Mapping the functional surface of domain 2 in the gelsolin superfamily

Regulatory role of the second gelsolin-like domain of Caenorhabditis elegans gelsolin-like protein 1 (GSNL-1) in its calcium-dependent conformation and actin-regulatory activities

Caenorhabditis elegans gelsolin-like protein-1 (GSNL-1) is an unconventional member of the gelsolin family of actin-regulatory proteins. Unlike typical gelsolin-related proteins with three or six G domains, GSNL-1 has four gelsolin-like (G) domains (G1-G4) and exhibits calcium-dependent actin filament severing and capping activities. The first G domain (G1) of GSNL-1 is necessary for its actin-regulatory activities. However, how other domains in GSNL-1 participate in regulation of its functions is not understood. Here, we report biochemical evidence that the second G domain (G2) of GSNL-1 has a regulatory role in its calcium-dependent conformation and actin-regulatory activities. Comparison of the sequences of gelsolin-related proteins from various species indicates that sequences of G2 are highly conserved. Among the conserved residues in G2, we focused on D162 of GSNL-1, since equivalent residues in gelsolin and severin are part of the calcium-binding sites and is a pathogenic mutation site in human gelsolin causing familial amyloidosis, Finish-type. The D162N mutation does not alter the inactive and fully calcium-activated states of GSNL-1 for actin filament severing (at 20 nM GSNL-1) and capping activities (at 50 nM GSNL-1). However, under these conditions, the mutant shows reduced calcium sensitivity for activation. By contrast, the D162N mutation strongly enhances susceptibility of GSNL-1 to chymotrypsin digestion only at high calcium concentrations but not at low calcium concentrations. The mutation also reduces affinity of GSNL-1 with actin monomers. These results suggest that G2 of GSNL-1 functions as a regulatory domain for its calcium-dependent actin-regulatory activities by mediating conformational changes of the GSNL-1 molecule.

Collagen remodeling by phagocytosis is determined by collagen substrate topology and calcium-dependent interactions of gelsolin with nonmuscle myosin IIA in cell adhesions

Cell adhesion to collagen presented on beads activates Ca²⁺ entry and promotes the formation of phagosomes enriched with NMMIIA and gelsolin. The Ca²⁺-dependent interaction of gelsolin and NMMIIA in turn enables actin remodeling and enhances collagen degradation by phagocytosis.

We examine how collagen substrate topography, free intracellular calcium ion concentration ([Ca²⁺]_i, and the association of gelsolin with nonmuscle myosin IIA (NMMIIA) at collagen adhesions are regulated to enable collagen phagocytosis. Fibroblasts plated on planar, collagen-coated substrates show minimal increase of [Ca²⁺]_i, minimal colocalization of gelsolin and NMMIIA in focal adhesions, and minimal intracellular collagen degradation. In fibroblasts plated on collagen-coated latex beads there are large increases of [Ca²⁺]_i, time- and Ca²⁺-dependent enrichment of NMMIIA and gelsolin at collagen adhesions, and abundant intracellular collagen degradation. NMMIIA knockdown retards gelsolin recruitment to adhesions and blocks collagen phagocytosis. Gelsolin exhibits tight, Ca²⁺-dependent binding to full-length NMMIIA. Gelsolin domains G4–G6 selectively require Ca²⁺ to interact with NMMIIA, which is restricted to residues 1339–1899 of NMMIIA. We conclude that cell adhesion to collagen presented on beads activates Ca²⁺ entry and promotes the formation of phagosomes enriched with NMMIIA and gelsolin. The Ca²⁺ -dependent interaction of gelsolin and NMMIIA in turn enables actin remodeling and enhances collagen degradation by phagocytosis.

Structure of d(CCGGGACCGG)4 as a four-way junction at 1.6 Å resolution: new insights into solvent interactions

The crystal structure of the decamer sequence d(CCGGGACCGG)(4) has previously been reported at 2.16 Å resolution as a four-way junction. Here, the structure of this sequence is reported at the significantly higher resolution of 1.6 Å, which is the highest resolution reported for a four-way junction. This allowed the unambiguous identification of an extensive hydration network with distinct patterns and solvent-mediated interactions that shed new light on the role of water in the formation and stabilization of junction structures.

Novel interactors and a role for supervillin in early cytokinesis

Supervillin, the largest member of the villin/gelsolin/flightless family, is a peripheral membrane protein that regulates each step of cell motility, including cell spreading. Most known interactors bind within its amino (N)-terminus. We show here that the supervillin carboxy (C)-terminus can be modeled as supervillin-specific loops extending from gelsolin-like repeats plus a villin-like headpiece. We have identified 27 new candidate interactors from yeast two-hybrid screens. The interacting sequences from 12 of these proteins (BUB1, EPLIN/LIMA1, FLNA, HAX1, KIF14, KIFC3, MIF4GD/SLIP1, ODF2/Cenexin, RHAMM, STARD9/KIF16A, Tks5/SH3PXD2A, TNFAIP1) co-localize with and mis-localize EGFP-supervillin in mammalian cells, suggesting associations in vivo. Supervillin-interacting sequences within BUB1, FLNA, HAX1, and MIF4GD also mimic supervillin over-expression by inhibiting cell spreading. Most new interactors have known roles in supervillin-associated processes, e.g. cell motility, membrane trafficking, ERK signaling, and matrix invasion; three (KIF14, KIFC3, STARD9/KIF16A) have kinesin motor domains; and five (EPLIN, KIF14, BUB1, ODF2/cenexin, RHAMM) are important for cell division. GST fusions of the supervillin G2-G3 or G4-G6 repeats co-sediment KIF14 and EPLIN, respectively, consistent with a direct association. Supervillin depletion leads to increased numbers of bi- and multi-nucleated cells. Cytokinesis failure occurs predominately during early cytokinesis. Supervillin localizes with endogenous myosin II and EPLIN in the cleavage furrow, and overlaps with the oncogenic kinesin, KIF14, at the midbody. We conclude that supervillin, like its interactors, is important for efficient cytokinesis. Our results also suggest that supervillin and its interaction partners coordinate actin and microtubule motor functions throughout the cell cycle.

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

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

Mapping the ADF/cofilin binding site on monomeric actin by competitive cross-linking and peptide array: evidence for a second binding site on monomeric actin

The binding sites for actin depolymerising factor (ADF) and cofilin on G-actin have been mapped by competitive chemical cross-linking using deoxyribonuclease I (DNase I), gelsolin segment 1 (G1), thymosin beta4 (Tbeta4), and vitamin D-binding protein (DbP). To reduce ADF/cofilin induced actin oligomerisation we used ADP-ribosylated actin. Both vitamin D-binding protein and thymosin beta4 inhibit binding by ADF or cofilin, while cofilin or ADF and DNase I bind simultaneously. Competition was observed between ADF or cofilin and G1, supporting the hypothesis that cofilin preferentially binds in the cleft between sub-domains 1 and 3, similar to or overlapping the binding site of G1. Because the affinity of G1 is much higher than that of ADF or cofilin, even at a 20-fold excess of the latter, the complexes contained predominantly G1. Nevertheless, cross-linking studies using actin:G1 complexes and ADF or cofilin showed the presence of low concentrations of ternary complexes containing both ADF or cofilin and G1. Thus, even with monomeric actin, it is shown for the first time that binding sites for both G1 and ADF or cofilin can be occupied simultaneously, confirming the existence of two separate binding sites. Employing a peptide array with overlapping sequences of actin overlaid by cofilin, we have identified five sequence stretches of actin able to bind cofilin. These sequences are located within the regions of F-actin predicted to bind cofilin in the model derived from image reconstructions of electron microscopical images of cofilin-decorated filaments. Three of the peptides map to the cleft region between sub-domains 1 and 3 of the upper actin along the two-start long-pitch helix, while the other two are in the DNase I loop corresponding to the site of the lower actin in the helix. In the absence of any crystal structures of ADF or cofilin in complex with actin, these studies provide further information about the binding sites on F-actin for these important actin regulatory proteins.

A CapG gain-of-function mutant reveals critical structural and functional determinants for actin filament severing

CapG is the only member of the gelsolin family unable to sever actin filaments. Changing amino acids 84-91 (severing domain) and 124-137 (WH2-containing segment) simultaneously to the sequences of gelsolin results in a mutant, CapG-sev, capable of severing actin filaments. The gain of severing function does not alter actin filament capping, but is accompanied by a higher affinity for monomeric actin, and the capacity to bind and sequester two actin monomers. Analysis of CapG-sev crystal structure suggests a more loosely folded inactive conformation than gelsolin, with a shorter S1-S2 latch. Calcium binding to S1 opens this latch and S1 becomes separated from a closely interfaced S2-S3 complex by an extended arm consisting of amino acids 118-137. Modeling with F-actin predicts that the length of this WH2-containing arm is critical for severing function, and the addition of a single amino acid (alanine or histidine) eliminates CapG-sev severing activity, confirming this prediction. We conclude that efficient severing utilizes two actin monomer-binding sites, and that the length of the WH2-containing segment is a critical functional determinant for severing.

Structural analysis of an Echinococcus granulosus actin-fragmenting protein by small-angle x-ray scattering studies and molecular modeling

The Echinococcus granulosus actin filament-fragmenting protein (EgAFFP) is a three domain member of the gelsolin family of proteins, which is antigenic to human hosts. These proteins, formed by three or six conserved domains, are involved in the dynamic rearrangements of the cytoskeleton, being responsible for severing and capping actin filaments and promoting nucleation of actin monomers. Various structures of six domain gelsolin-related proteins have been investigated, but little information on the structure of three domain members is available. In this work, the solution structure of the three domain EgAFFP has been investigated through small-angle x-ray scattering (SAXS) studies. EgAFFP exhibits an elongated molecular shape. The radius of gyration and the maximum dimension obtained by SAXS were, respectively, 2.52 +/- 0.01 nm and 8.00 +/- 1.00 nm, both in the absence and presence of Ca2+. Two different molecular homology models were built for EgAFFP, but only one was validated through SAXS studies. The predicted structure for EgAFFP consists of three repeats of a central beta-sheet sandwiched between one short and one long alpha-helix. Possible implications of the structure of EgAFFP upon actin binding are discussed.

Separate functions of gelsolin mediate sequential steps of collagen phagocytosis

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

The state of the filament

Movement is a defining characteristic of life. Macroscopic motion is driven by the dynamic interactions of myosin with actin filaments in muscle. Directed polymerization of actin behind the advancing membrane of a eukaryotic cell generates microscopic movement. Despite the fundamental importance of actin in these processes, the structure of the actin filament remains unknown. The Holmes model of the actin filament was published 15 years ago, and although it has been widely accepted, no high-resolution structural data have yet confirmed its veracity. Here, we review the implications of recently determined structures of F-actin-binding proteins for the structure of the actin filament and suggest a series of in silico tests for actin-filament models. We also review the significance of these structures for the arp2/3-mediated branched filament.

Cytoskeletal regulation: rich in lipids

Phosphorylated derivatives of the phospholipid phosphatidylinositol, or phosphoinositides, are implicated in many aspects of cell function. Binding of phosphoinositides that are localized within cell membranes to soluble protein ligands allows spatially selective regulation at the cytoplasm-membrane interface. Recently, studies that relate phosphoinositide production to membrane domains are converging with those that show effects of these lipids on the assembly of cellular actin, and are therefore linking membrane and cytoskeletal structures in new ways.

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.

Gelsolin domain 2 Ca2+ affinity determines susceptibility to furin proteolysis and familial amyloidosis of finnish type

Mutation of aspartic acid 187 to asparagine (D187N) or tyrosine (D187Y) in domain 2 of the actin-modulating protein gelsolin causes the neurodegenerative disease familial amyloidosis of Finnish type (FAF). These mutations render plasma gelsolin susceptible to aberrant proteolysis by furin in the trans-Golgi network, the initial proteolytic event in the formation of 71 and 53 residue fragments that assemble into amyloid fibrils. Ca(2+) binding stabilizes wild-type domain 2 gelsolin against denaturation and proteolysis, but the FAF variants are unable to bind and be stabilized by Ca(2+). Though the chain of events initiating FAF has been elucidated recently, uncertainty remains about the mechanistic details that allow the FAF variants to be processed. To test the hypothesis that impaired Ca(2+) binding in the D187 variants, but not other factors specific to residue 187, increases susceptibility to aberrant proteolysis and subsequent amyloidogenesis, we designed the gelsolin variant E209Q to remove a different Ca(2+) ligand from the same Ca(2+) site that is affected in the FAF variants. Here, we show that E209Q domain 2 does not bind Ca(2+) and is not stabilized against denaturation or furin proteolysis, analogous to the behavior exhibited by the FAF variants. Transfection of full-length E209Q into COS cells results in secretion of both the full-length and furin-processed fragments, as observed with D187N and D187Y. Mutation of the furin consensus sequence in D187N and E209Q gelsolin prevents cleavage during secretion, indicating that inhibition of proprotein convertases (furin) represents a viable therapeutic approach for the treatment of FAF. Mutations that diminish domain 2 Ca(2+) binding allow furin access to an otherwise protected cleavage site, initiating the proteolytic cascade that leads to gelsolin amyloidogenesis and FAF.

The gelsolin family of actin regulatory proteins: modular structures, versatile functions

This issue of FEBS Letters includes two manuscripts describing structural studies of gelsolin, the best-characterized member of a superfamily of actin binding proteins that sever, cap, and in some cases nucleate and bundle actin filaments. The manuscripts by Narayan et al. and Irobi et al. provide snapshots of gelsolin domains activated by calcium and in complex with the actin monomer, revealing new insights into the remarkable actin regulatory activities of this versatile protein. These studies build upon nearly a quarter of a century of research on gelsolin's effects on actin dynamics and its role in normal and diseased cells. In the following minireview, we summarize the structural studies that have provided insights into gelsolin's severing and capping activities and look to the future of work on this remarkable molecule.

From the first to the second domain of gelsolin: a common path on the surface of actin?1

We present the 2.6 A resolution crystal structure of a complex formed between G-actin and gelsolin fragment Met25-Gln160 (G1+). The structure differs from those of other gelsolin domain 1 (G1) complexes in that an additional six amino acid residues from the crucial linker region into gelsolin domain 2 (G2) are visible and are attached securely to the surface of actin. The linker segment extends away from G1 up the face of actin in a direction that infers G2 will bind along the same long-pitch helical strand as the actin bound to G1. This is consistent with a mechanism whereby G2 attaches gelsolin to the side of a filament and then directs G1 toward a position where it would disrupt actin-actin contacts. Alignment of the sequence of the structurally important residues within the G1-G2 linker with those of WH2 (WASp homology domain 2) domain protein family members (e.g. WASp (Wiscott-Aldridge syndrome protein) and thymosin beta4) suggests that the opposing activities of filament assembly and disassembly may exploit a common patch on the surface of actin.

Villidin, a novel WD-repeat and villin-related protein from Dictyostelium, is associated with membranes and the cytoskeleton

Villidin is a novel multidomain protein (190 kDa) from Dictyostelium amoebae containing WD repeats at its N-terminus, three PH domains in the middle of the molecule, and five gelsolin-like segments at the C-terminus, followed by a villin-like headpiece. Villidin mRNA and protein are present in low amounts during growth and early aggregation, but increase during development and reach their highest levels at the tipped mound stage. The protein is present in the cytosol as well as in the cytoskeletal and membrane fractions. GFP-tagged full-length villidin exhibits a similar distribution as native villidin, including a distinct colocalization with Golgi structures. Interestingly, GFP fusions with the gelsolin/villin-like region are uniformly dispersed in the cytoplasm, whereas GFP fusions of the N-terminal WD repeats codistribute with F-actin and are associated with the Triton-insoluble cytoskeleton. Strains lacking villidin because of targeted deletion of its gene grow normally and can develop into fruiting bodies. However, cell motility is reduced during aggregation and phototaxis is impaired in the mutant strains. We conclude that villidin harbors a major F-actin binding site in the N-terminal domain and not in the villin-like region as expected; association of villidin with vesicular membranes suggests that the protein functions as a linker between membranes and the actin cytoskeleton.

Gelsolin domains 4-6 in active, actin-free conformation identifies sites of regulatory calcium ions

Structural analysis of gelsolin domains 4-6 demonstrates that the two highest-affinity calcium ions that activate the molecule are in domains 5 and 6, one in each. An additional calcium site in domain 4 depends on subsequent actin binding and is seen only in the complex. The uncomplexed structure is primed to bind actin. Since the disposition of the three domains is similar in different crystal environments, either free or in complex with actin, the conformation in calcium is intrinsic to active gelsolin itself. Thus the actin-free structure shows that the structure with an actin monomer is a good model for an actin filament cap. The last 13 residues of domain 6 have been proposed to be a calcium-activated latch that, in the inhibited form only, links two halves of gelsolin. Comparison with the active structure shows that loosening of the latch contributes but is not central to activation. Calcium binding in domain 6 invokes a cascade of swapped ion-pairs. A basic residue swaps acidic binding partners to stabilise a straightened form of a helix that is kinked in inhibited gelsolin. The other end of the helix is connected by a loop to an edge beta-strand. In active gelsolin, an acidic residue in this helix breaks with its loop partner to form a new intrahelical ion-pairing, resulting in the breakage of the continuous sheet between domains 4 and 6, which is central to the inhibited conformation. A structural alignment of domain sequences provides a rationale to understand why the two calcium sites found here have the highest affinity amongst the five different candidate sites found in other gelsolin structures.

Actin binding proteins: regulation of cytoskeletal microfilaments

The actin cytoskeleton is a complex structure that performs a wide range of cellular functions. In 2001, significant advances were made to our understanding of the structure and function of actin monomers. Many of these are likely to help us understand and distinguish between the structural models of actin microfilaments. In particular, 1) the structure of actin was resolved from crystals in the absence of cocrystallized actin binding proteins (ABPs), 2) the prokaryotic ancestral gene of actin was crystallized and its function as a bacterial cytoskeleton was revealed, and 3) the structure of the Arp2/3 complex was described for the first time. In this review we selected several ABPs (ADF/cofilin, profilin, gelsolin, thymosin beta4, DNase I, CapZ, tropomodulin, and Arp2/3) that regulate actin-driven assembly, i.e., movement that is independent of motor proteins. They were chosen because 1) they represent a family of related proteins, 2) they are widely distributed in nature, 3) an atomic structure (or at least a plausible model) is available for each of them, and 4) each is expressed in significant quantities in cells. These ABPs perform the following cellular functions: 1) they maintain the population of unassembled but assembly-ready actin monomers (profilin), 2) they regulate the state of polymerization of filaments (ADF/cofilin, profilin), 3) they bind to and block the growing ends of actin filaments (gelsolin), 4) they nucleate actin assembly (gelsolin, Arp2/3, cofilin), 5) they sever actin filaments (gelsolin, ADF/cofilin), 6) they bind to the sides of actin filaments (gelsolin, Arp2/3), and 7) they cross-link actin filaments (Arp2/3). Some of these ABPs are essential, whereas others may form regulatory ternary complexes. Some play crucial roles in human disorders, and for all of them, there are good reasons why investigations into their structures and functions should continue.

The calcium activation of gelsolin: insights from the 3A structure of the G4-G6/actin complex

Gelsolin participates in the reorganization of the actin cytoskeleton that is required during such phenomena as cell movement, cytokinesis, and apoptosis. It consists of six structurally similar domains, G1-G6, which are arranged at resting intracellular levels of calcium ion so as to obscure the three actin-binding surfaces. Elevation of Ca(2+) concentrations releases latches within the constrained structure and produces large shifts in the relative positioning of the domains, permitting gelsolin to bind to and sever actin filaments. How Ca(2+) is able to activate gelsolin has been a major question concerning the function of this protein. We present the improved structure of the C-terminal half of gelsolin bound to monomeric actin at 3.0 A resolution. Two classes of Ca(2+)-binding site are evident on gelsolin: type 1 sites share coordination of Ca(2+) with actin, while type 2 sites are wholly contained within gelsolin. This structure of the complex reveals the locations of two novel metal ion-binding sites in domains G5 and G6, respectively. We identify both as type 2 sites. The absolute conservation of the type 2 calcium-ligating residues across the six domains of gelsolin suggests that this site exists in each of the domains. In total, gelsolin has the potential to bind eight calcium ions, two type 1 and six type 2. The function of the type 2 sites is to facilitate structural rearrangements within gelsolin as part of the activation and actin-binding and severing processes. We propose the novel type 2 site in G6 to be the critical site that initiates overall activation of gelsolin by releasing the tail latch that locks calcium-free gelsolin in a conformation unable to bind actin.

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

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

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.

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.

Furin initiates gelsolin familial amyloidosis in the Golgi through a defect in Ca(2+) stabilization

Hereditary familial amyloidosis of Finnish type (FAF) leading to amyloid in the peripheral and central nervous systems stems from deposition of a 71 residue fragment generated from the D187N/Y variants of plasma gelsolin by two sequential endoproteolytic events. We identify the protease accomplishing the first cleavage as furin, a proprotein convertase. Endoproteolysis of plasma gelsolin occurs in the trans-Golgi network due to the inability of the FAF variants to bind and be stabilized by Ca(2+). Secretion and processing of the FAF variants by furin can be uncoupled by blocking the convergence of the exocytic pathway transporting plasma gelsolin and the endocytic recycling of furin. We propose that coincidence of membrane trafficking pathways contributes to the development of proteolysis-initiated amyloid disease.

Destabilization of Ca2+-free gelsolin may not be responsible for proteolysis in Familial Amyloidosis of Finnish Type

Mutations at position 187 in secreted gelsolin enable aberrant proteolysis at the 172-173 and 243-244 amide bonds, affording the 71-residue amyloidogenic peptide deposited in Familial Amyloidosis of Finnish Type (FAF). Thermodynamic comparisons of two different domain 2 constructs were carried out to study possible effects of the mutations on proteolytic susceptibility. In the construct we consider to be most representative of domain 2 in the context of the full-length protein (134-266), the D187N FAF variant is slightly destabilized relative to wild type (WT) under the conditions of urea denaturation, but exhibits a T(m) identical to WT. The D187Y variant is less stable to intermediate urea concentrations and exhibits a T(m) that is estimated to be approximately 5 degrees C lower than WT (pH 7.4, Ca(2+)-free). Although the thermodynamic data indicate that the FAF mutations may slightly destabilize domain 2, these changes are probably not sufficient to shift the native to denatured state equilibrium enough to enable the proteolysis leading to FAF. Biophysical data indicate that these two FAF variants may have different native state structures and possibly different pathways of amyloidosis.

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.