Kinetics of the interaction of myosin subfragment-1 with G-actin. Effect of nucleotides and DNaseI

Mechanisms of the modulation of actin-myosin interactions by A1-type myosin light chains

BACKGROUND

The interaction of N-terminal extension of the myosin A1 essential light chain (A1 ELC) with actin is receiving increasing attention as a target in utilizing synthetic A1 ELC N-terminal-derived peptides in cardiac dysfunction therapy.

METHODS

To elucidate the mechanism by which these peptides regulate actin-myosin interaction, here we have investigated their effects on the myosin subfragment 1 (S1)-induced polymerization of G-actin.

RESULTS

The MLCF_pep and MLCS_pep peptides spanning the 3-12 of A1 ELC sequences from fast and slow skeletal muscle, respectively, increased the rate of actin polymerization not only by S1(A2) but also the rate of S1(A1)-induced actin polymerization, suggesting that they did not interfere with the direct binding of A1 ELC with actin. The efficiency of actin polymerization in the presence of the N-terminal ELC peptides depended on their sequence. Substitution of aspartic acid for neutral asparagine at position 5 of MLCF_pep dramatically enhanced its ability to stimulate S1-induced polymerization and enabled it to initiate polymerization of G-actin in the absence of S1.

CONCLUSIONS

These and other results presented in this work suggest that the modulation of myosin motor activity by N-terminal ELC peptides is exerted through a change in actin filament conformation rather than through blocking the A1 ELC-actin interaction.

GENERAL SIGNIFICANCE

The results imply the possibility of enhancing therapeutic effects of these peptides by modifications of their sequence.

The discovery of actin: "to see what everyone else has seen, and to think what nobody has thought"

Actin is among the most highly abundant and ubiquitous proteins in eukaryotic cells. The structure, dynamics and functional diversity of actin have continued to mesmerise cell and molecular biologists, biophysicists and physiologists for more than three quarters of a century. The discovery and initial characterization of actin, which took place in the laboratory of Albert Szent-Györgyi by Ilona Banga and Brúnó F. Straub during the second world war in Hungary, is a remarkable and inspiring moment in the history of science. Many of the early thoughts and ideas on the properties and functions of actin and particularly actomyosin, which are referred to in this short historical overview, resonate freshly even today.

A crosslinked and ribosylated actin trimer does not interact productively with myosin

A purified F-actin-derived actin trimer that interacts with end-binding proteins did not activate or bind the side-binding protein myosin under rigor conditions. Remodeling of the actin trimer by the binding of gelsolin did not rescue myosin binding, nor did the use of different means of inhibiting the polymerization of the trimer. Our results demonstrate that ADP-ribosylation on all actin subunits of an F-actin-derived trimer inhibits myosin binding and that the binding of DNase-I to the pointed end subunits of a crosslinked trimer also remodels the myosin binding site. Taken together, this work highlights the need for a careful balance between modification of actin subunits and maintaining protein–protein interactions to produce a physiologically relevant short F-actin complex.

Association between α4 integrin cytoplasmic tail and non-muscle myosin IIA regulates cell migration

α4β1 integrin regulates cell migration via cytoplasmic interactions. Here, we report an association between the cytoplasmic tail of α4 integrin (α4 tail) and non-muscle myosin IIA (MIIA), demonstrated by co-immunoprecipitation of the MIIA heavy chain (HC) with anti-α4-integrin antibodies and pull-down of MIIA-HC with recombinant α4 tail from cell lysates. The association between the α4 tail and MIIA does not require paxillin binding or phosphorylation at Ser988 in the α4 tail. We found that substituting Glu982 in the α4 tail with alanine (E982A) disrupts the α4-MIIA association without interfering with the paxillin binding or Ser988 phosphorylation. By comparing stably transfected CHO cells, we show that the E982A mutation reduces the ability of α4β1 integrin to mediate cell spreading and to promote front-back polarization. In addition, we show that E982A impairs shear-flow-induced migration of the α4-integrin-expressing CHO cells by reducing their migration speed and directional persistence. The E982A mutation also leads to defects in the organization of MIIA filament bundles. Furthermore, when cells are plated on fibronectin and simulated with shear flow, α4β1 integrin forms filament-like patterns that co-align with MIIA filament bundles. These results provide a new mechanism for linking integrins to the actomyosin cytoskeleton and for regulating cell migration by integrins and non-muscle myosin II.

The actin-myosin interface

In order to understand the mechanism of muscle contraction at the atomic level, it is necessary to understand how myosin binds to actin in a reversible way. We have used a novel molecular dynamics technique constrained by an EM map of the actin-myosin complex at 13-A resolution to obtain an atomic model of the strong-binding (rigor) actin-myosin interface. The constraining force resulting from the EM map during the molecular dynamics simulation was sufficient to convert the myosin head from the initial weak-binding state to the strong-binding (rigor) state. Our actin-myosin model suggests extensive contacts between actin and the myosin head (S1). S1 binds to two actin monomers. The contact surface between actin and S1 has increased dramatically compared with previous models. A number of loops in S1 and actin are involved in establishing the interface. Our model also suggests how the loop carrying the critical Arg 405 Glu mutation in S1 found in a familial cardiomyopathy might be functionally involved.

Mechanism of actin polymerization by myosin subfragment-1 probed by dynamic light scattering

Monomeric actin (G-actin) polymerizes spontaneously into helical filaments in the presence of inorganic salts. The slowest, rate-limiting step of the polymerization process is formation of actin trimers, the smallest oligomers that serve as nuclei for fast filament growth (filament elongation) by monomer addition at the filament ends. In low ionic-strength solutions, actin can be polymerized by myosin subfragment-1 (S1). In early works it has been suggested that G-actin-S1 1:1 complexes (GS) assemble into filaments according to the nucleation-filament elongation scheme. Subsequent studies indicated that one S1 molecule can bind two actin monomers, and that oligomerization of the initial complexes is a fast reaction. This has led to suggest an alternative mechanism, with a ternary G(2)S complex and its oligomers being predominant intermediates of S1-induced assembly of G-actin into filaments. We used dynamic light scattering to analyze the initial steps of S1-induced polymerization of actin. Our results suggest formation of GS complexes and their oligomers in the presence of S1 equimolar to or in excess over actin. We confirm formation of G(2)S complexes as intermediates of S1-induced polymerization in the presence of actin in excess over S1.

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

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

Formation and destabilization of actin filaments with tetramethylrhodamine-modified actin

Actin labeling at Cys(374) with tethramethylrhodamine derivatives (TMR-actin) has been widely used for direct observation of the in vitro filaments growth, branching, and treadmilling, as well as for the in vivo visualization of actin cytoskeleton. The advantage of TMR-actin is that it does not lock actin in filaments (as rhodamine-phalloidin does), possibly allowing for its use in investigating the dynamic assembly behavior of actin polymers. Although it is established that TMR-actin alone is polymerization incompetent, the impact of its copolymerization with unlabeled actin on filament structure and dynamics has not been tested yet. In this study, we show that TMR-actin perturbs the filaments structure when copolymerized with unlabeled actin; the resulting filaments are more fragile and shorter than the control filaments. Due to the increased severing of copolymer filaments, TMR-actin accelerates the polymerization of unlabeled actin in solution also at mole ratios lower than those used in most fluorescence microscopy experiments. The destabilizing and severing effect of TMR-actin is countered by filament stabilizing factors, phalloidin, S1, and tropomyosin. These results point to an analogy between the effects of TMR-actin and severing proteins on F-actin, and imply that TMR-actin may be inappropriate for investigations of actin filaments dynamics.

Cdc42/Rac1-dependent activation of the p21-activated kinase (PAK) regulates human platelet lamellipodia spreading: implication of the cortical-actin binding protein cortactin

Platelet activation by thrombin or thrombin receptor-activating peptide (TRAP) results in extensive actin reorganization that leads to filopodia emission and lamellae spreading concomitantly with activation of the Rho family small G proteins, Cdc42 and Rac1. Evidence has been provided that direct binding of Cdc42-guanosine triphosphate (GTP) and Rac1-GTP to the N-terminal regulatory domain of the p21-activated kinase (PAK) stimulates PAK activation and actin reorganization. In the present study, we have investigated the relationship between shape change and PAK activation. We show that thrombin, TRAP, or monoclonal antibody (MoAb) anti-Fc(gamma)RIIA IV.3 induces an activation of Cdc42 and Rac1. The GpVI ligand, convulxin (CVX), that forces platelets to lamellae spreading efficiently activates Rac1. Thrombin, TRAP, MoAb IV.3, and CVX stimulate autophosphorylation and kinase activity of PAK. Inhibition of Cdc42 and Rac1 with clostridial toxin B inhibits PAK activation and lamellae spreading. The cortical-actin binding protein, p80/85 cortactin, is constitutively associated with PAK in resting platelets and dissociates from PAK after thrombin stimulation. Inhibition of PAK autophosphorylation by toxin B prevents the dissociation of cortactin. These results suggest that Cdc42/Rac1-dependent activation of PAK may trigger early platelet shape change, at least in part through the regulation of cortactin binding to PAK.

Intrastrand cross-linked actin between Gln-41 and Cys-374. II. Properties of cross-linked oligomers

Actin filaments partially cross-linked with ANP (N-(4-azido-2-nitrophenyl)-putrescine between Gln-41 and Cys-374 on adjacent monomers in the long-pitch helix were depolymerized and fractionated into pools of longitudinal cross-linked dimers (s(o)20,w = 5.55 +/- 0.22 S), trimers (s(o)20,w = 6.93 +/- 0.12 S), and higher-order oligomers. Competition binding experiments of myosin subfragment (S1) to cross-linked dimers in the presence of pyrenyl G-actin revealed about 2 orders of magnitude stronger binding of the first than that of the second S1 molecule to actin dimer. Under similar conditions the unpolymerized cross-linked actin species activated the MgATPase of S1 only severalfold compared to 70-fold activation by F-actin. The cross-linked dimers, trimers, and oligomers were polymerized into filaments by MgCl2 faster than un-cross-linked actin. In electron micrographs these filaments appeared sometimes shorter and had greater tendency to bend than un-cross-linked actin filaments. Small amounts of cross-linked actin dimers nucleated S1-induced polymerization of actin, but the polymerization by S1 was inhibited for pure populations of cross-linked dimers, trimers, and oligomers. The cross-linked dimers did not decrease the kinetic difference between the polymerization of actin by S1 isozymes S1(A1) and S1(A2). According to electron microscopy evidence, cross-linked actin oligomers polymerized by S1 yielded much shorter arrowhead structures than the un-cross-linked actin. These results indicate the importance of lateral actin-actin interaction for the activation of myosin ATPase and the polymerization of actin by S1.

Kinetics of myosin subfragment-1-induced condensation of G-actin into oligomers, precursors in the assembly of F-actin-S1. Role of the tightly bound metal ion and ATP hydrolysis

In a low ionic strength buffer and in the absence of free ATP, the interaction of G-actin (G) with myosin subfragment-1 (S1) leads to the formation of arrowhead-decorated F-actin-S1 filaments, through a series of elementary steps. The initial formation of GS and G2S complexes is followed by their condensation into short oligomers. The kinetics of formation of G-actin-S1 oligomers have been monitored in a stopped-flow apparatus using a combination of light scattering and fluorescence of NBD-labeled actin. Oligomers appear more stable and are formed at a faster rate from MgATP-G-actin than from CaATP-G-actin. The actin-bound ATP is hydrolyzed when oligomers are formed from MgATP-G-actin, not when they are formed from CaATP-G-actin. The formation of oligomers is energetically favored in the presence of cytochalasin D. All data are consistent with the view that the actin-actin interactions which take place upon condensation of GS and G2S into oligomers are very similar to lateral actin-actin interactions along the short pitch helix of actin filaments, which are involved in actin nucleation. These interactions trigger ATP hydrolysis on actin.

Mechanism of myosin subfragment-1-induced assembly of CaG-actin and MgG-actin into F-actin-S1-decorated filaments

The kinetics and mechanism of myosin subfragment-1-induced polymerization of G-actin into F-actin-S1-decorated filaments have been investigated in low ionic strength buffer and in the absence of free ATP. The mechanism of assembly of F-actin-S1 differs from salt-induced assembly of F-actin. Initial condensation of G-actin and S1 into oligomers in reversible equilibrium is a prerequisite step in the formation of F-actin-S1 . Oligomers have a relatively low stability (10(6) M-1) and contain S1 in a molar ratio to actin close to 0.5. Increased binding of S1 up to a 1:1 molar ratio to actin is associated with further irreversible condensation of oligomers into large F-actin-S1 structures of very high stability. In contrast to salt-induced assembly of F-actin, no monomer-polymer equilibrium, characterized by a critical concentration, can be defined for F-actin-S1 assembly, and end-to-end annealing of oligomers is predominant over growth from nuclei in the kinetics. Simultaneous recordings of the changes in light scattering, pyrenyl- and NBD-actin fluorescence, ATP hydrolysis, and release of Pi during the polymerization process have been analyzed to propose a minimum kinetic scheme for assembly, within which several elementary steps, following oligomer formation, are required for assembly of F-actin-S1. ATP hydrolysis occurs before polymerization of MgATP-G-actin but not of CaATP-G-actin. The release of inorganic phosphate occurs on F-actin-S1 at the same rate as on F-actin.

Kinetics of association of myosin subfragment-1 to unlabeled and pyrenyl-labeled actin

The kinetics of reaction of myosin subfragment-1 (S1) with F-actin have been monitored by the changes in light scattering and in pyrenyl-actin fluorescence at 20 degrees C, pH 7.5, and physiological ionic strength. The association rate constant of S1 to F-actin decreases about 10-fold as the molar ratio of bound S1 increases from 0 to 1. This decrease in k+ is most likely due to the steric hindrance of available binding sites by initially bound S1. The apparent rate constant for association of S1 to bare filaments is 9 microM-1 s-1, a value 1 order of magnitude higher than the one previously estimated from experiments in which S1 was in excess over F-actin. The anticooperative binding kinetics of S1 to F-actin are consistent with the negative cooperativity displayed in the equilibrium binding curves of S1 to pyrenyl-F-actin. Fluorescence titration curves of partially labeled pyrenyl-F-actin by S1 are sigmoidal, consistent with a 4-fold higher affinity of S1 for unlabeled than for labeled action. This conclusion is strengthened by kinetic data of S1 binding to partially labeled F-actin, which exhibit a biphasic behavior due to the slower dissociation of S1 from unlabeled than from labeled actin.