Interaction of actin with divalent cations. 2. Characterization of protein-metal complexes

"Non-cytotoxic" doses of metal-organic framework nanoparticles increase endothelial permeability by inducing actin reorganization

Cytotoxicity of nanoparticles is routinely characterized by biochemical assays such as cell viability and membrane integrity assays. However, these approaches overlook cellular biophysical properties including changes in the actin cytoskeleton, cell stiffness, and cell morphology, particularly when cells are exposed to "non-cytotoxic" doses of nanoparticles. Zeolitic imidazolate framework-8 nanoparticles (ZIF-8 NPs), a member of metal-organic framework family, has received increasing interest in various fields such as environmental and biomedical sciences. ZIF-8 NPs may enter the blood circulation system after unintended oral and inhalational exposure or intended intravenous injection for diagnostic and therapeutic applications, yet the effect of ZIF-8 NPs on vascular endothelial cells is not well understood. Here, the biophysical impact of "non-cytotoxic" dose ZIF-8 NPs on human aortic endothelial cells (HAECs) is investigated. We demonstrate that "non-cytotoxic" doses of ZIF-8 NPs, pre-defined by a series of biochemical assays, can increase the endothelial permeability of HAEC monolayers by causing cell junction disruption and intercellular gap formation, which can be attributed to actin reorganization within adjacent HAECs. Nanomechanical atomic force microscopy and super resolution fluorescence microscopy further confirm that "non-cytotoxic" doses of ZIF-8 NPs change the actin structure and cell morphology of HAECs at the single cell level. Finally, the underlying mechanism of actin reorganization induced by the "non-cytotoxic" dose ZIF-8 NPs is elucidated. Together, this study indicates that the "non-cytotoxic" doses of ZIF-8 NPs, intentionally or unintentionally introduced into blood circulation, may still pose a threat to human health, considering increased endothelial permeability is essential to the progression of a variety of diseases. From a broad view of cytotoxicity evaluation, it is important to consider the biophysical properties of cells, since they can serve as novel and more sensitive markers to assess nanomaterial's cytotoxicity.

Cholate Conjugated Cationic Polymers for Regulation of Actin Dynamics

Cytoskeletal movement is a compulsory necessity for proper cell functioning and is largely controlled by actin filament dynamics. The actin dynamics can be finetuned by various natural and artificial materials...

Nano-ZnO leads to tubulin macrotube assembly and actin bundling, triggering cytoskeletal catastrophe and cell necrosis

Zinc is a crucial element in biology that plays chief catalytic, structural and protein regulatory roles. Excess cytoplasmic zinc is toxic to cells so there are cell-entry and intracellular buffering mechanisms that control intracellular zinc availability. Tubulin and actin are two zinc-scavenging proteins that are essential components of the cellular cytoskeleton implicated in cell division, migration and cellular architecture maintenance. Here we demonstrate how exposure to different ZnO nanostructures, namely ZnO commercial nanoparticles and custom-made ZnO nanowires, produce acute cytotoxic effects in human keratinocytes (HaCat) and epithelial cells (HeLa) triggering a dose-dependent cell retraction and collapse. We show how engulfed ZnO nanoparticles dissolve intracellularly, triggering actin filament bundling and structural changes in microtubules, transforming these highly dynamic 25 nm diameter polymers into rigid macrotubes of tubulin, severely affecting cell proliferation and survival. Our results demonstrate that nano-ZnO causes acute cytoskeletal collapse that triggers necrosis, followed by a late reactive oxygen species (ROS)-dependent apoptotic process.

Regulation of actin by ion-linked equilibria

Actin assembly, filament mechanical properties, and interactions with regulatory proteins depend on the types and concentrations of salts in solution. Salts modulate actin through both nonspecific electrostatic effects and specific binding to discrete sites. Multiple cation-binding site classes spanning a broad range of affinities (nanomolar to millimolar) have been identified on actin monomers and filaments. This review focuses on discrete, low-affinity cation-binding interactions that drive polymerization, regulate filament-bending mechanics, and modulate interactions with regulatory proteins. Cation binding may be perturbed by actin post-translational modifications and linked equilibria. Partial cation occupancy under physiological and commonly used in vitro solution conditions likely contribute to filament mechanical heterogeneity and structural polymorphism. Site-specific cation-binding residues are conserved in Arp2 and Arp3, and may play a role in Arp2/3 complex activation and actin-filament branching activity. Actin-salt interactions demonstrate the relevance of ion-linked equilibria in the operation and regulation of complex biological systems.

Identification of cation-binding sites on actin that drive polymerization and modulate bending stiffness

The assembly of actin monomers into filaments and networks plays vital roles throughout eukaryotic biology, including intracellular transport, cell motility, cell division, determining cellular shape, and providing cells with mechanical strength. The regulation of actin assembly and modulation of filament mechanical properties are critical for proper actin function. It is well established that physiological salt concentrations promote actin assembly and alter the overall bending mechanics of assembled filaments and networks. However, the molecular origins of these salt-dependent effects, particularly if they involve nonspecific ionic strength effects or specific ion-binding interactions, are unknown. Here, we demonstrate that specific cation binding at two discrete sites situated between adjacent subunits along the long-pitch helix drive actin polymerization and determine the filament bending rigidity. We classify the two sites as "polymerization" and "stiffness" sites based on the effects that mutations at the sites have on salt-dependent filament assembly and bending mechanics, respectively. These results establish the existence and location of the cation-binding sites that confer salt dependence to the assembly and mechanics of actin filaments.

Multiscale study of counterion-induced attraction and bundle formation of F-actin using an Ising-like mean-field model

An Ising-like counterion-binding model is developed and solved by a mean-field method. For G-actin, the calculated affinity constants of all the binding sites ranging from loose to tight binding match the experimental data. The model is used to calculate the interaction energy between two F-actin filaments. Within a certain counterion concentration range, a rapidly decaying attractive force between two parallel filaments is produced not only by the correlation of the counterion distributions on the two filaments, but also by the correlation of the configurations of the two filaments with fixed counterion positions, which has been ignored in previous calculations. The bundling energy depends strongly on the configuration of the filaments. Upon bundling, the tightly bound counterion site is not affected, but the medium and loosely bound ones are. The model reproduces the observed minimal divalent counterion concentration for bundling, and naturally predicts the resolubilization of bundles which is seen in recent experiments. At the optimal counterion concentration, we obtain a bundling energy of approximately -0.01 eV per monomer along the filament. The counterion valence strongly affects the optimal counterion concentration, but has only minor effects on the optimal bundling energy. We show that the attractive potential between filaments can be simplified as the sum of interactions between their monomers. This simplification makes it possible to calculate the exact free energy of a two-F-actin-filament system. We are thus able to probe the effects of filament length on F-actin bundling and obtain a critical length for bundling of 59 monomers at 1 microM monomer concentration and pH=7.2.

Chemical treatment of mica for atomic force microscopy can affect biological sample conformation

An important aspect in the preparation of substrate materials to use in atomic force microscopy lies in the question of interactions introduced by treatments designed to immobilize the sample over the substrate. Here we used a mica substrate that was chemically modified with cationic nickel to immobilize actin filaments (F-actin). Chemical modification could be followed quantitatively by measuring the interaction force between the scanning tip and the mica surface. This approach allowed us to observe polymeric F-actin in a structure that resembles an actin gel. It also improved sample throughput and conferred sample stability as well as repeatability from run to run.

Metal ions and human sperm mannose receptors

Zinc and lead concentrations were measured in seminal plasma from fertile donors, infertile men with varicocoele and men undergoing work-ups for in vitro fertilization. Ejaculated spermatozoa from these subjects were incubated in vitro with various metal ions and/or dibromoethane and dibromochloropropane. Mannose receptor expression was correlated with metal and toxicant levels. Sperm distributions of potassium channels were compared with lead ions and calcium channels with zinc ions. Mannose receptor expression by capacitated spermatozoa increased linearly with seminal plasma zinc levels, and correlated inversely with lead levels. Cobalt had no effect on mannose receptor expression, but nickel had a concentration-dependent biphasic effect. Mannose receptor expression was not affected by dibromoethane and dibromochloropropane if the cholesterol content of the sperm membrane was high, but mannose receptor expression was decreased in low cholesterol spermatozoa by exposures below estimated permissive exposure limits. Potassium channels and lead ions co-localized over the entire head of human spermatozoa, while both calcium channels and zinc ions were confined to the equatorial segment of the head. Mannose receptor expression on the external surface of the human sperm plasma membrane is a biomarker for the effects of transition and heavy metals and organic toxicants on sperm fertility potential.

The assembly of Ni2+-actin: some peculiarities

Nickel alters the organisation of highly dynamic cytoskeletal elements. In cultured cells Ni2+ causes microtubule aggregation and bundling as well as microfilament aggregation and redistribution. Here, we have analysed the effect(s) of Ni2+ on in vitro actin polymerisation. Using limited proteolysis by trypsin we have suggested that the regions around Arg-62 and Lys-68 change their conformation following Ni2+ binding to the single high-affinity site for divalent cations in the G-actin molecule. We have found that Ni2+ shortens the lag phase of actin polymerisation and increases the rate of assembly mainly because of an increased elongation rate. Ni2+ has no significant effect on the final plateau of actin polymerisation nor on the actin critical concentration. Electron microscopy revealed that actin filaments polymerised by 2 mM Ni2+ showed some tendency to lateral aggregation, being frequently formed by the cohesion of two or three filaments. Furthermore, they often appeared shorter than those of control as also confirmed by the larger amount of free filament ends as well as the faster depolymerisation rate than control.

Effect of replacement of the tightly bound Ca2+ by Ba2+ on actin polymerization

G-actin has a single tight-binding (high-affinity) site for divalent cations per mole of protein, whose occupancy is important for the stability of the molecule. Different tightly bound divalent cations differently influence the polymerization properties of actin. The tightly bound metal ion easily exchanges for free exogenous cations. Moreover, biochemical and structural evidence demonstrates that actin, in both the G- and F-forms, assumes different conformations depending on the metal ion bound with high affinity in the cleft between two main domains of the molecule. In this work, we used proteolytic susceptibility to detect possible local conformational alterations of the actin molecule following a brief incubation of Ca-G-actin with barium chloride and ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. We found that substitution of Ba2+ for the tightly bound Ca2+ affects the regions around Arg-62 and Lys-68 in subdomain 2 of G-actin, as judged from inhibition of tryptic cleavage at these residues. Using the fluorescent chelator Quin-2, we observed that about 0.95 mol of Ba2+ is released per 1 mol of actin. We also examined the effect of replacement of the tightly bound Ca2+ by Ba2+ on actin polymerization. With respect to Ca-actin, Ba-actin shows an increased polymerization rate, mainly due to its enhanced nucleation and a higher critical concentration.

Electrostatic effects of smooth muscle calponin on actin assembly

The contribution of electrostatic interactions to the effects of chicken gizzard calponin on the kinetics of actin polymerization and the bundling of F-actin were characterized by a combination of fluorescence, light-scattering, co-sedimentation, and electron-microscopic methods. Stoichiometric amounts of calponin accelerate actin polymerization in low-ionic-strength solutions, but this effect is diminished at [KCI] = 150 mM. At low ionic strengths, micromolar concentrations of calponin induce the formation of large bundles of actin filaments, and lower concentrations of calponin quench the fluorescence of pyrene-labeled F-actin. The latter effect is related to binding of calponin to F-actin rather than to bundling of the filaments. The concentration of calponin required to bundle a fixed concentration of actin filaments increases with increasing ionic strength, as the average diameter of the bundles decreases. Millimolar concentrations of ATP, GTP or ITP are equally efficient at dispersing actin bundles to single filaments or smaller aggregates, even though a significant fraction of calponin remains bound to F-actin. Our findings show that the binding of calponin to actin is determined at least in part by electrostatic interactions, and that the polycationic nature of calponin is primarily responsible for the formation of F-actin bundles via its ability to reduce the electrostatic repulsion between the negatively charged actin filaments.

Changes in molar volume and heat capacity of actin upon polymerization

We have used densimetry and microcalorimetry to measure the changes in molar volume and heat capacity of the actin molecule during Mg(2+)-induced polymerization. Molar volume is decreased by 720 ml/mol. This result is in contradiction with previous measurements by Ikkai and Ooi [(1966) Science 152, 1756-1757], and by Swezey and Somero [(1985) Biochemistry 24, 852-860]: both of these groups reported increases in actin volume during polymerization, of 391 ml/mol and 63 ml/mol respectively. We also observed a decrease in heat capacity of about 69.5 kJ.K-1.mol-1 during polymerization. This is in agreement with the concept of conformational fluctuation of proteins proposed by Lumry and Gregory [(1989) J.Mol. Liq. 42, 113-144]whereby either ligand binding by a protein or monomer-monomer interaction decreases the protein's conformational flexibility.

Zinc alters actin filaments in Madin-Darby canine kidney cells

Exposure of semiconfluent cultures of Madin-Darby canine kidney cells to 10 microM zinc leads to a change in the organization of the actin filament system. Most of the stress fibers at the basal end of the cell are lost and the actin associated with the lateral membrane and junctional regions appears to retract into the cytoplasm. In addition, at the base of the cell in regions of cell-substratum contact, dense, actin-rich plaques appear. These alterations in actin filaments are associated with a change in cell shape. Microtubules were unaffected by exposure to 10 microM zinc. At zinc concentrations greater than or equal to 50 microM the microtubules depolymerized. Exposure to cadmium alters the actin filaments as well but the effect is different from the change seen with zinc. When the cells are exposed simultaneously to zinc and cadmium the cells appear the same as they would if exposed to zinc alone. Exposure of MDCK cells to either metal, individually or in combination, results in a significant and similar increase in F-actin content as determined spectrofluorometrically. The changes in organization and amount of F-actin are associated with a reduction in the ability of the cells to remain attached to the substrate, a toxic effect of these metals with regard to epithelial function. The results indicate that zinc, an essential metal, and cadmium, a highly toxic metal, interact with the actin cytoskeleton in intact cells.

Tightly-bound divalent cation of actin

Actin is known to undergo reversible monomer-polymer transitions that coincide with various cell activities such as cell shape changes, locomotion, endocytosis and exocytosis. This dynamic state of actin filament self-assembly and disassembly is thought to be regulated by the properties of the monomeric actin molecule and in vivo by the influence of actin-associated proteins. Of major importance to the properties of the monomeric actin molecule are the presence of one tightly-bound ATP and one tightly-bound divalent cation per molecule. In vivo the divalent cation is thought to be Mg2+ (Mg-actin) but in vitro standard purification procedures result in the preparation of Ca-actin. The affinity of actin for a divalent cation at the tight binding site is in the nanomolar range, much higher than earlier thought. The binding kinetics of Mg2+ and Ca2+ at the high affinity site on actin are considered in terms of a simple competitive binding mechanism. This model adequately describes the published observations regarding divalent cation exchange on actin. The effects of the tightly-bound cation, Mg2+ or Ca2+, on nucleotide binding and exchange on actin, actin ATP hydrolysis activity and nucleation and polymerization of actin are discussed. From the characteristics that are reviewed, it is apparent that the nature of the bound divalent cation has a significant effect on the properties of actin.

Osmotically induced electrical signals from actin filaments

Actin filaments, F-actin, a major component of the cortical cytoskeleton, play an important role in a variety of cell functions. In this report we have assessed the role of osmotic stress on the electrochemical properties of F-actin. The spontaneous Donnan potential of a polymerized actin solution (5 mg/ml) was -3.93 +/- 1.84 mV, which was linearly reduced by osmotic stress on the order of 1-20 mOsm (0.28 +/- 0.06 mV/mM). Calculated surface charge density was reduced and eventually reversed by increasing the osmotic stress as expected for a phase transition behavior. The electro-osmotic behavior of F-actin disappeared at pH 5.5 and was dependent on its filamentous nature. Furthermore, osmotically stressed F-actin displayed a nonlinear electric response upon application of electric fields on the order of 500-2,000 V/cm. These data indicate that F-actin in solution may display nonideal electro-osmotic properties consistent with ionic "cable" behavior which may be of biological significance in the processing and conduction of electrical signals within the cellular compartment.

Zinc forms complexes with higher kinetical stability than calcium, 5-F-BAPTA as a good example

Increasing interest is focused on the role of zinc in biological systems. A rapidly growing family of DNA-binding proteins contains "zinc-fingers", where zinc is bound to cysteine or histidine residues. On the other hand zinc is able to displace calcium from its binding sites and in this way it may modify calcium-mediated cellular processes. In the present report dissociation rates of Zn2(+)- and Ca2(+)-complexes with 5-F-BAPTA, a widely used NMR-active calcium indicator, have been measured by two-dimensional 19F NMR exchange spectroscopic methods. The results show that the lifetime of the Zn2(+)-complex is more than five times longer than that of the Ca2(+)-complex. The longer lifetime, when combined with a higher thermodynamical stability of the Zn2+-complex, may explain why, in some cellular processes, Zn2+ can compete with Ca2+ in spite of a presumably high [Ca2+]/[Zn2+] free ion concentration ratio.

Biochemical and theoretical approach to localization of metal-ion-binding sites in the actin primary structure

The number of Ca2+ ions bound at sites other than the single high-affinity site in CaCl2-induced polymers of rabbit skeletal muscle, chicken gizzard, and bovine aorta actin was determined. The polymer of skeletal muscle and aorta actin contained 4 mol Ca2+/mol, whereas gizzard actin only 3 mol weakly bound Ca2+/mol monomer. This difference correlates with the deletion in smooth muscle gamma-actin of one out of four NH2-terminal acidic residues typical of skeletal and smooth muscle alpha-actin isoforms, suggesting that this additional acidic residue in alpha-actins is involved in the weak binding of cations which is essential for polymerization. This experimental result, as well as a theoretical analysis of the actin primary structure, argue against the implication of the NH2-terminal acidic residues in the high-affinity site for divalent cation. The analysis of the actin primary structure aimed at identification of sequences resembling the known Ca2+-binding patterns has revealed the absence of an EF-hand Ca2+-binding site. The best match was obtained between the sequence of the 292-301 segment and that of Ca2+ site in lectins. However, in the light of experimental data discussed, it is more plausible that the actual high-affinity Ca2+ site in actin involves sequentially distant residues from the NH2- and COOH-terminal portions of the polypeptide chain.

Polymerization of actin by positively charged liposomes

By cosedimentation, spectrofluorimetry, and electron microscopy, we have established that actin is induced to polymerize at low salt concentrations by positively charged liposomes. This polymerization occurs only at the surface of the liposomes, and thus monomers not in direct contact with the liposome remain monomeric. The integrity of the liposome membrane is necessary to maintain actin in its polymerized state since disruption of the liposome depolymerizes actin. Actin polymerized at the surface of the liposome is organized into two filamentous structures: sheets of parallel filaments in register and a netlike organization. Spectrofluorimetric analysis with the probe N-pyrenyl-iodoacetamide shows that actin is in the F conformation, at least in the environment of the probe. However, actin assembly induced by the liposome is not accompanied by full ATP hydrolysis as observed in vitro upon addition of salts.

The effect of divalent cations on the interaction between calf spleen profilin and different actins

The interaction between calf spleen profilin and actin depends critically on the status of the C-terminus of the actin, and in the case of profilin, the C-terminus is of great importance for the physiochemical behaviour of the protein. Both proteins easily lose their C-terminal amino acids during the preparation, and special care has to be taken to ensure the isolation of the proteins in the intact form. Another factor that may seriously influence the study of the interaction of profilin with actin is the presence of varying amounts of an activity that causes an apparent stabilization of the complex even at later stages of its purification. We have found conditions for the isolation of intact profilin and actin, and studied the interaction between the two proteins, including the determination of the Kdiss for the complex formed under various ionic conditions. The complex formed between profilin and actin from calf spleen was found to be significantly stronger (Kdiss less than or equal to 10(-8) M in 50 mM KCl, and Kdiss = 4.10(-7) M in 50 mM KCl, 1 mM MgCl2) than that formed between profilin and muscle alpha-actin (Kdiss = 10(-6) M in 50 mM KCl, +/- 1 mM MgCl2). The profilactin complex formed in the mammalian system was stronger than the complex formed between Acanthamoeba actin and the profilin-like protein isolated from this organism. Analysis of the formation of the calf spleen complex in the presence of varying concentrations of divalent cations gave evidence for the presence of a high-affinity divalent-cation-binding site on the spleen actin (beta, gamma) which appears to regulate the interaction with profilin.

Representing and defining patterns by graphs: applications to sol-gel patterns and to cytoskeleton

The problems of interdisciplinary interests--applying methods of graph theory to sol-gel patterns and to cytoskeleton--are discussed. The importance of sol-gel transition phenomena in living cells and the possibility of periodic sol-gel transition phenomena are briefly reviewed. Representing patterns by graphs and using graph probabilistic representation for calculating structure-property relationships are discussed and applied to sol-gel transition patterns.

Actin assembly by lithium ions

The ability of Li+ to promote the assembly of actin has been compared with the more common cations used in actin assembly assays, K+, Mg2+, and Ca2+. The principal assay of actin assembly utilized was fluorescence photobleaching recovery (FPR), from which it is possible to determine the fraction of actin protomers incorporated into filaments and the average diffusion coefficients of the filaments. In addition, critical concentrations of actin over a range of concentrations of all of these cations have been determined using an assay that involves sonication and dilution of assembled actin filaments containing trace amounts of pyrene-labeled actin. The results demonstrate that Li+ is a more potent promoter of actin assembly than is K+. The more rapid assembly of actin in the presence of Li+ is attributable to an increased rate of filament elongation. Filaments assembled in equivalent concentrations of Li+ or K+ have the same diffusion coefficients, and thus presumably the same average lengths. The critical concentration of actin is about three times less in the presence of Li+ than in the presence of an equal concentration of K+. Cytochalasin D accelerates the rate of Li+-promoted actin assembly and reduces slightly the total fraction of actin assembly. However, cytochalasin D causes less shortening of filaments in the presence of Li+ than in the presence of K+ or Mg2+. By the criteria of assembly kinetics and critical concentration, Li+ is much less potent as a promoter of actin assembly than either Mg2+ or Ca2+. These results are discussed in terms of the role of electrostatic forces in the actin assembly mechanism and in terms of possible relationships to therapeutic and toxicity mechanisms for Li+.

Divalent cation binding to the high- and low-affinity sites on G-actin

Metal binding to skeletal muscle G-actin has been assessed by equilibrium dialysis using 45Ca2+ and by kinetic measurements of the increase in the fluorescence of N-acetyl-N'-(5-sulfo-1-naphthyl)-ethylenediamine-labeled actin. Two classes of cation binding sites were found on G-actin which could be separated on the basis of their Ca2+ affinity: a single high-affinity site with a Kd considerably less than 1 microM and three identical moderate-affinity binding sites with a Kd of 18 microM. The data for the Mg2+-induced fluorescence enhancement of actin labeled with N-acetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine support a previously suggested mechanism [Frieden, C. (1982) J. Biol. Chem. 257, 2882-2886] in which Ca2+ is replaced by Mg2+ at the moderate affinity site(s), followed by a slow actin isomerization. This isomerization occurs independently of Ca2+ release from the high-affinity site. The fluorescence data do not support a mechanism in which this isomerization is directly related to Ca2+ release from the high-affinity site. Fluorescence changes of labeled actin associated with adding metal chelators are complex and do not reflect the same change induced by Mg2+ addition. Fluorescence changes in the labeled actin have also been observed for the addition of Cd2+ or Mn2+ instead of Mg2+. It is proposed actin may undergo a host of subtle conformational changes dependent on the divalent cation bound. We have also developed a method by which progress curves of a given reaction can be analyzed by nonlinear regression fitting of kinetic simulations to experimental reaction time courses.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence resonance energy transfer between sites in G-actin. The spatial relationship between Cys-10, Tyr-69, Cys-374, the high-affinity metal and the nucleotide

Intramonomer fluorescence resonance energy transfer spectroscopy was employed to investigate the spatial relationship between labels attached to the residues Cys-10, Tyr-69, Cys-374, the high-affinity metal binding site and the nucleotide binding site in G-actin. The separation between the fluorescence donor 5-(dimethylamino)naphthalene-1-sulphonyl (Dns) chloride (dansyl chloride) used to label Tyr-69 and the acceptor 4-dimethylaminophenylazophenyl-4'-maleimide (DABM) used to label Cys-374 was found to be 3.6 nm. The distance separating Dns on Tyr-69 from DABM on Cys-10 was found to be 2.7 nm. The distance separating the acceptor DABM bound to Cys-374 from the fluorescence donor formycin A 5'-triphosphate (FTP) occupying the nucleotide binding site was determined to be 3.0 nm. A slightly larger separation was determined between the FTP site and DABM attached to Cys-10. In this case a value of 3.2 nm was obtained. The distance separating Dns on Tyr-69 from Co2+ in the high-affinity metal binding site was determined to be 1.1 nm. Finally, the separation of FTP, now acting as donor, from the Dns molecule attached to Tyr-69 and acting as the acceptor was determined to be 2.1 nm. The likely relationship between these label sites on actin is represented by a model which is used to assist in the determination of the actin structure, with particular reference to the environment of the metal and nucleotide binding sites.

Effects of various amino acid replacements on the conformational stability of G-actin

Circular dichroic spectra of native, EDTA-treated and heat-denatured G-actin from chicken gizzard smooth muscle are virtually the same as those of rabbit skeletal muscle actin. The rates of changes produced by EDTA or heat in the secondary structure are, however, higher in the case of gizzard actin. Similar differences were found in the rates of inactivation as measured by loss of polymerizability during incubation with EDTA or Dowex 50. The results are explicable in terms of local differences in the conformation at specific site(s) important for maintaining the native state of actin monomer. Involvement of the ATP binding site was shown by measuring the equilibrium constant for the binding of ATP to the two actins. Difference in the conformation of some additional site(s) is indicated by a higher rate constant of inactivation of nucleotide-free actin observed for gizzard actin. No significant difference was found in the equilibrium constant for the binding of Ca2+ at the single high-affinity site in gizzard and skeletal muscle actin. Comparison of inactivation kinetics of actin from chicken gizzard, rabbit skeletal, bovine aorta, and bovine cardiac muscle suggests that the amino acid replacements Val-17----Cys-17 and/or Thr-89----Ser-89 have a destabilizing effect on the native conformation of G-actin. The results indicate that deletion of the acidic residue at position 1 of the amino acid sequence has no effect on the conformation of the ATP binding site and the high-affinity site for divalent cation as well.

The binding of Ca2+ to actin monomer is monitored by the fluorescence of actin-bound auramine O

The fluorescence of the cation auramine O was substantially enhanced by the presence of actin monomer. Titrations of this fluorescence enhancement indicated that actin monomer had two auramine O binding sites, each with a dissociation constant of approx. 20 microM. Calcium ions had no effect on the number of actin monomer-bound auramine O molecules or on the dissociation constant for that interaction. However, calcium ions increased the maximum change of fluorescence that occurs when actin monomer was fully saturated with auramine O. This effect of calcium was saturable and yielded a Ca2+ dissociation constant of 1.6 mM. It was concluded that auramine O bound to sites on actin monomer and independently monitored the binding of Ca2+ ion(s) to other site(s) on actin monomer. Further, the magnitude of the Ca2+ dissociation constant suggested that this Ca2+-binding site may be representative of the multiple bivalent cation-binding sites on actin monomer which are thought to be directly involved in actin polymerization. However, the exact relationship between these sites remains unclear.

Thin filament proteins and thin filament-linked regulation of vertebrate muscle contraction

Recent developments in the field of myofibrillar proteins will be reviewed. Consideration will be given to the proteins that participate in the contractile process itself as well as to those involved in Ca-dependent regulation of striated (skeletal and cardiac) and smooth muscle. The relation of protein structure to function will be emphasized and the relation of various physiologically and histochemically defined fiber types to the proteins found in them will be discussed.

Mechanism of K+-induced actin assembly

The assembly of highly purified actin from Dictyostelium discoideum amoebae and rabbit skeletal muscle by physiological concentrations of KCI proceeds through successive stages of (a) rapid formation of a distinct monomeric species referred to as KCI-monomer, (b) incorporation of KCI-monomers into an ATP-containing filament, and (c) ATP hydrolysis that occurs significantly after the incorporation event. KCI-monomer has a conformation which is distinct from that of either conventional G- or F-actin, as judged by UV spectroscopy at 210-220 nm and by changes in ATP affinity. ATP is not hydrolyzed during conversion of G-actin to KCI-monomer. KCI-monomer formation precedes filament formation and may be necessary for the assembly event. Although incorporation of KCI-monomers into filaments demonstrates lagphase kinetics by viscometry, both continuous absorbance monitoring at 232 nm and rapid sedimentation of filaments demonstrate hyperbolic assembly curves. ATP hydrolysis significantly lags the formation of actin filaments. When half of the actin has assembled, only 0.1 to 0.2 mole of ATP are hydrolyzed per mole of actin present as filaments.

Conformational studies of G-actin containing bound lanthanide

The presence of bound Ln ions gives rise to changes in the ultraviolet absorbance of G-actin. Previously, Ln ions have been shown to mobilize the adenosine moiety of ATP bound to the actin monomer. The changes in ultraviolet absorbance appear to be fully attributable to this mobilization. Any structural changes induced in G-actin by the substitution of Ln ions for Ca2+ must be small enough so as not to perturb the environment of any aromatic chromophores other than those exposed by the mobility of the nucleotide. This conclusion is supported by measurements of the rates of proteolytic digestion of actin with and without bound Ln ions. The effect of Ln ions on the actin-bound nucleotide is not simply due to the increase in positive charge on the actin monomer. This is demonstrated by the absence of any ultraviolet spectral changes on the addition of a fivefold excess of Ca2+. Analysis of the ultraviolet difference spectrum obtained when Ln ions bind to G-actin indicates that at least one tryptophan residue is partially exposed to the solvent coincident with mobilization of the nucleotide. Tb3+ luminescence enhancement studies indicate that there are no Trp, Tyr or Phe residues in the vicinity of the high-affinity cation site. These results suggest that the exposed Trp may be close to the nucleotide.

Effects of formamide on the polymerization and depolymerization of muscle actin

Formamide was found to interfere with the polymerization of electrophoretically pure rabbit skeletal muscle G-actin to F-actin in vitro. It decreased the rate as well as the extent of polymerization. However, this influence was dependent on the way the polymerization reaction was initiated. If polymerization of G-actin was induced by 2 mM MgCl2, formamide inhibited the rate and extent of the polymerization much less than if the polymerization was induced by either 2 mM CaCl2 or 0.1 M KCl. The critical protein concentration was increased when formamide was present. This effect was small in the presence of MgCl2 but an approximately tenfold increase in the critical value was observed for the CaCl2-induced or KCl-induced polymerization. Depolymerization of F-actin by molar amounts of formamide was faster and proceeded further when the polymer had been formed in the presence of KCl or CaCl2 than when it been formed in the presence of MgCl2. It is concluded that Mg2+ stabilizes the F-actin structure rendering it more resistant than either Ca2+ or K+ against the destabilizing action of formamide.

Effect of alpha-actinin on actin structure: viscosity studies

The effect of ATP on ability of alpha-actinin to increase viscosity of F-actin was measured in three different solutions: 100 mM KCl; 100 mM KCl/l mM Mg2+; and Mg2+ alone at concentrations of 1-6 mM. When ATP and Mg2+ are added at equimolar ratios or at added [ATP] to added [Mg2+] greater than equimolar, alpha-actinin has no effect on F-actin viscosity in the absence of KCl. ATP decreases viscosity of alpha-actinin/F-actin mixtures by 20% even in the presence of KCl, evidently because ATP affects the alpha-actinin-F-actin interaction. Molar ratios of 1 alpha-actinin to 49 actins increase specific viscosity of F-actin approx. 2-fold at 37 degrees C in the presence of 1 mM ATP, so ATP does not prevent the alpha-actinin-F-actin interaction.

The first step in the polymerisation of actin

In the presence of certain cations (e.g. K+ or Mg2+) actin polymerizes. Below a certain concentration (the critical concentration) the monomer G-actin does not polymerize on the addition of K+ or Mg2+. However, the proteolysis experiments of Rich and Estes [J. Mol. Biol. 104, 777--792 (1976)] strongly suggest that cations induce a change in conformation of G-actin leading to a novel form of actin, G*-actin. This conformational change may be the first step in the polymerization of actin. We have studied G*-actin induced by K+, by difference spectroscopy. We show that G*-actin is a monomer and we confirm that the bound ATP is not cleaved. We also studied the G-actin in equilibrium with G*-actin equilibrium at 4 degrees C as a function of K+ or Mg2+ concentration. With KCl, the transformation can be accounted for as a screening effect. The effect of Mg2+ is more specific and the change in conformation of the G-actin could result from the binding of two or three Mg2+ ions/molecule. We suggest that the G-actin in equilibrium with G*-actin transformation results from the neutralization of a polyanionic region on the actin surface and that this region could be the highly negatively charged N terminus.

Phosphorus-31 nuclear magnetic resonance studies of the adenosine 5'-triphosphate-calcium-G-actin complex

Phosphorus-31 nuclear magnetic resonance spectra of the adenosine 5'-triphosphate-calcium-G-actin complex were obtained, and the resonances of the three phosphates of the protein-bound ATP were detected. The exchange of the ATP between its protein-bound and free states were found to be slow on the NMR time scale, with an exchange rate of less than 480 s-1 at pH* 7.8, 4 degrees C. The line width of the protein-bound gamma-phosphate resonance (corrected for spin-spin splitting by the beta phosphate) was used to calculate a rotational correlation time for the G-actin-bound ATP. With the assumption that chemical shift anisotropy is the dominant relaxation mechanism at 109.29 MHz and that the chemical shielding tensor for pyrophosphate serves as a good model for the gamma phosphate of the bound ATP, a correlation time of 60 ns was estimated. Since the theoretical correlation time of a globular protein the size of G-actin is 36 ns, the line width of the bound gamma-phosphate resonance is consistent with that expected for ATP bound to G-actin without large-scale rapid internal mobility. The addition of 1.5 M urea to the ATP-Ca-G-actin complex caused exchange broadening of the gamma and beta phosphates, but no effect on the alpha phosphate. This indicates an increase in the rate of exchange for the beta and gamma phosphates between the protein-buried and solvent-exposed environments at 1.5 M urea. At 6 M urea, the intensities of the protein-bound ATP resonances were greatly reduced, and the intensities of the free ATP resonances were greatly increased, indicative of complete protein unfolding and liberation of protein-bound ATP.