The actin side-binding domain of gelsolin also caps actin filaments. Implications for actin filament severing

Molecular dynamics study of interactions between polymorphic actin filaments and gelsolin segment-1

The assembly of protein actin into double-helical filaments promotes many eukaryotic cellular processes that are regulated by actin-binding proteins (ABPs). Actin filaments can adopt multiple conformations, known as structural polymorphism, which possibly influences the interaction between filaments and ABPs. Gelsolin is a Ca²⁺ -regulated ABP that severs and caps actin filaments. Gelsolin binding modulates filament structure; however, it is not known how polymorphic actin filament structures influence an interaction of gelsolin S1 with the barbed-end of filament. Herein, we investigated how polymorphic structures of actin filaments affect the interactions near interfaces between the gelsolin segment 1 (S1) domain and the filament barbed-end. Using all-atom molecular dynamics simulations, we demonstrate that different tilted states of subunits modulate gelsolin S1 interactions with the barbed-end of polymorphic filaments. Hydrogen bonding and interaction energy at the filament-gelsolin S1 interface indicate distinct conformations of filament barbed ends, resulting in different interactions of gelsolin S1. This study demonstrates that filament's structural multiplicity plays important roles in the interactions of actin with ABPs.

Cholesterol-Dependent Phase-Demixing in Lipid Bilayers as a Switch for the Activity of the Phosphoinositide-Binding Cytoskeletal Protein Gelsolin

The lateral distribution of phosphatidylinositol 4,5-bisphosphate (PIP2) in lipid bilayers is affected both by divalent cation-mediated attractions and cholesterol-dependent phase demixing. The effects of lateral redistribution of PIP2 within a membrane on PIP2-protein interactions are explored with an N-terminal fragment of gelsolin (NtGSN) that severs actin in a Ca(2+)-insensitive manner. The extent of NtGSN inhibition by PIP2-containing large unilamellar vesicles (LUVs) depends on the lateral organization of the membrane as quantified by an actin-severing assay. At a fixed PIP2 mole fraction, the inhibition is largely enhanced by the segregation of liquid ordered/liquid disordered (Lo/Ld) phases that is induced by altering either cholesterol content or temperature, whereas the presence of Ca(2+) only slightly improves the inhibition. Inhibition of gelsolin induced by demixed LUVs is more effective with decreasing temperature, coincident with increasing membrane order as determined by Laurdan generalized polarization and is reversible as the temperature increases. This result suggests that PIP2-mediated inhibition of gelsolin function depends not only on changes in global concentration but also on lateral distribution of PIP2. These observations imply that gelsolin, and perhaps other PIP2-regulated proteins, can be activated or inactivated by the formation of nanodomains or clusters without changing PIP2 bulk concentration in the cell membrane.

Gelsolin-Like Domain 3 Plays Vital Roles in Regulating the Activities of the Lily Villin/Gelsolin/Fragmin Superfamily

The villin/gelsolin/fragmin superfamily is a major group of Ca2+-dependent actin-binding proteins (ABPs) involved in various cellular processes. Members of this superfamily typically possess three or six tandem gelsolin-like (G) domains, and each domain plays a distinct role in actin filament dynamics. Although the activities of most G domains have been characterized, the biochemical function of the G3 domain remains poorly understood. In this study, we carefully compared the detailed biochemical activities of ABP29 (a new member of this family that contains the G1-G2 domains of lily ABP135) and ABP135G1-G3 (which contains the G1-G3 domains of lily ABP135). In the presence of high Ca2+ levels in vitro (200 and 10 μM), ABP135G1-G3 exhibited greater actin severing and/or depolymerization and nucleating activities than ABP29, and these proteins had similar actin capping activities. However, in the presence of low levels of Ca2+ (41 nM), ABP135G1-G3 had a weaker capping activity than ABP29. In addition, ABP29 inhibited F-actin depolymerization, as shown by dilution-mediated depolymerization assay, differing from the typical superfamily proteins. In contrast, ABP135G1-G3 accelerated F-actin depolymerization. All of these results demonstrate that the G3 domain plays specific roles in regulating the activities of the lily villin/gelsolin/fragmin superfamily proteins.

Flightless I interacts with NMMIIA to promote cell extension formation, which enables collagen remodeling

The role of the actin-capping protein flightless I in collagen remodeling by mouse fibroblasts is examined. Flightless and nonmuscle myosin IIA cooperate to enable collagen phagocytosis.

We examined the role of the actin-capping protein flightless I (FliI) in collagen remodeling by mouse fibroblasts. FliI-overexpressing cells exhibited reduced spreading on collagen but formed elongated protrusions that stained for myosin10 and fascin and penetrated pores of collagen-coated membranes. Inhibition of Cdc42 blocked formation of cell protrusions. In FliI-knockdown cells, transfection with constitutively active Cdc42 did not enable protrusion formation. FliI-overexpressing cells displayed increased uptake and degradation of exogenous collagen and strongly compacted collagen fibrils, which was blocked by blebbistatin. Mass spectrometry analysis of FliI immunoprecipitates showed that FliI associated with nonmuscle myosin IIA (NMMIIA), which was confirmed by immunoprecipitation. GFP-FliI colocalized with NMMIIA at cell protrusions. Purified FliI containing gelsolin-like domains (GLDs) 1–6 capped actin filaments efficiently, whereas FliI GLD 2–6 did not. Binding assays showed strong interaction of purified FliI protein (GLD 1–6) with the rod domain of NMMIIA (k_D = 0.146 μM), whereas FliI GLD 2–6 showed lower binding affinity (k_D = 0.8584 μM). Cells expressing FliI GLD 2–6 exhibited fewer cell extensions, did not colocalize with NMMIIA, and showed reduced collagen uptake compared with cells expressing FliI GLD 1–6. We conclude that FliI interacts with NMMIIA to promote cell extension formation, which enables collagen remodeling in fibroblasts.

Pivotal and distinct role for Plasmodium actin capping protein alpha during blood infection of the malaria parasite

Accurate regulation of microfilament dynamics is central to cell growth, motility and response to environmental stimuli. Stabilizing and depolymerizing proteins control the steady-state levels of filamentous (F-) actin. Capping protein (CP) binds to free barbed ends, thereby arresting microfilament growth and restraining elongation to remaining free barbed ends. In all CPs characterized to date, alpha and beta subunits form the active heterodimer. Here, we show in a eukaryotic parasitic cell that the two CP subunits can be functionally separated. Unlike the beta subunit, the CP alpha subunit of the apicomplexan parasite Plasmodium is refractory to targeted gene deletion during blood infection in the mammalian host. Combinatorial complementation of Plasmodium berghei CP genes with the orthologs from Plasmodium falciparum verified distinct activities of CP alpha and CP alpha/beta during parasite life cycle progression. Recombinant Plasmodium CP alpha could be produced in Escherichia coli in the absence of the beta subunit and the protein displayed F-actin capping activity. Thus, the functional separation of two CP subunits in a parasitic eukaryotic cell and the F-actin capping activity of CP alpha expand the repertoire of microfilament regulatory mechanisms assigned to CPs.

Adseverin knockdown inhibits osteoclastogenesis in RAW264.7 cells

Osteoclastogenesis is a complex process that is highly dependent on the dynamic regulation of the actin cytoskeleton. Adseverin (Ads), a member of the gelsolin superfamily of actin-binding proteins, regulates actin remodeling by severing and capping actin filaments. The objective of the present study was to characterize the role of Ads during osteoclastogenesis by assessing Ads expression and using a knockdown strategy. Immunoblot analyses were used to examine Ads expression during osteoclastogenesis. A stable Ads knockdown macrophage cell line was generated using a retroviral shRNA construct. Osteoclast differentiation was morphologically examined via cell staining with osteoclast specific markers and light microscopy. The results showed that Ads expression was significantly increased in response to receptor activator of nuclear factor-κB ligand during osteoclastogenesis, and Ads was highly expressed in mature osteoclasts. Ads-knockdown macrophages showed major osteoclastogenesis defects, most likely caused by a pre-osteoclast fusion defect. These results indicate that Ads deficiency in monocytes inhibits osteoclastogenesis. Thus, in future studies it could be noteworthy to investigate the function of Ads in bone marrow monocytes during osteoclastogenesis.

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.

The human papillomavirus-16 E7 oncoprotein exerts antiapoptotic effects via its physical interaction with the actin-binding protein gelsolin

The oncoprotein E7 from human papillomavirus-16 (HPV-16 E7) plays a pivotal role in HPV postinfective carcinogenesis, and its physical interaction with host cell targets is essential to its activity. We identified a novel cellular partner for the viral oncoprotein: the actin-binding protein gelsolin (GSN), a key regulator of actin filament assembly and disassembly. In fact, biochemical analyses, generation of a 3D molecular interaction model and the use of specific HPV-16 E7 mutants provided clear cut evidence supporting the crucial role of HPV-16 E7 in affecting GSN integrity and function in human immortalized keratinocytes. Accordingly, functional analyses clearly suggested that stable HPV-16 E7 expression induced an imbalance between polymeric and monomeric actin in favor of the former. These events also lead to changes of cell cycle (increased S phase), to the inhibition of apoptosis and to the increase of cell survival. These results provide support to the hypotheses generated from the 3D molecular interaction model and encourage the design of small molecules hindering HPV-induced host cell reprogramming by specifically targeting HPV-16 E7-expressing cells.

Calcium-sensitive activity and conformation of Caenorhabditis elegans gelsolin-like protein 1 are altered by mutations in the first gelsolin-like domain

The gelsolin family of actin regulatory proteins is activated by Ca(2+) to sever and cap actin filaments. Gelsolin has six homologous gelsolin-like domains (G1-G6), and Ca(2+)-dependent conformational changes regulate its accessibility to actin. Caenorhabditis elegans gelsolin-like protein-1 (GSNL-1) has only four gelsolin-like domains (G1-G4) and still exhibits Ca(2+)-dependent actin filament-severing and -capping activities. We found that acidic residues (Asp-83 and Asp-84) in G1 of GSNL-1 are important for its Ca(2+) activation. These residues are conserved in GSNL-1 and gelsolin and previously implicated in actin-severing activity of the gelsolin family. We found that alanine mutations at Asp-83 and Asp-84 (D83A/D84A mutation) did not disrupt actin-severing or -capping activity. Instead, the mutants exhibited altered Ca(2+) sensitivity when compared with wild-type GSNL-1. The D83A/D84A mutation enhanced Ca(2+) sensitivity for actin severing and capping and its susceptibility to proteolytic digestion, suggesting a conformational change. Single mutations caused minimal changes in its activity, whereas Asp-83 and Asp-84 were required to stabilize Ca(2+)-free and Ca(2+)-bound conformations, respectively. On the other hand, the D83A/D84A mutation suppressed sensitivity of GSNL-1 to phosphatidylinositol 4,5-bisphosphate inhibition. The structure of an inactive form of gelsolin shows that the equivalent acidic residues are in close contact with G3, which may maintain an inactive conformation of the gelsolin family.

Distinct roles of four gelsolin-like domains of Caenorhabditis elegans gelsolin-like protein-1 in actin filament severing, barbed end capping, and phosphoinositide binding

Caenorhabditis elegans gelsolin-like protein-1 (GSNL-1) is a new member of the gelsolin family of actin regulatory proteins [Klaavuniemi, T., Yamashiro, S., and Ono, S. (2008) J. Biol. Chem. 283, 26071-26080]. It is an unconventional gelsolin-related protein with four gelsolin-like (G) domains (G1-G4), unlike typical gelsolin-related proteins with three or six G domains. GSNL-1 severs actin filaments and caps the barbed end in a calcium-dependent manner similar to that of gelsolin. In contrast, GSNL-1 has properties different from those of gelsolin in that it remains bound to F-actin and does not nucleate actin polymerization. To understand the mechanism by which GSNL-1 regulates actin dynamics, we investigated the domain-function relationship of GSNL-1 by analyzing activities of truncated forms of GSNL-1. G1 and the linker between G1 and G2 were sufficient for actin filament severing, whereas G1 and G2 were required for barbed end capping. The actin severing activity of GSNL-1 was inhibited by phosphatidylinositol 4,5-bisphosphate (PIP2), and a PIP2-sensitive domain was mapped to G1 and G2. At least two actin-binding sites were detected: a calcium-dependent G-actin-binding site in G1 and a calcium-independent G- and F-actin-binding site in G3 and G4. These results reveal both conserved and different utilization of G domains between C. elegans GSNL-1 and mammalian gelsolin for actin regulatory functions.

Villin severing activity enhances actin-based motility in vivo

Villin, an actin-binding protein associated with the actin bundles that support microvilli, bundles, caps, nucleates, and severs actin in a calcium-dependant manner in vitro. We hypothesized that the severing activity of villin is responsible for its reported role in enhancing cell plasticity and motility. To test this hypothesis, we chose a loss of function strategy and introduced mutations in villin based on sequence comparison with CapG. By pyrene-actin assays, we demonstrate that this mutant has a strongly reduced severing activity, whereas nucleation and capping remain unaffected. The bundling activity and the morphogenic effects of villin in cells are also preserved in this mutant. We thus succeeded in dissociating the severing from the three other activities of villin. The contribution of villin severing to actin dynamics is analyzed in vivo through the actin-based movement of the intracellular bacteria Shigella flexneri in cells expressing villin and its severing variant. The severing mutations abolish the gain of velocity induced by villin. To further analyze this effect, we reconstituted an in vitro actin-based bead movement in which the usual capping protein is replaced by either the wild type or the severing mutant of villin. Confirming the in vivo results, villin-severing activity enhances the velocity of beads by more than two-fold and reduces the density of actin in the comets. We propose a model in which, by severing actin filaments and capping their barbed ends, villin increases the concentration of actin monomers available for polymerization, a mechanism that might be paralleled in vivo when an enterocyte undergoes an epithelio-mesenchymal transition.

Interaction of phospholipase C-gamma1 with villin regulates epithelial cell migration

Tyrosine-phosphorylated villin regulates actin dynamics, cell morphology, and cell migration. Previously, we identified four tyrosine phosphorylation sites in the amino-terminal domain of villin. In this study we report six new sites in the carboxyl-terminal region of the villin core. With this study we document all phosphorylatable tyrosine residues in villin and map them to functions of villin. In this study, we identify for the first time the functional relevance of the carboxyl-terminal domains of the villin core. Expression of the carboxyl-terminal phosphorylation site mutant, as well as the villin truncation mutant S1-S3, inhibited cell migration in HeLa and Madin-Darby canine kidney Tet-Off cells, confirming the role of the carboxyl-terminal phosphorylation sites in villin-induced cell migration. The carboxyl-terminal phosphorylation sites were found to be critical for the interaction of villin with its ligand phospholipase C-gamma1 and for its localization to the developing lamellipodia in a motile cell. The results presented here elucidate the molecular basis for tyrosine-phosphorylated villin-induced changes in cell motility.

Towards gelsolin amyloid formation

Amyloid diseases result from protein misfolding and aggregation into fibrils. Some features of gelsolin amyloidogenic fragments comprised of residues 173-243 (G173-243) and residues 173-202 (G173-202) were investigated by the method of molecular dynamics (MD). The alpha-helical structure of G173-243 present in the whole protein unwinds during the course of MD simulation of the fragment G173-243, suggesting that the G173-243 structure is not stable and could unfold before becoming involved in gelsolin amyloid fibril formation. Twelve fragments of G173-202 were used to build a possible beta-fibril. During the course of the simulation, G173-202 fragments formed hydrogen bonds and tended to turn by an angle of 10 degrees -20 degrees towards each other.

Structure of the N-terminal half of gelsolin bound to actin: roles in severing, apoptosis and FAF

The actin filament-severing functionality of gelsolin resides in its N-terminal three domains (G1-G3). We have determined the structure of this fragment in complex with an actin monomer. The structure reveals the dramatic domain rearrangements that activate G1-G3, which include the replacement of interdomain interactions observed in the inactive, calcium-free protein by new contacts to actin, and by a novel G2-G3 interface. Together, these conformational changes are critical for actin filament severing, and we suggest that their absence leads to the disease Finnish-type familial amyloidosis. Furthermore, we propose that association with actin drives the calcium-independent activation of isolated G1-G3 during apoptosis, and that a similar mechanism operates to activate native gelsolin at micromolar levels of calcium. This is the first structure of a filament-binding protein bound to actin and it sets stringent, high-resolution limitations on the arrangement of actin protomers within the filament.

Functional dissection and molecular characterization of calcium-sensitive actin-capping and actin-depolymerizing sites in villin

All proteins of the villin superfamily, which includes the actin-capping and -severing proteins such as gelsolin, scinderin, and severin, are calcium-regulated actin-modifying proteins. Like some of these proteins, villin has morphologically distinct effects on actin assembly depending on the free calcium concentrations. At physiological calcium (Ca2+) villin nucleates and bundles actin, whereas at higher concentrations it caps (>50 microm) and severs (>200 microM) actin filaments. Although Ca(2+)-binding sites have been described in villin, the functional characterization of these sites has not been done previously. In the present study we functionally dissect the calcium-dependent actin-capping and -depolymerizing sites in villin. Our analysis reveals that villin binds Ca2+ with a Kd of 80.5 microM, a stoichiometry of 5.97, and a Hill's coefficient of 1.2. Using the NMR structure of villin 14T and the gelsolin-actin/Ca2+ crystal structure, six putative sites that result in Ca(2+)-induced conformational changes were identified in human villin and confirmed by mutational analysis. Molecular dynamics studies support the mutational analysis and provide a model for structural difference in the A93G mutant that prevents the calcium-induced conformational changes in the S1 domain of villin. Furthermore, we determined that villin expresses at least two types of Ca(2+)-sensitive sites that determine separate functional properties; site 1 (Glu-25, Asp-44, and Glu-74) regulates actin-capping, whereas sites 1 and 2 (Asp-86, Ala-93, and Asp-61), together with the intra-domain calcium-sensitive sites in villin, regulate actin depolymerization by villin. This is the first study that employs sequential mutagenesis to biochemically and functionally characterize the calcium-sensitive sites in villin. Such mutational analysis and functional characterization of the actin-capping and -depolymerizing sites are unknown for other proteins of the villin family.

Molecular dynamics study of a gelsolin-derived peptide binding to a lipid bilayer containing phosphatidylinositol 4,5-bisphosphate

Gelsolin is an actin-severing protein whose action is initiated by Ca(2+) and inhibited by binding to phosphorylated inositol lipid or phosphoinositides. The regions of gelsolin responsible for phosphoinositide binding are comprised of residues 150-169 (G150-169) and 135-142 (G135-142). The corresponding peptides possess similar binding potency as native gelsolin. Their common feature is the presence of arginine and lysine residues that can bind to negatively charged phosphate groups of phosphoinositides. In this work the binding of the G150-169 peptide to a phosphatidylinositol 4,5-bisphosphate (PIP2) cluster in a lipid membrane model was investigated by molecular dynamics calculations (MD) with the AMBER 4.1 force field, taking into account explicit solvent molecules. Initially the structure of G150-169 was simulated by using the electrostatically driven Monte Carlo (EDMC) and MD methods, and the resulting structure agreed within 3.7 A backbone-atom root mean square deviation with the corresponding experimentally derived structure (PDB code: 1SOL). Using this model for the peptide, a subsequent MD simulation of G150-169 in a periodic box containing a model of dimyristoyl-phosphatidylcholine (DMPC) lipids with a cluster of four PIP2 molecules was carried out. During the simulation G150-169 interacted strongly with PIP2 molecules, initially by formation of salt bridges between its N-terminal basic groups and the phosphate groups of PIP2, followed by formation of hydrophobic bonds between the hydrophobic side chains of the peptide and the fatty acid tail of the lipid. As a result of the formation of hydrophobic bonds, the PIP2 molecules were pulled out from the lipid bilayer. This mode of binding differs from those of other PIP2-binding protein motifs such as PH domains that interact solely with the hydrophilic head group of PIP2. These results suggest that dissociation of gelsolin from actin by PIP2 lipids may involve entering of the PIP2 molecules to the gelsolin-actin interface, thereby weakening the interactions between these proteins.

Fragmin60 encodes an actin-binding protein with a C2 domain and controls actin Thr-203 phosphorylation in Physarum plasmodia and sclerotia

We report the isolation of a cDNA clone encoding a 60-kDa protein termed fragmin60 that cross-reacts with fragmin antibodies. Unlike other gelsolin-related proteins, fragmin60 contains a unique N-terminal domain that shows similarity with C2 domains of aczonin, protein kinase C, and synaptotagmins. The fragmin60 C2 domain binds three calcium ions, one with nanomolar affinity and two with micromolar affinity. Actin binding by fragmin60 requires higher calcium concentrations than does binding of actin by a fragmin60 mutant lacking the C2 domain, suggesting that the C2 domain secures the actin binding moiety in a conformation preventing actin binding at low calcium concentrations. The fragmin60 C2 domain does not bind phospholipids but interacts with the endogenous homologue of Saccharomyces cerevisiae S-phase kinase-associated protein (Skp1), as shown by pull-down assays and co-expression in mammalian cells. Recombinant fragmin60 promotes in vitro phosphorylation of actin Thr-203 by the actin-fragmin kinase. We further show that in vivo phosphorylation of actin in the fragmin60-actin complex occurs in sclerotia, a dormant stage of Physarum development, as well as in plasmodia. Our findings indicate that we have cloned a novel type of gelsolin-related actin-binding protein that is involved in controlling regulation of actin phosphorylation in vivo.

Tropomyosin and gelsolin cooperate in controlling the microfilament system

Tropomyosin has been shown to cause annealing of gelsolin-capped actin filaments. Here we show that tropomyosin is highly efficient in transforming even the smallest gelsolin-actin complexes into long actin filaments. At low concentrations of tropomyosin, the effect of tropomyosin depends on the length of the actin oligomer, and the cooperative nature of the process is a direct indication that tropomyosin induces a conformational change in the gelsolin-actin complexes, altering the structure at the actin (+) end such that capping by gelsolin is abolished. At increased concentrations of tropomyosin, heterodimers, trimers, and tetramers are converted to actin filaments. In addition, evidence is presented demonstrating that gelsolin, once removed from the (+) end of the actin, can reassociate with the newly formed tropomyosin-decorated actin filaments. Interestingly, the binding of gelsolin to the tropomyosin-actin filament complexes saturates at 2 gelsolin molecules per 14 actin and 2 tropomyosins, i.e. two gelsolins per tropomyosin-regulatory unit along the filament. These observations support the view that both tropomyosin and gelsolin are likely to have important functions in addition to those proposed earlier.

Der f 16: a novel gelsolin-related molecule identified as an allergen from the house dust mite, Dermatophagoides farinae

Allergen from the house dust mite (Dermatophagoides sp.) is a major trigger factor of allergic disorders, and its characterization is crucial for the development of specific diagnosis or immunotherapy. Here we report the identification of a novel dust mite (Dermatophagoides farinae) antigen whose primary structure belongs to the gelsolin family, a group of actin cytoskeleton-regulatory proteins. Isolated mite cDNA, termed Der f 16, encodes 480 amino acids comprising a four-repeated gelsolin-like segmental structure, which is not seen in conventional gelsolin family members. Enzyme immunoassay indicated that recombinant Der f 16 protein, prepared using an Escherichia coli expression system, bound IgE from mite-allergic patients at 47% (8/17) frequency. This is the first evidence that the gelsolin family represents a new class of allergen recognizable by atopic patient IgE.

Binding of gelsolin domain 2 to actin. An actin interface distinct from that of gelsolin domain 1 and from ADF/cofilin

It is generally assumed that of the six domains that comprise gelsolin, domain 2 is primarily responsible for the initial contact with the actin filament that will ultimately result in the filament being severed. Other actin-binding regions within domains 1 and 4 are involved in gelsolin's severing and subsequent capping activity. The overall fold of all gelsolin repeated domains are similar to the actin depolymerizing factor (ADF)/cofilin family of actin-binding proteins and it has been proposed that there is a similarity in the actin-binding interface. Gelsolin domains 1 and 4 bind G-actin in a similar manner and compete with each other, whereas domain 2 binds F-actin at physiological salt concentrations, and does not compete with domain 1. Here we investigate the domain 2 : actin interface and compare this to our recent studies of the cofilin : actin interface. We conclude that important differences exist between the interfaces of actin with gelsolin domains 1 and 2, and with ADF/cofilin. We present a model for F-actin binding of domain 2 with respect to the F-actin severing and capping activity of the whole gelsolin molecule.

Elucidating the mechanism of familial amyloidosis- Finnish type: NMR studies of human gelsolin domain 2

Familial amyloidosis-Finnish type (FAF) results from a single mutation at residue 187 (D187N or D187Y) within domain 2 of the actin-regulating protein gelsolin. The mutation somehow allows a masked cleavage site to be exposed, leading to the first step in the formation of an amyloidogenic fragment. We have performed NMR experiments investigating structural and dynamic changes between wild-type (WT) and D187N gelsolin domain 2 (D2). On mutation, no significant structural or dynamic changes occur at or near the cleavage site. Areas in conformational exchange are observed between beta-strand 4 and alpha-helix 1 and within the loop region following beta-strand 5. Chemical shift differences are noted along the face of alpha-helix 1 that packs onto the beta-sheet, suggesting an altered conformation. Conformational changes within these areas can have an effect on actin binding and may explain why D187N gelsolin is inactive. [(1)H-(15)N] nuclear Overhauser effect and chemical shift data suggest that the C-terminal tail of D187N gelsolin D2 is less structured than WT by up to six residues. In the crystal structure of equine gelsolin, the C-terminal tail of D2 lies across a large cleft between domains 1 and 2 where the masked cleavage site sits. We propose that the D187N mutation destabilizes the C-terminal tail of D2 resulting in a more exposed cleavage site leading to the first proteolysis step in the formation of the amyloidogenic fragment.

Gelsolin as a calcium-regulated actin filament-capping protein

Various concentrations of gelsolin (25-100 nM) were added to 2 microM polymerized actin. The concentrations of free calcium were adjusted to 0.05-1.5 microM by EGTA/Ca2+ buffer. Following addition of gelsolin actin depolymerization was observed that was caused by dissociation of actin subunits from the pointed ends of treadmilling actin filaments and inhibition by gelsolin of polymerization at barbed ends. The time course of depolymerization revealed an initial lag phase that was followed by slow decrease of the concentration of polymeric actin to reach the final steady state polymer and monomer concentration. The initial lag phase was pronounced at low free calcium and low gelsolin concentrations. On the basis of quantitative analysis the kinetics of depolymerization could be interpreted as capping, i.e. binding of gelsolin to the barbed ends of actin filaments and subsequent inhibition of polymerization, rather than severing. The main argument for this conclusion was that even gelsolin concentrations (100 nM) that exceed the concentration of filament ends ( approximately 2 nM), cause the filaments to depolymerize at a rate that is similar to the rate of depolymerization of the concentration of pointed ends existing before addition of gelsolin. The rate of capping is directly proportional to the free calcium concentration. These experiments demonstrate that at micromolar and submicromolar free calcium concentrations gelsolin acts as a calcium-regulated capping protein but not as an actin filament severing protein, and that the calcium binding sites of gelsolin which regulate the various functions of gelsolin (capping, severing and monomer binding), differ in their calcium affinity.

Mapping the functional surface of domain 2 in the gelsolin superfamily

The crystal structure of the F-actin binding domain 2 of severin, the gelsolin homologue from Dictyostelium discoideum, has been determined by multiple isomorphous replacement and refined to 1.75 A resolution. The structure reveals an alpha-helix-beta-sheet sandwich similar to the domains of gelsolin and villin, and contains two cation-binding sites, as observed in other domain 1 and domain 2 homologues. Comparison of the structures of several gelsolin family domains has identified residues that may mediate F-actin binding in gelsolin domain 2 homologues. To assess the involvement of these residues in F-actin binding, three mutants of human gelsolin domain 2 were assayed for F-actin binding activity and thermodynamic stability. Two of the mutants, RRV168AAA and RLK210AAA, demonstrated a lowered affinity for F-actin, indicating a role for those residues in filament binding. Using both structural and biochemical data, we have constructed a model of the gelsolin domain 1-domain 2-F-actin complex. This model highlights a number of interactions that may serve as positive and negative determinants of filament end- and side-binding.

Villin function in the organization of the actin cytoskeleton. Correlation of in vivo effects to its biochemical activities in vitro

Villin is an actin-binding protein of the intestinal brush border that bundles, nucleates, caps, and severs actin in a Ca(2+)-dependent manner in vitro. Villin induces the growth of microvilli in transfected cells, an activity that requires a carboxyl-terminally located KKEK motif. By combining cell transfection and biochemical assays, we show that the capacity of villin to induce growth of microvilli in cells correlates with its ability to bundle F-actin in vitro but not with its nucleating activity. In agreement with its importance for microfilament bundling in cells, the KKEK motif of the carboxyl-terminal F-actin-binding site is crucial for bundling in vitro. In addition, substitutions of basic residues in a second site, located in the amino-terminal portion of villin, impaired its activity in cells and reduced its binding to F-actin in the absence of Ca(2+) as well as its bundling and severing activities in vitro. Altogether, these findings suggest that villin participates in the organization and stabilization of the brush border core bundle but does not initiate its assembly by nucleation of actin filaments.

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.

Domain analysis of supervillin, an F-actin bundling plasma membrane protein with functional nuclear localization signals

A growing number of actin-associated membrane proteins have been implicated in motile processes, adhesive interactions, and signal transduction to the cell nucleus. We report here that supervillin, an F-actin binding protein originally isolated from bovine neutrophil plasma membranes, contains functional nuclear targeting signals and localizes at or near vinculin-containing focal adhesion plaques in COS7-2 and CV1 cells. Overexpression of full-length supervillin in these cells disrupts the integrity of focal adhesion plaques and results in increased levels of F-actin and vinculin. Localization studies of chimeric proteins containing supervillin sequences fused with the enhanced green fluorescent protein indicate that: (1) the amino terminus promotes F-actin binding, targeting to focal adhesions, and limited nuclear localization; (2) the dominant nuclear targeting signal is in the center of the protein; and (3) the carboxy-terminal villin/gelsolin homology domain of supervillin does not, by itself, bind tightly to the actin cytoskeleton in vivo. Overexpression of chimeras containing both the amino-terminal F-actin binding site(s) and the dominant nuclear targeting signal results in the formation of large nuclear bundles containing F-actin, supervillin, and lamin. These results suggest that supervillin may contribute to cytoarchitecture in the nucleus, as well as at the plasma membrane.

Structural modules in actin-binding proteins: towards a new classification

The number of actin binding proteins for which (part of) the three-dimensional structure is known, is steadily increasing. This has led to a picture in which defined structural modules with actin binding capacity are shared between different actin binding proteins. A classification of these based on their common three-dimensional modules appears a logical future step and in this review we provide an initial list starting from the currently known structures. The discussed cases illustrate that a comparison of the similarities and variations within the common structural actin binding unit of different members of a particular class may ultimately provide shortcuts for defining their actin target site and for understanding their effect on actin dynamics. Within this concept, the multitude of possible interactions by an extensive, and still increasing, list of actin binding proteins becomes manageable because they can be presented as variations upon a limited number of structural themes. We discuss the possible evolutionary routes that may have produced the present array of actin binding modules.

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.

Murine adseverin (D5), a novel member of the gelsolin family, and murine adseverin are induced by interleukin-9 in T-helper lymphocytes

We identified a number of upregulated genes by differential screening of interleukin-9-stimulated T-helper lymphocytes. Interestingly, two of these messengers encode proteins that are similar to proteins of the gelsolin family. The first displays a typical structure of six homologous domains and shows a high level of identity (90%) with bovine adseverin (or scinderin) and may therefore be considered the murine adseverin homolog. The second encodes a protein with only five segments. Sequence comparison shows that most of the fifth segment and a short amino-terminal part of the sixth segment (amino acids 528 to 628 of adseverin) are missing, and thus, this form may represent an alternatively spliced product derived from the same gene. The corresponding protein is called mouse adseverin (D5). We expressed both proteins in Escherichia coli and show that mouse adseverin displays the typical characteristics of all members of the gelsolin family with respect to actin binding (capping, severing, and nucleation) and its regulation by Ca2+. In contrast, mouse adseverin (D5) fails to nucleate actin polymerization, although like mouse adseverin and gelsolin, it severs and caps actin filaments in a Ca2+-dependent manner. Adseverin is present in all of the tissues and most of the cell lines tested, although at low concentrations. Mouse adseverin (D5) was found only in blood cells and in cell lines derived from T-helper lymphocytes and mast cells, where it is weakly expressed. In a gel filtration experiment, we demonstrated that mouse adseverin forms a 1:2 complex with G actin which is stable only in the presence of Ca2+, while no stable complex was observed for mouse adseverin (D5).

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.

Analogous F-actin binding by cofilin and gelsolin segment 2 substantiates their structural relationship

Cofilin is representative for a family of low molecular weight actin filament binding and depolymerizing proteins. Recently the three-dimensional structure of yeast cofilin and of the cofilin homologs destrin and actophorin were resolved, and a striking similarity to segments of gelsolin and related proteins was observed (Hatanaka, H., Ogura, K., Moriyama, K., Ichikawa, S., Yahara, I., and Inagaka, F. (1996) Cell 85, 1047-1055; Fedorov, A. A., Lappalainen, P., Fedorov, E. V., Drubin, D. G., and Almo, S. C. (1997) Nat. Struct. Biol. 4, 366-369; Leonard, S. A., Gittis, A. G., Petrella, E. C., Pollard, T. D., and Lattman, E. E. (1997) Nat. Struct. Biol. 4, 369-373). Using peptide mimetics, we show that the actin binding site stretches over the entire cofilin alpha-helix 112-128. In addition, we demonstrate that cofilin and its actin binding peptide compete with gelsolin segments 2-3 for binding to actin filaments. Based on these competition data, we propose that cofilin and segment 2 of gelsolin use a common structural topology to bind to actin and probably share a similar target site on the filament. This adds a functional dimension to their reported structural homology, and this F-actin binding mode provides a basis to further enlighten the effect of members of the cofilin family on actin filament dynamics.

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.

Misakinolide A is a marine macrolide that caps but does not sever filamentous actin

We have investigated the biochemical properties of the marine natural product, misakinolide A, a 40-membered dimeric lactone macrolide that differs from swinholide A only in the size of the macrolide ring. Analytical ultracentrifugation and steady-state fluorescence experiments show that misakinolide A binds simultaneously to two actin subunits with virtually the same affinity as swinholide A, suggesting that the modification in the ring size does not change the actin-binding site. Sedimentation equilibrium experiments suggest that binding is independent at each binding site, with a Kd of approximately 50 nM. Remarkably, misakinolide A does not sever actin filaments like swinholide A; rather, it caps the barbed end of F-actin. When capped by misakinolide A, the elongation rate constant at the barbed end is reduced to zero; pointed end growth was affected only to the extent that the compound sequesters unpolymerized actin. Misakinolide A has essentially no effect on the off-rate of actin subunits leaving the barbed end. Energy-minimized models of misakinolide A and swinholide A are consistent with conservation of identical binding sites in both molecules, but a difference in orientation of one binding site relative to the other may explain why swinholide A has severing activity whereas misakinolide A only has capping activity.

Evidence for an actin binding helix in gelsolin segment 2; have homologous sequences in segments 1 and 2 of gelsolin evolved to divergent actin binding functions?

Gelsolin is built up of six homologous segments that perform different functions on actin. Segments 1 and 2, which are suggested to be highly similar in their overall folds, bind monomeric and filamentous actin respectively. A long alpha-helix in segment 1 forms the major contact site of this segment with actin. We show that sequence 197-226 of segment 2, equivalent to the region around the actin binding helix in segment 1, contains F-actin binding activity. Consequently, positionally similar parts of segment 1 and 2 are implicated in the actin contact and solvent exposed residues in these parts must have evolved differentially to meet their different actin binding properties.

Functions of [His321]gelsolin isolated from a flat revertant of ras-transformed cells

A mutant gelsolin, [His321]gelsolin, was isolated from R1, a flat revertant of human activated c-Ha-ras oncogene-transformed NIH/3T3 cells (EJ-NIH/3T3) produced by ethylmethanesulfonate treatment. [His321]Gelsolin has a histidine instead of a proline residue at position 321 and suppresses the tumorigenicity of EJ-NIH/3T3 cells when it is constitutively expressed [Müllauer, L., Fujita, H., Ishizaki, A. & Kuzumaki, N. (1993) Oncogene 8, 2531-2536]. To investigate the biochemical consequences of the amino acid substitution of His321, we expressed the [His321]gelsolin and wild-type gelsolin in Escherichia coli, purified them, and analyzed their effects on actin, polyphosphoinositol lipids and phospholipase C. [His321]Gelsolin has decreased actin-filament-severing activity and increased nucleating activity compared with wild-type gelsolin in vitro. Furthermore, compared to wild-type gelsolin both nucleation and severing by [His321]gelsolin are inhibited more strongly by the phosphoinositol lipids phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). In addition, [His321]gelsolin inhibits PtdInsP2 hydrolysis by phospholipase C gamma 1 more strongly than wild-type gelsolin in vitro because of its higher binding capacity for phosphoinositol lipid. Gelsolin has six homologous amino acid repeats called S1-S6. Our results suggest that the segment S3 which contains the mutation is functionally relevant for regulation of gelsolin's activities even though the relevant actin-binding domains are in segments 1, 2, and 4-6, and that the region around the residue 321 may contain a phosphoinositol-lipid-binding site. Altered functions of [His321]gelsolin might be important for the loss of tumorigenicity of the ras-transformed cells.

Swinholide A is a microfilament disrupting marine toxin that stabilizes actin dimers and severs actin filaments

Swinholide A, isolated from the marien sponge Theonella swinhoei, is a 44-carbon ring dimeric dilactone macrolide with a 2-fold axis of symmetry. Recent studies have elucidated its unusual structure and shown that it has potent cytotoxic activity. We now report that swinholide A disrupts the actin cytoskeleton of cells grown in culture, sequesters actin dimers in vitro in both polymerizing and non-polymerizing buffers with a binding stoichiometry of one swinholide A molecule per actin dimer, and rapidly severs F-actin in vitro with high cooperativity. These unique properties are sufficient to explain the cytotoxicity of swinholide A. They also suggest that swinholide A might be a model for studies of the mechanism of action of F-actin severing proteins and be therapeutically useful in conditions where filamentous actin contributes to pathologically high viscosities.

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.

Gain-of-function mutations conferring actin-severing activity to human macrophage cap G

Nonmuscle cell motility requires marked changes in the consistency and shape of the peripheral cytoplasm. These changes are regulated by a gel-sol transformation of the actin filament network, and actin filament-severing proteins are responsible for network solation. Macrophage Cap G, unlike all other proteins in the gelsolin family, caps but does not sever actin filaments. Two amino acid stretches in Cap G diverge markedly from the severing proteins: 84LNTLLGE and 124AFHKTS. Discrete mutations in Cap G have been generated to determine if these amino acid sequences are critical for actin filament severing. Conversion of 84LNTLLGE to the gelsolin actin-binding helix sequence (84LDDYLGG) renders Cap G capable of severing actin filaments (half-maximal severing, 1-2 microM). Adding a second set of mutations, converting 124AFHKTS to 124GFKHV, enhances severing by 10-fold (half-maximal severing, 0.1-0.2 microM). These experiments support a critical role for these two regions in actin filament severing and showcase the power of gain-of-function mutations in clarifying structure-function relationships.