Conformational changes in actin resulting from Ca2+/Mg2+ exchange as detected by proton NMR spectroscopy

Bound nucleotide can control the dynamic architecture of monomeric actin

Polymerization of actin into cytoskeletal filaments is coupled to its bound adenine nucleotides. The mechanism by which nucleotide modulates actin functions has not been evident from analyses of ATP- and ADP-bound crystal structures of the actin monomer. We report that NMR chemical shift differences between the two forms are globally distributed. Furthermore, microsecond–millisecond motions are spread throughout the molecule in the ATP form, but largely confined to subdomains 1 and 2, and the nucleotide binding site in the ADP form. Through these motions, the ATP- and ADP-bound forms sample different high-energy conformations. A deafness-causing, fast-nucleating actin mutant populates the high-energy conformer of ATP-actin more than the wild-type protein, suggesting that this conformer may be on the pathway to nucleation. Together, the data suggest a model in which differential sampling of a nucleation-compatible form of the actin monomer may contribute to control of actin filament dynamics by nucleotide.

NMR shows that ATP- and ADP-actin differ globally, including ground and excited state structures and dynamic architecture. Analyses of an actin mutant suggest the high-energy conformer of ATP-actin may be on the pathway to filament nucleation.

The structure of nonvertebrate actin: implications for the ATP hydrolytic mechanism

The structures of Saccharomyces cerevisiae, Dictyostelium, and Caenorhabditis elegans actin bound to gelsolin segment-1 have been solved and refined at resolutions between 1.9 and 1.75 A. These structures reveal several features relevant to the ATP hydrolytic mechanism, including identification of the nucleophilic water and the roles of Gln-137 and His-161 in positioning and activating the catalytic water, respectively. The involvement of these residues in the catalytic mechanism is consistent with yeast genetics studies. This work highlights both structural and mechanistic similarities with the small and trimeric G proteins and restricts the types of mechanisms responsible for the considerable enhancement of ATP hydrolysis associated with actin polymerization. The conservation of functionalities involved in nucleotide binding and catalysis also provide insights into the mechanistic features of members of the family of actin-related proteins.

Myosin II from rabbit skeletal muscle and Dictyostelium discoideum and its interaction with F-actin studied by (1)H NMR spectroscopy

Mg-F-actin occurs in two conformational states, I and M, where the N-terminal amino acids are either immobile or highly mobile. In the rigor or ADP complex of rabbit myosin S1 with Mg-F-actin the N-terminal acetyl group of actin stays in its highly mobile state. The same is true for the complexes with the myosin motor domain from Dictyostelium discoideum. This excludes a direct strong interaction of the N-terminal amino acids with myosin in the rigor state as suggested. An interaction of the N-terminus of F-actin with myosin is also not promoted by occupying its low-affinity binding site(s) with divalent ions. The N-terminal high-mobility region may be part of a structural system which has evolved for releasing inadequate stress applied to the actin filaments.

Thymosin-beta(4) changes the conformation and dynamics of actin monomers

Thymosin-beta(4) (Tbeta(4)) binds actin monomers stoichiometrically and maintains the bulk of the actin monomer pool in metazoan cells. Tbeta(4) binding quenches the fluorescence of N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine (AEDANS) conjugated to Cys(374) of actin monomers. The K(d) of the actin-Tbeta(4) complex depends on the cation and nucleotide bound to actin but is not affected by the AEDANS probe. The different stabilities are determined primarily by the rates of dissociation. At 25 degrees C, the free energy of Tbeta(4) binding MgATP-actin is primarily enthalpic in origin but entropic for CaATP-actin. Binding is coupled to the dissociation of bound water molecules, which is greater for CaATP-actin than MgATP-actin monomers. Proteolysis of MgATP-actin, but not CaATP-actin, at Gly(46) on subdomain 2 is >12 times faster when Tbeta(4) is bound. The C terminus of Tbeta(4) contacts actin near this cleavage site, at His(40). By tritium exchange, Tbeta(4) slows the exchange rate of approximately eight rapidly exchanging amide protons on actin. We conclude that Tbeta(4) changes the conformation and structural dynamics ("breathing") of actin monomers. The conformational change may reflect the unique ability of Tbeta(4) to sequester actin monomers and inhibit nucleotide exchange.

Actin-binding proteins in mouse C2 myoblasts and myotubes: a combination of affinity chromatography and two-dimensional gel electrophoresis

This paper analyzes proteins expressed in a mouse muscle precursor cell line (C2 myoblasts) and compares them with those observed in differentiated myotubes from the same cell line. We observed hundreds of proteins in myoblasts using IPG two-dimensional gel electrophoresis but this number is greatly reduced using Mini-Leak (divinylsulfone-activated agarose) affinity chromatography. Two kinds of affinity columns were prepared. One contained a chemically modified monomeric actin bound to the affinity matrix. The second matrix contained a high-affinity actin-binding protein (DNase I) which was bound to the actin Mini-Leak column to block specific sites on actin. Actin-binding proteins in homogenates of myoblasts or myotubes were passed through the affinity columns and eluted under high salt conditions. The Mini-Leak affinity medium itself appeared to have little ability to bind proteins. Our two-dimensional (2-D) gels identified a small number of proteins and we are currently focusing our attention on a particular protein spot which could correspond to cofilin. Comparison of myoblast and myotube proteins using affinity chromatography shows no qualitative, clearly identifiable differences but the analysis is still in progress. These findings are discussed in relation to reports in which the myoblast-myotube transformation was associated with the up-regulation or de novo synthesis of more than ten proteins.

Two-dimensional gel electrophoresis of actin-binding proteins isolated by affinity chromatography from human skeletal muscle

In muscle cells actin exists as a mixture of monomeric (G-actin) and filamentous actin (F-actin) and ionic conditions strongly favor the formation of F-actin. The existence of unpolymerized actin depends, among other factors, on proteins that bind to G-actin, the so-called G-actin-binding proteins (G-ABPs). We have coupled monomeric actin to divinylsulphone-activated agarose (Mini-Leak) to isolate G-ABPs in human skeletal muscle. Eluted proteins were analyzed by two-dimensional gel electrophoresis (2-DE), which shows that some proteins are selectively retained. Deoxyribonuclease I (DNase I) is known to bind residues at the "pointed end" of actin (subdomains 2 and 4) with a high affinity. When DNase I is bound to the actin Mini-Leak before applying the skeletal muscle extract, the 2-DE gels of the eluted proteins reveals differences when compared to gels of proteins eluted from actin-Mini-Leak and DNase I-Mini-Leak affinity columns. This strategy should detect ABPs which bind to sites other than the DNase I-binding site and some may prove to be novel.

Mobility of the N-terminal segment of rabbit skeletal muscle F-actin detected by 1H and 19F nuclear magnetic resonance spectroscopy

After polymerization filamentous actin (F-actin) still shows a number of rather narrow 1H NMR signals in its Mg2+ form which are quenched when Mg2+ is replaced by Ca2+. These resonances originate from mobile residues in F-actin. For assignment of these resonances three different strategies were used, the fluorine labeling of Cys-374 by 4-(perfluoro-tert-butyl)phenyliodoacetamide, binding studies with antibodies (Fab) against the seven N-terminal amino acids of actin, and two-dimensional 1H NMR spectroscopy of a highly concentrated F-actin sample. In contrast to the effects detected earlier by 1H NMR spectroscopy, 19F NMR spectroscopy of actin labeled at its C-terminal cysteine shows no significant spectral changes in dependence on the divalent ion present. In its G- (globular) form a strong, narrow 19F resonance can be observed at 15.06 ppm (relative to the external standard trifluoroacetic acid) which is broadened substantially after polymerization of actin. At 283 K the corresponding transverse relaxation time T2 decreases from 16.7 ms to approximately 0.6 ms. These data suggest that the highly mobile residues observed by 1H NMR spectroscopy do not originate from the C-terminus. Binding of Fab directed against the N-terminal amino acids of actin to Mg-F-actin leads to the disappearing of the 1H NMR resonances assigned to a mobile domain in F-actin. This indicates that the mobile region probably comprises the N-terminal amino acids. By homonuclear two-dimensional 1H NMR spectroscopy it was finally possible to sequentially assign the resonances of the mobile domain of F-actin. It turned out that amino acids 1-22 are in a highly mobile state in Mg-F-actin. The nuclear Overhauser effect data indicate that, rather surprisingly, in this high mobility state some of the beta-pleated structure is still conserved. The population of F-actin protomers in the M- (mobile) state can be obtained from the NMR spectra and was determined under different experimental conditions. In the presence of 150 mM KCl approximately half of the protomers in Mg-F-actin are in the M-state. This number is largely independent of the pH in the range studied (pH 7.2-7.8) and of the temperature in range studied (283-310 K). The equilibrium constant KMI for the equilibrium between the I- and M-states is approximately 1.3 under these conditions.

Compressibility and specific volume of actin decrease upon G to F transformation

We measured the densities as well as the sound velocities in solutions of G-actin, F-actin and the reconstituted thin filament. Using the data obtained, we determined their partial specific volumes and partial specific adiabatic compressibilities. The objectives were to investigate the volume change of actin upon polymerization and to detect the conformational change associated with the ca2+-binding to the reconstituted thin filament. The partial specific volume and the partial specific adiabatic compressibility of G-actin were 0.749 cm3/g and 9.3 x 10(-12) cm2/dyne, respectively. The results suggest that G-actin is a rather soft protein compared with other globular proteins. The partial specific volumes of F-actin were in a range of 0.63 -0.66 cm3/g depending on the solvent conditions. The partial specific adiabatic compressibilities of F-actin were negative (-(7-13) x 10(-12) cm3/dyne). These data indicate that the amount of hydration may increase by several times upon polymerization assuming that the size of the cavity remains constant. We detected little difference between the partial specific adiabatic compressibility of the reconstituted thin filament in a Ca2+-bound state and that in a Ca2+-unbound state. This suggests that the Ca2+ binding affected not the subunit itself but the inter-subunit junction.

Mobile segments in rabbit skeletal muscle F-actin detected by 1H nuclear magnetic resonance spectroscopy

Polymerization of actin by increasing the ionic strength leads to a quenching of almost all 1H NMR signals. Surprisingly, distinct signals with relatively small line widths can still be observed in actin filaments (F-actin) indicating the existence of mobile, NMR visible residues in the macromolecular structure. The intensity of the F-actin spectrum is much reduced if one replaces Mg2+ with Ca2+, and a moderate reduction of the signal intensity can also be obtained by increasing the ionic strength. These results can be explained in a two-state model of the actin promoters with a M- (mobile) state and a I- (immobile) state in equilibrium. In the M-state a number of residues in the actin promoter are mobile and give rise to observable NMR signals. This equilibrium is shifted towards the I-state specifically by replacing Mg2+ with Ca(2+)-ions and unspecifically by addition of monovalent ions such as K+. The binding of phalloidin to its high-affinity site in the filaments does not influence the equilibrium between M- and I-state. Phalloidin itself is completely immobilized in F-actin, its exchange with the solvent being slow on the NMR time scale.

Analysis of the interactions of actin depolymerizing factor with G- and F-actin

Chick actin depolymerizing factor (ADF) is an actin binding protein previously shown to rapidly depolymerize actin filaments in vitro, yielding a 1:1 complex of ADF and actin monomer. Here we show that ADF protects actin monomer from denaturation by EDTA by inhibiting the exchange of actin-bound nucleotide. Under low ionic strength conditions, the approximate dissociation constant (KD) for the ADF-actin complex determined from exchange of nucleotide (1,N6-etheno-ATP) is about 150 and is calcium-independent. Addition of ADF to monomeric actin inhibits actin assembly as well as the ATP hydrolysis that normally accompanies assembly. Complex formation is demonstrated between ADF and actin containing either ATP, ADP, or AMPPNP as the bound nucleotide. A KD of 0.1-0.2 microM was calculated for both the ADF-ATP-actin and ADF-AMPPNP-actin complexes, whereas the KD for the ADF-ADP-actin complex is about 1.3 microM. ADF can either depolymerize or cosediment with F-actin in a stoichiometric fashion, but these reciprocal activities are pH-dependent. At pHs between 6.5 and 7.1, ADF cosediments with F-actin and demonstrates only weak depolymerizing activity. ADF binding is cooperative and saturates at a 1:1 ADF:actin molar ratio. At pHs between 7.1 and 7.7, ADF shows increasing depolymerizing activity and less F-actin binding. At pH 8.0, ADF depolymerizes F-actin in a stoichiometric manner. Both the F-actin binding and the depolymerizing activities of ADF are inhibited by phalloidin.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical evidence for the existence of activated G-actin

Globular actin (G-actin) will polymerize to form filamentous actin (F-actin) under physiological ionic conditions, and is known to be regulated by univalent and bivalent cations, such as K+ and Mg2+. The current concept of this process involves four steps: activation, nucleation, elongation and annealing. Evidence for the existence of activated G-protein has been suggested by changes in the resistance to proteolysis [Rich & Estes (1976) J. Mol. Biol. 104, 777-792] and u.v.-light absorption [Rouayrenc & Travers (1981) Eur. J. Biochem. 116, 73-77]. More recently we [Liu et al. (1990) Biochem. J. 266, 453-459] have provided direct chemical evidence for extensive conformational changes during the transformation of G-actin into F-actin. In this study we now present direct chemical evidence for the existence of a short-lived species, an activated form of G-actin, which can be detected by changes in the accessibility of the free thiol groups on the G-actin molecule when modified by a specific thiol-group-targeted reagent, 7-dimethylamino-4-methyl-3-N-maleimidylcoumarin (DACM). The presence of K+ and/or Mg2+ ions caused a large increase in the accessibility of the thiol groups of Cys-217 and Cys-374, but not those of Cys-10 and Cys-257. Mg2+ effected relatively faster changes than did K+ ions. The results suggest that the function of these ions is to convert G-actin into an activated form, and further suggest that the change in conformation is mainly confined to the large domain. Such changes at least involve certain portions of the G-actin molecule that contain Cys-217 and Cys-374. On the other hand, little or no significant change could be observed in the small domain of G-actin as reflected by the accessibility of Cys-10. The bound nucleotide remained as ATP during the activation of G-actin and was hydrolysed to ADP on polymerization. The activated G-actin had a life-time of about 8 min or less depending on the concentration of G-actin. At higher protein concentration, its life-time was much shorter, probably owing to the earlier onset of polymerization, which apparently is governed by the concentration of the activated form. The life-time of this new species can be extended by lowering the temperature and is less affected by actin concentration. This new species is considered to be an activated form of G-actin, since polymerization renders all the thiol groups on actin inaccessible to the reagent DACM.

Detection of conformational changes in actin by fluorescence resonance energy transfer between tyrosine-69 and cysteine-374

The distance between 5-(dimethylamino)naphthalene-1-sulfonyl chloride (dansyl chloride or DNS-Cl) attached to Tyr-69 and N-[[4-[4-(dimethylamino)phenyl]azo]phenyl]maleimide (DABMI) or N-[4-(dimethylamino)-3,5-dinitrophenyl]maleimide (DDPM) attached to Cys-374 in an actin monomer was measured to be 2.51 nm or 2.27 +/- 0.04 nm, respectively, by fluorescence resonance energy transfer. This distance does not change significantly when the actin monomer binds DNase I, when the monomer is polymerized, when the polymer interacts with myosin subfragment 1, or when it interacts with tropomyosin-troponin in the presence and absence of Ca2+. Changes in the distance were within 0.1 nm. The results indicate that the structure of the region involving Tyr-69 and Cys-374 is substantially rigid. A large blue shift (about 15 nm) of the fluorescence spectrum and a large increase (about 80%) in the fluorescence intensity of DNS-actin were observed when DNS-actin was denatured upon addition of EDTA. On the other hand, a red shift (about 7 nm) of the fluorescence spectrum and a large decrease (about 50%) in the fluorescence intensity were observed when DNS-actin was completely unfolded in 8 M urea. The results indicate that dansyl chromophore becomes less exposed to the aqueous environment by EDTA denaturation in contradiction to the case of intrinsic tryptophan residues in G-actin. Resonance energy transfer measurements showed that the distance between probes attached to Tyr-69 and Cys-374 on an actin monomer changes by 0.37 nm during EDTA denaturation, but that the distance becomes longer than 4.0 nm in 8 M urea in which no energy transfer is observed.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Localization of the phalloidin and nucleotide-binding sites on actin

Phalloidin was found to block nucleotide exchange in F-actin, without interfering with nucleotide hydrolysis. This inhibition of nucleotide exchange occurs under conditions in which monomers are able to exchange. The distance separating a fluorescent chromophore attached to phalloidin from the nucleotide on actin was determined using fluorescence resonance energy-transfer spectroscopy. They are separated by less than 1.0 nm. Added confirmation of the close proximity of phalloidin to nucleotide was obtained by extracting a small peptide-ATP complex from an actin digest. The peptide comprises residues 114-118, which are from the same region as the residues that others have shown to crosslink to phalloidin [Vandekerckhove et al. (1985) EMBO J. 4, 2815-2818]. The results suggest that phalloidin has two major effects. It traps actin monomers in a conformation which appears to be distinct from G-actin and it stabilizes the structure of F-actin, an event accompanied by the trapping of ADP.

Effect of temperature on the mechanism of actin polymerization

The rate of the Mg2+-induced polymerization of rabbit skeletal muscle G-actin has been measured as as function of temperature at pH 8 by using various concentrations of Mg2+, Ca2+, and G-actin. A polymerization mechanism similar to that proposed at this pH [Frieden, C. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 6513-6517] was found to fit the data from 10 to 35 degrees C. From the kinetic data, no evidence for actin filament fragmentation was found at any temperature. Dimer formation is the most temperature-sensitive step, with the ratio of forward and reverse rate constants changing 4 orders of magnitude from 10 to 35 degrees C. Over this temperature change, all other ratios of forward and reverse rate constants change 7-fold or less, and the critical concentration remains nearly constant. The reversible Mg2+-induced isomerization of G-actin monomer occurs to a greater extent with increasing temperature, measured either by using N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine-labeled actin or by simulation of the full-time course of the polymerization reaction. This is partially due to Mg2+ binding becoming tighter, and Ca2+ binding becoming weaker, with increasing temperature. Elongation rates from the filament-pointed end, determined by using actin nucleated by plasma gelsolin, show a temperature dependence slightly larger than that expected for a diffusion-limited reaction.