms of Cdc42, or from stably transfected CHO cells, the induction of actin polymerization by phalloidin resulted in the incorporation of both IQGAP and Cdc42 into actin-containing complexes. Taken together, these findings are consistent with a model whereby IQGAP serves as a target for GTP-bound Cdc42 providing a direct link between the activated GTPase and the actin cytoskeleton.

Identification of the region in actin-binding protein that binds to the cytoplasmic domain of glycoprotein IBalpha

Actin-binding protein (ABP-280) is a component of the submembranous cytoskeleton and interacts with the glycoprotein (GP) Ibalpha subunit of the GP Ib-IX complex in platelets. In the present studies, we have identified the binding site for GP Ibalpha in ABP-280. A melanoma cell line lacking ABP-280 was stably transfected with the cDNAs coding for GP Ib-IX, then transiently transfected with cDNA coding for various carboxyl-truncates of ABP-280. Immunocapture assays and co-immunoprecipitation experiments from detergent-lysed cells showed that deletion of the carboxyl-terminal repeats 20-24 of ABP-280 had no effect on GP Ib-IX binding, but deletion of residues 2099 through 2136 within repeat 19 abolished binding. In the yeast two-hybrid system, an ABP-280 fragment comprising repeats 17-19 bound GP Ibalpha. Deletion from either end abolished binding. Individual or multiple repeats of ABP-280 were expressed as fusion protein in bacteria and purified; structural folding was evaluated, and binding to GP Ib-IX was assessed. Binding depended on the presence of repeats 17-19. None of the individual repeats were able to bind to GP Ib-IX. These findings demonstrate that residues 1850-2136 comprising repeats 17-19 contain the binding site for GP Ib-IX.

Energy-filtered electron microscopy reveals that talin is a highly flexible protein composed of a series of globular domains

Talin is a multidomain cytoskeletal protein containing discrete binding sites for acidic phospholipids, beta-integrin, actin and vinculin. Hence, it is thought to link microfilaments to the cytoplasmic membrane in cell-matrix adhesion sites, and this should critically depend on talin structure. To obtain more information on the latter, we used energy-filtered transmission electron microscopy of negatively stained talin purified from chicken smooth muscle. We show that in buffers of physiological ionic strength, talin adopts an elongated shape (56 +/- 7 nm in length), consisting of a series of globular masses. While these compact elements, arranged like beads on a string, were of rather uniform dimensions (3.8 nm in diameter), their center-to-center spacings varied, indicating the flexibility of the connecting strands. The ends of the elongated molecules frequently formed loops. The images obtained are consistent with the assumption that, under the conditions used, the majority of the talin molecules are monomeric. A minor fraction appeared as dimers, composed of two chains only partially intertwined, thus giving rise to Y-shaped particles. Electron micrographs revealed that the biochemically defined 50-kDa N-terminal talin head domain is composed of two globular subunits, while chemical cross-linking provided evidence that the C-terminal 220-kDa fragment is solely responsible for dimerization. These results imply that in the dimeric molecules, the polypeptide chains are arranged in parallel, in contrast to what has been described for human-platelet talin. In buffers of low ionic strength (0.02 M instead of 0.15 M KCl), the molecules collapsed into a compact shape. By showing the high flexibility and versatility of its morphology, our data favour the concept of talin as an important resilient link in microfilament-plasma-membrane attachment.

Actin interaction with purified dystrophin from electric organ of Torpedo marmorata: possible resemblance with filamin-actin interface

We have purified dystrophin from Torpedo marmorata electric tissue by means of alkaline extraction in conjunction with an affinity chromatography column using anti-peptide antibodies. Using solution (cosedimentation) and solid phase experiments (sedimentation with Sepharose filamentous actin and ELISA), we have demonstrated that purified dystrophin is able to bind filamentous and monomeric actin. Using ELISA coupled with biotin labelled peptides and taking advantage of strong affinity binding of streptavidin-biotin complex, we have identified two exposed sequences of the actin molecule implicated in dystrophin binding: fragment 40-113, further restricted to peptide 75-106 and peptide 360-372. In a previous study, we have shown that fragment 40-113 displays binding site(s) for filamin but probably not for alpha-actinin. Moreover, we have recently reported that alpha-actinin and filamin display divergent behaviours towards conformational changes of actin. In this study, we have demonstrated that, similarly to filamin, dystrophin binding is insensitive to the locking of actin in a monomeric conformation. Taken together, these results lead us to favour the idea that dystrophin could share properties in common with filamin in its binding of actin.

Isolation and properties of white skeletal muscle alpha-actinin from sea-trout (Salmo trutta) and bass (Dicentrarchus labrax)

Fish alpha-actinin purified from sea-trout and bass white muscle by means of two different extraction procedures was used to investigate the eventual presence of different muscle isoforms in Z-disks. These fish alpha-actinins have the same apparent molecular weight (100 kDa) and the same isoelectric point (pI = 5.6), and also have a total antigenic identity towards anti-bass and anti-chicken alpha-actinin antibodies, suggesting a single molecular species. The role of fish alpha-actinin as an anchorage site for thin actin filaments and elastic titin filaments in Z-bands was studied. Despite conservation of the actin-binding site, fish alpha-actinin has a better actin-binding ability (kD = 0.3 microM) than chicken smooth muscle alpha-actinin (kD = 1.6 microM). Several other structural and functional characteristics of fish alpha-actinin were also studied: conservation of sequence and domain structure, the role of divalent ions (Ca2+, Mg2+) and the dielectric constant of the medium in alpha-actinin-actin interaction. Although the reason for fish white muscle alpha-actinin's close affinity to actin was not clearly established, our results suggested that the physicochemical environment of the Z-filaments in Z-disks might be crucial.