The actin/actin interactions involving the N-terminus of the DNase-I-binding loop are crucial for stabilization of the actin filament

The Balance between Protealysin and Its Substrate, the Outer Membrane Protein OmpX, Regulates Serratia proteamaculans Invasion

Serratia are opportunistic bacteria, causing infections in plants, insects, animals and humans under certain conditions. The development of bacterial infection in the human body involves several stages of host-pathogen interaction, including entry into non-phagocytic cells to evade host immune cells. The facultative pathogen Serratia proteamaculans is capable of penetrating eukaryotic cells. These bacteria synthesize an actin-specific metalloprotease named protealysin. After transformation with a plasmid carrying the protealysin gene, noninvasive E. coli penetrate eukaryotic cells. This suggests that protealysin may play a key role in S. proteamaculans invasion. This review addresses the mechanisms underlying protealysin's involvement in bacterial invasion, highlighting the main findings as follows. Protealysin can be delivered into the eukaryotic cell by the type VI secretion system and/or by bacterial outer membrane vesicles. By cleaving actin in the host cell, protealysin can mediate the reversible actin rearrangements required for bacterial invasion. However, inactivation of the protealysin gene leads to an increase, rather than decrease, in the intensity of S. proteamaculans invasion. This indicates the presence of virulence factors among bacterial protealysin substrates. Indeed, protealysin cleaves the virulence factors, including the bacterial surface protein OmpX. OmpX increases the expression of the EGFR and β1 integrin, which are involved in S. proteamaculans invasion. It has been shown that an increase in the invasion of genetically modified S. proteamaculans may be the result of the accumulation of full-length OmpX on the bacterial surface, which is not cleaved by protealysin. Thus, the intensity of the S. proteamaculans invasion is determined by the balance between the active protealysin and its substrate OmpX.

The Serratia grimesii outer membrane vesicles-associated grimelysin triggers bacterial invasion of eukaryotic cells

Serratia grimesii are facultative pathogenic bacteria that can penetrate a wide range of host cells and cause infection, especially in immunocompromised patients. Previously, we have found that bacterial metalloprotease grimelysin is a potential virulence determinant of S. grimesii invasion (E. S. Bozhokina et al., (2011). Cell Biology International, 35(2), 111-118). Protease is characterized as an actin-hydrolyzing enzyme with a narrow specificity toward other cell proteins. It is not known, however, whether grimelysin is transported into eukaryotic cells. Here, we show, for the first time, that S. grimesii can generate outer membrane vesicles (OMVs) displayed specific proteolytic activity against actin, characteristic of grimelysin. The presence of grimelysin was also confirmed by the Western blot analysis of S. grimesii OMVs lysate. Furthermore, confocal microscopy analysis revealed that the S. grimesii grimelysin-containing OMVs attached to the host cell membrane. Finally, pretreatment of HeLa cells with S. grimesii OMVs before the cells were infected with bacteria increased the bacterial penetration several times. These data strongly suggest that protease grimelysin promotes S. grimesii internalization by modifying bacterial and/or host molecule(s) when it is delivered as a component of OMVs.

Bacterial Actin-Specific Endoproteases Grimelysin and Protealysin as Virulence Factors Contributing to the Invasive Activities of Serratia

The article reviews the discovery, properties and functional activities of new bacterial enzymes, proteases grimelysin (ECP 32) of Serratia grimesii and protealysin of Serratia proteamaculans, characterized by both a highly specific "actinase" activity and their ability to stimulate bacterial invasion. Grimelysin cleaves the only polypeptide bond Gly42-Val43 in actin. This bond is not cleaved by any other proteases and leads to a reversible loss of actin polymerization. Similar properties were characteristic for another bacterial protease, protealysin. These properties made grimelysin and protealysin a unique tool to study the functional properties of actin. Furthermore, bacteria Serratia grimesii and Serratia proteamaculans, producing grimelysin and protealysin, invade eukaryotic cells, and the recombinant Escherichia coli expressing the grimelysin or protealysins gene become invasive. Participation of the cellular c-Src and RhoA/ROCK signaling pathways in the invasion of eukaryotic cells by S. grimesii was shown, and involvement of E-cadherin in the invasion has been suggested. Moreover, membrane vesicles produced by S. grimesii were found to contain grimelysin, penetrate into eukaryotic cells and increase the invasion of bacteria into eukaryotic cells. These data indicate that the protease is a virulence factor, and actin can be a target for the protease upon its translocation into the host cell.

Tropomyosin isoforms regulate cofilin 1 activity by modulating actin filament conformation

Tropomyosin and cofilin are involved in the regulation of actin filament dynamic polymerization and depolymerization. Binding of cofilin changes actin filaments structure, leading to their severing and depolymerization. Non-muscle tropomyosin isoforms were shown before to differentially regulate the activity of cofilin 1; products of TPM1 gene stabilized actin filaments, but products of TPM3 gene promoted cofilin-dependent severing and depolymerization. Here, conformational changes at the longitudinal and lateral interface between actin subunits resulting from tropomyosin and cofilin 1 binding were studied using skeletal actin and yeast wild type and mutant Q41C and S265C actins. Cross-linking of F-actin and fluorescence changes in F-actin labeled with acrylodan at Cys41 (in D-loop) or Cys265 (in H-loop) showed that tropomyosin isoforms differentially regulated cofilin-induced conformational rearrangements at longitudinal and lateral filament interfaces. Tryptic digestion of F-Mg-actin confirmed the differences between tropomyosin isoforms in their regulation of cofilin-dependent changes at actin-actin interfaces. Changes in the fluorescence of AEDANS attached to C-terminal Cys of actin, as well as FRET between Trp residues in actin subdomain 1 and AEDANS, did not show differences in the conformation of the C-terminal segment of F-actin in the presence of different tropomyosins ± cofilin 1. Therefore, actin's D- and H-loop are the sites involved in regulation of cofilin activity by tropomyosin isoforms.

Cooperative effects of tropomyosin on the dynamics of the actin filament

Tropomyosin (Tpm) plays an important role in regulating the organisation and functions of the actin cytoskeleton. Here, we describe a new approach to analyse the effects of Tpm on actin dynamics. Using F-actin proteolytically modified within the DNase-binding loop (ECP-actin), we show that Tpm binding almost completely suppresses the increased subunit exchange intrinsic for this F-actin. The effect is both concentration-dependent and cooperative, with half-maximal inhibition observed at about a 1 : 50 Tpm : actin ratio. Tpm decreases not only the number concentration of ECP-actin filaments, but also the rate of the filament subunit exchange. Our data suggest that Tpm regulates the dynamics of actin filaments by an allosteric strengthening of intermonomer contacts in the actin filament, and that this mechanism may be involved in the modulation of cytoskeletal dynamics.

[New metalloendopeptidase of Morganella morganii ZM]

Proteolytic activity which is inhibited in the presence of o-phenanthroline was found in M. morganii ZM. Intracellular proteases of M. morganii ZM unlimited split musculoskeletal actin in contrast to grimelysin. Several proteolitic proteins of M. morganii ZM cells were identified by zymography with gelatin. Metalloproteinase of M. morganii ZM cell lysate was purified by hydrophobic chromatography fractionation. The molecular weight of the protein was 35 kDa.

Near-atomic resolution for one state of F-actin

Actin functions as a helical polymer, F-actin, but attempts to build an atomic model for this filament have been hampered by the fact that the filament cannot be crystallized and by structural heterogeneity. We have used a direct electron detector, cryo-electron microscopy, and the forces imposed on actin filaments in thin films to reconstruct one state of the filament at 4.7 Å resolution, which allows for building a reliable pseudo-atomic model of F-actin. We also report a different state of the filament where actin protomers adopt a conformation observed in the crystal structure of the G-actin-profilin complex with an open ATP-binding cleft. Comparison of the two structural states provides insights into ATP-hydrolysis and filament dynamics. The atomic model provides a framework for understanding why every buried residue in actin has been under intense selective pressure.

Structural states and dynamics of the D-loop in actin

Conformational changes induced by ATP hydrolysis on actin are involved in the regulation of complex actin networks. Previous structural and biochemical data implicate the DNase I binding loop (D-loop) of actin in such nucleotide-dependent changes. Here, we investigated the structural and conformational states of the D-loop (in solution) using cysteine scanning mutagenesis and site-directed labeling. The reactivity of D-loop cysteine mutants toward acrylodan and the mobility of spin labels on these mutants do not show patterns of an α-helical structure in monomeric and filamentous actin, irrespective of the bound nucleotide. Upon transition from monomeric to filamentous actin, acrylodan emission spectra and electron paramagnetic resonance line shapes of labeled mutants are blue-shifted and more immobilized, respectively, with the central residues (residues 43-47) showing the most drastic changes. Moreover, complex electron paramagnetic resonance line shapes of spin-labeled mutants suggest several conformational states of the D-loop. Together with a new (to our knowledge) actin crystal structure that reveals the D-loop in a unique hairpin conformation, our data suggest that the D-loop equilibrates in F-actin among different conformational states irrespective of the nucleotide state of actin.

Multiple forms of Spire-actin complexes and their functional consequences

Spire is a WH2 domain-containing actin nucleator essential for establishing an actin mesh during oogenesis. In vitro, in addition to nucleating filaments, Spire can sever them and sequester actin monomers. Understanding how Spire is capable of these disparate functions and which are physiologically relevant is an important goal. To study severing, we examined the effect of Drosophila Spire on preformed filaments in bulk and single filament assays. We observed rapid depolymerization of actin filaments by Spire, which we conclude is largely due to its sequestration activity and enhanced by its weak severing activity. We also studied the solution and crystal structures of Spire-actin complexes. We find structural and functional differences between constructs containing four WH2 domains (Spir-ABCD) and two WH2 domains (Spir-CD) that may provide insight into the mechanisms of nucleation and sequestration. Intriguingly, we observed lateral interactions between actin monomers associated with Spir-ABCD, suggesting that the structures built by these four tandem WH2 domains are more complex than originally imagined. Finally, we propose that Spire-actin mixtures contain both nuclei and sequestration structures.

Bacterial invasion of eukaryotic cells can be mediated by actin-hydrolysing metalloproteases grimelysin and protealysin

Earlier, we have shown that spontaneously isolated non-pathogenic bacteria Serratia grimesii and Serratia proteamaculans invade eukaryotic cells, provided that they synthesize thermolysin-like metalloproteases ECP32/grimelysin or protealysin characterized by high specificity towards actin. To address the question of whether the proteases are active players in entry of these bacteria into host cells, in this work, human larynx carcinoma Hep-2 cells were infected with recombinant Escherichia coli expressing grimelysin or protealysin. Using confocal and electron microscopy, we have found that the recombinant bacteria, whose extracts limitedly cleaved actin, were internalized within the eukaryotic cells residing both in vacuoles and free in cytoplasm. The E. coli-carrying plasmids without inserts of grimelysin or protealysin gene did not enter Hep-2 cells. Moreover, internalization of non-invasive E. coli was not observed in the presence of protealysin introduced into the culture medium. These results are consistent with the direct participation of ECP32/grimelysin and protealysin in entry of bacteria into the host cells. We assume that ECP32/grimelysin and protealysin mediate invasion being injected into the eukaryotic cell and that the high specificity of the enzyme towards actin may be a factor contributed to the bacteria internalization.

Structural basis for actin assembly, activation of ATP hydrolysis, and delayed phosphate release

Assembled actin filaments support cellular signaling, intracellular trafficking, and cytokinesis. ATP hydrolysis triggered by actin assembly provides the structural cues for filament turnover in vivo. Here, we present the cryo-electron microscopic (cryo-EM) structure of filamentous actin (F-actin) in the presence of phosphate, with the visualization of some α-helical backbones and large side chains. A complete atomic model based on the EM map identified intermolecular interactions mediated by bound magnesium and phosphate ions. Comparison of the F-actin model with G-actin monomer crystal structures reveals a critical role for bending of the conserved proline-rich loop in triggering phosphate release following ATP hydrolysis. Crystal structures of G-actin show that mutations in this loop trap the catalytic site in two intermediate states of the ATPase cycle. The combined structural information allows us to propose a detailed molecular mechanism for the biochemical events, including actin polymerization and ATPase activation, critical for actin filament dynamics.

Coarse-Grained Representations of Large Biomolecular Complexes from Low-Resolution Structural Data

High-resolution atomistic structures of many large biomolecular complexes have not yet been solved by experiments, such as X-ray crystallography or NMR. Often however low-resolution information is obtained by alternative techniques, such as cryo-electron microscopy or small-angle X-ray scattering. Coarse-grained (CG) models are an appropriate choice to computationally study these complexes given the limited resolution experimental data. One of the important questions therefore is how to define CG representations from these low-resolution density maps. This work provides a space-based essential dynamics coarse-graining (ED-CG) method to define a CG representation from a density map without detailed knowledge of its underlying atomistic structure and primary sequence information. This method is demonstrated on G-actin (both the atomic structure and its density map). It is then applied to the density maps of the Escherichia coli 70S ribosome and the microtubule. The results indicate that the method can define highly CG models that still preserve functionally important dynamics of large biomolecular complexes.

Specific cleavage of the DNase-I binding loop dramatically decreases the thermal stability of actin

Differential scanning calorimetry was used to investigate the thermal unfolding of actin specifically cleaved within the DNaseI-binding loop between residues Met47-Gly48 or Gly42-Val43 by two bacterial proteases, subtilisin or ECP32/grimelysin (ECP), respectively. The results obtained show that both cleavages strongly decreased the thermal stability of monomeric actin with either ATP or ADP as a bound nucleotide. An even more pronounced difference in the thermal stability between the cleaved and intact actin was observed when both actins were polymerized into filaments. Similar to intact F-actin, both cleaved F-actins were significantly stabilized by phalloidin and aluminum fluoride; however, in all cases, the thermal stability of the cleaved F-actins was much lower than that of intact F-actin, and the stability of ECP-cleaved F-actin was lower than that of subtilisin-cleaved F-actin. These results confirm that the DNaseI-binding loop is involved in the stabilization of the actin structure, both in monomers and in the filament subunits, and suggest that the thermal stability of actin depends, at least partially, on the conformation of the nucleotide-binding cleft. Moreover, an additional destabilization of the unstable cleaved actin upon ATP/ADP replacement provides experimental evidence for the highly dynamic actin structure that cannot be simply open or closed, but rather should be considered as being able to adopt multiple conformations.

Defining coarse-grained representations of large biomolecules and biomolecular complexes from elastic network models

Coarse-grained (CG) models of large biomolecular complexes enable simulations of these systems over long timescales that are not accessible for atomistic molecular dynamics (MD) simulations. A systematic methodology, called essential dynamics coarse-graining (ED-CG), has been developed for defining coarse-grained sites in a large biomolecule. The method variationally determines the CG sites so that key dynamic domains in the protein are preserved in the CG representation. The original ED-CG method relies on a principal component analysis (PCA) of a MD trajectory. However, for many large proteins and multi-protein complexes such an analysis may not converge or even be possible. This work develops a new ED-CG scheme using an elastic network model (ENM) of the protein structure. In this procedure, the low-frequency normal modes obtained by ENM are used to define dynamic domains and to define the CG representation accordingly. The method is then applied to several proteins, such as the HIV-1 CA protein dimer, ATP-bound G-actin, and the Arp2/3 complex. Numerical results show that ED-CG with ENM (ENM-ED-CG) is much faster than ED-CG with PCA because no MD is necessary. The ENM-ED-CG models also capture functional essential dynamics of the proteins almost as well as those using full MD with PCA. Therefore, the ENM-ED-CG method may be better suited to coarse-grain a very large biomolecule or biomolecular complex that is too computationally expensive to be simulated by conventional MD, or when a high resolution atomic structure is not even available.

Mutation of actin Tyr-53 alters the conformations of the DNase I-binding loop and the nucleotide-binding cleft

All but 11 of the 323 known actin sequences have Tyr at position 53, and the 11 exceptions have the conservative substitution Phe, which raises the following questions. What is the critical role(s) of Tyr-53, and, if it can be replaced by Phe, why has this happened so infrequently? We compared the properties of purified endogenous Dictyostelium actin and mutant constructs with Tyr-53 replaced by Phe, Ala, Glu, Trp, and Leu. The Y53F mutant did not differ significantly from endogenous actin in any of the properties assayed, but the Y53A and Y53E mutants differed substantially; affinity for DNase I was reduced, the rate of nucleotide exchange was increased, the critical concentration for polymerization was increased, filament elongation was inhibited, and polymerized actin was in the form of small oligomers and imperfect filaments. Growth and/or development of cells expressing these actin mutants were also inhibited. The Trp and Leu mutations had lesser but still significant effects on cell phenotype and the biochemical properties of the purified actins. We conclude that either Tyr or Phe is required to maintain the functional conformations of the DNase I-binding loop (D-loop) in both G- and F-actin, and that the conformation of the D-loop affects not only the properties that directly involve the D-loop (binding to DNase I and polymerization) but also allosterically modifies the conformation of the nucleotide-binding cleft, thus increasing the rate of nucleotide exchange. The apparent evolutionary "preference" for Tyr at position 53 may be the result of Tyr allowing dynamic modification of the D-loop conformation by phosphorylation (Baek, K., Liu, X., Ferron, F., Shu, S., Korn, E. D., and Dominguez, R. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 11748-11753) with effects similar, but not identical, to those of the Ala and Glu mutations.

F-actin structure destabilization and DNase I binding loop: fluctuations mutational cross-linking and electron microscopy analysis of loop states and effects on F-actin

The conformational dynamics of filamentous actin (F-actin) is essential for the regulation and functions of cellular actin networks. The main contribution to F-actin dynamics and its multiple conformational states arises from the mobility and flexibility of the DNase I binding loop (D-loop; residues 40-50) on subdomain 2. Therefore, we explored the structural constraints on D-loop plasticity at the F-actin interprotomer space by probing its dynamic interactions with the hydrophobic loop (H-loop), the C-terminus, and the W-loop via mutational disulfide cross-linking. To this end, residues of the D-loop were mutated to cysteines on yeast actin with a C374A background. These mutants showed no major changes in their polymerization and nucleotide exchange properties compared to wild-type actin. Copper-catalyzed disulfide cross-linking was investigated in equimolar copolymers of cysteine mutants from the D-loop with either wild-type (C374) actin or mutant S265C/C374A (on the H-loop) or mutant F169C/C374A (on the W-loop). Remarkably, all tested residues of the D-loop could be cross-linked to residues 374, 265, and 169 by disulfide bonds, demonstrating the plasticity of the interprotomer region. However, each cross-link resulted in different effects on the filament structure, as detected by electron microscopy and light-scattering measurements. Disulfide cross-linking in the longitudinal orientation produced mostly no visible changes in filament morphology, whereas the cross-linking of D-loop residues >45 to the H-loop, in the lateral direction, resulted in filament disruption and the presence of amorphous aggregates on electron microscopy images. A similar aggregation was also observed upon cross-linking the residues of the D-loop (>41) to residue 169. The effects of disulfide cross-links on F-actin stability were only partially accounted for by the simulations of current F-actin models. Thus, our results present evidence for the high level of conformational plasticity in the interprotomer space and document the link between D-loop interactions and F-actin stability.

Multidimensional degradomics identifies systemic autoantigens and intracellular matrix proteins as novel gelatinase B/MMP-9 substrates

The action radius of matrix metalloproteinases or MMPs is not restricted to massive extracellular matrix (ECM) degradation, it extends to the proteolysis of numerous secreted and membrane-bound proteins. Although many instances exist in which cells disintegrate, often in conjunction with induction of MMPs, the intracellular MMP substrate repertoire or degradome remains relatively unexplored. We started an unbiased exploration of the proteolytic modification of intracellular proteins by MMPs, using gelatinase B/MMP-9 as a model enzyme. To this end, multidimensional degradomics technology was developed by the integration of broadly available biotechniques. In this way, 100-200 MMP-9 candidate substrates were isolated, of which 69 were identified. Integration of these results with the known biological functions of the substrates revealed many novel MMP-9 substrates from the intracellular matrix (ICM), such as actin, tubulin, gelsolin, moesin, ezrin, Arp2/3 complex subunits, filamin B and stathmin. About 2/3 of the identified candidates were autoantigens described in multiple autoimmune conditions and in cancer (e.g. annexin I, nucleolin, citrate synthase, HMGB1, alpha-enolase, histidyl-tRNA synthetase, HSP27, HSC70, HSP90, snRNP D3). These findings led to the insight that MMPs and other proteases may have novel (immuno)regulatory properties by the clearance of toxic and immunogenic burdens of abundant ICM proteins released after extensive necrosis. In line with the extracellular processing of organ-specific autoantigens, proteolysis might also assist in the generation of immunodominant 'neo-epitopes' from systemic autoantigens. The study of proteolysis of ICM molecules, autoantigens, alarmins and other crucial intracellular molecules may result in the discovery of novel roles for proteolytic modification.

New aspects of the spontaneous polymerization of actin in the presence of salts

The mechanism of salt-induced actin polymerization involves the energetically unfavorable nucleation step, followed by filament elongation by the addition of monomers. The use of a bifunctional cross-linker, N,N'-(1,4-phenylene)dimaleimide, revealed rapid formation of the so-called lower dimers (LD) in which actin monomers are arranged in an antiparallel fashion. The filament elongation phase is characterized by a gradual LD decay and an increase in the yield of "upper dimers" (UD) characteristic of F-actin. Here we have used 90 degrees light scattering, electron microscopy, and N, N'-(1,4-phenylene)dimaleimide cross-linking to reinvestigate relationships between changes in filament morphology, LD decay, and increase in the yield of UD during filament growth in a wide range of conditions influencing the rate of the nucleation reaction. The results show irregularity and instability of filaments at early stages of polymerization under all conditions used, and suggest that an earlier documented coassembling of LD with monomeric actin contributes to the initial disordering of the filaments rather than to the nucleation of polymerization. The effects of the type of G-actin-bound divalent cation (Ca2+/Mg2+), nucleotide (ATP/ADP), and polymerizing salt on the relation between changes in filament morphology and progress in G-actin-to-F-actin transformation show that ligand-dependent alterations in G-actin conformation determine not only the nucleation rate but also the kinetics of ordering of the filament structure in the elongation phase. The time courses of changes in the yield of UD suggest that filament maturation involves cooperative propagation of "proper" interprotomer contacts. Acceleration of this process by the initially bound MgATP supports the view that the filament-destabilizing conformational changes triggered by ATP hydrolysis and Pi liberation during polymerization are constrained by the intermolecular contacts established between MgATP monomers prior to ATP hydrolysis. An important role of contacts involving the DNase-I-binding loop and the C-terminus of actin is proposed.

Connecting actin monomers by iso-peptide bond is a toxicity mechanism of the Vibrio cholerae MARTX toxin

The Gram-negative bacterium Vibrio cholerae is the causative agent of a severe diarrheal disease that afflicts three to five million persons annually, causing up to 200,000 deaths. Nearly all V. cholerae strains produce a large multifunctional-autoprocessing RTX toxin (MARTX(Vc)), which contributes significantly to the pathogenesis of cholera in model systems. The actin cross-linking domain (ACD) of MARTX(Vc) directly catalyzes a covalent cross-linking of monomeric G-actin into oligomeric chains and causes cell rounding, but the nature of the cross-linked bond and the mechanism of the actin cytoskeleton disruption remained elusive. To elucidate the mechanism of ACD action and effect on actin, we identified the covalent cross-link bond between actin protomers using limited proteolysis, X-ray crystallography, and mass spectrometry. We report here that ACD catalyzes the formation of an intermolecular iso-peptide bond between residues E270 and K50 located in the hydrophobic and the DNaseI-binding loops of actin, respectively. Mutagenesis studies confirm that no other residues on actin can be cross-linked by ACD both in vitro and in vivo. This cross-linking locks actin protomers into an orientation different from that of F-actin, resulting in strong inhibition of actin polymerization. This report describes a microbial toxin mechanism acting via iso-peptide bond cross-linking between host proteins and is, to the best of our knowledge, the only known example of a peptide linkage between nonterminal glutamate and lysine side chains.

A systematic methodology for defining coarse-grained sites in large biomolecules

Coarse-grained (CG) models of biomolecules have recently attracted considerable interest because they enable the simulation of complex biological systems on length-scales and timescales that are inaccessible for atomistic molecular dynamics simulation. A CG model is defined by a map that transforms an atomically detailed configuration into a CG configuration. For CG models of relatively small biomolecules or in cases that the CG and all-atom models have similar resolution, the construction of this map is relatively straightforward and can be guided by chemical intuition. However, it is more challenging to construct a CG map when large and complex domains of biomolecules have to be represented by relatively few CG sites. This work introduces a new and systematic methodology called essential dynamics coarse-graining (ED-CG). This approach constructs a CG map of the primary sequence at a chosen resolution for an arbitrarily complex biomolecule. In particular, the resulting ED-CG method variationally determines the CG sites that reflect the essential dynamics characterized by principal component analysis of an atomistic molecular dynamics trajectory. Numerical calculations illustrate this approach for the HIV-1 CA protein dimer and ATP-bound G-actin. Importantly, since the CG sites are constructed from the primary sequence of the biomolecule, the resulting ED-CG model may be better suited to appropriately explore protein conformational space than those from other CG methods at the same degree of resolution.

Control of the ability of profilin to bind and facilitate nucleotide exchange from G-actin

A major factor in profilin regulation of actin cytoskeletal dynamics is its facilitation of G-actin nucleotide exchange. However, the mechanism of this facilitation is unknown. We studied the interaction of yeast (YPF) and human profilin 1 (HPF1) with yeast and mammalian skeletal muscle actins. Homologous pairs (YPF and yeast actin, HPF1 and muscle actin) bound more tightly to one another than heterologous pairs. However, with saturating profilin, HPF1 caused a faster etheno-ATP exchange with both yeast and muscle actins than did YPF. Based on the -fold change in ATP exchange rate/K(d), however, the homologous pairs are more efficient than the heterologous pairs. Thus, strength of binding of profilin to actin and nucleotide exchange rate are not tightly coupled. Actin/HPF interactions were entropically driven, whereas YPF interactions were enthalpically driven. Hybrid yeast actins containing subdomain 1 (sub1) or subdomain 1 and 2 (sub12) muscle actin residues bound more weakly to YPF than did yeast actin (K(d) = 2 microm versus 0.6 microm). These hybrids bound even more weakly to HPF than did yeast actin (K(d) = 5 microm versus 3.2 microm). sub1/YPF interactions were entropically driven, whereas the sub12/YPF binding was enthalpically driven. Compared with WT yeast actin, YPF binding to sub1 occurred with a 5 times faster k(off) and a 2 times faster k(on). sub12 bound with a 3 times faster k(off) and a 1.5 times slower k(on). Profilin controls the energetics of its interaction with nonhybrid actin, but interactions between actin subdomains 1 and 2 affect the topography of the profilin binding site.

Grimelysin, a novel metalloprotease from Serratia grimesii, is similar to ECP32

Limited actin proteolysis is the hallmark of bacterial metalloprotease ECP32. While ECP32 has long been considered an Escherichia coli protein, the N-terminal amino acid sequence of the active enzyme described previously, could not been retrieved in the E. coli genome. We cloned, sequenced and characterized Serratia grimesii protease grimelysin and show that grimelysin is similar to the previously described protease ECP32.

Phosphorylation of actin Tyr-53 inhibits filament nucleation and elongation and destabilizes filaments

Dictyostelium actin was shown to become phosphorylated on Tyr-53 late in the developmental cycle and when cells in the amoeboid stage are subjected to stress but the phosphorylated actin had not been purified and characterized. We have separated phosphorylated and unphosphorylated actin and shown that Tyr-53 phosphorylation substantially reduces actin's ability to inactivate DNase I, increases actin's critical concentration, and greatly reduces its rate of polymerization. Tyr-53 phosphorylation substantially, if not completely, inhibits nucleation and elongation from the pointed end of actin filaments and reduces the rate of elongation from the barbed end. Negatively stained electron microscopic images of polymerized Tyr-53-phosphorylated actin show a variable mixture of small oligomers and filaments, which are converted to more typical, long filaments upon addition of myosin subfragment 1. Tyr-53-phosphorylated and unphosphorylated actin copolymerize in vitro, and phosphorylated and unphosphorylated actin colocalize in amoebae. Tyr-53 phosphorylation does not affect the ability of filamentous actin to activate myosin ATPase.

Crystal structure of polymerization-competent actin

All actin crystal structures reported to date represent actin complexed or chemically modified with molecules that prevent its polymerization. Actin cleaved with ECP32 protease at a single site between Gly42 and Val43 is virtually non-polymerizable in the Ca-ATP bound form but remains polymerization-competent in the Mg-bound form. Here, a crystal structure of the true uncomplexed ECP32-cleaved actin (ECP-actin) solved to 1.9 A resolution is reported. In contrast to the much more open conformation of the ECP-actin's nucleotide binding cleft in solution, the crystal structure of uncomplexed ECP-actin contains actin in a typical closed conformation similar to the complexed actin structures. This unambiguously demonstrates that the overall structure of monomeric actin is not significantly affected by a multitude of actin-binding proteins and toxins. The invariance of actin crystal structures suggests that the salt and precipitants necessary for crystallization stabilize actin in only one of its possible conformations. The asymmetric unit cell contains a new type of antiparallel actin dimer that may correspond to the "lower dimer" implicated in F-actin nucleation and branching. In addition, symmetry-related actin-actin contacts form a head to tail dimer that is strikingly similar to the longitudinal dimer predicted by the Holmes F-actin model, including a rotation of the monomers relative to each other not observed previously in actin crystal structures.

Allostery of actin filaments: molecular dynamics simulations and coarse-grained analysis

The structural and mechanical properties of monomeric actin (G-actin), the trimer nucleus, and actin filaments (F-actins) are determined as a function of the conformation of the DNase I-binding loop (DB loop) by using all-atom molecular dynamics simulations and coarse-grained (CG) analysis. Recent x-ray structures of ADP-bound G-actin (G-ADP) by Otterbein et al. [Otterbein, L. R., Graceffa, P. & Dominguez, R. (2001) Science 293, 708-711] and ATP-bound G-actin (G-ATP) by Graceffa and Dominguez [Graceffa, P. & Dominguez, R. (2003) J. Biol. Chem. 278, 34172-34180] indicate that the DB loop of actin does not have a well defined secondary structure in the ATP state but folds into an alpha-helix in the ADP state. MD simulations and CG analysis indicate that such a helical DB loop significantly weakens the intermonomer interactions of actin assemblies and thus leads to a wider, shorter, and more disordered filament. The computed persistence lengths of F-actin composed of G-ATP (16 microm) and of G-ADP (8.5 microm) agree well with the experimental values for the two states. Therefore, the loop-to-helix transition of the DB loop may be one of the factors that lead to the changes in structural and mechanical properties of F-actin after ATP hydrolysis. This result may provide a direct connection between the conformational changes of an actin monomer and the structural and mechanical properties of the cytoskeleton. The information provided by MD simulations also helps to understand the possible origin of the special features of actin dynamics.

Role of actin DNase-I-binding loop in myosin subfragment 1-induced polymerization of G-actin: implications for the mechanism of polymerization

Proteolytic cleavage of actin between Gly(42) and Val(43) within its DNase-I-binding loop (D-loop) abolishes the ability of Ca-G-actin to spontaneously polymerize in the presence of KCl. Here we show that such modified actin is assembled into filaments, albeit at a lower rate than unmodified actin, by myosin subfragment 1 (S1) carrying the A1 essential light chain but not by S1(A2). S1 titration of pyrene-G-actin showed a diminished affinity of cleaved actin for S1, but this could be compensated for by using S1 in excess. The most significant effect of the cleavage, revealed by measuring the fluorescence of pyrene-actin and light-scattering intensities as a function of actin concentration at saturating concentrations of S1, is strong inhibition of association of G-actin-S1 complexes into oligomers. Measurements of the fluorescence of dansyl cadaverine attached to Gln(41) indicate substantial inhibition of the initial association of G-actin-S1 into longitudinal dimers. The data provide experimental evidence for the critical role of D-loop conformation in both longitudinal and lateral, cross-strand actin-actin contact formation in the nucleation reaction. Electron microscopic analysis of the changes in filament-length distribution during polymerization of actin by S1(A1) and S1(A2) suggests that the mechanism of S1-induced polymerization is not substantially different from the nucleation-elongation scheme of spontaneous actin polymerization.

The DNase-I binding loop of actin may play a role in the regulation of actin-myosin interaction by tropomyosin/troponin

Various lines of evidence suggest that communication between tropomyosin and myosin in the regulation of vertebrate-striated muscle contraction involves yet unknown changes in actin conformation. Possible participation of loop 38-52 in this communication has recently been questioned based on unimpaired Ca(2+) regulation of myosin interaction, in the presence of the tropomyosin-troponin complex, with actin cleaved by subtilisin between Met(47) and Gly(48). We have compared the effects of actin cleavage by subtilisin and by protease ECP32, between Gly(42) and Val(43), on its interaction with myosin S1 in the presence and absence of tropomyosin or tropomyosin-troponin. Both individual modifications reduced activation of S1 ATPase by actin to a similar extent. The effect of ECP cleavage, but not of subtilisin cleavage, was partially reversed by stabilization of interprotomer contacts with phalloidin, indicating different pathways of signal transmission from the N- and C-terminal parts of loop 38-52 to myosin binding sites. ECP cleavage diminished the affinity to tropomyosin and reduced its inhibition of acto-S1 ATPase at low S1 concentrations, but increased the tropomyosin-mediated cooperative enhancement of the ATPase by S1 binding to actin. These effects were reversed by phalloidin. Subtilisin-cleaved actin more closely resembled unmodified actin than the ECP-modified actin. Limited proteolysis of the modified and unmodified F-actins revealed an allosteric effect of ECP cleavage on the conformation of the actin subdomain 4 region that is presumably involved in tropomyosin binding. Our results point to a possible role of the N-terminal part of loop 38-52 of actin in communication between tropomyosin and myosin through changes in actin structure.

Structure-based analysis of high pressure adaptation of alpha-actin

Deep-sea fishes occur to depths of several thousand meters, and at these abyssal depths encounter pressures that shallower living fishes cannot tolerate. Tolerance of abyssal pressures by deep-sea fish is likely to depend in part on adaptive modifications of proteins. However, the types of structural modifications to proteins that allow function at high pressure have not been discovered. To elucidate the mechanisms of protein adaptation to high pressure, we cloned the alpha-skeletal actin cDNAs from two abyssal Coryphaenoides species, C. armatus and C. yaquinae, and identified three amino acid substitutions, V54A or L67P, Q137K, and A155S, that distinguish these abyssal actins from orthologs of alpha-actin from non-abyssal Coryphaenoides. These substitutions, Q137K and A155S, prevent the dissociation reactions of ATP and Ca2+ from being influenced by high pressure. In particular, the lysine residue at position 137 results in a much smaller apparent volume change in the Ca2+ dissociation reaction. The V54A or L67P substitution reduces the volume change associated with actin polymerization and has a role in maintaining the DNase I activity of actin at high pressure. Together, these results indicate that a few amino acid substitutions in key functional positions can adaptively alter the pressure sensitivity of a protein.

Structural effects of cofilin on longitudinal contacts in F-actin

Structural effects of yeast cofilin on skeletal muscle and yeast actin were examined in solution. Cofilin binding to native actin was non-cooperative and saturated at a 1:1 molar ratio, with K(d)<or=0.05 microM for both CaATP-G-actin and F-actin. Cofilin binding enhanced the fluorescence of dansyl ethylenediamine (DED) attached to Gln41 on the DNase I binding loop of skeletal muscle F-actin and decreased the fluorescence of AEDANS at Cys41 on yeast Q41C/C374S mutant F-actin. However, cofilin had no effect on the spectral properties of DED or AEDANS on CaATP-G-actin. Fluorescence energy transfer (FRET) from tryptophan residues to DED at Gln41 on skeletal muscle actin and to AEDANS at Cys41 on yeast Q41C/C374S actin was decreased by cofilin binding to F- but not to G-actin. Cofilin inhibited strongly the rate of interprotomer disulfide cross-linking of Cys41 to Cys374 on yeast Q41C mutant F-actin. Binding of cofilin enhanced excimer formation between pyrene probes attached to Cys41 and Cys374 on Q41C F-actin. These results indicate that cofilin alters the interface between subdomains 1 and 2 and shifts the DNase I binding loop away from subdomain 1 of an adjacent actin protomer. Cofilin reduced FRET from tryptophan residues to 4-azido-2-nitrophenyl-putrescine (ANP) at Gln41 in skeletal muscle F-but not in G-actin. However, following the interprotomer cross-linking of Gln41 to Cys374 in F-actin by ANP, cofilin binding did not change FRET from the tryptophan residues to ANP. This suggests that cofilin binding and the conformational effect on F-actin are not coupled tightly. Overall, this study provides solution evidence for the weakening of longitudinal, subdomain 2/1 contacts in F-actin by cofilin.

The vasodilator-stimulated phosphoprotein promotes actin polymerisation through direct binding to monomeric actin

The vasodilator-stimulated phosphoprotein (VASP) functions as a cellular regulator of actin dynamics. VASP may initialise actin polymerisation, suggesting a direct interaction with monomeric actin. The present study demonstrates that VASP directly binds to actin monomers and that complex formation depends on a conserved four amino acid motif in the EVH2 domain. Point mutations within this motif drastically weaken VASP/G-actin interactions, thereby abolishing any actin-nucleating activity of VASP. Additionally, actin nucleation was found to depend on VASP oligomerisation since VASP monomers fail to induce the formation of actin filaments. Phosphorylation negatively affects VASP/G-actin interactions preventing VASP-induced actin filament formation.

Cofilin and DNase I affect the conformation of the small domain of actin

Cofilin binding induces an allosteric conformational change in subdomain 2 of actin, reducing the distance between probes attached to Gln-41 (subdomain 2) and Cys-374 (subdomain 1) from 34.4 to 31.4 A (pH 6.8) as demonstrated by fluorescence energy transfer spectroscopy. This effect was slightly less pronounced at pH 8.0. In contrast, binding of DNase I increased this distance (35.5 A), a change that was not pH-sensitive. Although DNase I-induced changes in the distance along the small domain of actin were modest, a significantly larger change (38.2 A) was observed when the ternary complex of cofilin-actin-DNase I was formed. Saturation binding of cofilin prevents pyrene fluorescence enhancement normally associated with actin polymerization. Changes in the emission and excitation spectra of pyrene-F actin in the presence of cofilin indicate that subdomain 1 (near Cys-374) assumes a G-like conformation. Thus, the enhancement of pyrene fluorescence does not correspond to the extent of actin polymerization in the presence of cofilin. The structural changes in G and F actin induced by these actin-binding proteins may be important for understanding the mechanism regulating the G-actin pool in cells.

Role of the DNase-I-binding loop in dynamic properties of actin filament

Effects of proteolytic modifications of the DNase-I-binding loop (residues 39-51) in subdomain 2 of actin on F-actin dynamics were investigated by measuring the rates of the polymer subunit exchange with the monomer pool at steady state and of ATP hydrolysis associated with it, and by determination of relative rate constants for monomer addition to and dissociation from the polymer ends. Cleavage of actin between Gly-42 and Val-43 by protease ECP32 resulted in enhancement of the turnover rate of polymer subunits by an order of magnitude or more, in contrast to less than a threefold increase produced by subtilisin cleavage between Met-47 and Gly-48. Probing the structure of the modified actins by limited digestion with trypsin revealed a correlation between the increased F-actin dynamics and a change in the conformation of subdomain 2, indicating a more open state of the filament subunits relative to intact F-actin. The cleavage with trypsin and steady-state ATPase were cooperatively inhibited by phalloidin, with half-maximal effects at phalloidin to actin molar ratio of 1:8 and full inhibition at a 1:1 ratio. The results support F-actin models in which only the N-terminal segment of loop 39-51 is involved in monomer-monomer contacts, and suggest a possibility of regulation of actin dynamics in the cell through allosteric effects on this segment of the actin polypeptide chain.

ATPase activity and conformational changes in the regulation of actin

The eukaryotic microfilament system is regulated in part through the nucleotide- and cation-dependent conformation of the actin molecule. In this review, recent literature on the crystal and solution structures of actin and other actin-superfamily proteins is summarized. Furthermore, the structure of the nucleotide binding cleft is discussed in terms of the mechanism of ATP hydrolysis and P(i) release. Two distinct domain movements are suggested to participate in the regulation of actin. (1) High-affinity binding of Mg(2+) to actin induces a rearrangement of side chains in the nucleotide binding site leading to an increased ATPase activity and polymerizability, as well as a rotation of subdomain 2 which is mediated by the hydroxyl of serine-14. (2) Hydrolysis of ATP and subsequent release of inorganic phosphate lead to a butterfly-like opening of the actin molecule brought about by a shearing in the interdomain helix 135-150. These domain rearrangements modulate the interaction of actin with a variety of different proteins, and conversely, protein binding to actin can restrict these conformational changes, with ultimate effects on the assembly state of the microfilament system.

The crystal structure of uncomplexed actin in the ADP state

The dynamics and polarity of actin filaments are controlled by a conformational change coupled to the hydrolysis of adenosine 5'-triphosphate (ATP) by a mechanism that remains to be elucidated. Actin modified to block polymerization was crystallized in the adenosine 5'-diphosphate (ADP) state, and the structure was solved to 1.54 angstrom resolution. Compared with previous ATP-actin structures from complexes with deoxyribonuclease I, profilin, and gelsolin, monomeric ADP-actin is characterized by a marked conformational change in subdomain 2. The successful crystallization of monomeric actin opens the way to future structure determinations of actin complexes with actin-binding proteins such as myosin.

Tropomyosin-troponin regulation of actin does not involve subdomain 2 motions

Dynamic properties of F-actin structure prompted suggestions (Squire, J. M., and Morris, E. P. (1998) FASEB J. 12, 761-771) that actin subdomain 2 movements play a role in thin-filament regulation. Using fluorescently labeled yeast actin mutants Q41C, Q41C/C374S, and D51C/C374S and azidonitrophenyl putrescine (ANP) Gln(41)-labeled alpha-actin, we monitored regulation-linked changes in subdomain 2. These actins had fully regulated acto-S1 ATPase activities, and emission spectra of regulated Q41C(AEDANS)/C374S and D51C(AEDANS)/C374S filaments did not reveal any calcium-dependent changes. Fluorescence energy transfer in these F-actins mostly occurred from Trp(340) and Trp(356) to 5-(2((acetyl)amino)ethyl)amino-naphthalene-1-sulfonate (AEDANS)-labeled Cys(41) or Cys(51) of adjacent same strand protomers. Our results show that fluorescence energy transfer between these residues is similar in the mostly blocked (-Ca(2+)) and closed (+Ca(2+)) states. Ca(2+) also had no effect on the excimer band in the pyrene-labeled Q41C-regulated actin, indicating virtually no change in the overlap of pyrenes on Cys(41) and Cys(374). ANP quenching of rhodamine phalloidin fluorescence showed that neither Ca(2+) nor S1 binding to regulated alpha-actin affects the phalloidin-probe distance. Taken together, our results indicate that transitions between the blocked, closed, and open regulatory states involve no significant subdomain 2 movements, and, since the cross-linked alpha-actin remains fully regulated, that subdomain 2 motions are not essential for actin regulation.

Functional specificity of actin isoforms

Actin, one of the main proteins of muscle and cytoskeleton, exists as a variety of highly conserved isoforms whose distribution in vertebrates is tissue-specific. Synthesis of specific actin isoforms is accompanied by their subcellular compartmentalization, with both processes being regulated by factors of cell proliferation and differentiation. Actin isoforms cannot substitute for each other, and the high-level synthesis of exogenous actins leads to alterations in cell organization and morphology. This indicates that the highly conserved actins are functionally specialized for the tissues in which they predominate. The first goal of this review is to analyze the data on the polymerizability of actin isoforms to show that cytoskeleton isoactins form less stable polymers than skeletal muscle actin. This difference correlates with the dynamics of actin microfilaments versus the stability of myofibrillar systems. The three-dimensional actin structure as well as progress in the analysis of conformational changes in both the actin monomer and the filament allows us to view the data on the structure and polymerization of isoactins in terms of structure-function relationships within the actin molecule. Most of the amino acid substitutions that distinguish actin isoforms are located apart from actin-actin contact sites in the polymer. We suggest that these substitutions can modulate the ability of actin monomers to form more or less stable polymers by long-range (allosteric) regulation of the contact sites.

Effect of intramolecular cross-linking between glutamine-41 and lysine-50 on actin structure and function

Subdomain 2 of actin is a dynamic segment of the molecule. The cross-linking of Gln-41 on subdomain 2 to Cys-374 on an adjacent monomer in F-actin inhibits actomyosin motility and force generation (Kim et al., 1998; Biochemistry 37, 17,801-17,809). To shed light on this effect, additional modifications of the Gln-41 site on actin were carried out. Both intact G-actin and G-actin cleaved by subtilisin between Met-47 and Gly-48 in the DNase 1 binding loop of subdomain 2 were treated with bacterial transglutaminase. According to the results of Edman degradation, transglutaminase introduced an intramolecular zero-length cross-linking between Gln-41 and Lys-50 in both intact and subtilisin cleaved actins. This cross-linking perturbs G-actin structure as shown by the inhibition of subtilisin and tryptic cleavage in subdomain 2, an allosteric inhibition of tryptic cleavage at the C-terminus and decrease of modification rate of Cys-374. The cross-linking increases while the subtilisin cleavage dramatically decreases the thermostability of F-actin. The Mg- and S1-induced polymerizations of both intact and subtilisin cleaved actins were only slightly influenced by the cross-linking. The activation of S1 ATPase by actin and the sliding speeds of actin filaments in the in vitro motility assays were essentially unchanged by the cross-linking. Thus, although intramolecular cross-linking between Gln-41 and Lys-50 perturbs the structure of the actin monomer, it has only a small effect on actin polymerization and its interaction with myosin. These results suggest that the new cross-linking does not alter the intermonomer interface in F-actin and that changes in actomyosin motility reported for the Gln-41-Cys-374 intrastrand cross-linked actin are not due to decreased flexibility of loop 38-52 but to constrains introduced into the F-actin structure and/or to perturbations at the actin's C-terminus.

Distinct structural changes detected by X-ray fiber diffraction in stabilization of F-actin by lowering pH and increasing ionic strength

Lowering pH or raising salt concentration stabilizes the F-actin structure by increasing the free energy change associated with its polymerization. To understand the F-actin stabilization mechanism, we studied the effect of pH, salt concentration, and cation species on the F-actin structure. X-ray fiber diffraction patterns recorded from highly ordered F-actin sols at high density enabled us to detect minute changes of diffraction intensities and to precisely determine the helical parameters. F-actin in a solution containing 30 mM NaCl at pH 8 was taken as the control. F-actin at pH 8, 30 to 90 mM NaCl or 30 mM KCl showed a helical symmetry of 2.161 subunits per turn of the 1-start helix (12.968 subunits/6 turns). Lowering pH from 8 to 6 or replacing NaCl by LiCl altered the helical symmetry to 2.159 subunits per turn (12.952/6). The diffraction intensity associated with the 27-A meridional layer-line increased as the pH decreased but decreased as the NaCl concentration increased. None of the solvent conditions tested gave rise to significant changes in the pitch of the left-handed 1-start helix (approximately 59.8 A). The present results indicate that the two factors that stabilize F-actin, relatively low pH and high salt concentration, have distinct effects on the F-actin structure. Possible mechanisms will be discussed to understand how F-actin is stabilized under these conditions.

Sodium channel activity in leukemia cells is directly controlled by actin polymerization

The actin cytoskeleton has been shown to be involved in the regulation of sodium-selective channels in non-excitable cells. However, the molecular mechanisms underlying the changes in channel function remain to be defined. In the present work, inside-out patch experiments were employed to elucidate the role of submembranous actin dynamics in the control of sodium channels in human myeloid leukemia K562 cells. We found that the application of cytochalasin D to the cytoplasmic surface of membrane fragments resulted in activation of non-voltage-gated sodium channels of 12 picosiemens conductance. Similar effects could be evoked by addition of the actin-severing protein gelsolin to the bath cytosol-like solution containing 1 microm [Ca(2+)](i). The sodium channel activity induced by disassembly of submembranous microfilaments with cytochalasin D or gelsolin could be abolished by intact actin added to the bath cytosol-like solution in the presence of 1 mm MgCl(2) to induce actin polymerization. In the absence of MgCl(2), addition of intact actin did not abolish the channel activity. Moreover, the sodium currents were unaffected by heat-inactivated actin or by actin whose polymerizability was strongly reduced by cleavage with specific Escherichia coli A2 protease ECP32. Thus, the inhibitory effect of actin on channel activity was observed only under conditions promoting rapid polymerization. Taken together, our data show that sodium channels are directly controlled by dynamic assembly and disassembly of submembranous F-actin.

The oxidation produced by hydrogen peroxide on Ca-ATP-G-actin

We report here that in vitro exposure of monomeric actin to hydrogen peroxide leads to a conversion of 6 of the 16 methionine residues to methionine sulfoxide residues. Although the initial effect of H2O2 on actin is the oxidation of Cys374, we have found that Met44, Met47, Met176, Met190, Met269, and Met355 are the other sites of the oxidative modification. Met44 and Met47 are the methionyl sites first oxidized. The methionine residues that are oxidized are not simply related to their accessibility to the external medium and are found in all four subdomains of actin. The conformations of subdomain 1, a region critical for the functional binding of different actin-binding proteins, and subdomain 2, which plays important roles in the polymerization process and stabilization of the actin filament, are changed upon oxidation. The conformational changes are deduced from the increased exposure of hydrophobic residues, which correlates with methionine sulfoxide formation, from the perturbations in tryptophan fluorescence, and from the decreased susceptibility to limited proteolysis of oxidized actin.

Intermolecular dynamics and function in actin filaments

Structural models of F-actin suggest that three segments in actin, the DNase I binding loop (residues 38-52), the hydrophobic plug (residues 262-274) and the C-terminus, contribute to the formation of an intermolecular interface between three monomers in F-actin. To test these predictions and also to assess the dynamic properties of intermolecular contacts in F-actin, Cys-374 pyrene-labeled skeletal alpha-actin and pyrene-labeled yeast actin mutants, with Gln-41 or Ser-265 replaced with cysteine, were used in fluorescence experiments. Large differences in Cys-374 pyrene fluorescence among copolymers of subtilisin-cleaved (between Met-47 and Gly-48) and uncleaved alpha-actin showed both intra- and intermolecular interactions between the C-terminus and loop 38-52 in F-actin. Excimer band formation due to intermolecular stacking of pyrene probes attached to Cys-41 and Cys-265, and Cys-41 and Cys-374, in mutant yeast F-actin confirmed the proximity of these residues on the paired sites (to within 18 A) in accordance with the models of F-actin structure. The dynamic properties of the intermolecular interface in F-actin formed by loop 38-52, plug 262-274 and the C-terminus may account for the observed cross-linking of these sites with reagents < 18 A. The functional importance of actin filament dynamics was demonstrated by the inhibition of the in vitro motility in the Gln-41-Cys-374 cross-linked actin filaments.

Deactivation of neutrophil NADPH oxidase by actin-depolymerizing agents in a cell-free system

The cell-free activation of human neutrophil NADPH oxidase (O(2)(-)-generating enzyme) is enhanced by actin [Morimatsu, Kawagoshi, Yoshida and Tamura (1997) Biochem. Biophys. Res. Commun. 230, 206--210]. In an attempt to elucidate the mechanism, we examined the effect of actin-depolymerizing agents on the duration of NADPH oxidase in a cell-free system. The addition of DNase I, an F-actin-depolymerizing protein, caused an accelerated deactivation of the oxidase. The deactivation was also facilitated by latrunculin A, a sponge toxin that depolymerizes F-actin. Exogenously added actin prevented the deactivation by DNase I or latrunculin A, whereas EDTA accelerated a dilution-induced deactivation of the oxidase and Mg(2+) ions retarded it. The stability in dilution was found to correlate well with free Mg(2+) concentration. Estimation of F-actin in the system showed that F-actin increased during the oxidase activation and that DNase I or EDTA decreased F-actin content in parallel with the activity. Treatment of the cell-free mixture with a chemical cross-linker prevented the deactivation and F-actin decrease by EDTA. Taken together, these results suggest that actin filaments which grow during the activation of NADPH oxidase prolong the lifetime of the oxidase.

Cross-linking constraints on F-actin structure

The DNase I binding loop (residues 38-52), the hydrophobic plug (residues 262-274), and the C terminus region are among the structural elements of monomeric (G-) actin proposed to form the intermonomer interface in F-actin. To test the proximity and interactions of these elements and to provide constraints on models of F-actin structure, cysteine residues were introduced into yeast actin either at residue 41 or 265. These mutations allowed for specific cross-linking of F-actin between C41 and C265, C265 and C374, and C41 and C265 using dibromobimane and disulfide bond formation. The cross-linked products were visualized on SDS-PAGE and by electron microscopy. Model calculations carried out for the cross-linked F-actins revealed that considerable flexibility or displacement of actin residues is required in the disulfide cross-linked segments to fit these filaments into model F-actin structures. The calculated, cross-linked structures showed a better fit to the Holmes rather than the refined Lorenz model of F-actin. It is predicted on the basis of such calculations that image reconstruction of electron micrographs of disulfide cross-linked C41-C374 F-actin should provide a conclusive test of these two similar models of F-actin structure.

Proteolytic cleavage of actin within the DNase-I-binding loop changes the conformation of F-actin and its sensitivity to myosin binding

Effects of subtilisin cleavage of actin between residues 47 and 48 on the conformation of F-actin and on its changes occurring upon binding of myosin subfragment-1 (S1) were investigated by measuring polarized fluorescence from rhodamine-phalloidin- or 1, 5-IAEDANS-labeled actin filaments reconstructed from intact or subtilisin-cleaved actin in myosin-free muscle fibers (ghost fibers). In separate experiments, polarized fluorescence from 1, 5-IAEDANS-labeled S1 bound to non-labeled actin filaments in ghost fibers was measured. The measurements revealed differences between the filaments of cleaved and intact actin in the orientation of rhodamine probe on the rhodamine-phalloidin-labeled filaments, orientation and mobility of the C-terminus of actin, filament flexibility, and orientation and mobility of the myosin heads bound to F-actin. The changes in the filament flexibility and orientation of the actin-bound fluorophores produced by S1 binding to actin in the absence of ATP were substantially diminished by subtilisin cleavage of actin. The results suggest that loop 38-52 plays an important role, not only in maintaining the F-actin structure, but also in the conformational transitions in actin accompanying the strong binding of the myosin heads that may be essential for the generation of force and movement during actin-myosin interaction.

Actin of Histriculus cavicola: characteristics of the highly divergent hypotrich ciliate actins

A macronuclear gene-sized molecule carrying an actin gene from the hypotrich ciliate, Histriculus cavicola, was characterized. Southern blot analysis using a coding region probe suggested that actin in H. cavicola is encoded by a single gene. A comparison of the promoter regions indicated that the H. cavicola actin gene has a TATA box in the 5' flanking region in a position identical to those in other oxytrich ciliates. The coding sequence of this gene is not interrupted by any introns, and codes for a protein of 375 amino acid residues. This protein shares a high degree of similarity with other oxytrichid actins, and a relatively low similarity with actins from other eukaryotes. Comparative analyses of sequences indicated that most of the amino acid substitutions in hypotrich actins are found in surface loops, while the core structures are well-conserved. The sites that interact with DNase I and several regions involved in actin-actin contact have diverged considerably in hypotrich actins, while nucleotide-binding sites are the best-conserved interaction motif.