Conformational change and cooperativity in actin filaments free of tropomyosin

Multiple Mechanisms to Regulate Actin Functions: "Fundamental" Versus Lineage-Specific Mechanisms and Hierarchical Relationships

Eukaryotic actin filaments play a central role in numerous cellular functions, with each function relying on the interaction of actin filaments with specific actin-binding proteins. Understanding the mechanisms that regulate these interactions is key to uncovering how actin filaments perform diverse roles at different cellular locations. Several distinct classes of actin regulatory mechanisms have been proposed and experimentally supported. However, these mechanisms vary in their nature and hierarchy. For instance, some operate under the control of others, highlighting hierarchical relationships. Additionally, while certain mechanisms are fundamental and ubiquitous across eukaryotes, others are lineage-specific. Here, we emphasize the fundamental importance and functional significance of the following actin regulatory mechanisms: the biochemical regulation of actin nucleators, the ATP hydrolysis-dependent aging of actin filaments, thermal fluctuation- and mechanical strain-dependent conformational changes of actin filaments, and cooperative conformational changes induced by actin-binding proteins.

Long-Range and Directional Allostery of Actin Filaments Plays Important Roles in Various Cellular Activities

A wide variety of uniquely localized actin-binding proteins (ABPs) are involved in various cellular activities, such as cytokinesis, migration, adhesion, morphogenesis, and intracellular transport. In a micrometer-scale space such as the inside of cells, protein molecules diffuse throughout the cell interior within seconds. In this condition, how can ABPs selectively bind to particular actin filaments when there is an abundance of actin filaments in the cytoplasm? In recent years, several ABPs have been reported to induce cooperative conformational changes to actin filaments allowing structural changes to propagate along the filament cables uni- or bidirectionally, thereby regulating the subsequent binding of ABPs. Such propagation of ABP-induced cooperative conformational changes in actin filaments may be advantageous for the elaborate regulation of cellular activities driven by actin-based machineries in the intracellular space, which is dominated by diffusion. In this review, we focus on long-range allosteric regulation driven by cooperative conformational changes of actin filaments that are evoked by binding of ABPs, and discuss roles of allostery of actin filaments in narrow intracellular spaces.

K336I mutant actin alters the structure of neighbouring protomers in filaments and reduces affinity for actin-binding proteins

Mutation of the Lys-336 residue of actin to Ile (K336I) or Asp (K336E) causes congenital myopathy. To understand the effect of this mutation on the function of actin filaments and gain insight into the mechanism of disease onset, we prepared and biochemically characterised K336I mutant actin from Dictyostelium discoideum. Subtilisin cleavage assays revealed that the structure of the DNase-I binding loop (D-loop) of monomeric K336I actin, which would face the adjacent actin-protomer in filaments, differed from that of wild type (WT) actin. Although K336I actin underwent normal salt-dependent reversible polymerisation and formed apparently normal filaments, interactions of K336I filaments with alpha-actinin, myosin II, and cofilin were disrupted. Furthermore, co-filaments of K336I and WT actins also exhibited abnormal interactions with cofilin, implying that K336I actin altered the structure of the neighbouring WT actin protomers such that interaction between cofilin and the WT actin protomers was prevented. We speculate that disruption of the interactions between co-filaments and actin-binding proteins is the primary reason why the K336I mutation induces muscle disease in a dominant fashion.

Cardiac leiomodin2 binds to the sides of actin filaments and regulates the ATPase activity of myosin

Leiomodin proteins are vertebrate homologues of tropomodulin, having a role in the assembly and maintenance of muscle thin filaments. Leiomodin2 contains an N-terminal tropomodulin homolog fragment including tropomyosin-, and actin-binding sites, and a C-terminal Wiskott-Aldrich syndrome homology 2 actin-binding domain. The cardiac leiomodin2 isoform associates to the pointed end of actin filaments, where it supports the lengthening of thin filaments and competes with tropomodulin. It was recently found that cardiac leiomodin2 can localise also along the length of sarcomeric actin filaments. While the activities of leiomodin2 related to pointed end binding are relatively well described, the potential side binding activity and its functional consequences are less well understood. To better understand the biological functions of leiomodin2, in the present work we analysed the structural features and the activities of Rattus norvegicus cardiac leiomodin2 in actin dynamics by spectroscopic and high-speed sedimentation approaches. By monitoring the fluorescence parameters of leiomodin2 tryptophan residues we found that it possesses flexible, intrinsically disordered regions. Leiomodin2 accelerates the polymerisation of actin in an ionic strength dependent manner, which relies on its N-terminal regions. Importantly, we demonstrate that leiomodin2 binds to the sides of actin filaments and induces structural alterations in actin filaments. Upon its interaction with the filaments leiomodin2 decreases the actin-activated Mg²⁺-ATPase activity of skeletal muscle myosin. These observations suggest that through its binding to side of actin filaments and its effect on myosin activity leiomodin2 has more functions in muscle cells than it was indicated in previous studies.

Actin binding domain of filamin distinguishes posterior from anterior actin filaments in migrating Dictyostelium cells

Actin filaments in different parts of a cell interact with specific actin binding proteins (ABPs) and perform different functions in a spatially regulated manner. However, the mechanisms of those spatially-defined interactions have not been fully elucidated. If the structures of actin filaments differ in different parts of a cell, as suggested by previous in vitro structural studies, ABPs may distinguish these structural differences and interact with specific actin filaments in the cell. To test this hypothesis, we followed the translocation of the actin binding domain of filamin (ABD_FLN) fused with photoswitchable fluorescent protein (mKikGR) in polarized Dictyostelium cells. When ABD_FLN-mKikGR was photoswitched in the middle of a polarized cell, photoswitched ABD_FLN-mKikGR rapidly translocated to the rear of the cell, even though actin filaments were abundant in the front. The speed of translocation (>3 μm/s) was much faster than that of the retrograde flow of cortical actin filaments. Rapid translocation of ABD_FLN-mKikGR to the rear occurred normally in cells lacking GAPA, the only protein, other than actin, known to bind ABD_FLN. We suggest that ABD_FLN recognizes a certain feature of actin filaments in the rear of the cell and selectively binds to them, contributing to the posterior localization of filamin.

Allosteric regulation by cooperative conformational changes of actin filaments drives mutually exclusive binding with cofilin and myosin

Heavy meromyosin (HMM) of myosin II and cofilin each binds to actin filaments cooperatively and forms clusters along the filaments, but it is unknown whether the two cooperative bindings are correlated and what physiological roles they have. Fluorescence microscopy demonstrated that HMM-GFP and cofilin-mCherry each bound cooperatively to different parts of actin filaments when they were added simultaneously in 0.2 μM ATP, indicating that the two cooperative bindings are mutually exclusive. In 0.1 mM ATP, the motor domain of myosin (S1) strongly inhibited the formation of cofilin clusters along actin filaments. Under this condition, most actin protomers were unoccupied by S1 at any given moment, suggesting that transiently bound S1 alters the structure of actin filaments cooperatively and/or persistently to inhibit cofilin binding. Consistently, cosedimentation experiments using copolymers of actin and actin-S1 fusion protein demonstrated that the fusion protein affects the neighboring actin protomers, reducing their affinity for cofilin. In reciprocal experiments, cofilin-actin fusion protein reduced the affinity of neighboring actin protomers for S1. Thus, allosteric regulation by cooperative conformational changes of actin filaments contributes to mutually exclusive cooperative binding of myosin II and cofilin to actin filaments, and presumably to the differential localization of both proteins in cells.

Cofilin-induced cooperative conformational changes of actin subunits revealed using cofilin-actin fusion protein

To investigate cooperative conformational changes of actin filaments induced by cofilin binding, we engineered a fusion protein made of Dictyostelium cofilin and actin. The filaments of the fusion protein were functionally similar to actin filaments bound with cofilin in that they did not bind rhodamine-phalloidin, had quenched fluorescence of pyrene attached to Cys374 and showed enhanced susceptibility of the DNase loop to cleavage by subtilisin. Quantitative analyses of copolymers made of different ratios of the fusion protein and control actin further demonstrated that the fusion protein affects the structure of multiple neighboring actin subunits in copolymers. Based on these and other recent related studies, we propose a mechanism by which conformational changes induced by cofilin binding is propagated unidirectionally to the pointed ends of the filaments, and cofilin clusters grow unidirectionally to the pointed ends following this path. Interestingly, the fusion protein was unable to copolymerize with control actin at pH 6.5 and low ionic strength, suggesting that the structural difference between the actin moiety in the fusion protein and control actin is pH-sensitive.

The role of structural dynamics of actin in class-specific myosin motility

The structural dynamics of actin, including the tilting motion between the small and large domains, are essential for proper interactions with actin-binding proteins. Gly146 is situated at the hinge between the two domains, and we previously showed that a G146V mutation leads to severe motility defects in skeletal myosin but has no effect on motility of myosin V. The present study tested the hypothesis that G146V mutation impaired rotation between the two domains, leading to such functional defects. First, our study showed that depolymerization of G146V filaments was slower than that of wild-type filaments. This result is consistent with the distinction of structural states of G146V filaments from those of the wild type, considering the recent report that stabilization of actin filaments involves rotation of the two domains. Next, we measured intramolecular FRET efficiencies between two fluorophores in the two domains with or without skeletal muscle heavy meromyosin or the heavy meromyosin equivalent of myosin V in the presence of ATP. Single-molecule FRET measurements showed that the conformations of actin subunits of control and G146V actin filaments were different in the presence of skeletal muscle heavy meromyosin. This altered conformation of G146V subunits may lead to motility defects in myosin II. In contrast, distributions of FRET efficiencies of control and G146V subunits were similar in the presence of myosin V, consistent with the lack of motility defects in G146V actin with myosin V. The distribution of FRET efficiencies in the presence of myosin V was different from that in the presence of skeletal muscle heavy meromyosin, implying that the roles of actin conformation in myosin motility depend on the type of myosin.

Cofilin-induced unidirectional cooperative conformational changes in actin filaments revealed by high-speed atomic force microscopy

High-speed atomic force microscopy was employed to observe structural changes in actin filaments induced by cofilin binding. Consistent with previous electron and fluorescence microscopic studies, cofilin formed clusters along actin filaments, where the filaments were 2-nm thicker and the helical pitch was ∼25% shorter, compared to control filaments. Interestingly, the shortened helical pitch was propagated to the neighboring bare zone on the pointed-end side of the cluster, while the pitch on the barbed-end side was similar to the control. Thus, cofilin clusters induce distinctively asymmetric conformational changes in filaments. Consistent with the idea that cofilin favors actin structures with a shorter helical pitch, cofilin clusters grew unidirectionally toward the pointed-end of the filament. Severing was often observed near the boundaries between bare zones and clusters, but not necessarily at the boundaries.

DOI:http://dx.doi.org/10.7554/eLife.04806.001

Actin is one of the most abundant proteins found inside all eukaryotic cells including plant, animal, and fungal cells. This protein is involved in a wide range of biological processes that are essential for an organism's survival. Actin proteins form long filaments that help cells to maintain their shape and also provide the force required for cells to divide and/or move.

Actin filaments are helical in shape and are made up of many actin subunits joined together. Actin filaments are changeable structures that continuously grow and shrink as new actin subunits are added to or removed from the ends of the filaments. One end of an actin filament grows faster than the other; the fast-growing end is known as the barbed-end, while the slow-growing end is referred to as the pointed-end.

Over 100 other proteins are known to bind to and work with actin to regulate its roles in cells and how it forms into filaments. Cofilin is one such protein that binds to and forms clusters on actin filaments and it can also sever actin filaments. Severing an actin filament can encourage the filament to disassemble, or it can help produce new barbed ends that can then grow into new filaments. Previous work had suggested that cofilin severs actin filaments at the junction between regions on the filament that are coated with cofilin and those that are not. It was also known that cofilin binding to a filament causes the filament to change shape, and that the shape change is propagated to neighboring sections of the filaments not coated with cofilin. However, the details of where cofilin binds and how changes in shape are propagated along an actin filament were not known. Furthermore, the findings of these previous studies were largely based on examining still images of actin filaments, which are unlike the constantly changing filaments of living cells.

Ngo, Kodera et al. have now analyzed what happens when cofilin binds to and forms clusters along actin filaments using a recently developed imaging technique called high-speed atomic force microscopy. This technique can be used to directly visualize individual proteins in action. Consistent with previous findings, Ngo, Kodera et al. observed that filaments coated with cofilin are thicker than those filaments without cofilin; and that cofilin binding also substantially reduces the helical twist of the filament. Ngo, Kodera et al. also found that these changes in shape are propagated along the filament but in only one direction—towards the pointed-end. Moreover, cofilin clusters also only grew towards the pointed-end of the actin filament—and the filaments were often severed near, but not exactly at, the junctions between cofilin-coated and uncoated regions. Such one-directional changes in shape of the actin filaments presumably help to regulate how other actin binding proteins can interact with the filament and consequently regulate the roles of the filaments themselves.

DOI:http://dx.doi.org/10.7554/eLife.04806.002

Stretching actin filaments within cells enhances their affinity for the myosin II motor domain

To test the hypothesis that the myosin II motor domain (S1) preferentially binds to specific subsets of actin filaments in vivo, we expressed GFP-fused S1 with mutations that enhanced its affinity for actin in Dictyostelium cells. Consistent with the hypothesis, the GFP-S1 mutants were localized along specific portions of the cell cortex. Comparison with rhodamine-phalloidin staining in fixed cells demonstrated that the GFP-S1 probes preferentially bound to actin filaments in the rear cortex and cleavage furrows, where actin filaments are stretched by interaction with endogenous myosin II filaments. The GFP-S1 probes were similarly enriched in the cortex stretched passively by traction forces in the absence of myosin II or by external forces using a microcapillary. The preferential binding of GFP-S1 mutants to stretched actin filaments did not depend on cortexillin I or PTEN, two proteins previously implicated in the recruitment of myosin II filaments to stretched cortex. These results suggested that it is the stretching of the actin filaments itself that increases their affinity for the myosin II motor domain. In contrast, the GFP-fused myosin I motor domain did not localize to stretched actin filaments, which suggests different preferences of the motor domains for different structures of actin filaments play a role in distinct intracellular localizations of myosin I and II. We propose a scheme in which the stretching of actin filaments, the preferential binding of myosin II filaments to stretched actin filaments, and myosin II-dependent contraction form a positive feedback loop that contributes to the stabilization of cell polarity and to the responsiveness of the cells to external mechanical stimuli.

Novel mode of cooperative binding between myosin and Mg2+ -actin filaments in the presence of low concentrations of ATP

Cooperative interaction between myosin and actin filaments has been detected by a number of different methods, and has been suggested to have some role in force generation by the actomyosin motor. In this study, we observed the binding of myosin to actin filaments directly using fluorescence microscopy to analyze the mechanism of the cooperative interaction in more detail. For this purpose, we prepared fluorescently labeled heavy meromyosin (HMM) of rabbit skeletal muscle myosin and Dictyostelium myosin II. Both types of HMMs formed fluorescent clusters along actin filaments when added at substoichiometric amounts. Quantitative analysis of the fluorescence intensity of the HMM clusters revealed that there are two distinct types of cooperative binding. The stronger form was observed along Ca(2+)-actin filaments with substoichiometric amounts of bound phalloidin, in which the density of HMM molecules in the clusters was comparable to full decoration. The novel, weaker form was observed along Mg(2+)-actin filaments with and without stoichiometric amounts of phalloidin. HMM density in the clusters of the weaker form was several-fold lower than full decoration. The weak cooperative binding required sub-micromolar ATP, and did not occur in the absence of nucleotides or in the presence of ADP and ADP-Vi. The G680V mutant of Dictyostelium HMM, which over-occupies the ADP-Pi bound state in the presence of actin filaments and ATP, also formed clusters along Mg(2+)-actin filaments, suggesting that the weak cooperative binding of HMM to actin filaments occurs or initiates at an intermediate state of the actomyosin-ADP-Pi complex other than that attained by adding ADP-Vi.

Muscle contraction as a Markov process. I: Energetics of the process

Force generation during muscle contraction can be understood in terms of cyclical length changes in segments of actin thin filaments moving through the three-dimensional lattice of myosin thick filaments. Recent anomalies discovered in connection with analysis of myosin step sizes in in vitro motility assays and with skinned fibres can be rationalized by assuming that ATP hydrolysis on actin accompanies these length changes. The paradoxically rapid regeneration of tension in quick release experiments, as well as classical energetic relationships, such as Hill's force-velocity curve, the Fenn effect, and the unexplained enthalpy of shortening, can be given mutually self-consistent explanations with this model. When muscle is viewed as a Markov process, the vectorial process of chemomechanical transduction can be understood in terms of lattice dependent transitions, wherein the phosphate release steps of the myosin and actin ATPases depend only on occurrence of allosteric changes in neighbouring molecules. Tropomyosin has a central role in coordinating the steady progression of these cooperative transitions along actin filaments and in gearing up the system in response to higher imposed loads.

Cooperative rigor binding of myosin to actin is a function of F-actin structure

Many aspects of cooperative behavior within pure F-actin filaments have now been described. We have used two myosin fragments, heavy meromyosin (HMM) and Subfragment 1 (S1), to look at the rigor binding to different forms of F-actin. With Ca2+ bound at the high-affinity metal binding site in actin, there is a very large cooperativity in the binding of HMM, but no cooperativity for S1. With Mg2+ bound at the high affinity site, or with conditions that stabilize the conformation of subdomain-2 of actin, there is no cooperativity seen with either HMM or S1. These results show that the two heads of HMM can induce structural changes in F-actin that are not observed with the single head of S1. They also support the notion that the binding of myosin to F-actin induces a conformational change in subdomain-2 of actin, and that under certain conditions this conformational change can be cooperatively propagated through an actin filament.

Actomyosin interactions in the presence of ATP and the N-terminal segment of actin

The binding of myosin subfragment 1 (S-1) to actin in the presence of ATP and the acto-S-1 ATPase activities of acto-S-1 complexes were determined at 5 degrees C under conditions of partial saturation of actin, up to 90%, by antibodies against the first seven N-terminal residues on actin. The antibodies [Fab(1-7)] inhibited strongly the acto-S-1 ATPase and the binding of S-1 to actin in the presence of ATP at low concentrations of S-1, up to 25 microM. Further increases in S-1 concentration resulted in a partial and cooperative recovery of both the binding of S-1 to actin and the acto-S-1 ATPase while causing only limited displacement of Fab(1-7) from actin. The extent to which the binding and the ATPase activity were recovered depended on the saturation of actin by Fab(1-7). The combined amounts of S-1 and Fab binding to actin suggested that the activation of the myosin ATPase activity was due to actin free of Fab. Examination of the acto-S-1 ATPase activities as a function of S-1 bound to actin at different levels of actin saturation by Fab(1-7) revealed that the antibodies inhibited the activation of the bound myosin. Thus, the binding of antibodies to the N-terminal segment of actin can act to inhibit both the binding of S-1 to actin in the presence of ATP and a catalytic step in ATP hydrolysis by actomyosin. The implications of these results to the regulation of actomyosin interaction are discussed.

Subtilisin cleavage of actin inhibits in vitro sliding movement of actin filaments over myosin

Subtilisin cleaved actin was shown to retain several properties of intact actin including the binding of heavy meromyosin (HMM), the dissociation from HMM by ATP, and the activation of HMM ATPase activity. Similar Vmax but different Km values were obtained for acto-HMM ATPase with the cleaved and intact actins. The ATPase activity of HMM stimulated by copolymers of intact and cleaved actin showed a linear dependence on the fraction of intact actin in the copolymer. The most important difference between the intact and cleaved actin was observed in an in vitro motility assay for actin sliding movement over an HMM coated surface. Only 30% of the cleaved actin filaments appeared mobile in this assay and moreover, the velocity of the mobile filaments was approximately 30% that of intact actin filaments. These results suggest that the motility of actin filaments can be uncoupled from the activation of myosin ATPase activity and is dependent on the structural integrity of actin and perhaps, dynamic changes in the actin molecule.

Cryoenzymic studies on actomyosin ATPase. Evidence that the degree of saturation of actin with myosin subfragment 1 affects the kinetics of the binding of ATP

The initial steps of actomyosin subfragment 1 (acto-S1) ATPase (dissociation and binding of ATP) were studied at -15 degrees C with 40% ethylene glycol as antifreeze. The dissociation kinetics were followed by light scattering in a stopped-flow apparatus, and the binding of ATP was followed by the ATP chase method in a rapid-flow quench apparatus. The data from the chase experiments were fitted to E + ATP in equilibrium (K1) E.ATP----(k2) E*ATP, where E is acto-S1 or S1. The kinetics of the binding of ATP to acto-S1 were sensitive to the degree of saturation of the actin with S1. There was a sharp transition with actin nearly saturated with S1: when the S1 to actin ratio was low, the kinetics were fast (K1 greater than 300 microM, k2 greater than 40 s-1); when it was high, they were slow (K1 = 14 microM, k2 = 2 s-1). With S1 alone K1 = 12 microM and k2 = 0.07 S-1. With acto heavy meromyosin (acto-HMM) the binding kinetics were the same as with saturated acto-S1, regardless of the HMM to actin ratio. The dissociation kinetics were independent of the S1 to actin ratio. Saturation kinetics were obtained with Kd = 460 microM and kd = 75 S-1. The data for the saturated acto-S1 could be fitted to a reaction scheme, but for lack of structural information the abrupt dependence of the ATP binding kinetics upon the S1 to actin ratio is difficult to explain.(ABSTRACT TRUNCATED AT 250 WORDS)

Separation and qualitative recovery of major proteins from a single sample of skeletal muscle

The major soluble and myofibrillar proteins of skeletal muscle were separated into five fractions by extracting a single sample with solutions of increasing ionic strength and pH. After separation of myoglobin, other soluble proteins, and myosin, an acetone powder was prepared from the residue; the extractions were continued to yield actin and the troponin-tropomyosin complex. From 200 g of skeletal muscle the average recoveries were: total sarcoplasmic proteins, 4.5 g; myoglobin, 0.55 g; myosin, 2.7 g; actin, 0.1 g; and troponin-tropomyosin complex, 17.5 mg. The method was designed for investigating the effects of physical or chemical treatment of whole muscle or whole animals by monitoring changes in individual muscle proteins. This is particularly desirable for comparisons of amino acid composition, since naturally occurring levels of methylated histidine and lysine vary in vertebrate muscle among species, among individual members of a species, and among muscle types.

Effect of troponin-tropomyosin complex and Ca2+ on conformational changes in F-actin induced by myosin subfragment-1

The interaction of regulated and unregulated actin of myosin-free ghost single fibre with myosin subfragment-1 free of 5,5'-dithiobis(2-nitrobenzoic acid) light chains was investigated by polarized microphotometry. The anisotropy of intrinsic tryptophan fluorescence of regulated actin is Ca2+-dependent and has a maximal value at low (pCa greater than or equal to 7) and a minimal value at high (pCa less than or equal to 6) concentrations of Ca2+. The interaction of myosin subfragment-1 with actin induces cooperative changes in actin structure, which manifest themselves in a decrease in the anisotropy of tryptophan fluorescence. The cooperativity of conformational changes in actin, induced by myosin subfragment-1, is high for regulated actin in the absence of Ca2+ and low for unregulated and regulated actin in the presence of Ca2+. The data obtained suggest that the decrease of the flexibility of actin filaments, induced by tropomyosin or by Ca-unsaturated troponin-tropomyosin complex, results in increased cooperativity of conformational changes of actin induced by myosin subfragment-1.

Kinetic model for the interaction of myosin subfragment 1 with regulated actin

A one-dimensional kinetic Ising model is developed to describe the binding of myosin subfragment 1 (SF-1) to regulated actin. The model allows for cooperative interactions between individual actin sites with bound SF-1 ligands rather than assuming that groups of actin monomer sites change their state in a cooperative fashion. With the triplet closure approximation, the model yields a set of 16 independent differential (master) equations which may be solved numerically to yield the extent of binding as a function of time. The predictions of the model are compared with experiments on the transient binding of SF-1 to regulated actin in the presence of Ca2+ and in the absence of Ca2+ with varying amounts of SF-1 prebound to the actin filament and on the equilibrium binding of SF-1 X ADP to regulated actin in the absence of Ca2+. In all cases, the calculations fit the data to within the experimental errors. In the case of SF-1 X ADP, the results suggest that a repulsive interaction exists between adjacently bound SF-1 at the ends of two neighboring seven-site actin units.

Effect of Ca2+ binding to 5,5'-dithiobis(2-nitrobenzoic acid) light chains on conformational changes of F-actin caused by myosin subfragment-1

The fluorescent ADP analogue, 1:N6-ethenoadenosine 5'-diphosphate, was incorporated into F-actin in a myosin-free ghost single fibre. Polarized fluorescence measurements of tryptophan residues and 1:N6-ethenoadenosine 5'-diphosphate were performed under a microspectrophotometer to investigate the conformation of F-actin and the changes induced in it by myosin subfragment-1 with 5,5'-dithiobis(2-nitrobenzoic acid) light chains and without them. A relation was found between the conformational state of F-actin and the presence of 5,5'-dithiobis(2-nitrobenzoic acid) light chains. The conformational changes were shown to be controlled by Ca2+ in the presence of 5,5'-dithiobis(2-nitrobenzoic acid) light chains.

Large-scale rotational motions of proteins detected by electron paramagnetic resonance and fluorescence

Direct spectroscopic measurements of rotational motions of proteins and large protein segments are crucial to understanding the molecular dynamics of protein function. Fluorescent probes and spin labels attached to proteins have proved to be powerful tools in the study of large-scale protein motions. Fluorescence depolarization and conventional electron paramagnetic resonance (EPR) are applicable to the study of rotational motions in the nanosecond-to-microsecond time range, and have been used to demonstrate segmental flexibility in an antibody and in myosin. Very slow rotational motions, occurring in the microsecond-to-millisecond time range, are particularly important in supramolecular assemblies, where protein motions are restricted by association with other molecules. Saturation transfer spectroscopy (ST-EPR), a recently developed electron paramagnetic resonance (EPR) technique that permits the detection of rotational correlation times as long as 1 ms, has been used to detect large-scale rotational motions of spin-labeled proteins in muscle filaments and in membranes, providing valuable insights into energy transduction mechanisms in these assemblies.

Study of actin and its interactions with heavy meromyosin and the regulatory proteins by the pulse fluorimetry in polarized light of a fluorescent probe attached to an actin cysteine

The decay of anisotropy of the N-iodoacetyl-N'-(5-sulfo-1-naphthyl)-ethylenediamine fluorescence attached to cysteine-373 of actin can be characterized by two correlation times theta1 and theta2. theta1 has a value of several nanoseconds and is thought to represent some local protein motion. theta2 is of the order of several hundreds of nanoseconds. Its value increases with actin concentration. It represents an average of the G and F actin correlation times. When actin interacts with heavy meromyosin, theta2 increases and becomes infinite at a molar ratio of one heavy meromyosin molecule per four actin protomers. It is concluded that a definite complex is then formed between F actin and heavy meromyosin. In the same time, G actin concentration becomes equal to zero. Finally, when F actin forms a complex with the regulatory proteins tropomyosin and troponin, the value of theta2 is greater in the absence than in the presence of Ca2+. This result indicates that micromolar concentrations of Ca2+ induces a conformation change of the complex of F actin with the regulatory proteins.

Molecular aspects of muscle contraction and regulation

Current views regarding some aspects of the contraction of muscle and its regulation are reviewed. Recent work bearing on the segmental flexibility of myosin is related to cross-bridge movements and ATP metabolism in living muscle. Interaction of troponin with calcium, the process that initiates activation of actin-myosin interaction, is discussed in terms of Ca-induced conformational changes and protein-protein interactions.

Permeability of a cell junction and the local cytoplasmic free ionized calcium concentration: a study with aequorin

A technique is devised to determine the spatial distribution of the free ionized cytoplasmic calcium concentration ([Ca2+]i) inside a cell: Chironomus salivary gland cells are loaded with aequorin, and hte Ca2+-dependent light emission of the aequorin is scanned with an image-intensifier/television system. With this technique, the [Ca2+]i is determined simultaneously with junctional electrical coupling when Ca2+ is microinjected into the cells, or when the cells are exposed to metabolic inhibitors, Ca-transporting ionophores, or Ca-free medium. Ca microinjections elevating the [Ca2+]i in the junctional locale produce depression of junctional membrane conductance. When the [Ca2+]i elevation is confined to the vicinity of one cell junction, the conductance of that junction alone is depressed; other junctions of the same cell are not affected. The depression sets in as the [Ca2+]i rises in the junctional locale, and reverses after the [Ca2+]i falls to baseline. When the [Ca2+]i elevation is diffuse throughout the cell, the conductances of all junctions of the cell are depressed. The Ca injections produce no detectable [Ca2+]i elevations in cells adjacent to the injected one; the Ca-induced change in junctional membrane permeability seems fast enough to block appreciable transjunctional flow of Ca2+. Control injections of Cl- or K+ do not affect junctional conductance. The Ca injections that elevate [Ca2+]i sufficiently to depress junctional conductance also produce under the usual conditions an increase in nonjunctional membrane conductance and, hence, depolarization. But injections that elevate [Ca2+]i at the junction while largely avoiding nonjunctional membrane cause depression of junctional conductance with little or no depolarization. Moreover, elevations of [Ca2+]i in cells clamped near resting potential produce the depression, too. On the other hand, complete depolarization in K medium does not produce the depression, unless accompanied by [Ca2+]i elevation. Thus, the depolarization is neither necessary nor sufficient for depression of junctional conductance. Treatment with cyanide, dinitrophenol and ionophores X537A or A23187 produces diffuse elevation of [Ca2+]i associated with depression of junctional conductance. Prolonged exposure to Ca-free medium leads to fluctuation in [Ca2+]i where rise and fall of [Ca2+]i correlate respectively with fall and rise in junctional conductance.

The interaction of actin monomers with myosin heads and other muscle proteins

The simplest interacting unit of actomyosin, viz., single myosin heads (subfragment 1) with actin monomers, has been studied at physiological ionic strength, by isolating the actin molecules from each other on a solid support. The interaction is characterized by a binding constant of 10(5) to 10(6) M-1 in the temperature range 4-30degrees C. It is endothermic with a standard enthalpy of 24 +/- 10 kcal mol-1, and a standard entropy of 110 +/- 40 eu. It is thus, like many protein-protein association processes, entropy-driven. Despite the high affinity of the association, which is comparable in its binding constant to that of subfragment 1 with F-actin, there is only very small activation of myosin ATPase. The ionic-strength dependence of the interaction shows unusual features. Binding of the proteins of the relaxing system to the monomeric actin was also examined: troponin binds both in the presence and absence of calcium ions, but neither tropomyosin nor the tropomyosin-troponin complex was found to bind significantly. Monomeric actin has also been examined as a function of ionic strength by spectroscopic methods; it appears that conformational differences between the G and the F state are the consequence of polymerization, and not of the change in ionic strength required to being the conversion about.