Ca-dependent binding of actin to gelsolin

Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities

Intellectual disability (ID) is a pathological condition characterized by limited intellectual functioning and adaptive behaviors. It affects 1–3% of the worldwide population, and no pharmacological therapies are currently available. More than 1000 genes have been found mutated in ID patients pointing out that, despite the common phenotype, the genetic bases are highly heterogeneous and apparently unrelated. Bibliomic analysis reveals that ID genes converge onto a few biological modules, including cytoskeleton dynamics, whose regulation depends on Rho GTPases transduction. Genetic variants exert their effects at different levels in a hierarchical arrangement, starting from the molecular level and moving toward higher levels of organization, i.e., cell compartment and functions, circuits, cognition, and behavior. Thus, cytoskeleton alterations that have an impact on cell processes such as neuronal migration, neuritogenesis, and synaptic plasticity rebound on the overall establishment of an effective network and consequently on the cognitive phenotype. Systems biology (SB) approaches are more focused on the overall interconnected network rather than on individual genes, thus encouraging the design of therapies that aim to correct common dysregulated biological processes. This review summarizes current knowledge about cytoskeleton control in neurons and its relevance for the ID pathogenesis, exploiting in silico modeling and translating the implications of those findings into biomedical research.

Myosin and gelsolin cooperate in actin filament severing and actomyosin motor activity

Actin is a major intracellular protein with key functions in cellular motility, signaling, and structural rearrangements. Its dynamic behavior, such as polymerization and depolymerization of actin filaments in response to intracellular and extracellular cues, is regulated by an abundance of actin binding proteins. Out of these, gelsolin is one of the most potent for filament severing. However, myosin motor activity also fragments actin filaments through motor-induced forces, suggesting that these two proteins could cooperate to regulate filament dynamics and motility. To test this idea, we used an in vitro motility assay, where actin filaments are propelled by surface-adsorbed heavy meromyosin (HMM) motor fragments. This allows studies of both motility and filament dynamics using isolated proteins. Gelsolin, at both nanomolar and micromolar Ca²⁺ concentration, appreciably enhanced actin filament severing caused by HMM-induced forces at 1 mM MgATP, an effect that was increased at higher HMM motor density. This finding is consistent with cooperativity between actin filament severing by myosin-induced forces and by gelsolin. We also observed reduced sliding velocity of the HMM-propelled filaments in the presence of gelsolin, providing further support of myosin-gelsolin cooperativity. Total internal reflection fluorescence microscopy–based single molecule studies corroborated that the velocity reduction was a direct effect of gelsolin binding to the filament and revealed different filament severing pattern of stationary and HMM propelled filaments. Overall, the results corroborate cooperative effects between gelsolin-induced alterations in the actin filaments and changes due to myosin motor activity leading to enhanced F-actin severing of possible physiological relevance.

Structure, regulation and related diseases of the actin-binding protein gelsolin

Gelsolin (GSN), one of the most abundant actin-binding proteins, is involved in cell motility, shape and metabolism. As a member of the GSN superfamily, GSN is a highly structured protein in eukaryotic cells that can be regulated by calcium concentration, intracellular pH, temperature and phosphatidylinositol-4,5-bisphosphate. GSN plays an important role in cellular mechanisms as well as in different cellular interactions. Because of its participation in immunologic processes and its interaction with different cells of the immune system, GSN is a potential candidate for various therapeutic applications. In this review, we summarise the structure of GSN as well as its regulating and functional roles, focusing on distinct diseases such as Alzheimer's disease, rheumatoid arthritis and cancer. A short overview of GSN as a therapeutic target in today's medicine is also provided.

Refrigerated storage of platelets initiates changes in platelet surface marker expression and localization of intracellular proteins

BACKGROUND

Platelets (PLTs) are currently stored at room temperature (22°C), which limits their shelf life, primarily due to the risk of bacterial growth. Alternatives to room temperature storage include PLT refrigeration (2-6°C), which inhibits bacterial growth, thus potentially allowing an extension of shelf life. Additionally, refrigerated PLTs appear more hemostatically active than conventional PLTs, which may be beneficial in certain clinical situations. However, the mechanisms responsible for this hemostatic function are not well characterized. The aim of this study was to assess the protein profile of refrigerated PLTs in an effort to understand these functional consequences.

STUDY DESIGN AND METHODS

Buffy coat PLTs were pooled, split, and stored either at room temperature (20-24°C) or under refrigerated (2-6°C) conditions (n = 8 in each group). PLTs were assessed for changes in external receptor expression and actin filamentation using flow cytometry. Intracellular proteomic changes were assessed using two-dimensional gel electrophoresis and Western blotting.

RESULTS

PLT refrigeration significantly reduced the abundance of glycoproteins (GPIb, GPIX, GPIIb, and GPIV) on the external membrane. However, refrigeration resulted in the increased expression of high-affinity integrins (αIIbβ3 and β1) and activation and apoptosis markers (CD62P, CD63, and phosphatidylserine). PLT refrigeration substantially altered the abundance and localization of several cytoskeletal proteins and resulted in an increase in actin filamentation, as measured by phalloidin staining.

CONCLUSION

Refrigerated storage of PLTs induces significant changes in the expression and localization of both surface-expressed and intracellular proteins. Understanding these proteomic changes may help to identify the mechanisms resulting in the refrigeration-associated alterations in PLT function and clearance.

The interaction of gelsolin with tropomyosin modulates actin dynamics

We have investigated the interactions between the actin-binding proteins gelsolin and tropomyosin, with special respect to any effects on the functional properties of gelsolin. Limited proteolysis indicated that the loop connecting the gelsolin domains G3 and G4 is involved in tropomyosin binding. Under nonpolymerizing conditions, binding of tropomyosin neither prevented the formation of a 2: 1actin-gelsolin complex, nor did it affect the nucleating activity of gelsolin in actin polymerization, likely as a result of competitive displacement of tropomyosin from gelsolin. To evaluate the effect of tropomyosin on the actin filament severing activity of gelsolin, we measured both filamentous actin (F-actin) viscosity and the relative number concentrations of filaments after fragmentation, either by gelsolin alone or by gelsolin-tropomyosin complexes. The interaction of gelsolin with tropomyosin caused a reduction in F-actin severing activity of up to 80% compared to gelsolin alone. Thus, being bound to gelsolin, tropomyosin prevented gelsolin from severing actin filaments. By contrast, the severing activity of gelsolin for F-actin/tropomyosin was similar to that for F-actin alone even at a tropomyosin : actin saturation ratio of 1: 7. Thus, when bound to actin filaments, tropomyosin did not significantly inhibit the severing of filaments by gelsolin. The interaction between gelsolin and tropomyosin was largely independent of the muscle actin and tropomyosin isoforms investigated. The results obtained in the present study suggest that tropomyosin is involved in the modulation of actin dynamics not via the protection of filaments against severing, but rather by binding gelsolin in solution to prevent it from severing and to promote the formation of new actin filaments.

Effects of cadmium on the actin cytoskeleton in renal mesangial cells

We provide an overview of our studies on cadmium and the actin cytoskeleton in mesangial cells, from earlier work on the effects of Cd2+ on actin polymerization in vivo and in vitro, to a role of disruption or stabilization of the cytoskeleton in apoptosis and apoptosis-like death. More recent studies implicate cadmium-dependent association of gelsolin and the Ca2+/calmodulin-dependent protein kinase II (CaMK-II) with actin filaments in cytoskeletal effects. We also present previously unpublished data concerning cadmium and the disruption of focal adhesions. The work encompasses studies on rat, mouse, and human mesangial cells. The major conclusions are that Cd2+ acts independently of direct effects on cellular Ca2+ levels to nevertheless activate Ca2+-dependent proteins that shift the actin polymerization–depolymerization in favour of depolymerization. Cadmium-dependent translocation of CaMK-IIδ, gelsolin, and a 50 kDa gelsolin cleavage fragment to the filamentous (F-)actin cytoskeleton appear to be involved. An intact filamentous actin cytoskeleton is required to initiate apoptotic and apoptotic-like death, but F-actin depolymerization is an eventual result.

A visualized observation of calcium-dependent gelsolin activity upon the surface coverage of fluorescent-tagged actin filaments

Gelsolin regulates the dynamics of F-actin by binding to F-actin to sever and cap. In the present study, a novel approach is introduced to observe gelsolin activity through the coverage of surface-bound F-actin. Gelsolin was immobilized on streptavidin coated surface using biotinylation and, as a result, the interaction between gelsolin and F-actin was visualized. Consequently, the coverage of F-actin reflects the activity of gelsolin as a function of free Ca(2+) concentrations. In order to prevent non-specific binding of F-actin, the combinations of BSA and Tween-20 as blocking agents were investigated. Moreover, the measurement of the length of F-actin with actin-gelsolin mixtures at various ratios provided the verification of gelsolin activity after biotinylation. The data shows the increase in Ca(2+) concentration leads to a proportional increase in F-actin coverage, giving to half-maximal coverage at ~2.9 μM. Furthermore, the length of bound F-actin was found to decrease along with increasing Ca(2+) concentration, and full-length F-actin was rarely observed. This may suggest that severing and capping activities of gelsolin occur without more additional Ca(2+) for subsequent activation after full-length gelsolin binds to a side of F-actin. This finding may provide a key to understand gelsolin activity.

N-terminal region of gelsolin induces apoptosis of activated hepatic stellate cells by a caspase-dependent mechanism

Activated hepatic stellate cells (HSCs) are the major source for alteration of extracellular matrix in fibrosis and cirrhosis. Conditioned medium (CM) collected from immortalized human hepatocytes (IHH) have earlier been shown to be responsible for apoptosis of HSCs. In this study, we have shown that antibodies raised against a peptide derived from a linear B-cell epitope in the N-terminal region of gelsolin identified a gelsolin fragment in IHH CM. Analysis of activated stellate cell death by CM collected from Huh7 cells transfected with plasmids encoding gelsolin deletion mutants suggested that the N-terminal half of gelsolin contained sequences which were responsible for stellate cell death. Further analysis determined that this activity was restricted to a region encompassing amino acids 1–70 in the gelsolin sequence; antibody directed to an epitope within this region was able to neutralize stellate cell death. Gelsolin modulation of cell death using this fragment involved upregulation of TRAIL-R1 and TRAIL-R2, and involved caspase 3 activation by extrinsic pathway. The apoptotic activity of N-terminal gelsolin fragments was restricted to activated but not quiescent stellate cells indicating its potential application in therapeutic use as an anti-fibrotic agent. Gelsolin fragments encompassing N-terminal regions in polypeptides of different molecular sizes were detected by N-terminal peptide specific antiserum in IHH CM immunoprecipitated with chronically HCV infected patient sera, suggesting the presence of autoantibodies generated against N-terminal gelsolin fragments in patients with chronic liver disease.

Gelsolin amyloidosis: genetics, biochemistry, pathology and possible strategies for therapeutic intervention

Protein misassembly into aggregate structures, including cross-β-sheet amyloid fibrils, is linked to diseases characterized by the degeneration of post-mitotic tissue. While amyloid fibril deposition in the extracellular space certainly disrupts cellular and tissue architecture late in the course of amyloid diseases, strong genetic, pathological and pharmacologic evidence suggests that the process of amyloid fibril formation itself, known as amyloidogenesis, likely causes these maladies. It seems that the formation of oligomeric aggregates during the amyloidogenesis process causes the proteotoxicity and cytotoxicity characteristic of these disorders. Herein, we review what is known about the genetics, biochemistry and pathology of familial amyloidosis of Finnish type (FAF) or gelsolin amyloidosis. Briefly, autosomal dominant D187N or D187Y mutations compromise Ca(2+) binding in domain 2 of gelsolin, allowing domain 2 to sample unfolded conformations. When domain 2 is unfolded, gelsolin is subject to aberrant furin endoproteolysis as it passes through the Golgi on its way to the extracellular space. The resulting C-terminal 68 kDa fragment (C68) is susceptible to extracellular endoproteolytic events, possibly mediated by a matrix metalloprotease, affording 8 and 5 kDa amyloidogenic fragments of gelsolin. These amyloidogenic fragments deposit systemically, causing a variety of symptoms including corneal lattice dystrophy and neurodegeneration. The first murine model of the disease recapitulates the aberrant processing of mutant plasma gelsolin, amyloid deposition, and the degenerative phenotype. We use what we have learned from our biochemical studies, as well as insight from mouse and human pathology to propose therapeutic strategies that may halt the progression of FAF.

Disarrangement of actin filaments and Ca²⁺ gradient by CdCl₂ alters cell wall construction in Arabidopsis thaliana root hairs by inhibiting vesicular trafficking

Cadmium (Cd), one of the most toxic heavy metals, inhibits many cellular and physiological processes in plants. Here, the involvement of cytoplasmic Ca²⁺ gradient and actin filaments (AFs) in vesicular trafficking, cell wall deposition and tip growth was investigated during root (hair) development of Arabidopsis thaliana in response to CdCl₂ treatment. Seed germination and root elongation were prevented in a dose- and time-dependent manner by CdCl₂ treatment. Fluorescence labelling and non-invasive detection showed that CdCl₂ inhibited extracellular Ca²⁺ influx, promoted intracellular Ca²⁺ efflux, and disturbed the cytoplasmic tip-focused Ca²⁺ gradient. In vivo labelling revealed that CdCl₂ modified actin organization, which subsequently contributed to vesicle trafficking. Transmission electron microscopy revealed that CdCl₂ induced cytoplasmic vacuolization and was detrimental to organelles such as mitochondria and endoplasmic reticulum (ER). Finally, immunofluorescent labelling and Fourier transform infrared (FTIR) analysis indicated that configuration/distribution of cell wall components such as pectins and cellulose was significantly altered in response to CdCl₂. Our results indicate that CdCl₂ induces disruption of Ca²⁺ gradient and AFs affects the distribution of cell wall components in root hairs by disturbing vesicular trafficking in A. thaliana.

On the mechanical stabilization of filopodia

We studied force-induced elongation of filopodia by coupling magnetic tweezers to the tip through the bacterial coat protein invasin, which couples the force generator to the actin bundles (through myosin X), thus impeding the growth of the actin plus end. Single force pulses (15-30 s) with amplitudes between 20 and 600 pN and staircase-like force scenarios (amplitudes, ∼50 pN; step widths, 30 s) were applied. In both cases, the responses consist of a fast viscoelastic deflection followed by a linear flow regime. The deflections are reversible after switching off the forces, suggesting a mechanical memory. The elongation velocity exhibits an exponential distribution (half-width <v(1/2)>, ∼0.02 μm s(-1)) and did not increase systematically with the force amplitudes. We estimate the bending modulus (0.4 × 10(-23) J m) and the number of actin filaments (∼10) by analyzing filopodium bending fluctuations. Sequestering of intracellular Ca(2+) by BAPTA caused a strong reduction in the amplitude of elongation, whereas latrunculin A resulted in loss of the elastic response. We attribute the force-independent velocity to the elongation of actin bundles enabled by the force-induced actin membrane uncoupling and the reversibility by the treadmilling mechanism and an elastic response.

BAPTA-AM, an intracellular calcium chelator, inhibits RANKL-induced bone marrow macrophages differentiation through MEK/ERK, p38 MAPK and Akt, but not JNK pathways

To examine the roles of intracellular calcium in RANKL-induced bone marrow macrophages (BMMs) differentiation, the effects of intracellular calcium chelator BAPTA-AM on RANKL-induced BMMs differentiation, and the activation of its relating signal proteins (MAPKs, and the PI3K/Akt) were studied. BMMs were cultured with various concentrations of BAPTA-AM in the presence of M-CSF (25 ng/ml) and RANKL (25 ng/ml) for 7 days, osteoclastogenic ability, cytosolic free Ca(2+) concentration, osteoclast survival and the expression of phosphorylated ERK1/2, SAPK/JNK, Akt and p38 MAPK were measured by TRAP staining, spectrofluorometer and Western blotting. BAPTA-AM inhibited osteoclastogenesis and osteoclast survival of BMMs by RANKL induction. In osteoclasts without the pretreatment of BAPTA-AM, the increased response of [Ca(2+)](i) was observed within 15 min and the maximum was about 1.2 times that of control. This response was sustained for 30 min and returned to the control level at 1h after RANKL-inducing, and the increased response of [Ca(2+)](i) was completely abolished and sustained to at least 8h by BAPTA-AM. Although immunoblotting data revealed that RANKL could activate the phosphorylation of ERK1/2, SAPK/JNK, Akt and p38 MAPK, the expression of ERK1/2, Akt and p38 MAPK phosphorylation was inhibited by BAPTA-AM dose-dependently. These results revealed that BAPTA-AM inhibit osteoclastogenic ability of BMMs via suppressing the increase of [Ca(2+)](i) which lead to inhibit RANKL-induced the phosphorylation of ERK, Akt and p38 MAPK, but not JNK. This finding may be useful in the development of an osteoclastic inhibitor that targets intracellular signaling factors.

Granzyme B-induced and caspase 3-dependent cleavage of gelsolin by mouse cytotoxic T cells modifies cytoskeleton dynamics

Granule-associated perforin and granzymes (gzms) are key effector molecules of cytotoxic T lymphocytes (Tc cells) and natural killer cells and play a critical role in the control of intracellular pathogens and cancer. Based on the notion that many gzms, including A, B, C, K, H, and M exhibit cytotoxic activity in vitro, all gzms are believed to serve a similar function in vivo. However, more recent evidence supports the concept that gzms are not unidimensional but, rather, possess non-cytotoxic potential, including stimulation of pro-inflammatory cytokines and anti-viral activities. The present study shows that isolated mouse gzmB cleaves the actin-severing mouse protein, cytoplasmic gelsolin (c-gelsolin) in vitro. However, when delivered to intact target cells by ex vivo immune Tc cells, gzmB mediates c-gelsolin proteolysis via activation of caspases 3/7. The NH(2)-terminal c-gelsolin fragment generated by either gzmB or caspase 3 in vitro constitutively severs actin filaments without destroying the target cells. The observation that gzmB secreted by Tc cells initiates a caspase cascade that disintegrates the actin cytoskeleton in target cells suggests that this intracellular process may contribute to anti-viral host defense.

The gelsolin:calponin complex nucleates actin filaments with distinct morphologies

Gelsolin and calponin are cytoskeletal and signalling proteins that form a tight 1:1 complex (GCC). We show that calponin within the GCC inhibits the rate of gelsolin mediated nucleation of actin polymerization. The actin-binding function of calponin is ablated within the GCC as the actin-binding site overlaps with one of the gelsolin binding sites. The structure of filaments that result from nucleation by GCC are different to those nucleated by gelsolin alone in that they are longer, loosely bundled and stain heterogeneously with phalloidin. GCC nucleated filaments appear contorted and wrap around each to form the loose bundles.

Exclusion of a proton ATPase from the apical membrane is associated with cell polarity and tip growth in Nicotiana tabacum pollen tubes

Polarized growth in pollen tubes results from exocytosis at the tip and is associated with conspicuous polarization of Ca(2+), H(+), K(+), and Cl(-) -fluxes. Here, we show that cell polarity in Nicotiana tabacum pollen is associated with the exclusion of a novel pollen-specific H(+)-ATPase, Nt AHA, from the growing apex. Nt AHA colocalizes with extracellular H(+) effluxes, which revert to influxes where Nt AHA is absent. Fluorescence recovery after photobleaching analysis showed that Nt AHA moves toward the apex of growing pollen tubes, suggesting that the major mechanism of insertion is not through apical exocytosis. Nt AHA mRNA is also excluded from the tip, suggesting a mechanism of polarization acting at the level of translation. Localized applications of the cation ionophore gramicidin A had no effect where Nt AHA was present but acidified the cytosol and induced reorientation of the pollen tube where Nt AHA was absent. Transgenic pollen overexpressing Nt AHA-GFP developed abnormal callose plugs accompanied by abnormal H(+) flux profiles. Furthermore, there is no net flux of H(+) in defined patches of membrane where callose plugs are to be formed. Taken together, our results suggest that proton dynamics may underlie basic mechanisms of polarity and spatial regulation in growing pollen tubes.

Two distinct sites of interaction form the calponin: gelsolin complex and two calcium switches control its activity

Gelsolin and calponin are well characterized actin-binding proteins that form a tight gelsolin:calponin complex (GCC). We show here that the GCC is formed through two distinct interfaces. One of these is formed between 144-182 of calponin and 25-150 of gelsolin (G1). The second is a calcium-sensitive site centred on calponin's CH domain, and the C-terminal half of gelsolin (G4-6). The behaviour of this second interface is dependent on the presence of calcium and so it is possible that potential GCC-binding partners may be selected by calcium availability. Actin is one such GCC-binding partner and we show that a larger complex is formed with monomeric actin in calcium. The stoichiometry of this complex is determined to be 1 gelsolin/1 calponin/2 G-actins (GCA(2)). Both actin monomers bind the GCC through gelsolin. Both calponin and gelsolin are reported to play signaling roles in addition to their better-characterized actin-binding properties and it is possible that the GCC regulates both of these functions.

Involvement of gelsolin in cadmium-induced disruption of the mesangial cell cytoskeleton

Cadmium (Cd2+) is known to cause a selective disruption of the filamentous actin cytoskeleton in the smooth muscle-like renal mesangial cell. We examined the effect of Cd2+ on the distribution of the actin-severing protein, gelsolin. Over 8 h, CdCl2 (10 microM) caused a progressive shift of gelsolin from a diffuse perinuclear and cytoplasmic distribution to a pattern decorating F-actin filaments. Over this time filaments were decreased in number in many cells, and membrane ruffling was initiated. Western blotting and 125I-F-actin gel overlays demonstrated an increase in actin-binding gelsolin activity in the cytoskeletal fraction of cell extracts following Cd2+ treatment. In in vitro polymerization assays, gelsolin acted as a nucleating factor and increased the rate of polymerization. Cytosolic extracts also increased the polymerization rate. Addition of Cd2+ together with gelsolin further increased the rate of polymerization. Gelsolin enhanced depolymerization of purified actin, and Cd2+ partially suppressed this effect. However, cytoskeletal extracts from Cd2+-treated cells also markedly increased depolymerization, suggesting further that Cd2+ may activate cellular component(s) such as gelsolin for actin binding. We conclude that a major effect of Cd2+ on the mesangial cell cytoskeleton is manifest through activating the association of gelsolin with actin, with gelsolin's severing properties predominating under conditions found in Cd2+-treated cells.

Gelsolin secretion in interleukin-4-treated bronchial epithelia and in asthmatic airways

RATIONALE

The airway surface liquid, the thin layer of liquid covering the airways, is essential for mucociliary clearance and as a barrier against microbial and other noxious agents. Proteins secreted into the airway surface liquid by epithelial and nonepithelial cells may be important in innate immunity and to improve the fluidity of mucous secretions.

OBJECTIVES

We aimed to identify proteins specifically secreted into the airway surface liquid by human bronchial epithelial cells, in resting conditions and after treatment with interleukin 4 (IL-4), a cytokine released in asthma.

METHODS AND MAIN RESULTS

By using a proteomics approach, we found that one of the most abundant proteins was gelsolin, which breaks down actin filaments. Gelsolin mRNA and protein secretion were increased threefold in the airway surface liquid of epithelia treated with IL-4. These results were confirmed at the functional level by measuring actin depolymerization using a fluorescence assay. Gelsolin protein was also upregulated in the airways of subjects with asthma.

CONCLUSIONS

Our findings indicate that gelsolin is released by epithelial cells into the airways and that its secretion is increased by IL-4 in vitro. In addition, we found that the concentration of both IL-4 and gelsolin were raised in the bronchoalveolar lavage of patients with asthma. These results suggest that gelsolin might improve the fluidity of airway surface liquid in asthma by breaking down filamentous actin that may be released in large amounts by dying cells during inflammation.

Topological assignment of the N-terminal extension of plasma gelsolin to the gelsolin surface

The actin-binding protein gelsolin is highly conserved in vertebrates and exists in two isoforms, a cytoplasmic and an extracellular variant, generated by alternative splicing. In mammals, these isoforms differ only by an N-terminal extension in plasma gelsolin, a short sequence of up to 25 amino acids. Cells and tissues may contain both variants, as plasma gelsolin is secreted by many cell types. The tertiary structure of equine plasma gelsolin has been elucidated, but without any information on the N-terminal extension. In this paper, we present topographical data on the N-terminal extension, derived using a biochemical and immunological approach. For this purpose, a monoclonal antibody was generated that exclusively recognizes cytoplasmic gelsolin but not the extracellular variant and thus allows isoform-specific immunodetection and quantification of cytoplasmic gelsolin in the presence of plasma gelsolin. Using limited proteolysis and pepscan analysis, we mapped the binding epitope and localized it within two regions in segment 1 of the cytoplasmic gelsolin sequence: Tyr34-Ile45 and Leu64-Ile78. In the tertiary structure of the cytoplasmic variant, these sequences are mutually adjacent and located in the proximity of the N-terminus. We therefore conclude that the binding site of the antibody is covered by the N-terminal extension in plasma gelsolin and thus sterically hinders antibody binding. Our results allow for a topological model of the N-terminal extension on the surface of the gelsolin molecule, which was unknown previously.

Co-operation of domain-binding and calcium-binding sites in the activation of gelsolin

Gelsolin is an abundant calcium dependent actin filament severing and capping protein. In the absence of calcium the molecule is compact but in the presence of calcium, as its six similar domains alter their relative position, a generally more open configuration is adopted to reveal the three actin binding sites. It is generally held that a 'helical-latch' at the C-terminus of gelsolin's domain 6 (G6), binds domain 2 (G2) to keep gelsolin in the calcium-free compact state, and that the crutial calcium binding site(s) reside in the C-terminal half of gelsolin perhaps involving the C-terminal helix itself has to be bound to release this latch. Here we provide evidence for a calcium dependent conformational change within G2 (Kd = approximately 15 micro m). We also report a calcium dependent binding site for the C-terminus (G4-6) within G2 and delimit this further to a specific region formed by residues 203-225 and 159-193. It is known that the activation of gelsolin involves multiple calcium binding events (around 6) the first of which (in G6) may release the latch. We propose that the calcium-dependent conformational change in G2 may be a subsequent step that is necessary for the dissociation of G2 from G4-6, and that this movement occurs in sympathy with calcium induced conformational changes within G6 by the physical coupling of the two calcium binding sites within G2 and G6. Additional calcium binding in other domains then result in the complete opening and activation of the gelsolin molecule.