Isolation and properties of two actin-binding domains in gelsolin

Global shapes of F-actin depolymerization-competent minimal gelsolins: insight into the role of g2-g3 linker in pH/Ca2+ insensitivity of the first half

Because of its ability to rapidly depolymerize F-actin, plasma gelsolin has emerged as a therapeutic molecule in different disease conditions. High amounts of exogenous gelsolin are, however, required to treat animal models of different diseases. Knowing that the F-actin depolymerizing property of gelsolin resides in its N terminus, we made several truncated versions of plasma gelsolin. The smaller versions, particularly the one composed of the first 28-161 residues, depolymerized the F-actin much faster than the native gelsolin and other truncates at the same molar ratios. Although G1-G3 loses its dependence on Ca(2+) or low pH for the actin depolymerization function, interestingly, G1-G2 and its smaller versions were found to regain this requirement. Small angle x-ray scattering-based shape reconstructions revealed that G1-G3 adopts an open shape in both the presence and the absence of Ca(2+) as well as low pH, whereas G1-G2 and residues 28-161 prefer collapsed states in Ca(2+)-free conditions at pH 8. The mutations in the g2-g3 linker resulted in the calcium sensitivity of the mutant G1-G3 for F-actin depolymerization activity, although the F-actin-binding sites remained exposed in the mutant G1-G3 as well as in the smaller truncates even in the Ca(2+)-free conditions at pH 8. Furthermore, unlike wild type G1-G3, calcium-sensitive mutants of G1-G3 acquired closed shapes in the absence of free calcium, implying a role of g2-g3 linker in determining the open F-actin depolymerizing-competent shape of G1-G3 in this condition. We demonstrate that the mobility of the G1 domain, essential for F-actin depolymerization, is indirectly regulated by the gelsolin-like sequence of g2-g3 linker.

A llama-derived gelsolin single-domain antibody blocks gelsolin-G-actin interaction

RNA interference has tremendously advanced our understanding of gene function but recent reports have exposed undesirable side-effects. Recombinant Camelid single-domain antibodies (VHHs) provide an attractive means for studying protein function without affecting gene expression. We raised VHHs against gelsolin (GsnVHHs), a multifunctional actin-binding protein that controls cellular actin organization and migration. GsnVHH-induced delocalization of gelsolin to mitochondria or the nucleus in mammalian cells reveals distinct subpopulations including free gelsolin and actin-bound gelsolin complexes. GsnVHH 13 specifically recognizes Ca(2+)-activated gelsolin (K (d) approximately 10 nM) while GsnVHH 11 binds gelsolin irrespective of Ca(2+) (K (d) approximately 5 nM) but completely blocks its interaction with G-actin. Both GsnVHHs trace gelsolin in membrane ruffles of EGF-stimulated MCF-7 cells and delay cell migration without affecting F-actin severing/capping or actin nucleation activities by gelsolin. We conclude that VHHs represent a potent way of blocking structural proteins and that actin nucleation by gelsolin is more complex than previously anticipated.

Gelsolin and diseases

Gelsolin is a calcium-activated actin filament severing and capping protein found in many cell types and as a secreted form in the plasma of vertebrates. Mutant mice for gelsolin as well as clinical studies have shown that gelsolin is linked to a number of pathological conditions such as inflammation, cancer and amyloidosis. The tight regulation of gelsolin by calcium is crucial for its physiological role and constitutive activation leads to apoptosis. In the following we will give an overview on how gelsolin is regulated by calcium, and which clinical conditions have been linked to lack or misregulation of gelsolin.

Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

The actin cytoskeleton is one of the major structural components of the cell. It often undergoes rapid reorganization and plays crucial roles in a number of dynamic cellular processes, including cell migration, cytokinesis, membrane trafficking, and morphogenesis. Actin monomers are polymerized into filaments under physiological conditions, but spontaneous depolymerization is too slow to maintain the fast actin filament dynamics observed in vivo. Gelsolin, actin-depolymerizing factor (ADF)/cofilin, and several other actin-severing/depolymerizing proteins can enhance disassembly of actin filaments and promote reorganization of the actin cytoskeleton. This review presents advances as well as a historical overview of studies on the biochemical activities and cellular functions of actin-severing/depolymerizing proteins.

Separate functions of gelsolin mediate sequential steps of collagen phagocytosis

Collagen phagocytosis is a critical mediator of extracellular matrix remodeling. Whereas the binding step of collagen phagocytosis is facilitated by Ca2+-dependent, gelsolin-mediated severing of actin filaments, the regulation of the collagen internalization step is not defined. We determined here whether phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] regulation of gelsolin is required for collagen internalization. In gelsolin null fibroblasts transfected with gelsolin severing mutants, actin severing and collagen binding were strongly impaired but internalization and actin monomer addition at collagen bead sites were much less affected. PI(4,5)P2 accumulated around collagen during internalization and was associated with gelsolin. Cell-permeable peptides mimicking the PI(4,5)P2 binding site of gelsolin blocked actin monomer addition, the association of gelsolin with actin at phagosomes, and collagen internalization but did not affect collagen binding. Collagen beads induced recruitment of type 1 gamma phosphatidylinositol phosphate kinase (PIPK1gamma661) to internalization sites. Dominant negative constructs and RNA interference demonstrated a requirement for catalytically active PIPK1gamma661 for collagen internalization. We conclude that separate functions of gelsolin mediate sequential stages of collagen phagocytosis: Ca2+-dependent actin severing facilitates collagen binding, whereas PI(4,5)P2-dependent regulation of gelsolin promotes the actin assembly required for internalization of collagen fibrils.

Gelsolin binds to polyphosphoinositide-free lipid vesicles and simultaneously to actin microfilaments

Gelsolin is a calcium-, pH- and lipid-dependent actin filament severing/capping protein whose main function is to regulate the assembly state of the actin cytoskeleton. Gelsolin is associated with membranes in cells, and it is generally assumed that this interaction is mediated by PPIs (polyphosphoinositides), since an interaction with these lipids has been characterized in vitro. We demonstrate that non-PPI lipids also bind gelsolin, especially at low pH. The data suggest further that gelsolin becomes partially buried in the lipid bilayer under mildly acidic conditions, in a manner that is not dependent of the presence of PPIs. Our data also suggest that lipid binding involves a number of sites that are spread throughout the gelsolin molecule. Linker regions between gelsolin domains have been implicated by other work, notably the linker between G1 and G2 (gelsolin domains 1 and 2 respectively), and we postulate that the linker region between the N-terminal and C-terminal halves of gelsolin (between G3 and G4) is also involved in the interaction with lipids. This region is compatible with other studies in which additional binding sites have been located within G4-6. The lipid-gelsolin interactions reported in the present paper are not calcium-dependent, and are likely to involve significant conformational changes to the gelsolin molecule, as the chymotryptic digest pattern is altered by the presence of lipids under our conditions. We also report that vesicle-bound gelsolin is capable of binding to actin filaments, presumably through barbed end capping. Gelsolin bound to vesicles can nucleate actin assembly, but is less active in severing microfilaments.

Structure of an F-actin trimer disrupted by gelsolin and implications for the mechanism of severing

Stable oligomers of filamentous actin were obtained by cross-linking F-actin with 1,4-N,N'-phenylenedimaleimide and depolymerization with excess segment-1 of gelsolin. Segment-1-bound and cross-linked actin oligomers containing either two or three actin subunits were purified and shown to nucleate actin assembly. Kinetic assembly data from mixtures of monomeric actin and the actin oligomers fit a nucleation model where cross-linked actin dimer or trimer reacts with an actin monomer to produce a competent nucleus for filament assembly. We report the three-dimensional structure of the segment-1-actin hexamer containing three actin subunits, each with a tightly bound ATP. Comparative analysis of this structure with twelve other actin structures provides an atomic level explanation for the preferential binding of ATP by the segment-1-complexed actin. Although the structure of segment-1-bound actin trimer is topologically similar to the helical model of F-actin (1), it has a distorted symmetry compared with that of the helical model. This distortion results from intercalation of segment-1 between actin protomers that increase the rise per subunit and rotate each of the actin subunits relative to their positions in F-actin. We also show that segment-1 of gelsolin is able to sever actin filaments, although the severing activity of segment-1 is significantly lower than full-length gelsolin.

Dissecting the Gelsolin–Polyphosphoinositide Interaction and Engineering of a Polyphosphoinositide-sensitive Gelsolin C-terminal Half Protein

Gelsolin and other proteins in the villin/gelsolin family are regulated by polyphosphoinositides (PPIs), and manipulation of cellular PIP(2) levels alters the structure of the actin cytoskeleton coincident with the dissociation of gelsolin-actin complexes. This work explores the structure-function relationship of the gelsolin-PPI interaction. Circular dichroism experiments show that upon binding to PPIs, the PPI-sensitive N-terminal half of gelsolin undergoes significant secondary and tertiary structural changes that do not occur in the structurally homologous but PPI-insensitive C-terminal half. Secondary structure modeling algorithms predict an alpha-helical conformation for one of the gelsolin PPI-binding sites, P2, which differs from the conformation of P2 in the structure of gelsolin determined by X-ray crystallography, whereas structure prediction of the C-terminal homolog of P2 agrees well with the X-ray crystallography structure. Simulation of a change to helical conformation for P2 using molecular modeling indicates that such a structural transition will destabilize the F-actin-binding sites in domain 2. A hypothesis is proposed that PPIs initiate conformational changes at the PPI-binding site(s) that destabilize the protein structure, and subsequently disrupt the actin-binding sites. To further evaluate the role of P2 in the gelsolin-PPI interaction, a Ct mutant P2Ct is constructed by inserting P2 in place of its C-terminal homologous site. P2Ct interacts with actin in the same way as the wild-type protein. In contrast to Ct, however, P2Ct interacts strongly with PPIs, and its monomeric actin-binding activity becomes regulated by PPIs. It is concluded that the P2 site is sufficient for PPI-sensitivity in gelsolin. Furthermore, the P2 site in P2Ct and the actin-binding sites of Ct do not overlap, suggesting that PPIs regulate actin binding of P2Ct through induction of structural changes, rather than through direct competition.

Purification of native myosin filaments from muscle

Analysis of the structure and function of native thick (myosin-containing) filaments of muscle has been hampered in the past by the difficulty of obtaining a pure preparation. We have developed a simple method for purifying native myosin filaments from muscle filament suspensions. The method involves severing thin (actin-containing) filaments into short segments using a Ca(2+)-insensitive fragment of gelsolin, followed by differential centrifugation to purify the thick filaments. By gel electrophoresis, the purified thick filaments show myosin heavy and light chains together with nonmyosin thick filament components. Contamination with actin is below 3.5%. Electron microscopy demonstrates intact thick filaments, with helical cross-bridge order preserved, and essentially complete removal of thin filaments. The method has been developed for striated muscles but can also be used in a modified form to remove contaminating thin filaments from native smooth muscle myofibrils. Such preparations should be useful for thick filament structural and biochemical studies.

Stress fiber formation is required for matrix reorganization in a corneal myofibroblast cell line

Corneal wound healing fibroblasts (myofibroblasts) develop a muscle-like contractile apparatus composed of prominent microfilament bundles (stress fibers) and express alpha-smooth muscle actin (alpha-SMA). In this study, gelsolin, an actin filament-severing protein, was overexpressed in a alpha-SMA-expressing corneal myofibroblast cell line (TRK43) to assess whether intact stress fibers are required for in vitro matrix organization and wound contraction. Stably integrated gelsolin was introduced by electroporation of an expression construct (pREPCG8) into cultured cells. Thirty-seven clones were isolated with half of the clones showing a fibroblastic phenotype while the remaining half appeared epithelioid. One fibroblastic clone, GS56, and one epithelioid clone, GS44, were selected for detailed characterization. The GS56 cells appeared highly elongated and spindle-shaped and had prominent stress fibers and focal adhesions. GS44 cells showed disruption of stress fibers and a cortical f-actin organization as well as the down regulation of alpha-SMA expression by immunocytochemistry and Western blotting. Both phenotypes showed enhanced gelsolin expression; however, fractionation of cell extracts demonstrated differences in the subcellular distribution of gelsolin with GS44 cells having markedly reduced and GS56 cells having markedly increased cytoskeletal gelsolin. In an in vitro wound contraction assay, epithelioid GS44 cells showed a significantly impaired ability to contract a collagen matrix compared to that of TRK43 cells, CT9 or GS56 transfectants. Loss of stress fibers in GS44 cells also correlated with enhanced cell motility. Together, these results demonstrate that the ability to form microfilament bundles or stress fibers is required for matrix organization and contraction by corneal myofibroblasts. Although no clear explanation is available, we suspect that differences in gene insertion of the gelsolin overexpression vector may have led to differential intercellular localization of gelsolin and its effect on stress fiber formation in the two cell lines.

Ca2+ regulation of gelsolin by its C-terminal tail

Gelsolin is activated by Ca(2+) to sever actin filaments. Ca(2+) regulation is conferred on the N-terminal half by the C-terminal half. This paper seeks to understand how Ca(2+) regulates gelsolin by testing the "tail helix latch hypothesis," which is based on the structural data showing that gelsolin has a C-terminal tail helix that contacts the N-terminal half in the absence of Ca(2+). Ca(2+) activation of gelsolin at 37 degrees C occurs in three steps, with apparent K(d) for Ca(2+) of 0.1, 0.3, and 6.4 x 10(-6) m. Tail helix truncation decreases the apparent Ca(2+) requirement for severing to 10(-7) m and eliminates the conformational change observed at 10(-6) m Ca(2+). The large decrease in Ca(2+) requirement for severing is not due to a change in Ca(2+) binding nor to Ca(2+)-independent activation of the C-terminal half per se. Thus, the tail helix latch is primarily responsible for transmitting micromolar Ca(2+) information from the gelsolin C-terminal half to the N-terminal half. Occupation of submicromolar Ca(2+)-binding sites primes gelsolin for severing, but gelsolin cannot sever because the tail latch is still engaged. Unlatching the tail helix by 10(-6) m Ca(2+) releases the final constraint to initiate the severing cascade.

Artificial phosphorylation removes Gelsolin's dependence on calcium

Gelsolin is one of the best known actin-binding proteins with several distinct activities regulated by calcium. Using a kinase fraction isolated from mitotic HeLa cells, we found that the plasma form of gelsolin can be phosphorylated at a site located within the NH2-terminus region which does not exist in the cytoplasmic form. After this phosphorylation, gelsolin no longer requires Ca2+ for activity; it severs and subsequently caps actin filaments, and nucleates filament formation in Ca2+-free solution. These findings may clarify the mechanism of gelsolin regulation by Ca2+, and indicate that changes in electrical interactions between the NH2- and COOH-terminal ends are important for this regulation. Moreover, since only a single site is phosphorylated, and since the phosphorylated region does not contribute to this protein's own activity, the results suggest that a single chemical charge modification at a site away from the protein's core structure, such as this phosphorylation site, is sufficient to alter the protein's function.

Fluorescent phosphoinositide derivatives reveal specific binding of gelsolin and other actin regulatory proteins to mixed lipid bilayers

Fluorescent derivatives of phosphatidyl inositol (PtdIns)-(4,5)-P2 were synthesized and used to test the effects of the PtdIns-(4, 5)-P2-regulated proteins gelsolin, tau, cofilin, and profilin on labeled PtdIns-(4,5)-P2 that was either in micellar form or mixed with phosphatidylcholine (PtdCho) in bilayer vesicles. Gelsolin increased the fluorescence of 7-nitrobenz-2-oxa-1,3-diazole (NBD)- or pyrene-labeled PtdIns-(4,5)-P2 and NBD-PtdIns-(3,4,5)-P3. Cofilin and profilin produced no detectable change at equimolar ratios to PtdIns-(4,5)-P2, while tau decreased NBD-PtdIns-(4,5)-P2 fluorescence. Fluorescence enhancement by gelsolin of NBD-PtdIns-(4, 5)-P2 in mixed lipid vesicles depended on the mole fraction of PtdIns-(4,5)-P2 in the bilayer. Specific enhancement of 3% NBD-PtdIns-(4,5)-P2 : 97% PtdCho was much lower than that of 10% PtdIns-(4,5)-P2 : 90% PtdCho, but the enhancement of 3% NBD-PtdIns-(4,5)-P2 could be increased by addition of 7% unlabeled PtdIns-(4,5)-P2. The gelsolin-dependent increase in NBD-PtdIns-(4, 5)-P2 fluorescence was reversed by addition of Ca2+ or G-actin. Significant, but weaker, fluorescence enhancement was observed with the gelsolin N-terminal domain (residues 1-160) and a peptide comprised of gelsolin residues 150-169. Fluorescence energy transfer from gelsolin to pyrene-PtdIns-(4,5)-P2 was much stronger with intact gelsolin than the N-terminal region of gelsolin containing the PtdIns-(4,5)-P2 binding sites, suggesting that PtdIns-(4,5)-P2 may bind near a site formed by the juxtaposition of the N- and C-terminal domains of gelsolin.

Co-operative binding of Ca2+ ions to the regulatory binding sites of gelsolin

The rate of association of actin with gelsolin was measured at various Ca2+ and ATP concentrations. The fraction of Ca2+-activated gelsolin was determined by quantitative evaluation of the association rates thereby assuming that Ca2+-binding gelsolin associates with actin and Ca2+-free gelsolin does not. A plot of the fraction of Ca2+-activated gelsolin vs. the free Ca2+ concentration revealed a sigmoidal shape suggesting that co-operative binding of Ca2+ ions is required for activation of gelsolin. A good fit of the experimental data by calculated binding curves was obtained if two Ca2+ ions were assumed to bind to actin in a highly co-operative manner. ATP decreased the rate of association of gelsolin with actin and bound to gelsolin at a low affinity (Kd = 32 microm for Ca2+-free and Kd = 400 microm for Ca2+-activated gelsolin). In contrast, a 1 : 1 gelsolin-actin complex was found to be activated for association with actin by a single Ca2+ ion in a non-co-operative manner.

Phototactic migration of Dictyostelium cells is linked to a new type of gelsolin-related protein

The molecular and functional characterization of a 125-kDa Ca2+-extractable protein of the Triton X-100-insoluble fraction of Dictyostelium cells identified a new type of a gelsolin-related molecule. In addition to its five gelsolin segments, this gelsolin-related protein of 125 kDa (GRP125) reveals a number of unique domains, two of which are predicted to form coiled-coil regions. Another distinct attribute of GRP125 concerns the lack of sequence elements known to be essential for characteristic activities of gelsolin-like proteins, i.e. the severing, capping, or nucleation of actin filaments. The subcellular distribution of GRP125 to vesicular compartments suggests an activity of GRP125 different from actin-binding, gelsolin-related proteins. GRP125 expression is tightly regulated and peaks at the transition to the multicellular pseudoplasmodial stage of Dictyostelium development. GRP125 was found indispensable for slug phototaxis, because slugs fail to correctly readjust their orientation in the absence of GRP125. Analysis of the GRP125-deficient mutant showed that GRP125 is required for coupling photodetection to the locomotory machinery of slugs. We propose that GRP125 is essential in the natural environment for the propagation of Dictyostelium spores. We also present evidence for further representatives of the GRP125 type in Dictyostelium, as well as in heterologous cells from lower to higher eukaryotes.

The actin-binding proteins adseverin and gelsolin are both highly expressed but differentially localized in kidney and intestine

To understand the distinct functions of the closely related actin-severing proteins adseverin and gelsolin, we examined the expression of these proteins in detail during mouse and human development using a new highly sensitive and specific set of antibody reagents. Immunoblot analysis demonstrated that adseverin was highly expressed in mouse kidney and intestine at all stages of development and in human fetal and adult kidney. In contrast and as reported previously, gelsolin was expressed much more widely in both murine and human tissues. Immunohistochemistry on murine kidney sections revealed a predominantly differential localization of adseverin and gelsolin. Adseverin was expressed in peripolar cells, thin limbs, thick ascending limbs, and principal cells of cortical and medullary collecting ducts where it was diffusely localized in the cytoplasm. Gelsolin was expressed in the distal convoluted tubule, intercalated cells and principal cells of cortical and medullary collecting ducts, and in ureter. In the distal convoluted tubule, gelsolin showed a diffuse distribution and in principal cells of collecting ducts a localization at the basolateral pole. In intercalated cells, gelsolin localization was heterogeneous, either at the apical pole or diffusely in the cytoplasm. In human fetal and adult kidney, adseverin was expressed only in collecting ducts whereas gelsolin was expressed in thick ascending limbs and collecting ducts. In mouse and human intestine adseverin was expressed in enterocytes with a gradient of increasing expression from the duodenum to the colon, and from the crypt to the villus. The observations indicate high level expression of adseverin in specific cells of the kidney and colon, and suggest a previously unrecognized function of adseverin in epithelial cell function.

Caspase-3-induced gelsolin fragmentation contributes to actin cytoskeletal collapse, nucleolysis, and apoptosis of vascular smooth muscle cells exposed to proinflammatory cytokines

Gelsolin, an 80 kDa actin-severing protein, has been recently identified as a substrate for the cell death-promoting cysteinyl protease caspase-3 (CPP32/apopain/YAMA). We investigated the role of gelsolin and its cleavage product in apoptosis of vascular smooth muscle cells (SMC) induced by the proinflammatory cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha). Treatment with a combination of IFN-gamma and TNF-alpha reduced viability of SMC in a time- and concentration-dependent manner. Immunoblotting revealed that SMC treated with the cytokines generated a 41 kDa gelsolin fragment. The gelsolin fragmentation required activation of caspase-3, as the caspase-3 inhibitor diminished cytokine-induced cell death as well as the fragmentation. Gelsolin cleavage was accompanied by a reduction in F-actin content and by a marked disruption of cell structure. Adenovirus-mediated transfection of this N-terminal gelsolin fragment into SMC altered cell morphology, reduced cell viability, increased the number of TUNEL-positive cells, and promoted internucleosomal DNA fragmentation. Compared to wild-type cells, gelsolin-deficient SMC showed resistance to apoptosis induced by the inflammatory cytokines. These results suggest a mechanistic role for gelsolin cleavage during SMC apoptosis, a process implicated in vessel development as well as stability of atherosclerotic plaque.

Identification of the binding partners for flightless I, A novel protein bridging the leucine-rich repeat and the gelsolin superfamilies

Flightless-I (fliI) is a novel member of the gelsolin family that is important for actin organization during Drosophila embryogenesis and myogenesis. Drosophila fliI and the human homolog FLI both contain the classic gelsolin 6-fold segmental repeats and an amino-terminal extension of 16 tandem leucine-rich repeats (LRR). LRR repeats form amphipathic beta-alpha structural units that mediate protein-protein interactions. Although there are close to 100 known LRR domain-containing proteins, only a few binding pairs have been identified. In this paper, we used biochemical and genetic approaches to identify proteins that interact with human FLI. In vitro synthesized FLI bound to actin-Sepharose and binding was reduced by competition with excess soluble actin. Actin binding was mediated through the gelsolin-like domain and not the LRR domain. Although the FLI LRR module is most closely related to the LRR domains of Ras-interactive proteins, FLI does not associate with Ras, selected Ras effectors, or other Ras-related small GTPases. Two-hybrid screens using FLI LRR as bait identified a novel LRR binding partner. The 0.65-kilobase pair (kb) clone from the screen survived additional rounds of stringent two-hybrid pairwise assays, establishing a specific interaction. Binding to FLI LRR was corroborated by co-immunoprecipitation with FLI LRR. The translated sequence of the FLI LRR associated protein (FLAP) encodes a novel protein not represented in the data base. Northern blot analyses revealed four FLAP messages of approximately 2.7, 2.9, 3.3, and 5.1 kb, which are differentially expressed in the tissues tested. Skeletal and cardiac muscles are particularly rich in the 3.3-kb FLAP message, and the FLI message as well. Full-length FLAP clones were isolated from a mouse skeletal muscle cDNA library. They have an open reading frame which encodes for a protein containing 626 amino acids. Sequence analyses predict that the FLAP protein is rich in alpha-helices and contains stretches of dimeric coiled coil in its middle region and COOH terminus. The identification of actin and FLAP as the binding ligands for the gelsolin-like domain and the LRR domain, respectively, suggests that FLI may link the actin cytoskeleton to other modules implicated in intermolecular recognition and structural organization.

Probing the effects of calcium on gelsolin

Gelsolin is a calcium-regulated actin severing and capping protein that binds two calcium ions and has three sites for actin; two recognize monomeric actin and one attaches to the sides of filaments. It contains six repeating sequence segments (G1-6). Here, we have analyzed the effects of calcium ions on (i) limited proteolysis of bacterially expressed human gelsolin by plasmin and (ii) dynamic light scattering and circular dichroism of gelsolin and various of its subdomains. Following cleavage of gelsolin in the absence of calcium between Lys150 and His151 (the junction between G1 and G2), the molecule does not fall apart, nor does it bind actin without added calcium. This same molecule can be reconstituted by mixing an excess of G1 with G2-6 in EGTA. The noncovalently linked form of gelsolin shows three actin binding sites in calcium and requires 3 microM calcium for 50% activation of actin binding. Measurements of light scattering and circular dichroism revealed structural changes in response to calcium for intact gelsolin and a number of its actin-binding subdomains. Many of these changes occurred at calcium concentrations below 100 nM. These results are discussed in relation to the calcium control of gelsolin function and its three-dimensional structure (Burtnick et al.(1997) Cell 90, 661-670). Nanomolar concentrations of calcium initiate the unlatching of structural constraints that maintain the inaccessibility of the actin binding sites, but actin binding occurs only after additional micromolar calcium sites in both the N-terminal and C-terminal halves of the molecule are occupied.

Characterization of gelsolin truncates that inhibit actin depolymerization by severing activity of gelsolin and cofilin

Gelsolin is a calcium-activated actin-binding protein with six subdomains. The N-terminal (G1) domain is essential for actin-filament-severing activity while other domains within G2-3 position the protein on the filament side allowing G1 to sever. In order to generate reagents capable of competitively inhibiting endogenous gelsolin and, potentially, other actin filament regulatory protein, we expressed several truncates of gelsolin in Escherichia coli, and analyzed how they affected the in vitro activity of two different actin-binding proteins, gelsolin and cofilin. A Ca2+-sensitive truncate containing G2-6 inhibited the F-actin-depolymerizing activities of both gelsolin and cofilin, while a G2-3 truncate was less effective. Using two independent assays, our results support the idea that gelsolin truncates inhibit actin filament severing and do not markedly affect actin subunit dissociation kinetics. Cosedimentation assays in the presence of calcium demonstrate that the G2-6 truncate binds to F-actin more strongly than the G2-3 truncate consistent with a protection mechanism by conformational change of F-actin and/or competitive binding to actin filaments which depends upon the presence of actin filament binding domains.

The crystal structure of plasma gelsolin: implications for actin severing, capping, and nucleation

The structure of gelsolin has been determined by crystallography and comprises six structurally related domains that, in a Ca2+-free environment, pack together to form a compact globular structure in which the putative actin-binding sequences are not sufficiently exposed to enable binding to occur. We propose that binding Ca2+ can release the connections that join the N- and C-terminal halves of gelsolin, enabling each half to bind actin relatively independently. Domain shifts are proposed in response to Ca2+ as bases for models of how gelsolin acts to sever, cap, or nucleate F-actin filaments. The structure also invites discussion of polyphosphoinositide binding to segment 2 and suggests how mutation at Asp-187 could initiate a series of events that lead to deposition of amyloid plaques, as observed in victims of familial amyloidosis (Finnish type).

Gelsolin binding to phosphatidylinositol 4,5-bisphosphate is modulated by calcium and pH

The actin cytoskeleton of nonmuscle cells undergoes extensive remodeling during agonist stimulation. Lamellipodial extension is initiated by uncapping of actin nuclei at the cortical cytoplasm to allow filament elongation. Many actin filament capping proteins are regulated by phosphatidylinositol 4,5-bisphosphate (PIP2), which is hydrolyzed by phospholipase C. It is hypothesized that PIP2 dissociates capping proteins from filament ends to promote actin assembly. However, since actin polymerization often occurs at a time when PIP2 concentration is decreased rather than increased, capping protein interactions with PIP2 may not be regulated solely by the bulk PIP2 concentration. We present evidence that PIP2 binding to the gelsolin family of capping proteins is enhanced by Ca2+. Binding was examined by equilibrium and nonequilibrium gel filtration and by monitoring intrinsic tryptophan fluorescence. Gelsolin and CapG affinity for PIP2 were increased 8- and 4-fold, respectively, by microM Ca2+, and the Ca2+ requirement was reduced by lowering the pH from 7.5 to 7.0. Studies with the NH2- and COOH-terminal halves of gelsolin showed that PIP2 binding occurred primarily at the NH2-terminal half, and Ca2+ exposed its PIP2 binding sites through a change in the COOH-terminal half. Mild acidification promotes PIP2 binding by directly affecting the NH2-terminal sites. Our findings can explain increased PIP2-induced uncapping even as the PIP2 concentration drops during cell activation. The change in gelsolin family PIP2 binding affinity during cell activation can impact divergent PIP2-dependent processes by altering PIP2 availability. Cross-talk between these proteins provides a multilayered mechanism for positive and negative modulation of signal transduction from the plasma membrane to the cytoskeleton.

Refined structure of villin 14T and a detailed comparison with other actin-severing domains

Villin 14T is the amino terminal actin monomer binding domain from the actin-severing and bundling protein villin. Its structure has been determined in solution using heteronuclear multidimensional nuclear magnetic resonance (NMR) spectroscopy (Markus MA, Nakayama T, Matsudaira P, Wagner G. 1994. Solution structure of villin 14T, a domain conserved among actin-severing proteins. Protein Science 3:70-81). An additional nuclear Overhauser effect (NOE) spectroscopy data set, acquired using improved gradient techniques, and further detailed analysis of existing data sets, produced an additional 601 NOE restraints for structure calculations. The overall fold does not change significantly with the additional NOE restraints but the definition of the structure is improved, as judged by smaller deviations among an ensemble of calculated structures that adequately satisfy the NMR restraints. Some of the side chains, especially those in the hydrophobic core of the domain, are much more defined. This improvement in the detail of the solution structure of villin 14T makes it interesting to compare the structure with the crystal structure of gelsolin segment 1, which shares 58% sequence identity with villin 14T, in an effort to gain insight into villin 14T's weaker affinity for actin monomers. Villin 14T has smaller side chains at several positions that make hydrophobic contacts with actin in the context of gelsolin segment 1. The structure is also compared with the structure of the related actin-severing domain, severin domain 2.

Conformational and functional studies of three gelsolin subdomain-1 synthetic peptides and their implication in actin polymerization

Gelsolin, a calcium and inositol phospholipid-sensitive protein, regulates actin filament length. Its activity is complex (capping, severing, etc.) and is supported by several functional domains. The N-terminal domain alone (S1), in particular, is able to impede actin polymerization. Our investigations were attempted to precise this inhibitory process by using synthetic peptides as models mimicking gelsolin S1 activity. Three peptides issued from S1 and located in gelsolin-actin interfaces were synthesized. The peptides (15-28, 42-55, and 96-114 sequences) were tested for their conformational and actin binding properties. Although the three peptides interact well with actin, only peptide 42-55 affects actin polymerization. A detailed kinetic study shows that the latter peptide essentially inhibits the nucleation step during actin polymerization. In conclusion, the present work shows that the binding of a synthetic peptide to a small sequence located outside the actin-actin interface is essential in the actin polymerization process.

Identification of two sites in gelsolin with different sensitivities to adenine nucleotides

The affinity of monomeric actin for several actin-binding proteins, including gelsolin, depends on adenine nucleotides. Gelsolin binds faster and with higher affinity to ADP-actin than to ATP-actin. Here, we show that the C-terminal actin-binding domain of gelsolin, which is required for filament nucleating activity but not for filament severing activity, contains the site that distinguishes between ATP-actin and ADP-actin monomers. In contrast, actin binding to the N-terminal half of gelsolin depends on solution ATP concentrations, but not on the nucleotide (ATP or ADP) tightly bound in the cleft of the actin monomer. Binding is stronger in the absence of free nucleotide or in the presence of 0.5 mM ADP than in solutions containing 0.5 mM ATP. Complexes formed using different nucleotide concentrations differ in their filament-severing activities as well as in their abilities to increase the fluorescence of 4-chloro-7-nitrobenzeno-2-oxa-1,3-diazole-labeled actin monomers. These results suggest that, at physiologic concentrations of nucleotides, both free and actin-bound ATP may affect the binding of actin to its accessory proteins and that gelsolin, actin, or the gelsolin-actin complex, contains a low-affinity nucleotide-binding site.

Sequences of actin implicated in the polymerization process: a simplified mathematical approach to probe the role of these segments

Regulation of actin polymerization and depolymerization is essential for the functions of actin in non-muscle cells and is mediated by a large number of heterologous actin-binding proteins which questions their true impact on the polymerization process. As a model, we report here the modulating effect of monospecific antibody fragments (Fab) as in vitro effectors on actin polymerization kinetics. Polymerization curves were obtained through fluorescence measurements. They were fitted using analytical equations derived from classical models describing the actin polymerization process with the aim of identifying kinetic steps potentially altered by the effectors. The study was limited to three short segments bore by the 300-328 sequence which is located in actin subdomain 3 and implicated in one of the monomer-monomer interfaces. We observed that antibodies which inhibited actin polymerization reacted with both G- and F-actins, modulated both nucleation and elongation steps, enhanced actin monomer dissociation from the filament and apparently did not act as capping or sequestering proteins. Among the antibody populations specific for a restricted and selected sequence in subdomain 3 of actin (sequence 300-326), only those directed to epitopes located near Met 305 and 325 were effective. In contrast, antibodies directed towards the alpha-helix located between the two preceding epitopes had no effect. All the results analyzed here emphasize the important role of some discrete regions and their conformational state in regulation of the interconversion between monomeric and polymeric actins which could be controlled in different ways by the various actin-binding proteins.

The actin-binding properties of the Physarum actin-fragmin complex. Regulation by calcium, phospholipids, and phosphorylation

The actin-binding properties of the actin-fragmin complex from Physarum polycephalum microplasmodia were investigated with respect to regulation by Ca2+, phospholipids, and phosphorylation of the actin subunit by the endogenous actin-fragmin kinase. Fragmin possesses two high affinity actin-binding sites and probably also a third, low affinity site. Its nucleating and F-actin severing activities are inhibited by phosphatidylinositol 4,5-bisphosphate (PIP2). Actin-fragmin specifically binds PIP2 which competes with actin for the Ca(2+)-sensitive site. However, PIP2 cannot dissociate the actin-fragmin complex nor the actin2-fragmin trimer. Efficient F-actin nucleating activity by actin-fragmin is only observed with unphosphorylated actin-fragmin, in the absence of PIP2 and at high Ca2+ (> microM) concentrations. In the presence of PIP2, actin-fragmin only caps actin filaments when unphosphorylated. The results suggest that in the cell, hydrolysis of PIP2, concomitant with the increase of cytosolic Ca2+, could promote subcortical actin polymerization.

Identification of an actin binding region in aldolase

Fragmentation of the actin binding glycolytic enzyme, aldolase, with cyanogen bromide yields an 18K actin binding fragment which corresponds to residues 1-164 of the aldolase sequence. Within this fragment there is a region of sequence (residues 32-52) which is highly homologous to a region of sequence near the C-terminus of actin itself and which is also found in the actin binding domains of a number of other actin binding proteins. A synthetic peptide corresponding to the aldolase sequence 32-52 encompassing this region of homology binds to F-actin and specifically competes with native aldolase for binding to this cytoskeletal protein.

Horse plasma gelsolin labelled with fluorescein isothiocyanate responds to calcium and actin

Reaction between horse plasma gelsolin and fluorescein-5-isothiocyanate (FITC) resulted in incorporation of 4.8 +/- 0.6 fluorescein groups/gelsolin molecule. The sites of modification were not clustered in any one portion of the gelsolin polypeptide chain; all major peptides produced by proteolytic digestion with alpha-chymotrypsin exhibited a fluorescence characteristic of fluorescein. FITC-gelsolin has a peptide-backbone circular dichroism spectrum at 20 degrees C that is indistinguishable from that of native gelsolin, but FITC-gelsolin is considerably more resistant than native gelsolin to thermally induced precipitation. FITC-gelsolin is fully able to carry out severing of F-actin filaments, the prime function of gelsolin in plasma. An opening up of the structure of gelsolin on binding Ca2+ is evident from an increased susceptibility of FITC-gelsolin to quenching by I-. Ca2+ dependence of the interaction between gelsolin and actin is evident in titrations both of intensity and polarization of the fluorescence of FITC-gelsolin solutions. A Ca(2+)-sensitive interaction between gelsolin and tropomyosin also is observed.

Dynamic changes in chromaffin cell cytoskeleton as prelude to exocytosis

Earlier work by us as well as others has demonstrated that filamentous actin is mainly localized in the cortical surface of chromaffin cell. This F-actin network acts as a barrier to the chromaffin granules, impeding their contact with the plasma membrane. Chromaffin granules contain alpha-actinin, an anchorage protein that mediates F-actin association with these vesicles. Consequently, chromaffin granules crosslink and stabilize F-actin networks. Stimulation of chromaffin cell produces disassembly of F-actin and removal of the barrier. This interpretation is based on: (1) Cytochemical experiments with rhodamine-labeled phalloidin indicated that in resting chromaffin cells, the F-actin network is visualized as a strong cortical fluorescent ring; (2) Nicotinic receptor stimulation produced fragmentation of this fluorescent ring, leaving chromaffin cell cortical areas devoid of fluorescence; and (3) These changes are accompanied by a decrease in F-actin, a concomitant increase in G-actin, and a decrease in the F-actin associated with the chromaffin cell cytoskeleton (DNAse I assay). We also have demonstrated the presence in chromaffin cells of gelsolin and scinderin, two Ca(2+)-dependent actin filament-severing proteins, and suggested that chromaffin cell stimulation activates scinderin with the consequent disruption of F-actin networks. Scinderin, a protein recently isolated in our laboratory, is restricted to secretory cells and is present mainly in the cortical chromaffin cell cytoplasm. Scinderin, which is structurally different from gelsolin (different pIs, amino acid composition, peptide maps, and so on), decreases the viscosity of actin gels as a result of its F-actin-severing properties, as demonstrated by electron microscopy. Stimulation of chromaffin cells either by nicotine (10 microM) or high K+ (56 mM) produces a redistribution of subplasmalemmal scinderin and actin disassembly, which preceded exocytosis. The redistribution of scinderin and exocytosis is Ca(2+)-dependent and is not mediated by muscarinic receptors. Furthermore, our cytochemical experiments demonstrate that chromaffin cell stimulation produces a concomitant and similar redistribution of scinderin (fluorescein-labeled antibody) and F-actin (rhodamine phalloidin fluorescence), suggesting a functional interaction between these two proteins. Stimulation-induced redistribution of scinderin and F-actin disassembly would produce subplasmalemmal areas of decreased cytoplasmic viscosity and increased mobility for chromaffin granules. Exocytosis sites, evaluated by antidopamine-beta-hydroxylase (anti-D beta H) surface staining, are preferentially localized in plasma membrane areas devoid of F-actin.

The amino-terminal fragment of gelsolin is cross-linked to Cys-374 of actin in the EGTA-resistant actin-gelsolin complex

It has been shown that the EGTA-resistant actin, one of the two actin molecules associated to gelsolin, can be predominantly cross-linked to gelsolin by benzophenone-4-maleimide (BPM), a photoaffinity-labeling reagent, which was conjugated to Cys-374 of actin prior to cross-linking (Doi, Y., Banba, M. and Vertut-Doï, A. (1991) Biochemistry 30, 5769-5777). When a chymotryptic digest of gelsolin containing the amino-terminal 15-kDa fragment was mixed with BPM-actin (42 kDa) and irradiated for cross-linking, a band of 58 kDa appeared on SDS-PAGE which was shown to contain actin molecule by using fluorescently labeled actin. The amino-terminal sequence of the 58-kDa complex was identical to that of gelsolin, confirming that the amino-terminal segment (residues 1-133) of pig plasma gelsolin lies closely to Cys-374 of actin in the EGTA-resistant complex.

Schistosoma mansoni: characterization and identification of calcium-binding proteins associated with the apical plasma membrane and envelope

Calcium-binding proteins (CaBPs) of Schistosoma mansoni were purified by hydrophobic affinity chromatography. Metabolically labeled CaBPs were characterized using SDS-polyacrylamide gel electrophoresis followed by fluorography. A number of CaBPs were detected in total tissue extracts, apical plasma membrane, and soluble fractions of the apical bilayer complex, ranging from 15 to 205 kDa in their molecular masses. No CaBPs were discerned in the envelope of the apical bilayer complex. Two CaBPs were positively identified as calmodulin and gelsolin via immunoblot analyses. The possible role of CaBPs in surface signal transduction mechanisms has also been briefly discussed.

65-kilodalton protein phosphorylated by interleukin 2 stimulation bears two putative actin-binding sites and two calcium-binding sites

We have previously characterized a 65-kilodalton protein (p65) as an interleukin 2 stimulated phosphoprotein in human T cells and showed that three endopeptide sequences of p65 are present in the sequence of l-plastin [Zu et al. (1990) Biochemistry 29, 1055-1062]. In this paper, we present the complete primary structure of p65 based on the cDNA isolated from a human T lymphocyte (KUT-2) cDNA library. Analysis of p65 sequences and the amino acid composition of cleaved p65 N-terminal peptide indicated that the deduced p65 amino acid sequence exactly coincides with that of l-plastin over the C-terminal 580 residues [Lin et al. (1988) Mol. Cell. Biol. 8, 4659-4668] and has a 57-residue extension at the N-terminus to l-plastin. Computer-assisted structural analysis revealed that p65 is a multidomain molecule involving at least three intriguing functional domains: two putative calcium-binding sites along the N-terminal 80 amino acid residues; a putative calmodulin-binding site following the calcium-binding region; and two tandem repeats of putative actin-binding domains in its middle and C-terminal parts, each containing approximately 240 amino acid residues. These results suggest that p65 belongs to actin-binding proteins.

Weak binding of divalent cations to plasma gelsolin

Calcium binding of swine plasma gelsolin was examined. When applied to ion-exchange chromatography, its elution volume was drastically altered depending on the free Ca2+ concentration of the medium. The presence of two classes of Ca2+ binding sites, high-affinity sites (Kd = 7 microM) and low-affinity sites (Kd = 1 mM), was suggested from the concentration dependence of the elution volume. The tight binding sites were specific for Ca2+. The weakly bound Ca2+ could be replaced by Mg2+ once the tight binding sites were occupied with Ca2+. The binding of metal ions was totally reversible. Circular dichroism measurement of plasma gelsolin indicated that most change in secondary structure was associated with Ca2+ binding to the high-affinity sites. Binding of Mg2+ to the low-affinity sites caused a secondary structural change different from that caused by Ca2+ bound to the high-affinity sites. Gel permeation chromatography exhibited a small change in Stokes radius with and without Ca2+. Microheterogeneity revealed by isoelectric focusing did not relate to the presence of two classes of Ca2+ binding sites. These results indicated that plasma gelsolin drastically altered its surface charge property due to binding of Ca2+ or Ca2+, Mg2+ with a concomitant conformational change.