Evidence for the Direct Interaction Between Tightly Bound Divalent Metal Ion and ATP on Actin

Gender Differences With Ibutilide Effectiveness and Safety in Cardioversion of Atrial Fibrillation

INTRODUCTION

Few studies have examined the use of ibutilide in noncardiac surgical populations. Our study considered the effectiveness and safety of ibutilide in cardioversion of atrial fibrillation (AF) in medical and surgical intensive care patients.

METHODS

A retrospective chart review was performed for patients with a confirmed diagnosis of AF who were hemodynamically stable and received ibutilide after the initial diagnosis. Patients were administered 1 mg of ibutilide fumarate intravenous for 10 min with a second dose administered if AF persisted after 30 min. Patients were pretreated with intravenous magnesium sulfate if their blood magnesium level was <2 mg/dL.

RESULTS

Fifty seven total female patients and 99 male patients received ibutilide. Females had an 88% conversion rate to normal sinus rhythm (NSR) compared to 68% in males (P = 0.008). A 70% successful return to NSR was observed in patients from all groups pretreated with magnesium sulfate (P = 0.045). One year after discharge, 74% of the patients stayed in the NSR.

CONCLUSIONS

Within our population, pretreatment with magnesium sulfate followed by ibutilide was associated with increased conversion to NSR. Additionally, we noted that females had a higher conversion rate to NSR compared to males, regardless of whether they were pretreated with magnesium sulfate.

Individual actin filaments in a microfluidic flow reveal the mechanism of ATP hydrolysis and give insight into the properties of profilin

A novel microfluidic approach allows the analysis of the dynamics of individual actin filaments, revealing both their local ADP/ADP-Pi-actin composition and that Pi release is a random mechanism.

The hydrolysis of ATP associated with actin and profilin-actin polymerization is pivotal in cell motility. It is at the origin of treadmilling of actin filaments and controls their dynamics and mechanical properties, as well as their interactions with regulatory proteins. The slow release of inorganic phosphate (Pi) that follows rapid cleavage of ATP gamma phosphate is linked to an increase in the rate of filament disassembly. The mechanism of Pi release in actin filaments has remained elusive for over 20 years. Here, we developed a microfluidic setup to accurately monitor the depolymerization of individual filaments and determine their local ADP-Pi content. We demonstrate that Pi release in the filament is not a vectorial but a random process with a half-time of 102 seconds, irrespective of whether the filament is assembled from actin or profilin-actin. Pi release from the depolymerizing barbed end is faster (half-time of 0.39 seconds) and further accelerated by profilin. Profilin accelerates the depolymerization of both ADP- and ADP-Pi-F-actin. Altogether, our data show that during elongation from profilin-actin, the dissociation of profilin from the growing barbed end is not coupled to Pi release or to ATP cleavage on the terminal subunit. These results emphasize the potential of microfluidics in elucidating actin regulation at the scale of individual filaments.

Actin proteins assemble into microfilaments that control cell shape and movement by polymerizing or depolymerizing. These actin monomers can bind ATP or ADP molecules. The incorporation of an ATP-actin monomer into a growing filament results in rapid cleavage of ATP into ADP and inorganic phosphate (Pi), followed by a slower release of Pi. As a consequence, actin filaments are composed mainly of ADP- and ADP-Pi-actin subunits, which have different depolymerization kinetics and mechanical properties, and can be targeted specifically by regulatory proteins that affect filament function. Hence, the understanding of many cellular processes requires a knowledge of the ADP/ADP-Pi composition of actin filaments at a molecular scale. This has so far remained elusive because traditional studies rely on measuring an average over many filaments in solution. To address this issue, we developed a microfluidics setup to monitor individual filaments with light microscopy while rapidly changing their chemical environment. We find that depolymerization accelerates progressively and corresponds to an exponential ADP-Pi-actin profile in the filament, meaning that each subunit releases its Pi with the same rate. Our method also provides novel insight into the function of profilin, a protein important for regulation of actin dynamics in cells, thus demonstrating the method's potential in the functional analysis of actin regulators.

Energetic requirements for processive elongation of actin filaments by FH1FH2-formins

Formin-homology (FH) 2 domains from formin proteins associate processively with the barbed ends of actin filaments through many rounds of actin subunit addition before dissociating completely. Interaction of the actin monomer-binding protein profilin with the FH1 domain speeds processive barbed end elongation by FH2 domains. In this study, we examined the energetic requirements for fast processive elongation. In contrast to previous proposals, direct microscopic observations of single molecules of the formin Bni1p from Saccharomyces cerevisiae labeled with quantum dots showed that profilin is not required for formin-mediated processive elongation of growing barbed ends. ATP-actin subunits polymerized by Bni1p and profilin release the gamma-phosphate of ATP on average >2.5 min after becoming incorporated into filaments. Therefore, the release of gamma-phosphate from actin does not drive processive elongation. We compared experimentally observed rates of processive elongation by a number of different FH2 domains to kinetic computer simulations and found that actin subunit addition alone likely provides the energy for fast processive elongation of filaments mediated by FH1FH2-formin and profilin. We also studied the role of FH2 structure in processive elongation. We found that the flexible linker joining the two halves of the FH2 dimer has a strong influence on dissociation of formins from barbed ends but only a weak effect on elongation rates. Because formins are most vulnerable to dissociation during translocation along the growing barbed end, we propose that the flexible linker influences the lifetime of this translocative state.

The real-time monitoring of the thermal unfolding of tetramethylrhodamine-labeled actin

Modification of actin at Cys (374) with tetramethylrhodamine maleimide (TMR-actin) has been used for visualization of actin filaments and to produce high-resolution crystal structures of actin. We show that TMR-actin exhibits a 21% decrease in absorbance at 557 nm upon thermal unfolding, likely due to the movement of TMR to a more hydrophobic environment upon rapid unfolding and protein aggregation. We took advantage of this property to test models of actin protein unfolding. A transition temperature ( T m) of 60.2 +/- 0.2 degrees C for Ca (2+).ATP.TMR-actin was determined using A 557 and agreed with our own determinations employing different techniques and previous work with unlabeled actin. Our data show that the dependence of TMR-actin thermal stability on the bound nucleotide and cations follows a trend of Ca (2+).ATP > Mg (2+).ATP > Ca (2+).ADP > Mg (2+).ADP. The activation energies and frequency factors for the thermal unfolding of TMR-actin determined with two methods were in good agreement with those previously determined for unlabeled actin. We observed a biphasic trend in the T m of TMR-actin with increasing nucleotide concentrations, supporting a two-pathway model for actin protein unfolding where one pathway dominates at different concentrations of nucleotide. Additionally, TMR-actin bound by DNase I or gelsolin segment-1 exhibited elevated transition temperatures.

Functional characterization of proteins regulating actin assembly

A very large, ever-increasing repertoire of actin-binding proteins regulates the assembly dynamics and the spatial organization of actin filaments, thus orchestrating the motile behavior of the cell. The authors describe a series of biochemical functional assays that allow one to characterize the function of a putative actin-binding protein in actin filament dynamics. These tests allow the characterization of three types of actin-binding proteins: G-actin-sequestering proteins, profilin-like proteins, and barbed-end capping proteins. Biochemical tests include the use of sedimentation of actin filaments, polymerization assays at the barbed or pointed end of actin filaments derived from fluorescently labeled actin, thermodynamic measurements of actin assembly at steady state and during turnover of actin filaments, measurements of nucleotide exchange on G-actin, and the use of the intrinsic or extrinsic fluorescence of actin to measure direct binding of different protein ligands to G-actin.

How ATP hydrolysis controls filament assembly from profilin-actin: implication for formin processivity

Formins catalyze rapid filament growth from profilin-actin, by remaining processively bound to the elongating barbed end. The sequence of elementary reactions that describe filament assembly from profilin-actin at either free or formin-bound barbed ends is not fully understood. Specifically, the identity of the transitory complexes between profilin and actin terminal subunits is not known; and whether ATP hydrolysis is directly or indirectly coupled to profilin-actin assembly is not clear. We have analyzed the effect of profilin on actin assembly at free and FH1-FH2-bound barbed ends in the presence of ADP and non-hydrolyzable CrATP. Profilin blocked filament growth by capping the barbed ends in ADP and CrATP/ADP-Pi states, with a higher affinity when formin is bound. We confirm that, in contrast, profilin accelerates depolymerization of ADP-F-actin, more efficiently when FH1-FH2 is bound to barbed ends. To reconcile these data with effective barbed end assembly from profilin-MgATP-actin, the nature of nucleotide bound to both terminal and subterminal subunits must be considered. All data are accounted for quantitatively by a model in which a barbed end whose two terminal subunits consist of profilin-ATP-actin cannot grow until ATP has been hydrolyzed and Pi released from the penultimate subunit, thus promoting the release of profilin and allowing further elongation. Formin does not change the activity of profilin but simply uses it for its processive walk at barbed ends. Finally, if profilin release from actin is prevented by a chemical cross-link, formin processivity is abolished.

ATP hydrolysis on actin-related protein 2/3 complex causes debranching of dendritic actin arrays

Extension of lamellipodia, an important dissipative process in cell motility, is driven by the turnover of a polarized dendritic array of actin filaments. Motility is driven by catalytic cycles of filament attachment to Wiskott-Aldrich syndrome protein (WASP)-activated actin-related protein (Arp)2/3 complex at the leading edge, branch formation, and detachment, allowing subsequent growth of branched filaments. The morphology, mechanical strength, and lifetime of the array are determined by the processes of filament branching, debranching, and treadmilling. All three processes are controlled by ATP hydrolysis. ATP hydrolysis on F-actin is known to be at the origin of treadmilling. Here, by using radiolabeled ATP covalently bound to Arp2/3, we show that ATP is hydrolyzed on Arp2, not on Arp3, after a delay following filament branching. Hydrolysis of ATP on Arp2 promotes debranching of filaments and acts as a clock that controls the stability of dendritic actin arrays in lamellipodia. Finally, we propose that hydrolysis of ATP on G-actin in the ternary G-actin-WASP-Arp2/3 complex on branch formation destabilizes the WASP-actin interface and energetically facilitates the detachment step in the branching reaction.

A cross-linked profilin-actin heterodimer interferes with elongation at the fast-growing end of F-actin

Profilin and beta/gamma-actin from calf thymus were covalently linked using the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide in combination with N-hydroxysuccinimide, yielding a single product with an apparent molecular mass of 60 kDa. Sequence analysis and x-ray crystallographic investigations showed that the cross-linked residues were glutamic acid 82 of profilin and lysine 113 of actin. The cross-linked complex was shown to bind with high affinity to deoxyribonuclease I and poly(l-proline). It also bound and exchanged ATP with kinetics close to that of unmodified profilin-actin and inhibited the intrinsic ATPase activity of actin. This inhibition occurred even in conditions where actin normally forms filaments. By these criteria the cross-linked profilin-actin complex retains the characteristics of unmodified profilin-actin. However, the cross-linked complex did not form filaments nor copolymerized with unmodified actin, but did interfere with elongation of actin filaments in a concentration-dependent manner. These results support a polymerization mechanism where the profilin-actin heterodimer binds to the (+)-end of actin filaments, followed by dissociation of profilin, and ATP hydrolysis and P(i) release from the actin subunit as it assumes its stable conformation in the helical filament.

Functional specificity of actin isoforms

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

Actin depolymerizing factor (ADF/cofilin) enhances the rate of filament turnover: implication in actin-based motility

Actin-binding proteins of the actin depolymerizing factor (ADF)/cofilin family are thought to control actin-based motile processes. ADF1 from Arabidopsis thaliana appears to be a good model that is functionally similar to other members of the family. The function of ADF in actin dynamics has been examined using a combination of physical-chemical methods and actin-based motility assays, under physiological ionic conditions and at pH 7.8. ADF binds the ADP-bound forms of G- or F-actin with an affinity two orders of magnitude higher than the ATP- or ADP-Pi-bound forms. A major property of ADF is its ability to enhance the in vitro turnover rate (treadmilling) of actin filaments to a value comparable to that observed in vivo in motile lamellipodia. ADF increases the rate of propulsion of Listeria monocytogenes in highly diluted, ADF-limited platelet extracts and shortens the actin tails. These effects are mediated by the participation of ADF in actin filament assembly, which results in a change in the kinetic parameters at the two ends of the actin filament. The kinetic effects of ADF are end specific and cannot be accounted for by filament severing. The main functionally relevant effect is a 25-fold increase in the rate of actin dissociation from the pointed ends, while the rate of dissociation from the barbed ends is unchanged. This large increase in the rate-limiting step of the monomer-polymer cycle at steady state is responsible for the increase in the rate of actin-based motile processes. In conclusion, the function of ADF is not to sequester G-actin. ADF uses ATP hydrolysis in actin assembly to enhance filament dynamics.

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.

The effect of the S14A mutation on the conformation and thermostability of Saccharomyces cerevisiae G-actin and its interaction with adenine nucleotides

The actin Ser14 hydroxyl is one of a number of ligands that binds to the gamma-phosphate of ATP thereby stabilizing the actin.ATP complex. In yeast actin, conversion of Ser14 to Ala (S14A), causes a temperature-sensitive phenotype in vivo and temperature-sensitive polymerization defects in vitro (Chen, X., and Rubenstein, P. A. (1995) J. Biol. Chem. 270, 11406-11414). Here, using a new luciferase-based procedure, we show that the mutation results in a 40-60-fold decrease in actin's affinity for ATP. The mutation causes a decrease in the intrinsic ATPase activity of both Ca- and Mg-G-actin at 30 degrees C and alters the protease susceptibility of sites on subdomain 2. Ca-S14A-actin but not Mg-S14A-actin binds etheno-ATP at 37 degrees C. Intrinsic tryptophan fluorescence measurements show that at 37 degrees C, Mg-S14A-actin but not the calcium form unfolds. CD measurements show the mutation causes a decrease in the apparent denaturation temperature for Ca-actin from 57 to 45 degrees C and for the magnesium form a decrease from 52 to 40 degrees C. Based on a re-examination of actin's crystal structure coordinates, we propose that the Ser14 hydroxyl forms a polar bridge between the ATP gamma-phosphate and the amide nitrogen of Gly74, thus conferring additional stability on the actin small domain.

Binding of divalent cation and nucleotide to G-actin in the presence of profilin

The effect of profilin, a G-actin binding protein, on the mechanism of exchange of the tightly bound metal ion and nucleotide on G-actin, has been investigated. 1) In low ionic strength buffer, profilin increases the rates of Ca2+ and Mg2+ dissociation from G-actin 250- and 50-fold, respectively. On the profilin-actin complex as well as on G-actin alone, nucleotide exchange is dependent on the concentration of divalent metal ion and is kinetically limited, at low concentration of metal ion, by the dissociation of the metal ion. 2) Under physiological ionic conditions, nucleotide exchange on G-actin is 1 order of magnitude faster than at low ionic strength. The rate of MgATP dissociation is increased by profilin from 0.05 s-1 to 2 s-1, the rate of MgADP dissociation is increased from 0.2 s-1 to 24 s-1. The dependences of the exchange rates on profilin concentration are consistent with a high affinity (5 x 10(6) to 10(7) M-1) of profilin for ATP-G-actin, and a 20-fold lower affinity for ADP-G-actin. Profilin binding to actin lowers the affinity of metal-nucleotide by about 1 order of magnitude. These results restrain the possible roles of profilin in actin assembly in vivo.

Sequence 18-29 on actin: antibody and spectroscopic probing of conformational changes

Experimental evidence for the involvement of the 18-29 site within actin subdomain-1 in the actomyosin weak binding interface includes the inhibition of actomyosin ATPase activity by specific peptide antibodies [Adams, S., & Reisler, E. (1993) Biochemistry 32, 5051-5056] and by the Dictyostelium actin mutant D24H/D25H [Johara, M., et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 2127-2131]. In this work, the effect of the 18-29 peptide antibodies on the polymerization and conformation of actin has been characterized. Binding of antibody to the 18-29 site strongly inhibited the MgCl2-induced polymerization of G-actin, had a much weaker impact on the CaCl2 polymerization of actin, and showed very little effect on the NaCl polymerization of G-actin. These observations were linked to the binding of the 18-29 antibody to the different forms of actin. In sedimentation assays, the (18-29) IgG bound more strongly to Mg-F- and Mg-G-actins than to Ca-F- and Ca-G-actins, respectively. The binding of IgG to F-actin decreased sharply with an increase in ionic strength. Antibody binding to the 18-29 site induced conformational changes within the nucleotide cleft, both slowing the rate of nucleotide exchange and increasing the fluorescence intensity of actin-bound epsilon ATP. The increased fluorescence of a dansyl probe attached to Gln-41 and a pyrene probe attached to Cys-374 demonstrated that antibody binding also caused local perturbations in the DNase I loop of subdomain-2 and at the C-terminus of actin. These results are discussed in terms of actin plasticity and its implications for actomyosin interactions.

Actin-bound nucleotide/divalent cation interactions

At this point, it may be worthwhile to list, in summary form, the important aspects of divalent cation and nucleotide binding to actin that have been reviewed here: 1) High affinity divalent cation binding to actin is very tight, with equilibrium dissociation constant KCa approximately 1 nM and KMg approximately 5 nM at pH 7.0. 2) The binding kinetics of Ca++ are diffusion limited. Dissociation is slow, with k-Ca approximately 0.015 sec at pH 7.0 (and low ionic strength). 3) The binding kinetics of Mg++ are limited by the characteristics of the Mg++ aquo-ion and are much slower than for Ca++; k-Mg approximately 0.0012 at pH 7.0. 4) Increase in pH or ionic strength weakens divalent cation binding at the high affinity site, primarily by increasing k-Ca and k-Mg. 5) Exchange of Mg++ for Ca++ (or vice versa) at the high affinity site is by a competitive pseudo-first order process with an apparent rate constant (kapp) intermediate between k-Ca and k-Mg and dependent upon the cation concentration ratio [Ca]/[Mg] present. 6) High affinity ATP binding is modulated by the high affinity divalent cation. The cation concentration range over which this modulation occurs is about 100-fold higher for Mg++ than for Ca++, again because of the different characteristics of the Mg++ and Ca++ aquo-ions. 7) At low divalent cation concentrations, ATP dissociation from actin is limited by dissociation of the tightly-bound divalent cation. 8) At high divalent cation concentrations, ATP dissociation probably occurs via dissociation of the divalent cation-nucleotide complex and is quite slow, with dissociation rate constant approximately 0.0005 sec-1. 9) Competitive nucleotide exchange on actin may be described by a pseudo-first order model analogous to that for divalent cation exchange. The pseudo-first order rate constants depend upon the divalent cation concentration. The overall nucleotide exchange rate constant kex depends upon these constants and the solution nucleotide concentration ratio, e.g. [ATP]/[ADP]. The following circumstances develop from the characteristics of the high affinity binding of divalent cation and nucleotide to actin: 1) The standard methods for actin preparation convert in vivo Mg-actin into Ca-actin. 2) Converting Ca-actin back to Mg-actin is not easy. A very low ratio of [Ca]/[Mg] is necessary, which usually requires the use of Ca-cheltors, and a long time (5-10 min) must be allowed for complete exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Actin polymerization: regulation by divalent metal ion and nucleotide binding, ATP hydrolysis and binding of myosin

Actin filaments are major dynamic components of the cytoskeleton of eukaryotic cells. Assembly of filaments from monomeric actin occurs with expenditure of energy, the tightly bound ATP being irreversibly hydrolyzed during polymerization. This dissipation of energy perturbs the laws of reversible helical polymerization defined by Oosawa and Asakura (1975), and affects the dynamics of actin filaments. We have shown that ATP hydrolysis destabilizes actin-actin interactions in the filament. The destabilization is linked to the liberation of Pi that follows cleavage of gamma-phosphate. Pi release therefore plays the role of a conformational switch. Because ATP hydrolysis is uncoupled from polymerization, the nucleotide content of the filaments changes during the polymerization process, and filaments grow with a stabilizing "cap" of terminal ADP-Pi subunits. The fact that the dynamic properties of F-actin are affected by ATP hydrolysis results in a non-linear dependence of the rate of filament elongation on monomer concentration. Possible modes of regulation of filament assembly may be anticipated from the basic properties of actin. We have shown that the tightly bound divalent metal ion (Ca2+ or Mg2+) interacts with the beta- and gamma-phosphates of ATP bound to actin, and that the Me-ATP bidentate chelate is bound to G-actin in the A configuration. The nature of the bound metal ion affects the conformation of actin and the rate of ATP hydrolysis. In motile living cells, a large pool of actin is maintained unpolymerized by interaction with G-actin binding proteins such as thymosin beta 4 and its variants or profilin. Part of this pool is released to increase the F-actin pool upon cell stimulation. The role of G-actin polymerizing proteins may be crucial in defining the patterns of filament assembly in these situations. The myosin head (myosin subfragment-1) may be considered as a model actin polymerizing protein, may be the closest model to the short tailed myosin I family. The mechanism of assembly of decorated filaments from G-actin and myosin subfragment-1 has therefore been examined.

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

Actin can be specifically cleaved between residues 42 and 43 with a novel protease from Escherichia coli A2 strain (ECP) [Khaitlina, S. Y., Collins, J. H., Kuznetsova, I.M., Pershina, V.P., Synakevich, I.G., Turoverov, K.K. & Usmanova, A.M. (1991) FEBS Lett. 279, 49-51]. The resulting C-terminal and N-terminal fragments remained associated to one another in the presence of either Ca2+ or Mg2+. The protease-treated actin was, however, neither able to spontaneously assemble into filaments nor to copolymerize with intact actin unless its tightly bound Ca2+ was replaced with Mg2+. Substitution of Mg2+ for the bound Ca2+ was also necessary to partially restore the ability of the protease-treated actin to inhibit the DNase I activity. The critical concentration for KCl-induced polymerization of ECP-treated ATP-Mg-G-actin, determined by measuring the fluorescence of pyrenyl label, was approximately threefold higher than that for actin cleaved between residues 47 and 48 using subtilisin, and 36-fold higher than the critical concentration for polymerization of intact actin under the same conditions. Morphologically, the filaments of ECP-treated actin were indistinguishable from those of intact actin. Comparison of the fluorescence spectra of pyrenyl-labelled actins and chemical cross-linking with N,N'-1,2-phenylenebismaleimide have, however, revealed structural differences between the filaments assembled from ECP-treated actin and those of intact as well as subtilisin-treated actin. Moreover, the filaments of ECP-treated actin were easily disrupted by centrifugal forces or shearing stress unless they were stabilized by phalloidin. The results are consistent with the direct participation of the region around residues 42 and 43 in the monomer/monomer interactions as predicted from the atomic model of F-actin [Holmes, K.C., Popp, D., Gebhard, W. & Kabsch, W. (1990) Nature 347, 44-49] and suggest that the interactions involving this region are of primary importance for stabilization of the actin filament. The mechanism of the regulation of actin polymerization by the tightly bound divalent cation is also discussed.

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.

Clinical correlates of the molecular and cellular actions of magnesium on the cardiovascular system

Magnesium is a ubiquitous element that participates in metabolic processes essential for life. Magnesium acts as a metallic cofactor in more than 300 enzymatic reactions; notably it is essential for all reactions requiring ATP. Magnesium also functions as a transmembrane and intracellular modulator of other ions. Altered magnesium homeostasis, particularly a deficiency, can cause alterations in metabolic functions that result in clinically recognizable events. Recognition of magnesium deficiency is problematic, since there is no test that will reliably and consistently detect this condition. A high index of suspicion for magnesium deficiency is necessary and treatment should be given when indicated. This article reviews the molecular and cellular actions of magnesium and correlates these basic scientific findings with clinically recognized cardiovascular events in humans. In addition, management guidelines are delineated.

Thermodynamics of actin polymerization; influence of the tightly bound divalent cation and nucleotide

Previous work by this laboratory has shown that the tightly bound divalent cation of actin affects the enthalpy of the polymerization reaction for ATP-actin (Selden et al. (1986) J. Muscle Res. Cell Motil. 7, 215-224). In the present study, we have measured the temperature dependence of polymerization for actin containing ATP or ADP as the bound nucleotide and Mg2+ or Ca2+ (Mg-actin or Ca-actin) as the tightly bound divalent cation. In contrast to the marked effect of the tightly bound divalent cation on enthalpy and entropy changes for the polymerization of ATP-actin, ADP-actin polymerization is affected very little by the tightly bound divalent cation. The Arrhenius and van't Hoff plots for polymerization of Ca-ATP-, Mg-ADP- and Ca-ADP-actin were found to be non-linear. The free energy data for actin polymerization have been analyzed as a second order function of absolute temperature (Osborne et al. (1976) Biochemistry 15, 317-320). The values of the enthalpy change and activation enthalpy change for Ca-ATP-, Mg-ADP- and Ca-ADP-actin polymerization were found to be temperature-dependent, in contrast to those for Mg-ATP-actin, which were nearly constant over the temperature range studied. These results suggest that (1) polymerization of actin which does not contain both Mg2+ and ATP may be a multi-step reaction including a rate-limiting step and (2) Mg-ATP-actin has a unique conformation which enhances its ability to polymerize.

Nucleotide hydrolysis in cytoskeletal assembly

Two major polymers of the cytoskeleton, actin filaments and microtubules, are assembled with expenditure of energy: the ATP/GTP tightly bound to actin/tubulin is irreversibly hydrolyzed to ADP/GTP during the assembly process, and liberation of Pi in the medium occurs subsequent to the incorporation of subunits in the polymer. Pi release acts as a switch, causing the destabilization of protein-protein interactions in the polymer, therefore regulating the dynamics of these fibres. An understanding of this regulation in vivo requires that progress be made in four areas: the chemistry of the NTPase reaction; the structure of the intermediates in nucleotide hydrolysis and the nature of the conformational switch; the regulation of parameters involved in dynamic instability of microtubules; and the possible involvement of nucleotide hydrolysis in the macroscopic organization of these polymers in highly concentrated solutions, compared with the simple case of a equilibrium polymers. Progress made along these lines will define trends for future investigation.

Time-resolved cryo-electron microscopy of vitrified muscular components

Biological objects may be arrested in defined stages of their activity by fast freezing and may then be structurally examined. If the time between the start of activity and freezing is controlled, structural rearrangements due to biological function can be determined. Cryo-electron microscopy shows great potential for the study of such time-dependent phenomena. This study examines the actin polymerization process using cryo-electron microscopy of vitrified specimens. Actin filaments are shown to undergo a structural change during polymerization. In the early stages of the polymerization process (t less than 2 min), filaments exhibit a pronounced structural variation and frequently show a central low-density area. In the later stages of the polymerization, F-actin-ADP filaments have a more uniform appearance and rarely display a central low-density area. These findings, analysed on the basis of a previously proposed polymerization model, suggest that polymerization intermediates (F-actin-ATP and more probably F-actin-ADP-Pi) and filaments at steady state (F-actin-ADP) have different structures. To investigate the physiological relevance of these results at the cellular level, the potential of cryo-substitution in preserving the structure of muscular fibre was assessed. Optical diffraction patterns of relaxed and contracted frog cutaneous muscle are similar to the corresponding X-ray diffraction patterns. The resolution of the images extends to about 7 nm. These results show that dynamic study of muscle contraction is possible using cryo-substitution.

Atomic structure of the actin:DNase I complex

The atomic models of the complex between rabbit skeletal muscle actin and bovine pancreatic deoxyribonuclease I both in the ATP and ADP forms have been determined by X-ray analysis at an effective resolution of 2.8 A and 3A, respectively. The two structures are very similar. The actin molecule consists of two domains which can be further subdivided into two subdomains. ADP or ATP is located in the cleft between the domains with a calcium ion bound to the beta- or beta- and gamma-phosphates, respectively. The motif of a five-stranded beta sheet consisting of a beta meander and a right handed beta alpha beta unit appears in each domain suggesting that gene duplication might have occurred. These sheets have the same topology as that found in hexokinase.