Bovine serum brevin. Purification by hydrophobic chromatography and properties

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.

The role of F-actin cytoskeleton-associated gelsolin in the guinea pig capacitation and acrosome reaction

The acrosomal reaction (AR) is a regulated sperm exocytotic process that involves fusion of the plasma membrane (PM) with the outer acrosomal membrane (OAM). Our group has described F-actin cytoskeletons associated to these membranes. It has been proposed that in regulated exocytosis, a cortical cytoskeleton acts as a barrier that obstructs membrane fusion, and must be disassembled for exocytosis to occur. Actin-severing proteins from the gelsolin family have been considered to break this barrier. The present study attempted to determine if gelsolin has a function in guinea pig sperm capacitation and AR. By indirect immunofluorescence (IIF), gelsolin was detected in the apical and postacrosomal regions of the head and in the flagellum in both capacitated and non-capacitated guinea pig spermatozoa. By Western blotting, gelsolin was detected in isolated PM and OAM of non-capacitated spermatozoa. Gelsolin and actin were detected in a mixture of PM-OAM obtained by sonication, and both proteins were absent in membranes of capacitated spermatozoa. Inhibition of three different pathways of PIP2 hydrolysis during capacitation did not cancel gelsolin loss from membranes. Gelsolin was detected by Western blotting associated to membrane cytoskeletons obtained after phalloidin F-actin stabilization and Triton-X treatment; additionally, by immunoprecipitation, it was shown that gelsolin is associated with actin. By electron microscopy we observed that skeletons disassemble during capacitation, but phalloidin prevents disassembly. A three-dimensional skeleton was observed that apparently joins PM with OAM. Exogenous gelsolin stimulates AR assayed in a permeabilized spermatozoa model. Results suggest that gelsolin disassembles F-actin cytoskeletons during capacitation, promoting AR.

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.

Gelsolin inhibits apoptosis by blocking mitochondrial membrane potential loss and cytochrome c release

Apoptotic cell death, characterized by chromatin condensation, nuclear fragmentation, cell membrane blebbing, and apoptotic body formation, is also accompanied by typical mitochondrial changes. The latter includes enhanced membrane permeability, fall in mitochondrial membrane potential (Deltapsi(m)) and release of cytochrome c into the cytosol. Gelsolin, an actin regulatory protein, has been shown to inhibit apoptosis, but when cleaved by caspase-3, a fragment that is implicated as an effector of apoptosis is generated. The mechanism by which the full-length form of gelsolin inhibits apoptosis is unclear. Here we show that the overexpression of gelsolin inhibits the loss of Deltapsi(m) and cytochrome c release from mitochondria resulting in the lack of activation of caspase-3, -8, and -9 in Jurkat cells treated with staurosporine, thapsigargin, and protoporphyrin IX. These effects were corroborated in vitro using recombinant gelsolin protein on isolated rat mitochondria stimulated with Ca(2+), atractyloside, or Bax. This protective function of gelsolin, which was not due to simple Ca(2+) sequestration, was inhibited by polyphosphoinositide binding. In addition we confirmed that gelsolin, besides its localization in the cytosol, is also present in the mitochondrial fraction of cells. Gelsolin thus acts on an early step in the apoptotic signaling at the level of mitochondria.

Capping and dynamic relation between domains 1 and 2 of gelsolin

Gelsolin is a protein that severs and caps actin filaments. The two activities are located in the N-terminal half of the gelsolin molecules. Severing and subsequent capping requires the binding of domains 2 and 3 (S2-3) to the side of the filaments to position the N-terminal domain 1 (S1) at the barbed end of actin (actin subdomains 1 and 3). The results provide a structural basis for the gelsolin capping mechanism. The effects of a synthetic peptide derived from the sequence of a binding site located in gelsolin S2 on actin properties have been studied. CD and IR spectra indicate that this peptide presented a secondary structure in solution which would be similar to that expected for the native full length gelsolin molecule. The binding of the synthetic peptide induces conformational changes in actin subdomain 1 and actin oligomerization. An increase in the polymerization rate was observed, which could be attributed to a nucleation kinetics effect. The combined effects of two gelsolin fragments, the synthetic peptide derived from an S2 sequence and the purified segment 1 (S1), were also investigated as a molecule model. The two fragments induced nucleation enhancement and inhibited actin depolymerization, two characteristic properties of capping. In conclusion, for the first time it is reported that the binding of a small synthetic fragment is sufficient to promote efficient capping by S1 at the barbed end of actin filaments.

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.

Localization of a calcium sensitive binding site for gelsolin on actin subdomain I: implication for severing process

The binding of the N-terminal domain (S1) of gelsolin to monomeric actin has been extensively documented. In contrast, the location of the C-terminal calcium dependent domains (S4-6) interacting with the actin filament during the severing process remains uncertain. In this study, we have identified a new interface that supports calcium dependent gelsolin binding to actin. This site is located in a critical position towards actin-actin contact in the filament and in the vicinity of the phalloidin site. Using specific antibody and synthetic peptides derived from actin sequence within 105-132 residues, this interface was finally ascribed to the segment 112-120 on the actin subdomain-1.

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.

Moesin, ezrin, and p205 are actin-binding proteins associated with neutrophil plasma membranes

Actin-binding proteins in bovine neutrophil plasma membranes were identified using blot overlays with 125I-labeled F-actin. Along with surface-biotinylated proteins, membranes were enriched in major actin-binding polypeptides of 78, 81, and 205 kDa. Binding was specific for F-actin because G-actin did not bind. Further, unlabeled F-actin blocked the binding of 125I-labeled F-actin whereas other acidic biopolymers were relatively ineffective. Binding also was specifically inhibited by myosin subfragment 1, but not by CapZ or plasma gelsolin, suggesting that the membrane proteins, like myosin, bind along the sides of the actin filaments. The 78- and 81-kDa polypeptides were identified as moesin and ezrin, respectively, by co-migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoprecipitation with antibodies specific for moesin and ezrin. Although not present in detectable amounts in bovine neutrophils, radixin (a third and closely related member of this gene family) also bound 125I-labeled F-actin on blot overlays. Experiments with full-length and truncated bacterial fusion proteins localized the actin-binding site in moesin to the extreme carboxy terminus, a highly conserved sequence. Immunofluorescence micrographs of permeabilized cells and cell "footprints" showed moesin co-localization with actin at the cytoplasmic surface of the plasma membrane, consistent with a role as a membrane-actin-linking protein.

Variant plasma gelsolin responsible for familial amyloidosis (Finnish type) has defective actin severing activity

Familial amyloidosis, Finnish type is caused by a single base mutation in gelsolin, an actin filament severing and capping protein that is present in most tissues and in blood plasma. The mutation replaces aspartic acid with asparagine at residue 187 of the plasma sequence. This renders the gelsolin susceptible to proteolysis as a consequence of which amyloid protein is formed. Here it is shown that the mutant protein in plasma from a patient homozygous for this mutation lacks both actin severing and nucleating activities. Evidence is presented that the cleaved mutant gelsolin has dissociated under non-denaturing conditions and that the resultant 65,000 and 55,000 M(r) C-terminal fragments aggregate.

Definition of the EGTA-independent interface involved in the serum gelsolin-actin complex

The gelsolin-actin complex in the presence of Ca2+ revealed at least three interacting sites on the gelsolin molecule located in the S1, S2-3, and S4-6 domains. In the presence of EGTA, the N-terminal domain of gelsolin is known to be involved. However, the corresponding site on the surface of actin is poorly defined. The present result locates the Ca(2+)-independent plasma gelsolin-binding site on the actin surface. Natural and synthetic actin peptides were tested for their possible interaction with gelsolin and monitored by fluorescence anisotropy measurements and e.l.i.s.a. The interface was thus located within the 360-372 actin sequence near the C-terminal extremity. In addition, we used a chymotryptic digest of gelsolin and determined that its N-terminal domain (S1) was implicated in this interface. We conclude that the interaction of the 41-126 region of plasma gelsolin is the counterpart of the 360-372 sequence in subdomain 1 of actin.

Influence of Ca-activated brevin on the mechanical properties of skinned smooth muscle

Solutions of purified brevin were applied to skinned thin bundles or isolated fibres of smooth muscle. This produced a sharp drop of isometric tension, an effect due to the severing effect of brevin on actin filaments, partially depleted from tropomyosin in skinned preparations. On skinned single fibres, brevin accelerates the speed of unloaded shortening. As no effect was detected on the myofibrillar ATPase turnover rate, brevin was thought to affect the viscosity of the cytoplasm. This was confirmed by analysis of the cytoplasm stiffness which decreased in the presence of brevin. It is proposed that Ca-activated brevin acts on actin-filamin gels, set in parallel to the contractile apparatus.

Complex stiffness of smooth muscle cytoplasm in the presence of Ca-activated brevin

Brevin, an F-actin severing protein, regulates actin gel-sol transformation in a Ca(2+)-dependent way. Here, we tested its effect on the stiffness of the cytoplasm of skinned smooth muscle, in the absence of actin-myosin interaction (inhibited myosin ATPase). Complex stiffness was measured by imposing sinusoidal stretches and releases at different frequencies (1-50 Hz). In the presence of Ca-activated brevin, the stiffness decreased by about 30%, at all frequencies, from its initial values in Ca-free, relaxing solution. This decrease reflected a fall in both elasticity and viscosity of the cytoplasm. We propose that brevin specifically operates on an actin network in parallel with the contractile apparatus, e.g. on the actin-filamin gel.

The binding of terbium ions to gelsolin reveals two classes of metal ion binding sites

Spectroscopically active terbium ions have been used to probe the Ca2+ ion-binding sites on human plasma gelsolin. The luminescence of Tb3+ ions bound to gelsolin is markedly enhanced when excited indirectly at 295 nm due to Förster type dipole-dipole energy transfer from neighboring tryptophan residues. Titration of this luminescence with increasing concentrations of Tb3+ ions was saturable although the shape of this titration curve was complex indicating the involvement of multiple classes of sites. Luminescence lifetime measurements (obtained by indirect excitation at 295 nm) demonstrate the presence of two classes of sites characterized by a major lifetime of 1.0-1.1 ms and a minor lifetime of 0.7-0.8 ms. However, while the amplitude of the minor lifetime showed a hyperbolic dependence on the Tb3+ ion concentration, the amplitude of the major lifetime showed a strongly sigmoidal dependence. Different classes of Tb3+ ion binding sites can also be distinguished by the different Ca2+ ion concentrations needed to displace Tb3+ ions from these sites on gelsolin. It is proposed that the occupancy of one class of Tb3+ ion binding sites on gelsolin causes a conformational change in gelsolin which then allows a second class of cryptic Tb3+ ion binding sites to be expressed. The implications of these results in terms of the binding of Ca2+ ions to gelsolin and the regulation of the activities of gelsolin by calcium are discussed.

Definition of a Ca2(+)-sensitive interface in the plasma gelsolin-actin complex

Gelsolin is a Ca2(+)-dependent protein which severs actin filaments, caps their fast-growing ends and promotes nucleation. We report here results that delimit one of the interfaces between serum gelsolin and actin monomer. An actin-derived synthetic peptide (amino acids 305-326 of actin) coupled to a hydrophilic resin was tested for its possible interaction with gelsolin. We selected this sequence because it corresponds to a region implicated in the gelsolin-actin complex in a previous work [Boyer, Feinberg, Hue, Capony, Benyamin & Roustan (1987) Biochem. J. 248, 359-364]. We showed that this actin sequence is located at the surface of the actin molecule and observed a Ca2(+)-sensitive binding of gelsolin to this actin-derived peptide. In addition, by using a chymotryptic digest of gelsolin, we reported that only the C-terminal half of gelsolin interacts with the actin-(305-326)-peptide. These results that the Ca2(+)-sensitive interface includes both amino acids 305-326 of actin and probably amino acids 660-738 of gelsolin.

gCap39, a calcium ion- and polyphosphoinositide-regulated actin capping protein

The polymerization of actin filaments is involved in growth, movement, and cell division. It has been shown that actin polymerization is controlled by gelsolin, whose interactions with actin are activated by calcium ion (Ca2+) and inhibited by membrane polyphosphoinositides (PPI). A smaller Ca2(+)- and PPI-regulated protein, gCap39, which has 49% sequence identity with gelsolin, has been identified by cDNA cloning and protein purification. Like gelsolin, gCap39 binds to the fast-growing (+) end of actin filaments. However, gCap39 does not sever actin filaments and can respond to Ca2+ and PPI transients independently, under conditions in which gelsolin is ineffective. The coexistence of gCap39 with gelsolin should allow precise regulation of actin assembly at the leading edge of the cell.

Gel electrophoresis of native gelsolin and gelsolin-actin complexes

BHK gelsolin migrated on non-denaturing 8-25% polyacrylamide gels with an apparent molecular mass of 80 kDa. In the absence of Ca2+ no complex formation occurred between BHK gelsolin and actin. In the presence of Ca2+ two complex species were found: a ternary complex, GA2, of apparent molecular mass of 210 kDa at gelsolin:actin ratio of 1:2, and a novel quaternary complex, GA3, of apparent molecular mass of 247 kDa when actin was in excess. Both cytoplasmic and plasma gelsolin species form GA3 with skeletal muscle actin. No complexes larger than GA3 were observed. The formation of GA3 involves the binding of a third actin to the gelsolin molecule at the site previously assumed to be masked, rather than to the actin molecules already present in GA2. In preference to GA3, GA2 was incorporated into actin filaments stabilized with phalloidin. On chelation of free Ca2+, both GA2 and GA3 dissociated to form the EGTA stable binary complex (GA) with an apparent molecular mass of 140 kDa and free actin.

The action of brevin, an F-actin severing protein, on the mechanical properties and ATPase activity of skinned smooth muscle

Brevin is a protein which regulates the actin gel-sol transformation: it severs F-actin filaments into shorter ones. This action is Ca-dependent and is prevented by tropomyosin. We tested the effect of brevin on isometric contractions of skinned smooth muscle (taenia coli) and noted a dramatic loss of tension that possibly reflects some F-actin fragmentation. This effect is tentatively attributed to a partial loss of tropomyosin in the skinning procedure. We also studied the effect of brevin on unloaded shortenings of skinned preparations: thin bundles and enzymatically dissociated cells. We observed a marked increase of the velocity of shortening in the presence of brevin. This effect cannot be attributed to an increased ATPase activity as the latter is slightly reduced in the presence of brevin. We interpret this result as reflecting a decrease in internal resistance to movement, possibly by solation of an actin-filamin domain.

Modulation of gelsolin content in rat aortic smooth muscle cells during development, experimental intimal thickening and culture. An immunohistochemical and biochemical study

Gelsolin is a Ca2+ and polyphosphoinositol-phospholipid regulated modulator of actin polymerization present in most mammalian cells and in plasma. Cytoplasmic gelsolin was first described in highly motile cells such as leukocytes, where actin polymerization is dynamic; however arterial smooth muscle cells (SMC), despite their stabilized actin bundles, express high levels of gelsolin. We have investigated gelsolin modulation in rat aortic SMC by immunohistochemistry, Western blotting and Northern hybridization using three models which are known to show modulation of actin isoform expression: development, aortic intimal thickening after experimental endothelial injury, and growth in culture. When related to the protein and mRNA content of adult aortic SMC, gelsolin is expressed about 50% in aortic SMC of five-day-old rats, 20-30% in SMC of intimal thickening 15 days after endothelial injury (when SMC are actively replicating) and in SMC growing in culture; in intimal thickening 60 days after injury (when SMC have returned to quiescence), the gelsolin content becomes similar to that of control SMC. The high level of gelsolin in smooth muscle (SM) tissues and the down regulation with proliferation and migration raises the question as to whether gelsolin in these cells has functions other than the dynamic control of actin filament length. The similar modulation patterns of gelsolin and alpha-SM actin suggest a preferential interaction between these two proteins.

Simple and rapid purification of brevin

Brevin or plasma gelsolin, a calcium dependent actin-binding and actin-severing protein, was purified from bovine plasma by a very rapid and simple procedure; ammonium sulfate fractionation and only one step of anion exchange column chromatography by a convenient use. It takes only 24 hrs to complete all the procedure. The purity of brevin prepared by this method was more than 95% on SDS-PAGE and total recovery was much better than previous preparation methods. This brevin preparation has about 8 isomers on 2-D PAGE and strong severing activity on F-actin under electron microscopic observation.

Microfilament organization correlates with increased cellular content of gelsolin

The relative amounts of intracellular gelsolin were determined in a number of human somatic cell hybrids and parental cell lines which greatly differ in their microfilament organizations. In contrast to the disruptive effect of gelsolin on actin filament formation in vitro, there is a correlation between the degree of microfilament organization and the amount of gelsolin within these cell lines.

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.

Distribution of a gelsolin-like 74,000 mol. wt protein in neural and endocrine tissues

A Ca2(+)-dependent actin binding protein with a molecular weight of 74,000, was purified from bovine adrenal medulla by using deoxyribonuclease I affinity chromatography followed by ion-exchange chromatography and gel filtration. This protein broke actin filaments into fragments and promoted nucleation of actin polymerization in a Ca2(+)-dependent manner as effectively as gelsolin. Proteolytic and immunological comparison with gelsolin which is widely distributed actin-severing protein, indicated that the 74,000 mol. wt protein is a distinct protein, but its domain structure resembles that of gelsolin. Immunoblotting using antibody against this protein showed a tissue-specific distribution. The protein was detected in various endocrine, neuroendocrine and nervous tissues, but not in muscle tissues and plasma which contained relatively large amounts of cytoplasmic and plasma gelsolin. This fact might indicate that this actin-severing protein is involved in the regulation of the secretory process of endocrine and nervous tissues. In the exocytotic process regulated by Ca2+, this protein probably plays a role to free secretory organelles like vesicles from the cytoskeletal network, mainly F-actin, which prevents the movement of secretory vesicles in the resting state.

Isolation and characterization of gelsolin from cultured BHK cells

The Ca2+-dependent actin-polymerization nucleating protein of the cytoplasmic fraction of Baby hamster kidney (BHK) C13 cells has been isolated by anion-exchange, hydroxyapatite and gel-filtration chromatography. This protein has been identified as a cytoplasmic gelsolin by the following criteria: molecular mass of 90 kDa on SDS-PAGE, immunocrossreactivity with pig plasma gelsolin and similar actin-binding properties to gelsolins purified from other sources. BHK gelsolin forms a 1:2 ternary complex with rabbit muscle actin that is dependent on the presence of Ca2+. The ternary complex is dissociated on chelation of Ca2+ with EGTA to a binary complex and free actin. BHK gelsolin nucleates the polymerization of pyrene-labelled G-actin in a Ca2+-dependent manner. The proportion of unpolymerized monomer is increased in the presence of BHK gelsolin by an amount consistent with capping of the positive filament ends. The rate of actin depolymerization induced by diluting F-actin to below its critical concentration (Cc) is unaffected by the presence of BHK gelsolin in EGTA. However, in the presence of Ca2+ the rate of depolymerization is increased indicating that BHK gelsolin severs actin filaments in a Ca2+-dependent manner.

An electrophoretic procedure for detecting proteins that bind actin monomers

The electrophoretic mobility of fluorescently labeled G-actin in polyacrylamide gels under nondenaturing conditions is altered by the formation of complexes with actin-binding proteins. This effect offers a convenient method for detecting and quantitating such proteins in tissue fractions and for monitoring their purification. When followed by second-dimension electrophoresis in the presence of sodium dodecyl sulfate, the method also gives the apparent molecular weights of the actin-binding components and the stoichiometry of the complexes. The method has also been used to identify actin-binding fragments in digests of actin-binding proteins, to investigate the formation of multicomponent complexes, and to determine the calcium-sensitivity of complexes.

Microheterogeneity of actin gels formed under controlled linear shear

The diffusion coefficients and fluorescence polarization properties of actin subjected to a known shear have been determined both during and after polymerization, using a modification of a cone-plate Wells-Brookfield rheometer that allows monitoring of samples with an epifluorescence microscope. Fluorescence polarization and fluorescence photobleaching recovery experiments using rhodamine-labeled actin as a tracer showed that under conditions of low shear (shear rates of 0.05 s-1), a spatial heterogeneity of polymerized actin was observed with respect to fluorescence intensity and the diffusion coefficients with actin mobility becoming quite variable in different regions of the sample. In addition, complex changes in fluorescence polarization were noted after stopping the shear. Actin filaments of controlled length were obtained using plasma gelsolin (gelsolin/actin molar ratios of 1:50 to 1:300). At ratios of 1:50, neither spatial heterogeneity nor changes in polarization were observed on subjecting the polymerized actin to shear. At ratios of approximately 1:100, a decrease on the intensity of fluorescence polarization occurs on stopping the shear. Longer filaments exhibit spatial micro-heterogeneity and complex changes in fluorescence polarization. In addition, at ratios of 1:100 or 1:300, the diffusion coefficient decreases as the total applied shear increased. This behavior is interpreted as bundling of filaments aligned under shear. We also find that the F-actin translational diffusion coefficients decrease as the total applied shear increases (shear rates between 0.05 and 12.66 s-1), as expected for a cumulative process. When chicken gizzard filamin was added to gelsolin-actin filaments (at filamin/actin molar ratios of 1:300 to 1:10), a similar decrease in the diffusion coefficients was observed for unsheared samples. Spatial microheterogeneity might be related to the effects of the shear field in the alignment of filaments, and the balance between a three-dimensional network and a microheterogeneous system (containing bundles or anisotropic phases) appears related to both shear and the presence of actin-binding proteins.

A 35-kilodalton fragment from gizzard smooth muscle caldesmon that induces F-actin bundles

Specific thrombin proteolysis of native 120-kDa gizzard caldesmon gave rise to a major cleavage into an N-terminal 90-kDa and a C-terminal 35-kDa fragment. Fluorescent labeling, cosedimentation, passage through an affinity column, and carbodiimide crosslinking with actin revealed that the 35-kDa purified segment of the molecule contains the actin and the calcium-calmodulin binding regions. Electron microscopic analysis of its actin complex demonstrated that the 35-kDa segment possesses the bundling properties of the intact molecule. Thus, a possible pathway for the expression of the caldesmon regulatory function during smooth muscle contraction would be a conformational change twisting the helicoïdal structure of the actin filament, which occurs when the 35-kDa caldesmon portion binds to it.

Localization and mobility of gelsolin in cells

To investigate the physiologic role of gelsolin in cells, we have studied the location and mobility of gelsolin in a mouse fibroblast cell line (C3H). Gelsolin was localized by immunofluorescence of fixed and permeabilized cells and by fluorescent analog cytochemistry of living cells and cells that were fixed and/or permeabilized. Overall, the images show that in living cells gelsolin has a diffuse cytoplasmic distribution, but in fixed cells a minor fraction is associated with regions of the cell that are rich in actin filaments. The latter fraction is more prominent after permeabilization of the fixed cells because some diffuse gelsolin is not fixed and is therefore lost during permeabilization, confirmed by immunoblots. To determine quantitatively whether gelsolin is bound to actin filaments in living cells, we measured the mobility of microinjected fluorescent gelsolin by fluorescence photobleaching recovery. Gelsolin is fully mobile with a diffusion coefficient similar to that of control proteins. As a positive control, fluorescent phalloidin, which binds actin filaments, is totally immobile. These results are supported by immunoblots on cells permeabilized with detergent. All the endogenous gelsolin is extracted, and the half-time for the extraction is approximately 5 s, which is about the rate predicted for diffusion. Therefore, gelsolin is not tightly bound to actin filaments in cells. The most likely interpretation of the difference between living and fixed cells is that fixation traps a fraction of gelsolin that is associated with actin filaments in short-lived complexes.

Antigenic probes locate a serum-gelsolin-interaction site on the C-terminal part of actin

The implication of part of the C-terminal of actin (within the 285-375 sequence) in the interaction of serum gelsolin was investigated by the use of specific antibodies. These antibodies were directed against two or three discrete epitopes, one of which was specific for skeletal-muscle actin. Some of these epitopes were found to be near the serum gelsolin-actin interface. Thus it can be assumed that part of the C-terminal of actin is exposed at the barbed end of the actin filament. The interaction between tropomyosin and actin was also studied.

Anti-actin antibodies. Detection and quantitation of total and skeletal muscle actin in human plasma using a competitive ELISA

A competitive ELISA has been used to titrate skeletal muscle and total actins in human plasma. Specific antibodies directed against the variable N-terminal 1-7 sequence and conserved sequences respectively were used. The N-terminus of actin appears to be accessible in native and brevin-complexed actins. The skeletal muscle actin isoform represents about 1% of the total circulating actin (mean: 50 micrograms/ml plasma), but is markedly increased after severe muscle tissue injuries.

Microinjection of gelsolin into living cells

Gelsolins are actin-binding proteins that cap, nucleate, and sever actin filaments. Microinjection of cytoplasmic or plasma gelsolin into living fibroblasts and macrophages did not affect the shape, actin distribution, deformability, or ruffling activity of the cells. Gelsolin requires calcium for activity, but the NH2-terminal half is active without calcium. Microinjection of this proteolytic fragment had marked effects: the cells rounded up, stopped ruffling, became soft, and stress fibers disappeared. These changes are similar to those seen with cytochalasin, which also caps barbed ends of actin filaments. Attempts to raise the cytoplasmic calcium concentration and thereby activate the injected gelsolin were unsuccessful, but the increases in calcium concentration were minimal or transient and may not have been sufficient. Our interpretation of these results is that at the low calcium concentrations normally found in cells, gelsolin does not express the activities observed in vitro at higher calcium concentrations. We presume that gelsolin may be active at certain times or places if the calcium concentration is elevated to a sufficient level, but we cannot exclude the existence of another molecule that inhibits gelsolin. Microinjection of a 1:1 gelsolin/actin complex had no effect on the cells. This complex is stable in the absence of calcium and has capping activity but no severing and less nucleation activity as compared with either gelsolin in calcium or the NH2-terminal fragment. The NH2-terminal fragment-actin complex also has capping and nucleating activity but no severing activity. On microinjection it had the same effects as the fragment alone. The basis for the difference between the two complexes is unknown. The native molecular weight of rabbit plasma gelsolin is 82,500, and the extinction coefficient at 280 nm is 1.68 cm2/mg. A new simple procedure for purification of plasma gelsolin is described.

Modulation of gelsolin function by phosphatidylinositol 4,5-bisphosphate

The actin-binding protein gelsolin requires micromolar concentrations of calcium ions to sever actin filaments, to potentiate its binding to the end of the filament and to promote the polymerization of monomeric actin into filaments. Because transient increases in both intracellular [Ca2+] and actin polymerization accompany the cellular response to certain stimuli, it has been suggested that gelsolin regulates the reversible assembly of actin filaments that accompanies such cellular activations. But other evidence suggests that these activities do not need increased cytoplasmic [Ca2+] and that once actin-gelsolin complexes form in the presence of Ca2+ in vitro, removal of free Ca2+ causes dissociation of only one of two bound actin monomers from gelsolin and the resultant binary complexes cannot sever actin filaments. The finding that cellular gelsolin-actin complexes can be dissociated suggests that a Ca2+-independent regulation of gelsolin also occurs. Here we show that, like the dissociation of profilin-actin complexes, phosphatidylinositol 4,5-bisphosphate, which undergoes rapid turnover during cell stimulation, strongly inhibits the actin filament-severing properties of gelsolin, inhibits less strongly the nucleating ability of this protein and restores the potential for filament-severing activity to gelsolin-actin complexes.

Preparation and characterization of pig plasma and platelet gelsolins

Pig plasma gelsolin has been prepared by a revised method involving poly(ethylene glycol) precipitation, chromatography on CM-cellulose and affinity chromatography on actin-Sepharose. Pig platelet gelsolin has been prepared by chromatography on DEAE-cellulose and actin-Sepharose. Partial chemical and proteolytic cleavage shows that the two proteins are closely related in their fragmentation patterns. The amino acid sequences are identical at the N-terminus of the platelet protein, but the plasma protein has an additional nine residues on the N-terminal side of the common sequence. Calcium binding studies show that the plasma protein has similar calcium binding properties to both macrophage and platelet gelsolins.

Effects of serum vitamin-D-binding protein on actin in the presence of plasma gelsolin

The interaction of serum vitamin-D-binding protein (DBP) and plasma gelsolin with actin was studied using fluorescent 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole-actin or N-pyrenylcarboxyamidomethyl-actin. DBP and gelsolin formed very tight complexes with one or two monomeric actin subunits respectively. Kd values of about 10 nm have been found for both complexes. When DBP and gelsolin were added simultaneously to G-actin no ternary complex was observed and the DBP-actin complex was formed preferentially. In the presence of CaCl2 no transfer of actin occurred from the 1:2 gelsolin:actin molar complex to free DBP while in the presence of EGTA one actin monomer could be transferred. DBP did not affect the severing activity of gelsolin. The effects of a mixture of gelsolin and DBP on F-actin suggest that only individual interactions of these two plasma proteins with actin occurred in solution.

Plasma and cytoplasmic gelsolins are encoded by a single gene and contain a duplicated actin-binding domain

Gelsolin is representative of a class of actin-modulating proteins found in lower eukaryotes to mammals, which sever actin filaments. Gelsolin found in the cytoplasm of cells is functionally similar to a mammalian plasma protein of similar size, originally called ADF or brevin. Human plasma and rabbit macrophage gelsolins differ by the presence of a 25-amino-acid residue extension on plasma gelsolin which appears to account for the difference in relative molecular mass (Mr) between the proteins as assessed by SDS-polyacrylamide gel electrophoresis (PAGE), 93,000 (93K) and 90K, respectively. Here we report the isolation of full-length human plasma gelsolin complementary DNA clones from a HepG2 library. The inferred amino-acid sequence reveals the presence of a signal peptide, a long tandem repeat that matches the actin-binding domains of gelsolin, a tetrapeptide present in actin and extended regions of identical sequence with rabbit macrophage gelsolin. Southern blot analysis indicates that a single gene in the haploid genome encodes both protein forms.

The actin filament-severing domain of plasma gelsolin

Gelsolin, a multifunctional actin-modulating protein, has two actin-binding sites which may interact cooperatively. Native gelsolin requires micromolar Ca2+ for optimal binding of actin to both sites, and for expression of its actin filament-severing function. Recent work has shown that an NH2-terminal chymotryptic 17-kD fragment of human plasma gelsolin contains one of the actin-binding sites, and that this fragment binds to and severs actin filaments weakly irrespective of whether Ca2+ is present. The other binding site is Ca2+ sensitive, and is found in a chymotryptic peptide derived from the COOH-terminal two-thirds of plasma gelsolin; this fragment does not sever F-actin or accelerate the polymerization of actin. This paper documents that larger thermolysin-derived fragments encompassing the NH2-terminal half of gelsolin sever actin filaments as effectively as native plasma gelsolin, although in a Ca2+-insensitive manner. This result indicates that the NH2-terminal half of gelsolin is the actin-severing domain. The stringent Ca2+ requirement for actin severing found in intact gelsolin is not due to a direct effect of Ca2+ on the severing domain, but indirectly through an effect on domains in the COOH-terminal half of the molecule to allow exposure of both actin-binding sites.

Characterization of the Ca2+-induced conformational changes in gelsolin and identification of interaction regions between actin and gelsolin

Serum gelsolin, a Ca2+-dependent protein regulating the length of actin filaments, undergoes conformational changes upon binding Ca2+. These were detected and analyzed by several approaches including ultraviolet difference spectroscopy, circular dichroism studies, analytical ultracentrifugation, thiol group titration, and limited proteolytic digestions. The effect of Ca2+ binding on the UV absorption difference spectrum and the near-UV circular dichroism spectrum was consistent with changes in the environments of tyrosine and phenylalanine residues. In the presence of Ca2+, the S0(20),w value decreased from 5.3 to 4.7. This latter result implies a transformation to a more asymmetric molecular shape. Gelsolin contained only two accessible thiol groups per mole of protein, one of which was titratable in the native protein; it was more accessible to 5,5'-dithiobis(2-nitrobenzoic acid) in the absence than in the presence of Ca2+. The limited digestion of gelsolin from serum and bovine aorta smooth muscle by two different proteases, chymotrypsin and trypsin, proceeded much faster in the presence of Ca2+ than in its absence with the production of three main fragments of about 40K, 32K, and 21K. This fragment mixture was found still able to shorten F-actin in a Ca2+-dependent manner; this severing activity was expressed by the isolated 40K peptide. Gelsolin was cross-linked to F- and G-actin by the zero-length cross-linker 1-ethyl-3-[3-(dimethylamino)propyl]-carbodiimide (EDC), generating a covalent 130K binary complex (actin1-gelsolin1) followed by a covalent 180K ternary complex (actin2-gelsolin1).(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of plasma gelsolin with actin. Isolation and characterization of binary and ternary plasma-gelsolin-actin complexes

We have studied the interactions between plasma gelsolin and actin: firstly the complex formation between both proteins, secondly the effects of gelsolin and its complexes on G-actin polymerization and F-actin fragmentation. Complex formation has been studied by high-performance gel permeation chromatography; plasma gelsolin alone elutes at an Mr of about 77000 and a Stokes radius of 3.7 nm; complex formation occurs in the presence of Ca2+: by chromatography in the presence of EGTA, a binary complex is obtained with an Mr of 134000 and a Stokes radius of 4.7 nm; and by chromatography in the presence of Ca2+, a ternary complex is obtained with an Mr of 173000 and a Stokes radius of 5.2 nm. The binary complex is EGTA-stable. In relation to this stability of the binary complex, the depolymerizing function of gelsolin is not reversed upon chelation of Ca2+. The effects of plasma gelsolin and its complexes on both G-actin polymerization and F-actin fragmentation, and their Ca2+ dependence have been examined by viscometry and electron microscopy. The main conclusions of these studies are the following: the fast processes are the formation of ternary complex, which acts as a heteronucleus for G-actin polymerization, and the severing function of gelsolin, these fast processes are Ca2+-dependent; the slow processes are related to the capping ability of gelsolin or its complexes and are Ca2+-independent.