Expression of a nonpolymerizable actin mutant in Sf9 cells

Comprehensive Cell Biological Investigation of Cytochalasin B Derivatives with Distinct Activities on the Actin Network

In search of a more comprehensive structure-activity relationship (SAR) regarding the inhibitory effect of cytochalasin B (2) on actin polymerization, a virtual docking of 2 onto monomeric actin was conducted. This led to the identification of potentially important functional groups of 2 (i.e., the NH group of the isoindolone core (N-2) and the hydroxy groups at C-7 and C-20) involved in interactions with the residual amino acids of the binding pocket of actin. Chemical modifications of 2 at positions C-7, N-2, and C-20 led to derivatives 3-6, which were analyzed for their bioactivities. Compounds 3-5 exhibited reduced or no cytotoxicity in murine L929 fibroblasts compared to that of 2. Moreover, short- and long-term treatments of human osteosarcoma cells (U-2OS) with 3-6 affected the actin network to a variable extent, partially accompanied by the induction of multinucleation. Derivatives displaying acetylation at C-20 and N-2 were subjected to slow intracellular conversion to highly cytotoxic 2. Together, this study highlights the importance of the hydroxy group at C-7 and the NH function at N-2 for the potency of 2 on the inhibition of actin polymerization.

The EV71 2A protease occupies the central cleft of SETD3 and disrupts SETD3-actin interaction

SETD3 is an essential host factor for the replication of a variety of enteroviruses that specifically interacts with viral protease 2A. However, the interaction between SETD3 and the 2A protease has not been fully characterized. Here, we use X-ray crystallography and cryo-electron microscopy to determine the structures of SETD3 complexed with the 2A protease of EV71 to 3.5 Å and 3.1 Å resolution, respectively. We find that the 2A protease occupies the V-shaped central cleft of SETD3 through two discrete sites. The relative positions of the two proteins vary in the crystal and cryo-EM structures, showing dynamic binding. A biolayer interferometry assay shows that the EV71 2A protease outcompetes actin for SETD3 binding. We identify key 2A residues involved in SETD3 binding and demonstrate that 2A's ability to bind SETD3 correlates with EV71 production in cells. Coimmunoprecipitation experiments in EV71 infected and 2A expressing cells indicate that 2A interferes with the SETD3-actin complex, and the disruption of this complex reduces enterovirus replication. Together, these results reveal the molecular mechanism underlying the interplay between SETD3, actin, and viral 2A during virus replication.

Dilated cardiomyopathy-associated skeletal muscle actin (ACTA1) mutation R256H disrupts actin structure and function and causes cardiomyocyte hypocontractility

Skeletal muscle actin (ACTA1) mutations are a prevalent cause of skeletal myopathies consistent with ACTA1's high expression in skeletal muscle. Rare de novo mutations in ACTA1 associated with combined cardiac and skeletal myopathies have been reported, but ACTA1 represents only ~20% of the total actin pool in cardiomyocytes, making its role in cardiomyopathy controversial. Here we demonstrate how a mutation in an actin isoform expressed at low levels in cardiomyocytes can cause cardiomyopathy by focusing on a unique ACTA1 mutation, R256H. We previously identified this mutation in multiple family members with dilated cardiomyopathy (DCM), who had reduced systolic function without clinical skeletal myopathy. Using a battery of multiscale biophysical tools, we show that R256H has potent functional effects on ACTA1 function at the molecular scale and in human cardiomyocytes. Importantly, we demonstrate that R256H acts in a dominant manner, where the incorporation of small amounts of mutant protein into thin filaments is sufficient to disrupt molecular contractility, and that this effect is dependent on the presence of troponin and tropomyosin. To understand the structural basis of this change in regulation, we resolved a structure of R256H filaments using Cryo-EM, and we see alterations in actin's structure that have the potential to disrupt interactions with tropomyosin. Finally, we show that ACTA1R256H/+ human induced pluripotent stem cell cardiomyocytes demonstrate reduced contractility and sarcomeric disorganization. Taken together, we demonstrate that R256H has multiple effects on ACTA1 function that are sufficient to cause reduced contractility and establish a likely causative relationship between ACTA1 R256H and clinical cardiomyopathy.

Pointed-end processive elongation of actin filaments by Vibrio effectors VopF and VopL

According to the cellular actin dynamics paradigm, filaments grow at their barbed ends and depolymerize predominantly from their pointed ends to form polar structures and do productive work. We show that actin can elongate at the pointed end when assisted by Vibrio VopF/L toxins, which act as processive polymerases. In cells, processively moving VopF/L speckles are inhibited by factors blocking the pointed but not barbed ends. Multispectral single-molecule imaging confirmed that VopF molecules associate with the pointed end, actively promoting its elongation even in the presence of profilin. Consequently, VopF/L can break the actin cytoskeleton's polarity by compromising actin-based cellular processes. Therefore, actin filament design allows processive growth at both ends, which suggests unforeseen possibilities for cellular actin organization, particularly in specialized cells and compartments.

A solution to the long-standing problem of actin expression and purification

Actin is the most abundant protein in the cytoplasm of eukaryotic cells and interacts with hundreds of proteins to perform essential functions, including cell motility and cytokinesis. Numerous diseases are caused by mutations in actin, but studying the biochemistry of actin mutants is difficult without a reliable method to obtain recombinant actin. Moreover, biochemical studies have typically used tissue-purified α-actin, whereas humans express six isoforms that are nearly identical but perform specialized functions and are difficult to obtain in isolation from natural sources. Here, we describe a solution to the problem of actin expression and purification. We obtain high yields of actin isoforms in human Expi293F cells. Experiments along the multistep purification protocol demonstrate the removal of endogenous actin and the functional integrity of recombinant actin isoforms. Proteomics analysis of endogenous vs. recombinant actin isoforms confirms the presence of native posttranslational modifications, including N-terminal acetylation achieved after affinity-tag removal using the actin-specific enzyme Naa80. The method described facilitates studies of actin under fully native conditions to determine differences among isoforms and the effects of disease-causing mutations that occur in all six isoforms.

Characterization of pseudokinase ILK-mediated actin assembly

Pseudokinase ILK is a key protein involved in regulating focal adhesion assembly and cell-extracellular matrix adhesion. By forming a tight trimeric complex (IPP) with adaptor proteins PINCH and Parvin that both contain an actin binding WH2 motif, IPP can promote the formation of unique actin bundles thereby linking the focal adhesions and actin cytoskeleton and triggering cytoskeleton reassembly and dynamic cell adhesion processes such as cell spreading and migration. This chapter describes detailed characterization of the IPP-mediated actin binding and bundle formation.

Higher-order phosphatase-substrate contacts terminate the integrated stress response

Many regulatory PPP1R subunits join few catalytic PP1c subunits to mediate phosphoserine and phosphothreonine dephosphorylation in metazoans. Regulatory subunits engage the surface of PP1c, locally affecting flexible access of the phosphopeptide to the active site. However, catalytic efficiency of holophosphatases towards their phosphoprotein substrates remains unexplained. Here we present a cryo-EM structure of the tripartite PP1c-PPP1R15A-G-actin holophosphatase that terminates signaling in the mammalian integrated stress response (ISR) in the pre-dephosphorylation complex with its substrate, translation initiation factor 2α (eIF2α). G-actin, whose essential role in eIF2α dephosphorylation is supported crystallographically, biochemically and genetically, aligns the catalytic and regulatory subunits, creating a composite surface that engages the N-terminal domain of eIF2α to position the distant phosphoserine-51 at the active site. Substrate residues that mediate affinity for the holophosphatase also make critical contacts with eIF2α kinases. Thus, a convergent process of higher-order substrate recognition specifies functionally antagonistic phosphorylation and dephosphorylation in the ISR.

Insights into Actin Polymerization and Nucleation Using a Coarse-Grained Model

We studied actin filament polymerization and nucleation with molecular dynamics simulations and a previously established coarse-grained model having each residue represented by a single interaction site located at the C_α atom. We approximate each actin protein as a fully or partially rigid unit to identify the equilibrium structural ensemble of interprotein complexes. Monomers in the F-actin configuration bound to both barbed and pointed ends of a short F-actin filament at the anticipated locations for polymerization. Binding at both ends occurred with similar affinity. Contacts between residues of the incoming subunit and the short filament were consistent with expectation from models based on crystallography, x-ray diffraction, and cryo-electron microscopy. Binding at the barbed and pointed end also occurred at an angle with respect to the polymerizable bound structure, and the angle range depended on the flexibility of the D-loop. Additional barbed end bound states were seen when the incoming subunit was in the G-actin form. Consistent with an activation barrier for pointed end polymerization, G-actin did not bind at an F-actin pointed end. In all cases, binding at the barbed end also occurred in a configuration similar to the antiparallel (lower) dimer. Individual monomers bound each other in a short-pitch helix complex in addition to other configurations, with several of them apparently nonproductive for polymerization. Simulations with multiple monomers in the F-actin form show assembly into filaments as well as transient aggregates at the barbed end. We discuss the implications of these observations on the kinetic pathway of actin filament nucleation and polymerization and possibilities for future improvements of the coarse-grained model.

Engineered SUMO/protease system identifies Pdr6 as a bidirectional nuclear transport receptor

Cleavage of affinity tags by specific proteases can be exploited for highly selective affinity chromatography. The SUMO/SENP1 system is the most efficient for such application but fails in eukaryotic expression because it cross-reacts with endogenous proteases. Using a novel selection system, we have evolved the SUMOEu/SENP1Eu pair to orthogonality with the yeast and animal enzymes. SUMOEu fusions therefore remain stable in eukaryotic cells. Likewise, overexpressing a SENP1Eu protease is nontoxic in yeast. We have used the SUMOEu system in an affinity-capture-proteolytic-release approach to identify interactors of the yeast importin Pdr6/Kap122. This revealed not only further nuclear import substrates such as Ubc9, but also Pil1, Lsp1, eIF5A, and eEF2 as RanGTP-dependent binders and thus as export cargoes. We confirmed that Pdr6 functions as an exportin in vivo and depletes eIF5A and eEF2 from cell nuclei. Thus, Pdr6 is a bidirectional nuclear transport receptor (i.e., a biportin) that shuttles distinct sets of cargoes in opposite directions.

Differential regulation of actin-activated nucleotidyl cyclase virulence factors by filamentous and globular actin

Several bacterial pathogens produce nucleotidyl cyclase toxins to manipulate eukaryotic host cells. Inside host cells they are activated by endogenous cofactors to produce high levels of cyclic nucleotides (cNMPs). The ExoY toxin from Pseudomonas aeruginosa (PaExoY) and the ExoY-like module (VnExoY) found in the MARTX (Multifunctional-Autoprocessing Repeats-in-ToXin) toxin of Vibrio nigripulchritudo share modest sequence similarity (~38%) but were both recently shown to be activated by actin after their delivery to the eukaryotic host cell. Here, we further characterized the ExoY-like cyclase of V. nigripulchritudo. We show that, in contrast to PaExoY that requires polymerized actin (F-actin) for maximum activation, VnExoY is selectively activated by monomeric actin (G-actin). These two enzymes also display different nucleotide substrate and divalent cation specificities. In vitro in presence of the cation Mg2+, the F-actin activated PaExoY exhibits a promiscuous nucleotidyl cyclase activity with the substrate preference GTP>ATP≥UTP>CTP, while the G-actin activated VnExoY shows a strong preference for ATP as substrate, as it is the case for the well-known calmodulin-activated adenylate cyclase toxins from Bordetella pertussis or Bacillus anthracis. These results suggest that the actin-activated nucleotidyl cyclase virulence factors despite sharing a common activator may actually display a greater variability of biological effects in infected cells than initially anticipated.

Non-catalytic signaling by pseudokinase ILK for regulating cell adhesion

Dynamic communication between integrin-containing complexes (focal adhesions, FAs) and actin filaments is critical for regulating cell adhesion. Pseudokinase ILK plays a key role in this process but the underlying mechanism remains highly elusive. Here we show that by recruiting FA adaptors PINCH and Parvin into a heterotrimeric complex (IPP), ILK triggers F-actin filament bundling - a process known to generate force/mechanical signal to promote cytoskeleton reassembly and dynamic cell adhesion. Structural, biochemical, and functional analyses revealed that the F-actin bundling is orchestrated by two previously unrecognized WASP-Homology-2 actin binding motifs within IPP, one from PINCH and the other from Parvin. Strikingly, this process is also sensitized to Mg-ATP bound to the pseudoactive site of ILK and its dysregulation severely impairs stress fibers formation, cell spreading, and migration. These data identify a crucial mechanism for ILK, highlighting its uniqueness as a pseudokinase to transduce non-catalytic signal and regulate cell adhesion.

The Molecular Mechanisms of Mutations in Actin and Myosin that Cause Inherited Myopathy

The discovery that mutations in myosin and actin genes, together with mutations in the other components of the muscle sarcomere, are responsible for a range of inherited muscle diseases (myopathies) has revolutionized the study of muscle, converting it from a subject of basic science to a relevant subject for clinical study and has been responsible for a great increase of interest in muscle studies. Myopathies are linked to mutations in five of the myosin heavy chain genes, three of the myosin light chain genes, and three of the actin genes. This review aims to determine to what extent we can explain disease phenotype from the mutant genotype. To optimise our chances of finding the right mechanism we must study a myopathy where there are a large number of different mutations that cause a common phenotype and so are likely to have a common mechanism: a corollary to this criterion is that if any mutation causes the disease phenotype but does not correspond to the proposed mechanism, then the whole mechanism is suspect. Using these criteria, we consider two cases where plausible genotype-phenotype mechanisms have been proposed: the actin “A-triad” and the myosin “mesa/IHD” models.

Mutations in actin used for structural studies partially disrupt β-thymosin/WH2 domains interaction

Understanding the structural basis of actin cytoskeleton remodeling requires stabilization of actin monomers, oligomers, and filaments in complex with partner proteins, using various biochemical strategies. Here, we report a dramatic destabilization of the dynamic interaction with a model β-thymosin/WH2 domain induced by mutations in actin. This result underlines that mutant actins should be used with prudence to characterize interactions with intrinsically disordered partners as destabilization of dynamic interactions, although identifiable by NMR, may be invisible to other structural techniques. It also highlights how both β-thymosin/WH2 domains and actin tune local structure and dynamics in regulatory processes involving intrinsically disordered domains.

Utilization of paramagnetic relaxation enhancements for structural analysis of actin-binding proteins in complex with actin

Actin cytoskeleton dynamics are controlled by various actin binding proteins (ABPs) that modulate the polymerization of the monomeric G-actin and the depolymerization of filamentous F-actin. Although revealing the structures of the actin/ABP complexes is crucial to understand how the ABPs regulate actin dynamics, the X-ray crystallography and cryoEM methods are inadequate to apply for the ABPs that interact with G- or F-actin with lower affinity or multiple binding modes. In this study, we aimed to establish the alternative method to build a structural model of G-actin/ABP complexes, utilizing the paramagnetic relaxation enhancement (PRE) experiments. Thymosin β4 (Tβ4) was used as a test case for validation, since its structure in complex with G-actin was reported recently. Recombinantly expressed G-actin, containing a cysteine mutation, was conjugated with a nitroxyl spin label at the specific site. Based on the intensity ratio of the ¹H-¹⁵N HSQC spectra of Tβ4 in the complex with G-actin in the paramagnetic and diamagnetic states, the distances between the amide groups of Tβ4 and the spin label of G-actin were estimated. Using the PRE-derived distance constraints, we were able to compute a well-converged docking structure of the G-actin/Tβ4 complex that shows great accordance with the reference structure.

Kindlin-2 directly binds actin and regulates integrin outside-in signaling

Bledzka et al. show that kindlin-2 binds actin via its F0 domain, and mutation of this site diminishes cell spreading, revealing a new mechanism by which kindlin-2 regulates cellular responses.

Reduced levels of kindlin-2 (K2) in endothelial cells derived from K2^+/− mice or C2C12 myoblastoid cells treated with K2 siRNA showed disorganization of their actin cytoskeleton and decreased spreading. These marked changes led us to examine direct binding between K2 and actin. Purified K2 interacts with F-actin in cosedimentation and surface plasmon resonance analyses and induces actin aggregation. We further find that the F0 domain of K2 binds actin. A mutation, LK⁴⁷/AA, within a predicted actin binding site (ABS) of F0 diminishes its interaction with actin by approximately fivefold. Wild-type K2 and K2 bearing the LK⁴⁷/AA mutation were equivalent in their ability to coactivate integrin αIIbβ3 in a CHO cell system when coexpressed with talin. However, K2-LK⁴⁷/AA exhibited a diminished ability to support cell spreading and actin organization compared with wild-type K2. The presence of an ABS in F0 of K2 that influences outside-in signaling across integrins establishes a new foundation for considering how kindlins might regulate cellular responses.

On the acquisition and analysis of microscale thermophoresis data

A comprehensive understanding of the molecular mechanisms underpinning cellular functions is dependent on a detailed characterization of the energetics of macromolecular binding, often quantified by the equilibrium dissociation constant, KD. While many biophysical methods may be used to obtain KD, the focus of this report is a relatively new method called microscale thermophoresis (MST). In an MST experiment, a capillary tube filled with a solution containing a dye-labeled solute is illuminated with an infrared laser, rapidly creating a temperature gradient. Molecules will migrate along this gradient, causing changes in the observed fluorescence. Because the net migration of the labeled molecules will depend on their liganded state, a binding curve as a function of ligand concentration can be constructed from MST data and analyzed to determine KD. Herein, simulations demonstrate the limits of KD that can be measured in current instrumentation. They also show that binding kinetics is a major concern in planning and executing MST experiments. Additionally, studies of two protein-protein interactions illustrate challenges encountered in acquiring and analyzing MST data. Combined, these approaches indicate a set of best practices for performing and analyzing MST experiments. Software for rigorous data analysis is also introduced.

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.

Three-color single molecule imaging shows WASP detachment from Arp2/3 complex triggers actin filament branch formation

During cell locomotion and endocytosis, membrane-tethered WASP proteins stimulate actin filament nucleation by the Arp2/3 complex. This process generates highly branched arrays of filaments that grow toward the membrane to which they are tethered, a conflict that seemingly would restrict filament growth. Using three-color single-molecule imaging in vitro we revealed how the dynamic associations of Arp2/3 complex with mother filament and WASP are temporally coordinated with initiation of daughter filament growth. We found that WASP proteins dissociated from filament-bound Arp2/3 complex prior to new filament growth. Further, mutations that accelerated release of WASP from filament-bound Arp2/3 complex proportionally accelerated branch formation. These data suggest that while WASP promotes formation of pre-nucleation complexes, filament growth cannot occur until it is triggered by WASP release. This provides a mechanism by which membrane-bound WASP proteins can stimulate network growth without restraining it. DOI:http://dx.doi.org/10.7554/eLife.01008.001.

Structural basis of actin filament nucleation by tandem W domains

Spontaneous nucleation of actin is very inefficient in cells. To overcome this barrier, cells have evolved a set of actin filament nucleators to promote rapid nucleation and polymerization in response to specific stimuli. However, the molecular mechanism of actin nucleation remains poorly understood. This is hindered largely by the fact that actin nucleus, once formed, rapidly polymerizes into filament, thus making it impossible to capture stable multisubunit actin nucleus. Here, we report an effective double-mutant strategy to stabilize actin nucleus by preventing further polymerization. Employing this strategy, we solved the crystal structure of AMPPNP-actin in complex with the first two tandem W domains of Cordon-bleu (Cobl), a potent actin filament nucleator. Further sequence comparison and functional studies suggest that the nucleation mechanism of Cobl is probably shared by the p53 cofactor JMY, but not Spire. Moreover, the double-mutant strategy opens the way for atomic mechanistic study of actin nucleation and polymerization.

Functional characterization of the human α-cardiac actin mutations Y166C and M305L involved in hypertrophic cardiomyopathy

Inherited cardiomyopathies are caused by point mutations in sarcomeric gene products, including α-cardiac muscle actin (ACTC1). We examined the biochemical and cell biological properties of the α-cardiac actin mutations Y166C and M305L identified in hypertrophic cardiomyopathy (HCM). Untagged wild-type (WT) cardiac actin, and the Y166C and M305L mutants were expressed by the baculovirus/Sf9-cell system and affinity purified by immobilized gelsolin G4-6. Their correct folding was verified by a number of assays. The mutant actins also displayed a disturbed intrinsic ATPase activity and an altered polymerization behavior in the presence of tropomyosin, gelsolin, and Arp2/3 complex. Both mutants stimulated the cardiac β-myosin ATPase to only 50 % of WT cardiac F-actin. Copolymers of WT and increasing amounts of the mutant actins led to a reduced stimulation of the myosin ATPase. Transfection of established cell lines revealed incorporation of EGFP- and hemagglutinin (HA)-tagged WT and both mutant actins into cytoplasmic stress fibers. Adenoviral vectors of HA-tagged WT and Y166C actin were successfully used to infect adult and neonatal rat cardiomyocytes (NRCs). The expressed HA-tagged actins were incorporated into the minus-ends of NRC thin filaments, demonstrating the ability to form hybrid thin filaments with endogenous actin. In NRCs, the Y166C mutant led after 72 h to a shortening of the sarcomere length when compared to NRCs infected with WT actin. Thus our data demonstrate that a mutant actin can be integrated into cardiomyocyte thin filaments and by its reduced mode of myosin interaction might be the basis for the initiation of HCM.

Direct redox regulation of F-actin assembly and disassembly by Mical

Different types of cell behavior, including growth, motility, and navigation, require actin proteins to assemble into filaments. Here, we describe a biochemical process that was able to disassemble actin filaments and limit their reassembly. Actin was a specific substrate of the multidomain oxidation-reduction enzyme, Mical, a poorly understood actin disassembly factor that directly responds to Semaphorin/Plexin extracellular repulsive cues. Actin filament subunits were directly modified by Mical on their conserved pointed-end, which is critical for filament assembly. Mical posttranslationally oxidized the methionine 44 residue within the D-loop of actin, simultaneously severing filaments and decreasing polymerization. This mechanism underlying actin cytoskeletal collapse may have broad physiological and pathological ramifications.

Biochemical and cell biological analysis of actin in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has long been a useful model organism for muscle research. Its body wall muscle is obliquely striated muscle and exhibits structural similarities with vertebrate striated muscle. Actin is the core component of the muscle thin filaments, which are highly ordered in sarcomeric structures in striated muscle. Genetic studies have identified genes that regulate proper organization and function of actin filaments in C. elegans muscle, and sequence of the worm genome has revealed a number of conserved candidate genes that may regulate actin. To precisely understand the functions of actin-binding proteins, such genetic and genomic studies need to be complemented by biochemical characterization of these actin-binding proteins in vitro. This article describes methods for purification and biochemical characterization of actin from C. elegans. Although rabbit muscle actin is commonly used to characterize actin-binding proteins from many eukaryotic organisms, we detect several quantitative differences between C. elegans actin and rabbit muscle actin, highlighting that use of actin from an appropriate source is important in some cases. Additionally, we describe probes for cell biological analysis of actin in C. elegans.

Design, overexpression, and purification of polymerization-blocked yeast αβ-tubulin mutants

Microtubule dynamics play essential roles in intracellular organization and cell division. They result from structural and biochemical properties of αβ-tubulin heterodimers and how these polymerizing subunits interact with themselves and with regulatory proteins. A broad understanding of the underlying mechanisms has been established, but fundamental questions remain unresolved. The lack of routine access to recombinant αβ-tubulin represents an obstacle to deeper insight into αβ-tubulin structure, biochemistry, and recognition. Indeed, the widespread reliance on animal brain αβ-tubulin means that very few in vitro studies have taken advantage of powerful and ordinarily routine techniques like site-directed mutagenesis. Here we report new methods for purifying wild-type or mutant yeast αβ-tubulin from inducibly overexpressing strains of Saccharomyces cerevisiae. Inducible overexpression is an improvement over existing approaches that rely on constitutive expression: it provides higher yields while also allowing otherwise lethal mutants to be purified. We also designed and purified polymerization-blocked αβ-tubulin mutants. These "blocked" forms of αβ-tubulin give a dominant lethal phenotype when expressed in cells; they cannot form microtubules in vitro and when present in mixtures inhibit the polymerization of wild-type αβ-tubulin. The effects of blocking mutations are very specific, because purified mutants exhibit normal hydrodynamic properties, bind GTP, and interact with a tubulin-binding domain. The ability to overexpress and purify wild-type αβ-tubulin, or mutants like the ones we report here, creates new opportunities for structural studies of αβ-tubulin and its complexes with regulatory proteins, and for biochemical and functional studies of microtubule dynamics and its regulation.

Phosducin and monomeric β-actin have common epitope recognized by anti-phosducin antibodies

Phosducin family proteins are regulators of cytoplasmic processes. The main function ascribed to phosducin is the binding and sequestration of the β subunit of heterotrimeric G proteins. Phosducin-like protein 1, longer than phosducin by 37 amino-acids, is involved in chaperoning of newly synthesized proteins. β-Actin, a component of the cytoskeleton, participates in cell movement. There is no apparent evolutionary relationship between phosducin and β-actin nor structure similarity. Nevertheless we obtained the polyclonal antibodies named ap33, originally directed against a phosducin-derived peptide (SQSLEEDFEGQATHTGPK), that also recognized β-actin. The epitope on the β-actin molecule was characterized. It is a conformational epitope grouping some of the L-D-F-E-Q-A-T-K amino-acids found in the peptide originally used to obtain the antibodies. The main part of the epitope is localized on the actin-actin interface of polymerized actin, so it is accessible only on monomeric actin. The existence of a common epitope on the molecules of phosducin and β-actin may reflect a topological similarity of a small region of their surfaces.

ADP-ribosylation of cross-linked actin generates barbed-end polymerization-deficient F-actin oligomers

Actin filament subunit interfaces are required for the proper interaction between filamentous actin (F-actin) and actin binding proteins (ABPs). The production of small F-actin complexes mimicking such interfaces would be a significant advance toward understanding the atomic interactions between F-actin and its many binding partners. We produced actin lateral dimers and trimers derived from F-actin and rendered polymerization-deficient by ADP-ribosylation of Arg-177. The degree of modification resulted in a moderate reduction in thermal stability. Calculated hydrodynamic radii were comparable to theoretical values derived from recent models of F-actin. Filament capping capabilities were retained and yielded pointed-end dissociation constants similar those of wild-type actin, suggesting native or near-native interfaces on the oligomers. Changes in DNase I binding affinity under low and high ionic strength suggested a high degree of conformational flexibility in the dimer and trimer. Polymer nucleation activity was lost upon ADP-ribosylation and rescued upon enzyme-mediated deADP-ribosylation, or upon binding to gelsolin, suggesting that interactions with actin binding proteins can overcome the inhibiting activities of ADP-ribosylation. The combined strategy of chemical cross-linking and ADP-ribosylation provides a minimalistic and reversible approach to engineering polymerization-deficient F-actin oligomers that are able to act as F-actin binding protein scaffolds.

Conformational dynamics of actin: effectors and implications for biological function

Actin is a protein abundant in many cell types. Decades of investigations have provided evidence that it has many functions in living cells. The diverse morphology and dynamics of actin structures adapted to versatile cellular functions is established by a large repertoire of actin-binding proteins. The proper interactions with these proteins assume effective molecular adaptations from actin, in which its conformational transitions play essential role. This review attempts to summarise our current knowledge regarding the coupling between the conformational states of actin and its biological function.

Actin polymerization is controlled by residue size at position 204

Previous work has shown that purified double mutant A204C/C374A yeast actin is polymerization-deficient in vitro under physiological concentrations. To understand the importance of the 204 residue in subdomain 4, a series of actin proteins with a single mutation at this position were created with Cys-374 retained. Only yeast expressing A204G-, A204S-, or A204C-actin were viable. The A204G and A204S strains were sensitive to cold temperature and hyperosmolarity, whereas the A204C strain showed more profound effects on growth under these conditions. Cells expressing A204C-actin exhibited anomalies previously observed for A204C/C374A actin, including abnormal actin structures. A204G- and A204S-actin proteins had 12- and 13-fold increased critical concentrations, respectively, relative to wild-type. Only at very high concentrations could A204C actin polymerize when ATP was bound; when hydrolyzed, the ADP-containing A204C filaments depolymerized, demonstrating a profound difference in critical concentration between ATP and ADP states with A204C actin. A correlation between size of the residue substituted at position 204 and energy minimization of actin filament models was observed. We propose that the region surrounding residue 204 is involved in interactions that change depending on the phosphorylation state of the bound nucleotide that might reflect different conformations of F-actin subunits.

Cysteine engineering of actin self-assembly interfaces

The Holmes model of filamentous actin (F-actin) and recent structural studies suggest specific atomic interactions between F-actin subunits. We tested these interactions through a cysteine-engineering approach with the goal of inhibiting filament formation by introducing chemical groups at sites important for polymerization. We substituted surface amino acids on the actin molecule with cysteine residues and tested the effect of producing these actin mutant proteins in a yeast expression system. The intrinsic folding and polymerization characteristics of the cysteine-engineered actin proteins were measured. The effect of chemical modification of the introduced cysteine residues on the polymerization of the actin mutant proteins was also examined. Modification of cysteine residues with large hydrophobic reagents resulted in polymerization inhibition. We examined the finding that the D288C actin protein does not polymerize under oxidizing conditions and forms protein aggregates when magnesium and EGTA are present. Chemical crosslinking experiments revealed the presence of a lower dimer when only D288C actin was present. When both D288C and A204C actin were present, crosslinking experiments support the proximity of Asp288 on the barbed end of one subunit to Ala204 on the pointed end of a neighboring subunit in the Holmes model of F-actin.

Purification of cytoplasmic actin by affinity chromatography using the C-terminal half of gelsolin

A new rapid method of the cytoplasmic actin purification, not requiring the use of denaturants or high concentrations of salt, was developed, based on the affinity chromatography using the C-terminal half of gelsolin (G4-6), an actin filament severing and capping protein. When G4-6 expressed in Escherichia coli was added to the lysate of HeLa cells or insect cells infected with a baculovirus encoding the beta-actin gene, in the presence of Ca(2+) and incubated overnight at 4 degrees C, actin and G4-6 were both detected in the supernatant. Following the addition of Ni-Sepharose beads to the mixture, only actin was eluted from the Ni-NTA column by a Ca(2+)-chelating solution. The functionality of the cytoplasmic actins thus purified was confirmed by measuring the rate of actin polymerization, the gliding velocity of actin filaments in an in vitro motility assay on myosin V-HMM, and the ability to activate the ATPase activity of myosin V-S1.

The nature of the globular- to fibrous-actin transition

Actin plays crucial parts in cell motility through a dynamic process driven by polymerization and depolymerization, that is, the globular (G) to fibrous (F) actin transition. Although our knowledge about the actin-based cellular functions and the molecules that regulate the G- to F-actin transition is growing, the structural aspects of the transition remain enigmatic. We created a model of F-actin using X-ray fibre diffraction intensities obtained from well oriented sols of rabbit skeletal muscle F-actin to 3.3 A in the radial direction and 5.6 A along the equator. Here we show that the G- to F-actin conformational transition is a simple relative rotation of the two major domains by about 20 degrees. As a result of the domain rotation, the actin molecule in the filament is flat. The flat form is essential for the formation of stable, helical F-actin. Our F-actin structure model provides the basis for understanding actin polymerization as well as its molecular interactions with actin-binding proteins.

Recombinant alpha-actin for specific fluorescent labeling

Until recently, actin was thought to act merely as a passive track for its motility partner, myosin, during actomyosin interactions. Yet a recent report having observed dynamical conformational changes in labeled skeletal muscle alpha-actin suggests that actin has a more active role. Because the labeling technique was still immature, however, conclusions regarding the significance of the different conformations are difficult to make. Here, we describe the preparation of fully active alpha-actin obtained from a baculovirus expression system. We developed alpha-actin recombinants, of which subdomains 1 and 2 have specific sites for fluorescent probes. This specific labeling technique offers to significantly expand the information acquired from actin studies.

Crystal structures of monomeric actin bound to cytochalasin D

The fungal toxin cytochalasin D (CD) interferes with the normal dynamics of the actin cytoskeleton by binding to the barbed end of actin filaments. Despite its widespread use as a tool for studying actin-mediated processes, the exact location and nature of its binding to actin have not been previously determined. Here we describe two crystal structures of an expressed monomeric actin in complex with CD: one obtained by soaking preformed actin crystals with CD, and the other obtained by cocrystallization. The binding site for CD, in the hydrophobic cleft between actin subdomains 1 and 3, is the same in the two structures. Polar and hydrophobic contacts play equally important roles in CD binding, and six hydrogen bonds stabilize the actin-CD complex. Many unrelated actin-binding proteins and marine toxins target this cleft and the hydrophobic pocket at the front end of the cleft (viewing actin with subdomain 2 in the upper right corner). CD differs in that it binds to the back half of the cleft. The ability of CD to induce actin dimer formation and actin-catalyzed ATP hydrolysis may be related to its unique binding site and the necessity to fit its bulky macrocycle into this cleft. Contacts with residues lining this cleft appear to be crucial to capping and/or severing. The cocrystallized actin-CD structure also revealed changes in actin conformation. An approximately 6 degrees rotation of the smaller actin domain (subdomains 1 and 2) with respect to the larger domain (subdomains 3 and 4) results in small changes in crystal packing that allow the D-loop to adopt an extended loop structure instead of being disordered, as it is in most crystal structures of actin. We speculate that these changes represent a potential conformation that the actin monomer can adopt on the pathway to polymerization or in the filament.

Effect of short-range forces on the length distribution of fibrous cytoskeletal proteins

The length distribution of cytoskeletal filaments is an important physical parameter, which can modulate physiological cell functions. In both eukaryotic and prokaryotic cells various biological cytoskeletal polymers form supramolecular structures due to short-range forces induced mainly by molecular crowding or cross linking proteins, but their in vivo length distribution remains difficult to measure. In general, based on experimental evidence and mathematical modeling of actin filaments in aqueous solutions, the steady state length distribution of fibrous proteins is believed to be exponential. We performed in vitro TIRF- and electron-microscopy to demonstrate that in the presence of short-range forces, which are an integral part of any living cell, the steady state length distributions of the eukaryotic cytoskeletal biopolymer actin, its prokaryotic homolog ParM and microtubule homolog FtsZ deviate from the classical exponential and are either double-exponential or Gaussian, as recent theoretical modeling predicts. Double exponential or Gaussian distributions opposed to exponential can change for example the visco-elastic properties of actin networks within the cell, influence cell motility by decreasing the amount of free ends at the leading edge of the cell or effect the assembly of FtsZ into the bacterial Z-ring thus modulating membrane constriction.

Quantitative immunochemical evaluation of fish metallothionein upon exposure to cadmium

Efficient implementation of an environmental biomarker requires multi-annual comparability over a wide geographical range. The present study improved the comparability of a quantitative competitive metallothionein (MT) enzyme-linked-immuno-sorbent-assay (ELISA) in the sentinel fish Lithognathus mormyrus by introducing to the assay recombinant MT and beta-actin standards. Commercial antibodies for cod MT and mammalian actin were implemented. In addition, a sensitive anti L. mormyrus MT antibody was produced, adequate only for solid phase immunochemical assays. Cadmium was applied to the fish through injection and feeding to serve as a testing platform of the ELISA. The results demonstrated high potential protective capacity of the liver against toxic levels of transition metals through increasing MT levels. MT transcript levels were evaluated also from fish sampled at polluted and relatively clean natural sites, indicating applicability of MT as biomarker of exposure to a multi-factorial pollution, in comparison to its low revealed sensitivity to controlled cadmium exposure.

Dual roles of Gln137 of actin revealed by recombinant human cardiac muscle alpha-actin mutants

The actin filament is quite dynamic in the cell. To determine the relationship between the structure and the dynamic properties of the actin filament, experiments using actin mutants are indispensable. We focused on Gln(137) to understand the relationships between two activities: the conformational changes relevant to the G- to F-actin transition and the activation of actin ATPase upon actin polymerization. To elucidate the function of Gln(137) in these activities, we characterized Gln(137) mutants of human cardiac muscle alpha-actin. Although all of the single mutants, Q137E, Q137K, Q137P, and Q137A, as well as the wild type were expressed by a baculovirus-based system, only Q137A and the wild type were purified to high homogeneity. The CD spectrum of Q137A was similar to that of the wild type, and Q137A showed the typical morphology of negatively stained Q137A F-actin images. However, Q137A had an extremely low critical concentration for polymerization. Furthermore, we found that Q137A polymerized 4-fold faster, cleaved the gamma-phosphate group of bound ATP 4-fold slower, and depolymerized 5-fold slower, as compared with the wild-type rates. These results suggest that Gln(137) plays dual roles in actin polymerization, in both the conformational transition of the actin molecule and the mechanism of ATP hydrolysis.

Functional effects of nemaline myopathy mutations on human skeletal alpha-actin

Mutations in human alpha-skeletal actin have been implicated in causing congenital nemaline myopathy, a disease characterized histopathologically by nemaline bodies in skeletal muscle and manifested in the patient as skeletal muscle weakness. Here we investigate the functional effects of three severe nemaline myopathy mutations (V43F, A138P, and R183G) in human alpha-skeletal actin. Wild-type and mutant actins were expressed and purified from the baculovirus/insect cell expression system. The mutations are located in different subdomains of actin; Val-43 is located in a flexible loop of subdomain 2, Ala-138 is near a hydrophobic cleft in the "hinge" region between subdomains 1 and 3, and Arg-183 is near the nucleotide-binding site. None of the three mutations affected the folding of the actin monomer, the velocity at which skeletal myosin moves actin in an in vitro motility assay, or the relative average isometric force supported by F-actin. Defects in fundamental actomyosin interactions are, therefore, unlikely to account for the muscle weakness observed in affected patients. There were, however, significant changes observed in the polymerization kinetics of V43F and A138P and in the rate of nucleotide release for V43F. No detectable defect was found for R183G. If these subtle changes in polymerization observed in vitro are amplified in the context of the sarcomere, it could in principle be one of the primary insults that triggers the development of nemaline myopathy.

Single molecule polymerization, annealing and bundling dynamics of SipA induced actin filaments

Salmonella bacteria cause more than three million deaths each year. They hijack cells and inject among other proteins SipA via a "molecular syringe" into the cell, which can tether actin subunits in opposing strands to form mechanically stabilized filaments which rapidly reshape the cells surface into extended ruffles, leading to bacterial internalization. Exactly how these ruffles form at a single filament level remains unknown. Our real time total internal fluorescence microscopy observations show that both bidirectional elongation of actin by SipA as well as end-to-end annealing of SipA-actin filaments are rapid processes. Complementary electron microscopy investigations demonstrate that crowding agents in vitro readily induce stiff bundles of SipA-actin filaments. Taken together these three effects, rapid SipA induced actin polymerization, filament annealing and bundle formation due to molecular crowding can explain how Salmonella invades cells at molecular level.

Overexpression of cardiac actin with baculovirus is promoter dependent

The influence of the promoter and an N-terminal hexahistidine tag on human cardiac actin (ACTC) expression and function was investigated using four baculovirus constructs. It was found that both non-tagged ACTC and hisACTC expression from the p10 promoter was higher than from the polh promoter. Characterization showed that an N-terminal hexahistidine tag has a negative effect on ACTC. Recombinant ACTC inhibits DNase-I and binds myosin S1, indicative of proper folding. Our data support the hypothesis that the actin protein down-regulates the polh promoter.

The structure of bacterial ParM filaments

Bacterial ParM is a homolog of eukaryotic actin and is involved in moving plasmids so that they segregate properly during cell division. Using cryo-EM and three-dimensional reconstruction, we show that ParM filaments have a different structure from F-actin, with very different subunit-subunit interfaces. These interfaces result in the helical handedness of the ParM filament being opposite to that of F-actin. Like F-actin, ParM filaments have a variable twist, and we show that this involves domain-domain rotations within the ParM subunit. The present results yield new insights into polymorphisms within F-actin, as well as the evolution of polymer families.

Yeast actin with a subdomain 4 mutation (A204C) exhibits increased pointed-end critical concentration

Characterizing mutants of actin that do not polymerize will advance our understanding of the mechanism of actin polymerization and will be invaluable for the production of short F-actin structures for structural studies. To circumvent the problem of expressing dominant lethal nonpolymerizing actin in yeast, we adopted a cysteine engineering strategy. Here we report the characterization of a mutant of yeast actin, AC-actin, possessing a single pointed-end mutation, A204C. Expression of this mutant in yeast results in actin-polymerization-deficient phenotypes. When copolymerized with wild-type actin, ATP-AC-actin is incorporated into filaments. ADP-AC-actin participates in the nucleation and elongation of wild-type filaments only at very high concentrations. At low concentrations, ADP-AC-actin appears to participate only in the nucleation of wild-type filaments, suggesting that Ala-204 is involved in modulating the critical concentration of the pointed end of actin.

A novel system for expressing toxic actin mutants in Dictyostelium and purification and characterization of a dominant lethal yeast actin mutant

We have developed a novel system for expressing recombinant actin in Dictyostelium. In this system, the C terminus of actin is fused to thymosin beta via a glycine-based linker. The fusion protein is purified using a His tag attached to the thymosin beta moiety and then cleaved by chymotrypsin immediately after the native final residue of actin to yield intact actin. Wild-type actin prepared in this way was functionally normal in terms of its polymerization kinetics and muscle myosin-mediated motility. We expected that this system would be particularly useful for expressing toxic actin mutants, because the actin moiety of the fusion protein is unlikely to interact with the actin cytoskeleton of the host cells. We therefore chose to express the E206A/R207A/E208A mutant, which appears to be dominant lethal in yeast, as a model case of a toxic actin mutant that is difficult to express. We found that the E206A/R207A/E208A mutant could be expressed and purified with a yield comparable to the wild-type molecule (3-4 mg/20 g cells), even though green fluorescent protein-fused actin carrying the E206A/R207A/E208A mutation was expressed at a much lower level than wild-type actin. Purified E206A/R207A/E208A actin did not polymerize, even in the presence of muscle actin; however, it accelerated polymerization of muscle actin and inhibited the nucleating and severing activities of gelsolin. Given that the location of the substituted residues is near the pointed end face of the mutant, we suggest that E206A/R207A/E208A actin behaves like a weak pointed end-capping protein that perturbs the actin cytoskeleton of the host cells.

Biochemical implications of a three-dimensional model of monomeric actin bound to magnesium-chelated ATP

Actin structure is of intense interest in biology due to its importance in cell function and motility mediated by the spatial and temporal regulation of actin monomer-filament interconversions in a wide range of developmental and disease states. Despite this interest, the structure of many functionally important actin forms has eluded high-resolution analysis. Due to the propensity of actin monomers to assemble into filaments structural analysis of Mg-bound actin monomers has proven difficult, whereas high-resolution structures of actin with a diverse array of ligands that preclude polymerization have been quite successful. In this work, we provide a high-resolution structural model of the Mg-ATP-actin monomer using a combination of computational methods and experimental footprinting data that we have previously published. The key conclusion of this study is that the structure of the nucleotide binding cleft defined by subdomains 2 and 4 is essentially closed, with specific contacts between two subdomains predicted by the data.

Crystal structures of expressed non-polymerizable monomeric actin in the ADP and ATP states

Actin filament growth and disassembly, as well as affinity for actin-binding proteins, is mediated by the nucleotide-bound state of the component actin monomers. The structural differences between ATP-actin and ADP-actin, however, remain controversial. We expressed a cytoplasmic actin in Sf9 cells, which was rendered non-polymerizable by virtue of two point mutations in subdomain 4 (A204E/P243K). This homogeneous monomer, called AP-actin, was crystallized in the absence of toxins, binding proteins, or chemical modification, with ATP or ADP at the active site. The two surface mutations do not perturb the structure. Significant differences between the two states are confined to the active site region and sensor loop. The active site cleft remains closed in both states. Minor structural shifts propagate from the active site toward subdomain 2, but dissipate before reaching the DNase binding loop (D-loop), which remains disordered in both the ADP and ATP states. This result contrasts with previous structures of actin made monomeric by modification with tetramethylrhodamine, which show formation of an alpha-helix at the distal end of the D-loop in the ADP-bound but not the ATP-bound form (Otterbein, L. R., Graceffa, P., and Dominguez, R. (2001) Science 293, 708-711). Our reanalysis of the TMR-modified actin structures suggests that the nucleotide-dependent formation of the D-loop helix may result from signal propagation through crystal packing interactions. Whereas the observed nucleotide-dependent changes in the structure present significantly different surfaces on the exterior of the actin monomer, current models of the actin filament lack any actin-actin interactions that involve the region of these key structural changes.

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.

Functional Consequences of a Mutation in an Expressed Human α-Cardiac Actin at a Site Implicated in Familial Hypertrophic Cardiomyopathy

Point mutations in human alpha-cardiac actin cause familial hypertrophic cardiomyopathy. Functional characterization of these actin mutants has been limited by the lack of a high level expression system for human cardiac actin. Here, wild-type (WT) human alpha-cardiac actin and a mutant E99K actin have been expressed and purified from the baculovirus/insect cell expression system. Glu-99 in subdomain 1 of actin is thought to interact with a positively charged cluster located in the lower 50-kDa domain of the myosin motor domain. Actin-activated ATPase measurements using the expressed actins and beta-cardiac myosin showed that the mutation increased the K(m) for actin 4-fold (4.7 +/- 0.7 mum for WT versus 19.1 +/- 3.0 mum for the mutant), whereas the V(max) values were similar. The mutation slightly decreased the affinity of actin for S1 in the absence of nucleotide, which can partly be accounted for by a slower rate of association. The in vitro motility for the E99K mutant was consistently lower than WT over a range of ionic strengths, which is likely related to the lower average force supported by the mutant actin. The thermal stability of the E99K was comparable to that of WT-actin, implying no folding defects. The lower density of negative charge in subdomain 1 of actin therefore weakens the actomyosin interaction sufficiently to decrease the force and motion generating capacity of E99K actin, thus providing the primary insult that ultimately leads to the disease phenotype.

Expression of actin mutants to study their roles in cardiomyopathyThis paper is one of a selection of papers published this Special Issue, entitled Young Investigator's Forum

Mutations in the human cardiac actin gene (ACTC) have been implicated in the development of hypertrophic or dilated cardiomyopathy in humans. To determine the molecular mechanism for the disease development, a system for the expression of mutant cardiac actin proteins that may be lethal to eukaryotic cells must be developed. Here, we explore some of the advantages and disadvantages of human ACTC expression in yeast and insect cells. We show that human ACTC is incapable of rescuing a yeast endogenous actin (ACT1) - knockout in yeast cells and that coexpression of human ACTC in yeast results in slower growth, making yeast an unsuitable expression system. However, we show that it is possible for yeast cells to express a polymerization-deficient ACT1 mutant, thereby allowing us to examine the cell biology of this mutation in the future. Finally, mutant forms of human cardiac actin can be expressed in and purified from insect cells in a properly folded and functional form, permitting important characterization of the biochemical mechanisms responsible for cardiomyopathy development in humans. These studies allow for further research into the biochemical characteristics of previously untenable actin mutant proteins.

A method for rapidly screening functionality of actin mutants and tagged actins

Recombinant production and biochemical analysis of actin mutants has been hampered by the fact that actin has an absolute requirement for the eukaryotic chaperone CCT to reach its native state. We therefore have developed a method to rapidly screen the folding capacity and functionality of actin variants, by combining in vitro expression of labelled actin with analysis on native gels, band shift assays or copolymerization tests. Additionally, we monitor, using immuno-fluorescence, incorporation of actin variants in cytoskeletal structures in transfected cells. We illustrate the method by two examples. In one we show that tagged versions of actin do not always behave native-like and in the other we study some of the molecular defects of three beta-actin mutants that have been associated with diseases.