Orientation of intermediate nucleotide states of indane dione spin-labeled myosin heads in muscle fibers

Structural Dynamics of Protein Interactions Using Site-Directed Spin Labeling of Cysteines to Measure Distances and Rotational Dynamics with EPR Spectroscopy

Here we review applications of site-directed spin labeling (SDSL) with engineered cysteines in proteins, to study the structural dynamics of muscle and non-muscle proteins, using and developing the electron paramagnetic resonance (EPR) spectroscopic techniques of dipolar EPR, double electron electron resonance (DEER), saturation transfer EPR (STEPR), and orientation measured by EPR. The SDSL technology pioneered by Wayne Hubbell and collaborators has greatly expanded the use of EPR, including the measurement of distances between spin labels covalently attached to proteins and peptides. The Thomas lab and collaborators have applied these techniques to elucidate dynamic interactions in the myosin-actin complex, myosin-binding protein C, calmodulin, ryanodine receptor, phospholamban, utrophin, dystrophin, β-III-spectrin, and Aurora kinase. The ability to design and engineer cysteines in proteins for site-directed covalent labeling has enabled the use of these powerful EPR techniques to measure distances, while showing that they are complementary with optical spectroscopy measurements.

Adjusting Rotational Behavior of Molecular Rotors by a Rational Tuning of Molecular Structure

Many crystalline molecular rotors have been developed in the past decades. However, manipulating the rotational gesture that intrinsically controls the physical performance of materials remains a challenge. Herein, we report a series of crystalline rotors whose rotational gestures can be modulated by modifying the structures of molecular stators. In these dynamic crystals, the ox^2- (ox^2- = oxalate anion) behave as molecular rotators performing axial-free rotation in cavities composed of five complex cations, [M^II(en)₃]²⁺ (en = ethylenediamine). The structure of [M^II(en)₃]²⁺ that serves as a molecular stator can be tuned by varying the metal center with different ionic radii, consequently altering the chemical environment around the molecular rotator. Owing to the quasi-transverse isotropy of ox^2- and multiple hydrogen-bond interactions around it, the molecular rotator exhibits unusual motional malleability, i.e., it can rotate either longitudinally in the compound of Zn^II, or with a tilt angle of 42° in the compound of Fe^II, or even laterally in the compound of Cd^II. The atypical dynamic behavior demonstrated here provides a new chance for the development of exquisite crystalline molecular rotors with advanced tunable functionalities.

Mechanistic analysis of actin-binding compounds that affect the kinetics of cardiac myosin-actin interaction

Actin-myosin mediated contractile forces are crucial for many cellular functions, including cell motility, cytokinesis, and muscle contraction. We determined the effects of ten actin-binding compounds on the interaction of cardiac myosin subfragment 1 (S1) with pyrene-labeled F-actin (PFA). These compounds, previously identified from a small-molecule high-throughput screen (HTS), perturb the structural dynamics of actin and the steady-state actin-activated myosin ATPase activity. However, the mechanisms underpinning these perturbations remain unclear. Here we further characterize them by measuring their effects on PFA fluorescence, which is decreased specifically by the strong binding of myosin to actin. We measured these effects under equilibrium and steady-state conditions, and under transient conditions, in stopped-flow experiments following addition of ATP to S1-bound PFA. We observed that these compounds affect early steps of the myosin ATPase cycle to different extents. They increased the association equilibrium constant K₁ for the formation of the strongly bound collision complex, indicating increased ATP affinity for actin-bound myosin, and decreased the rate constant k₊₂ for subsequent isomerization to the weakly bound ternary complex, thus slowing the strong-to-weak transition that actin-myosin interaction undergoes early in the ATPase cycle. The compounds' effects on actin structure allosterically inhibit the kinetics of the actin-myosin interaction in ways that may be desirable for treatment of hypercontractile forms of cardiomyopathy. This work helps to elucidate the mechanisms of action for these compounds, several of which are currently used therapeutically, and sets the stage for future HTS campaigns that aim to discover new drugs for treatment of heart failure.

Looking for Targets to Restore the Contractile Function in Congenital Myopathy Caused by Gln147Pro Tropomyosin

We have used the technique of polarized microfluorimetry to obtain new insight into the pathogenesis of skeletal muscle disease caused by the Gln¹⁴⁷Pro substitution in β-tropomyosin (Tpm2.2). The spatial rearrangements of actin, myosin and tropomyosin in the single muscle fiber containing reconstituted thin filaments were studied during simulation of several stages of ATP hydrolysis cycle. The angular orientation of the fluorescence probes bound to tropomyosin was found to be changed by the substitution and was characteristic for a shift of tropomyosin strands closer to the inner actin domains. It was observed both in the absence and in the presence of troponin, Ca²⁺ and myosin heads at all simulated stages of the ATPase cycle. The mutant showed higher flexibility. Moreover, the Gln¹⁴⁷Pro substitution disrupted the myosin-induced displacement of tropomyosin over actin. The irregular positioning of the mutant tropomyosin caused premature activation of actin monomers and a tendency to increase the number of myosin cross-bridges in a state of strong binding with actin at low Ca²⁺.

The Primary Causes of Muscle Dysfunction Associated with the Point Mutations in Tpm3.12; Conformational Analysis of Mutant Proteins as a Tool for Classification of Myopathies

Point mutations in genes encoding isoforms of skeletal muscle tropomyosin may cause nemaline myopathy, cap myopathy (Cap), congenital fiber-type disproportion (CFTD), and distal arthrogryposis. The molecular mechanisms of muscle dysfunction in these diseases remain unclear. We studied the effect of the E173A, R90P, E150A, and A155T myopathy-causing substitutions in γ-tropomyosin (Tpm3.12) on the position of tropomyosin in thin filaments, and the conformational state of actin monomers and myosin heads at different stages of the ATPase cycle using polarized fluorescence microscopy. The E173A, R90P, and E150A mutations produced abnormally large displacement of tropomyosin to the inner domains of actin and an increase in the number of myosin heads in strong-binding state at low and high Ca²⁺, which is characteristic of CFTD. On the contrary, the A155T mutation caused a decrease in the amount of such heads at high Ca²⁺ which is typical for mutations associated with Cap. An increase in the number of the myosin heads in strong-binding state at low Ca²⁺ was observed for all mutations associated with high Ca²⁺-sensitivity. Comparison between the typical conformational changes in mutant proteins associated with different myopathies observed with α-, β-, and γ-tropomyosins demonstrated the possibility of using such changes as tests for identifying the diseases.

The primary cause of muscle disfunction associated with substitutions E240K and R244G in tropomyosin is aberrant behavior of tropomyosin and response of actin and myosin during ATPase cycle

Using the polarized photometry technique we have studied the effects of two amino acid replacements, E240K and R244G, in tropomyosin (Tpm1.1) on the position of Tpm1.1 on troponin-free actin filaments and the spatial arrangement of actin monomers and myosin heads at various mimicked stages of the ATPase cycle in the ghost muscle fibres. E240 and R244 are located in the C-terminal, seventh actin-binding period, in f and b positions of the coiled-coil heptapeptide repeat. Actin, Tpm1.1, and myosin subfragment-1 (S1) were fluorescently labeled: 1.5-IAEDANS was attached to actin and S1, 5-IAF was bound to Tpm1.1. The labeled proteins were incorporated in the ghost muscle fibres and changes in polarized fluorescence during the ATPase cycle have been measured. It was found that during the ATPase cycle both mutant tropomyosins occupied a position close to the inner domain of actin. The relative amount of the myosin heads in the strongly-bound conformations and of the switched on actin monomers increased at mimicking different stages of the ATPase cycle. This might be one of the reasons for muscle dysfunction in congenital fibre type disproportion caused by the substitutions E240K and R244G in tropomyosin.

The reason for a high Ca2+-sensitivity associated with Arg91Gly substitution in TPM2 gene is the abnormal behavior and high flexibility of tropomyosin during the ATPase cycle

Substitution of Arg for Gly residue in 91th position in β-tropomyosin caused by a point mutation in TPM2 gene is associated with distal arthrogryposis, characterized by a high Ca²⁺-sensitivity of myofilament and contracture syndrome. To understand the mechanisms of this defect, we studied multistep changes in mobility and spatial arrangement of tropomyosin, actin and myosin heads during the ATPase cycle in reconstituted ghost fibres, using the polarized fluorescence microscopy. The mutation was shown to markedly decrease the bending stiffness of β-tropomyosin in the thin filaments. In the absence of the myosin heads the mutation did not alter the ability of troponin to shift tropomyosin to the blocked position and to switch actin monomers off at low Ca²⁺. During the ATPase cycle the movement of the mutant tropomyosin is restrained, it is located near the open position, which allows strong binding of the myosin heads to actin even at low Ca²⁺. This may be the reason for both high Ca²⁺-sensitivity and contractures associated with the Arg91Gly mutation. The use of reagents that decrease the Ca²⁺sensitivity of the troponin complex may not be appropriate to restore muscle function in patients with this mutation.

Molecular mechanisms of deregulation of the thin filament associated with the R167H and K168E substitutions in tropomyosin Tpm1.1

Point mutations R167H and K168E in tropomyosin Tpm1.1 (TM) disturb Ca²⁺-dependent regulation of the actomyosin ATPase. To understand mechanisms of this defect we studied multistep changes in mobility and spatial arrangement of tropomyosin, actin and myosin heads during the ATPase cycle in reconstituted ghost fibres using the polarized fluorescence microscopy. It was found that both mutations disturbed the mode of troponin operation in the fibres. At high Ca²⁺, troponin increased the fraction of actin monomers that were in the "switched on" state, but both mutant tropomyosins were shifted toward the outer actin domains, which decreased the fraction of strongly bound myosin heads throughout the ATPase cycle. At low Ca²⁺, the R167H-TM was located close to the outer actin domains, which reduced the number of strongly-bound myosin heads. However, under these conditions troponin increased the number of actin monomers that were switched on. The K168E-TM was displaced far to the outer actin domains and troponin binding decreased the fraction of switched on actin monomers, but the proportion of the strongly bound myosin heads was abnormally high. Thus, the mutations differently disturbed transmission of conformational changes between troponin, tropomyosin and actin, which is essential for the Са²⁺-dependent regulation of the thin filament.

Abnormal movement of tropomyosin and response of myosin heads and actin during the ATPase cycle caused by the Arg167His, Arg167Gly and Lys168Glu mutations in TPM1 gene

Amino acid substitutions: Arg167His, Arg167Gly and Lys168Glu, located in a consensus actin-binding site of the striated muscle tropomyosin Tpm1.1 (TM), were used to investigate mechanisms of the thin filament regulation. The azimuthal movement of TM strands on the actin filament and the responses of the myosin heads and actin subunits during the ATPase cycle were studied using fluorescence polarization of muscle fibres. The recombinant wild-type and mutant TMs labelled with 5-IAF, 1,5-IAEDANS-labelled S1and FITC-phalloidin F-actin were incorporated into the ghost muscle fibres to acquire information on the orientation of the probes relative to the fibre axis. The substitutions Arg167Gly and Lys168Glu shifted TM strands into the actin filament centre, whereas Arg167His moved TM towards the periphery of the filament. In the presence of Arg167Gly-TM and Lys168Glu-TM the fraction of actin monomers that were switched on and the number of the myosin heads strongly bound to F-actin were abnormally high even under conditions close to relaxation. In contrast, Arg167His-TM decreased the fraction of switched on actin and reduced the formation of strongly bound myosin heads throughout the ATPase cycle. We concluded that the altered TM-actin contacts destabilized the thin filament and affected the actin-myosin interactions.

Aberrant movement of β-tropomyosin associated with congenital myopathy causes defective response of myosin heads and actin during the ATPase cycle

We have investigated the effect of the E41K, R91G, and E139del β-tropomyosin (TM) mutations that cause congenital myopathy on the position of TM and orientation of actin monomers and myosin heads at different mimicked stages of the ATPase cycle in troponin-free ghost muscle fibers by polarized fluorimetry. A multi-step shifting of wild-type TM to the filament center accompanied by an increase in the amount of switched on actin monomers and the strongly bound myosin heads was observed during the ATPase cycle. The R91G mutation shifts TM further towards the inner and outer domains of actin at the strong- and weak-binding stages, respectively. The E139del mutation retains TM near the inner domains, while the E41K mutation captures it near the outer domains. The E41K and R91G mutations can induce the strong binding of myosin heads to actin, when TM is located near the outer domains. The E139del mutation inhibits the amount of strongly bound myosin heads throughout the ATPase cycle.

The E117K mutation in β-tropomyosin disturbs concerted conformational changes of actomyosin in muscle fibers

The effect of the skeletal myopathy-causing E117K mutation in human β-tropomyosin on actomyosin structure during the ATPase cycle was studied using fluorescent probes bound to actin subdomain 1 and the myosin head. Multistep changes in flexural rigidity of actin filament and in spatial arrangement of actin subdomain 1 and myosin SH1 helix in troponin-free ghost muscle fibers were revealed. During the ATPase cycle E117K tropomyosin inhibited the rotation of subdomain 1 by 46% and the tilt of the SH1 helix by 49% compared with wild-type. At strong-binding stages the proportion of strong binding sub-states in the actomyosin population is decreased by the mutation. At weak-binding stages abnormally high numbers of switched-on actin monomers were observed, thus indicating a disturbance in concerted conformational changes of actomyosin. These structural alterations are likely to underlie the contractile deficit observed with this mutation.

Gly126Arg substitution causes anomalous behaviour of α-skeletal and β-smooth tropomyosins during the ATPase cycle

To investigate how TM stabilization induced by the Gly126Arg mutation in skeletal α-TM or in smooth muscle β-TM affects the flexibility of TMs and their position on troponin-free thin filaments, we labelled the recombinant wild type and mutant TMs with 5-IAF and F-actin with FITC-phalloidin, incorporated them into ghost muscle fibres and studied polarized fluorescence at different stages of the ATPase cycle. It has been shown that in the myosin- and troponin-free filaments the Gly126Arg mutation causes a shift of TM strands towards the outer domain of actin, reduces the number of switched on actin monomers and decreases the rigidity of the C-terminus of α-TM and increases the rigidity of the N-terminus of β-TMs. The binding of myosin subfragment-1 to the filaments shifted the wild type TMs towards the inner domain of actin, decreased the flexibility of both terminal parts of TMs, and increased the number of switched on actin monomers. Multistep alterations in the position of α- and β-TMs and actin monomers in the filaments and in the flexibility of TMs and F-actin during the ATPase cycle were observed. The Gly126Arg mutation uncouples a correlation between the position of TM and the number of the switched on actin monomers in the filaments.

The nemaline myopathy-causing E117K mutation in β-tropomyosin reduces thin filament activation

The effect of the nemaline myopathy-causing E117K mutation in β-tropomyosin (TM) on the structure and function of this regulatory protein was studied. The E117K mutant was found to have indistinguishable actin affinity compared with wild-type (WT) and similar secondary structure as measured by circular dichroism. However the E117K mutation significantly lowered maximum activation of actomyosin ATPase. To explain the molecular mechanism of impaired ATPase activation, WT and E117K TMs were covalently labeled at Cys-36 with 5-iodoacetimido-fluorescein and incorporated into ghost muscle fibers. The changes in the position and flexibility of tropomyosin strands on the thin filaments were observed at simulation of weak and strong binding states of actomyosin at high or low Ca(2+) by polarized fluorescence techniques. The E117K mutation was found to shift the tropomyosin strands towards the closed position and restrict the tropomyosin displacement during the transformation of actomyosin from weak to strong binding state thus leading to a reduction in thin filament activation.

Twitchin can regulate the ATPase cycle of actomyosin in a phosphorylation-dependent manner in skinned mammalian skeletal muscle fibres

The effect of twitchin, a thick filament protein of molluscan muscles, on the actin-myosin interaction at several mimicked sequential steps of the ATPase cycle was investigated using the polarized fluorescence of 1.5-IAEDANS bound to myosin heads, FITC-phalloidin attached to actin and acrylodan bound to twitchin in the glycerol-skinned skeletal muscle fibres of mammalian. The phosphorylation-dependent multi-step changes in mobility and spatial arrangement of myosin SH1 helix, actin subunit and twitchin during the ATPase cycle have been revealed. It was shown that nonphosphorylated twitchin inhibited the movements of SH1 helix of the myosin heads and actin subunits and decreased the affinity of myosin to actin by freezing the position and mobility of twitchin in the muscle fibres. The phosphorylation of twitchin reverses this effect by changing the spatial arrangement and mobility of the actin-binding portions of twitchin. In this case, enhanced movements of SH1 helix of the myosin heads and actin subunits are observed. The data imply a novel property of twitchin incorporated into organized contractile system: its ability to regulate the ATPase cycle in a phosphorylation-dependent fashion by changing the affinity and spatial arrangement of the actin-binding portions of twitchin.

The effect of the Asp175Asn and Glu180Gly TPM1 mutations on actin-myosin interaction during the ATPase cycle

Hypertrophic cardiomyopathy (HCM), characterized by cardiac hypertrophy and contractile dysfunction, is a major cause of heart failure. HCM can result from mutations in the gene encoding cardiac α-tropomyosin (TM). To understand how the HCM-causing Asp175Asn and Glu180Gly mutations in α-tropomyosin affect on actin-myosin interaction during the ATPase cycle, we labeled the SH1 helix of myosin subfragment-1 and the actin subdomain-1 with the fluorescent probe N-iodoacetyl-N'-(5-sulfo-1-naphtylo)ethylenediamine. These proteins were incorporated into ghost muscle fibers and their conformational states were monitored during the ATPase cycle by measuring polarized fluorescence. For the first time, the effect of these α-tropomyosins on the mobility and rotation of subdomain-1 of actin and the SH1 helix of myosin subfragment-1 during the ATP hydrolysis cycle have been demonstrated directly by polarized fluorimetry. Wild-type α-tropomyosin increases the amplitude of the SH1 helix and subdomain-1 movements during the ATPase cycle, indicating the enhancement of the efficiency of the work of cross-bridges. Both mutant TMs increase the proportion of the strong-binding sub-states, with the effect of the Glu180Gly mutation being greater than that of Asp175Asn. It is suggested that the alteration in the concerted conformational changes of actomyosin is likely to provide the structural basis for the altered cardiac muscle contraction.

The effect of the dilated cardiomyopathy-causing Glu40Lys TPM1 mutation on actin-myosin interactions during the ATPase cycle

Dilated cardiomyopathy (DCM), characterized by cardiac dilatation and contractile dysfunction, is a major cause of heart failure. DCM can result from mutations in the gene encoding cardiac α-tropomyosin (TM). In order to understand how the dilated cardiomyopathy-causing Glu40Lys mutation in TM affects actomyosin interactions, thin filaments have been reconstituted in muscle ghost fibers by incorporation of labeled Cys707 of myosin subfragment-1 and Cys374 of actin with fluorescent probe 1.5-IAEDANS and α-tropomyosin (wild-type or Glu40Lys mutant). For the first time, the effect of these α-tropomyosins on the mobility and rotation of subdomain-1 of actin and the SH1 helix of myosin subfragment-1 during the ATP hydrolysis cycle have been demonstrated directly by polarized fluorimetry. The Glu40Lys mutant TM inhibited these movements at the transition from AM(∗∗)·ADP·Pi to AM state, indicating a decrease of the proportion of the strong-binding sub-states in the actomyosin population. These structural changes are likely to underlie the contractile deficit observed in human dilated cardiomyopathy.

Inhibitors of myosin, but not actin, alter transport through Tradescantia plasmodesmata

Actin and myosin are components of plasmodesmata, the cytoplasmic channels between plant cells, but their role in regulating these channels is unclear. Here, we investigated the role of myosin in regulating plasmodesmata in a well-studied, simple system comprising single filaments of cells which form stamen hairs in Tradescantia virginiana flowers. Effects of myosin inhibitors were assessed by analysing cell-to-cell movement of fluorescent tracers microinjected into treated cells. Incubation in the myosin inhibitor, 2,3-butanedione monoxime (BDM) or injection of anti-myosin antibodies increased cell-cell transport of fluorescent dextrans, while treatment with the myosin inhibitor N-ethylmaleimide (NEM) decreased cell-cell transport. Pretreatment with the callose synthesis inhibitor, deoxy-D: -glucose (DDG), enhanced transport induced by BDM treatment or injection of myosin antibodies but did not relieve NEM-induced reduction in transport. In contrast to the myosin inhibitors, cell-to-cell transport was unaffected by treatment with the actin polymerisation inhibitor, latrunculin B, after controlling for callose synthesis with DDG. Transport was increased following azide treatment, and reduced after injection of ATP, as in previous studies. We propose that myosin detachment from actin, induced by BDM, opens T. virginiana plasmodesmata whereas the firm attachment of myosin to actin, promoted by NEM, closes them.

A new property of twitchin to restrict the "rolling" of mussel tropomyosin and decrease its affinity for actin during the actomyosin ATPase cycle

A new evidence on the regulatory function of twitchin, a titin-like protein of molluscan muscles, at muscle contraction has been obtained at studying the movements of IAF-labeled mussel tropomyosin in skeletal ghost fibers during the ATP hydrolysis cycle simulated using nucleotides and non-hydrolysable ATP analogs. For the first time, myosin-induced multistep changes in mobility and in the position of mussel tropomyosin strands on the surface of the thin filament during the ATP hydrolysis cycle have been demonstrated directly. Unphosphorylated twitchin shifts the tropomyosin towards the position typical for muscle relaxation, decreases the tropomyosin affinity to actin and inhibits its movements during the ATPase cycle. Phosphorylation of twitchin by the catalytic subunit of protein kinase A reverses this effect. These data imply that twitchin is a thin filament regulator that controls actin-myosin interaction by "freezing" tropomyosin in the blocked position, resulting in the inhibition of the transformation of weak-binding states into strong-binding ones during ATPase cycle.

Caldesmon inhibits the rotation of smooth actin subdomain-1 and alters its mobility during the ATP hydrolysis cycle

Smooth muscle thin filaments have been reconstituted in muscle ghost fibers by incorporation of smooth muscle actin, tropomyosin and caldesmon. For the first time, rotation of subdomain-1 and changes of its mobility in IAEDANS-labeled actin during the ATP hydrolysis cycle simulated using nucleotides and non-hydrolysable ATP analogs have been demonstrated directly. Binding of caldesmon altered the mobility and inhibited the rotation of actin subdomain-1 during the transition from AM * *.ADP.Pi to AM state, resulting in inhibition of both strong and weak-binding intermediate states. These new results imply that regulation of actomyosin interaction by caldesmon during the ATPase cycle is fulfilled via the inhibition of actin subdomain-1 rotation toward the periphery of the thin filament, which decreases the area of the specific binding between actin and myosin molecules and is likely to underlie at least in part the mechanism of caldesmon-induced contractility suppression.

The effect of the dilated cardiomyopathy-causing mutation Glu54Lys of alpha-tropomyosin on actin-myosin interactions during the ATPase cycle

In order to understand how the Glu54Lys mutation of alpha-tropomyosin affects actomyosin interactions, we labeled SH1 helix of myosin subfragment-1 (S1) and the actin subdomain-1 with fluorescent probes. These proteins were incorporated into ghost muscle fibers and their conformational states were monitored during the ATPase cycle by measuring polarized fluorescence. The addition of wild-type alpha-tropomyosin to actin filaments increases the amplitude of the SH1 helix and subdomain-1 movements during the ATPase cycle, indicating the enhancement of the efficiency of work of each cross-bridge. The Glu54Lys mutation inhibits this effect. The Glu54Lys mutation also results in the coupling of the weak-binding sub-state of S1 to the strong-binding sub-state of actin thus altering the concerted conformational changes during the ATPase cycle. We suggest that these alterations will result in reduced force production, which is likely to underlie at least in part the contractile deficit observed in human dilated cardiomyopathy.

Dilated cardiomyopathy mutations in alpha-tropomyosin inhibit its movement during the ATPase cycle

The Glu40Lys and Glu54Lys mutations in alpha-tropomyosin cause dilated cardiomyopathy (DCM). Functional analysis has demonstrated that both mutations decrease thin filament Ca2+-sensitivity and that Glu40Lys reduces maximum activation. To understand the molecular mechanism underlying these changes, we labeled wild type alpha-tropomyosin and both mutants at Cys190 with 5-iodoacetamide-fluorescein and incorporated the labeled proteins into ghost muscle fibers. Using the polarized fluorimetry, the position of the labeled tropomyosins on the thin filament and their affinity for actin were measured and the change in these parameters at different stages of the ATPase cycle determined. Both DCM mutations were found to shift tropomyosin towards the periphery of thin filament and to change the affinity of tropomyosin for actin; during the ATPase cycle the amplitude of tropomyosin movement was reduced and at some stages of the cycle even reversed. The correlation of these structural changes with the observed function effects is discussed.

Modulation of the effects of tropomyosin on actin and myosin conformational changes by troponin and Ca2+

The molecular mechanisms by which troponin (TN)-tropomyosin (TM) regulates the myosin ATPase cycle were investigated using fluorescent probes specifically bound to Cys36 of TM, Cys707 of myosin subfragment-1, and Cys374 of actin incorporated into ghost muscle fibers. Intermediate states of actomyosin were simulated by using nucleotides and non-hydrolysable ATP analogs. Multistep changes in mobility and spatial arrangement of SH1 helix of myosin motor domain and actin subdomain-1 during the ATPase cycle were observed. Each intermediate state of actomyosin induced a definite conformational state and specific position of TM strands on the surface of thin filament. TM increased the amplitude of myosin SH1 helix and actin subdomain-1 movements at transition from weak- to strong-binding states shifting to the center of thin filament at strong-binding and to the periphery of thin filament at weak-binding states. TN modulated those movements in a capital ES, Cyrillicsmall a, Cyrillic(2+)-dependent manner. At high-Ca(2+), TN enhanced the effect of TM on SH1 helix and subdomain-1 movements by transferring TM further to the center of thin filament at strong-binding states. In contrast, at low-Ca(2+), TN inhibited the effect of TM movements, "freezing" actin structure in "OFF" state and TM in the position typical for weak-binding states, resulting in disturbing the interplay of actin and myosin.

Caldesmon freezes the structure of actin filaments during the actomyosin ATPase cycle

Hybrid contractile apparatus was reconstituted in skeletal muscle ghost fibers by incorporation of skeletal muscle myosin subfragment 1 (S1), smooth muscle tropomyosin and caldesmon. The spatial orientation of FITC-phalloidin-labeled actin and IAEDANS-labeled S1 during sequential steps of the acto-S1 ATPase cycle was studied by measurement of polarized fluorescence in the absence or presence of nucleotides conditioning the binding affinity of both proteins. In the fibers devoid of caldesmon addition of nucleotides evoked unidirectional synchronous changes in the orientation of the fluorescent probes attached to F-actin or S1. The results support the suggestion on the multistep rotation of the cross-bridge (myosin head and actin monomers) during the ATPase cycle. The maximal cross-bridge rotation by 7 degrees relative to the fiber axis and the increase in its rigidity by 30% were observed at transition between A**.M**.ADP.Pi (weak binding) and A--.M--.ADP (strong binding) states. When caldesmon was present in the fibers (OFF-state of the thin filament) the unidirectional changes in the orientation of actin monomers and S1 were uncoupled. The tilting of the myosin head and of the actin monomer decreased by 29% and 90%, respectively. It is suggested that in the "closed" position caldesmon "freezes" the actin filament structure and induces the transition of the intermediate state of actomyosin towards the weak-binding states, thereby inhibiting the ATPase activity of the actomyosin.

Orientation and mobility of actin in different intermediate states of the ATP hydrolysis cycle

Using polarization fluorimetry, we have investigated conformational changes of FITC-phalloidin-labeled F-actin in ghost muscle fibers. These changes were induced by myosin subfragment-1 (S1) in the absence and presence of MgADP, MgAMP-PNP, MgATPgammaS, or MgATP. Modeling of various intermediate states was accompanied by discrete changes in actomyosin orientation and mobility of fluorescent dye dipoles. This suggests multistep changes of orientation and mobility of actin monomers during the ATPase cycle. The most pronounced differences in orientation (~4 degrees ) and in mobility (~43%) of actin were found between the actomyosin states induced by MgADP and MgATP.

Effect of nucleotides on the orientation and mobility of myosin subfragment-1 in ghost muscle fiber

Using polarization fluorimetry, the orientation and mobility of 1,5-IAEDANS specifically bound to Cys707 of myosin subfragment-1 (S1) were studied in ghost muscle tropomyosin-containing fibers in the absence and in the presence of MgADP, MgAMP-PNP, MgATPgammaS, or MgATP. Modeling of various intermediate states was accompanied by discrete changes in actomyosin orientation and mobility of fluorescent dye dipoles. This suggests multistep changes in the structural state of the myosin head during the ATPase cycle. Maximal differences in the probe orientation by 4 degrees and its mobility by 30% were found between actomyosin states in the presence of MgADP and MgATP. It is suggested that interaction of S1 with F-actin induces nucleotide-dependent rotation of the whole motor domain of the myosin head or only the dye-binding site and also change in the head mobility.

Structural dynamics of actin during active interaction with myosin: different effects of weakly and strongly bound myosin heads

We have used optical spectroscopy (transient phosphorescence anisotropy, TPA, and fluorescence resonance energy transfer, FRET) to detect the effects of weakly bound myosin S1 on actin during the actomyosin ATPase cycle. The changes in actin were reported by (a) a phosphorescent probe (ErIA) attached to Cys 374 and (b) a FRET donor-acceptor pair, IAEDANS attached to Cys 374 and a nucleotide analogue (TNPADP) in the nucleotide-binding cleft. Strong interactions were detected in the absence of ATP, and weak interactions were detected in the presence of ATP or its slowly hydrolyzed analogue ATP-gamma-S, under conditions where a significant fraction of weakly bound acto-S1 complex was present and the rate of nucleotide hydrolysis was low enough to enable steady-state measurements. The results show that actin in the weakly bound complex with S1 assumes a new structural state in which (a) the actin filament has microsecond rotational dynamics intermediate between that of free actin and the strongly bound complex and (b) S1-induced changes are not propagated along the actin filament, in contrast to the highly cooperative changes due to the strongly bound complex. We propose that the transition on the acto-myosin interface from weak to strong binding is accompanied by transitions in the structural dynamics of actin parallel to transitions in the dynamics of interacting myosin heads.

Changes in actin and myosin structural dynamics due to their weak and strong interactions

Figure 3 summarizes the effects of actomyosin binding on the internal and global dynamics of either protein, as discussed in this chapter. These effects depend primarily on the strength of the interaction; which in turn depends on the state of the nucleotide at the myosin active site. When either no nucleotide or ADP is bound, the interaction is strong and the effect on each protein is maximal. When the nucleotide is ATP or ADP.Pi, or the equivalent nonhydrolyzable analogs, the interaction is weak and the effect on molecular dynamics of each protein is minimal. The weaker effects in weak-binding states are not simply the reflection of lower occupancy of binding sites--the molecular models in Fig. 3 illustrate the effects of the formation of the ternary complex, after correction for the free actin and myosin in the system. Thus EPR on myosin (Berger and Thomas 1991; Thomas et al. 1995) and pyrene fluorescence studies on actin (Geeves 1991) have shown that the formation of a ternary complex has a negligible effect on the internal dynamics of both [figure: see text] proteins (left side of Fig. 3, white arrows). As shown by both EPR (Baker et al. 1998; Roopnarine et al. 1998) and phosphorescence (Ramachandran and Thomas 1999), both domains of myosin are dynamically disordered in weak-binding states, and this is essentially unaffected by the formation of the ternary complex (left side of Fig. 3, indicated by disordered myosin domains). The only substantial effect of the formation of the weak interaction that has been reported is the EPR-detected (Ostap and Thomas 1991) restriction of the global dynamics of actin upon weak myosin binding (left column of Fig. 3, gray arrow). The effects of strong actomyosin formation are much more dramatic. While substantial rotational dynamics, both internal and global, exist in both myosin and actin in the presence of ADP or the absence of nucleotides, spin label EPR, pyrene fluorescence, and phosphorescence all show dramatic restrictions in these motions upon formation of the strong ternary complex (right column of Fig. 3). One implication of this is that the weak-to-strong transition is accompanied by a disorder-to-order transition in both actin and myosin, and this is itself an excellent candidate for the structural change that produces force (Thomas et al. 1995). Another clear implication is that the crystal structures obtained for isolated myosin and actin are not likely to be reliable representations of structures that exist in ternary complexes of these proteins (Rayment et al. 1993a and 1993b; Dominguez et al. 1998; Houdusse et al. 1999). This is clearly true of the strong-binding states, since the spectroscopic studies indicate consistently that substantial changes occur in both proteins upon strong complex formation. For the weak complexes, the problem is not that complex formation induces large structural changes, but that the structures themselves are dynamically disordered. This is probably why so many different structures have been obtained for myosin S1 with nucleotides bound--each crystal is selecting one of the many different substates represented by the dynamic ensemble. Finally, there is the problem that the structures of actomyosin complexes are probably influenced strongly by their mechanical coupling to muscle protein lattice (Baker at al. 2000). Thus, even if co-crystals of actin and myosin are obtained in the future, an accurate description of the structural changes involved in force generation will require further experiments using site-directed spectroscopic probes of both actin and myosin, in order to detect the structural dynamics of these ternary complexes under physiological conditions.

Reversible changes in size of cell nuclei isolated from Amoeba proteus: Role of the cytoskeleton

Micrurgically isolated interphasal nuclei of Amoeba proteus, which preserve F-actin cytoskeletal shells on their surface, shrink after perfusion with imidazole buffer without ATP, and expand to about 200% of their cross-sectional area upon addition of pyrophosphate. These changes in size may be reproduced several times with the same nucleus. The shrunken nuclei are insensitive to the osmotic effects of sugars and distilled water, whereas the expanded ones react only to the distilled water, showing further swelling. The shrinking-expansion cycles are partially inhibited by cytochalasins. They are attributed to the state of actomyosin complex in the perinuclear cytoskeleton, which is supposed to be in the rigor state in the imidazole buffer without ATP, and to dissociate in the presence of pyrophosphate. Inflow of external medium to the nuclei during dissociation of the myosin from the perinuclear F-actin may be due to colloidal osmosis depending on other macromolecular components of the karyoplasm.Key words: Amoeba proteus, isolated nuclei, perinuclear cytoskeleton.

The C-terminus of tubulin increases cytoplasmic dynein and kinesin processivity

In motor movement on microtubules, the anionic C-terminal of tubulin has been implicated as a significant factor. Our digital analyses of movements of cytoplasmic dynein- and kinesin-coated beads on microtubules have revealed dramatic changes when the C-terminal region (2-4-kDa fragment) of tubulin was cleaved by limited subtilisin digestion of assembled microtubules. For both motors, bead binding to microtubules was decreased threefold, bead run length was decreased over fourfold, and there was a dramatic 20-fold decrease in diffusional movements of cytoplasmic dynein beads on microtubules (even with low motor concentrations where the level of bead motile activity was linear with motor concentration). The velocity of active bead movements on microtubules was unchanged for cytoplasmic dynein and slightly decreased for kinesin. There was also a decrease in the frequency of bead movements without a change in velocity when the ionic strength was raised. However, with high ionic strength there was not a decrease in run length or any selective inhibition of the diffusional movement. The C-terminal region of tubulin increased motor run length (processivity) by inhibiting "detachment" but without affecting velocity. Because the major motor binding sites of microtubules are not on the C-terminal tail of tubulin (), we suggest that the changes are the result of the compromise of a weakly attached state that is the lowest affinity step in both motors' ATPase cycles and is not rate limiting.

Synthesis of a spin-labeled photoaffinity ATP analogue, and its use to specifically photolabel myosin cross-bridges in skeletal muscle fibers

A spin-labeled photoaffinity ATP analogue 3'(2')-O-[4-[4-oxo-(4-amido-2,2,6,6-tetramethyl-piperidino-1-oxyl)]-benz oyl]benzoyl adenosine 5'-triphosphate (SL-Bz2ATP) was synthesized and used to photolabel myosin in muscle fibers. Previous work has shown that 3'(2')-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (Bz2ATP) photolabeled Ser-324 of the 50 kDa tryptic fragment of skeletal S1 heavy chain. In this work, [alpha-32P]SL-Bz2ATP was hydrolyzed and trapped as the diphosphate analogue with Co2+ and orthovanadate at the active site of myosin in rabbit psoas muscle fibers. After UV irradiation, the myosin heavy chain was the only protein band found to be significantly photolabeled as assayed by gel electrophoresis and radioactivity counting. The labeling was localized after brief trypsin digestion by SDS-PAGE to be on the 50 kDa tryptic fragment of the S1 heavy chain. Ca. 35% of the myosin in fibers was covalently photolabeled. The fibers photolabeled with SL-Bz2ATP had the same active tension and maximum shortening velocity as the control fibers. The resulting spin label on myosin was too mobile to report the orientation of the heads in fibers. Nonetheless, this is the first work to show the feasibility of utilizing active site binding and photoaffinity labeling to place covalent spectroscopic probes at the myosin active site in fibers with high specificity and yield without affecting mechanical function.

Myosin light-chain domain rotates upon muscle activation but not ATP hydrolysis

We have studied the correlation between myosin structure, myosin biochemistry, and muscle force. Two distinct orientations of the myosin light-chain domain were previously resolved using electron paramagnetic resonance (EPR) spectroscopy of spin-labeled regulatory light chains in scallop muscle fibers. In the present study, we measured isometric force during EPR spectral acquisition, in order to define how these two light-chain domain orientations are coupled to force and the myosin ATPase cycle. When muscle fibers are partially activated with increasing amounts of calcium, the distribution between the two light-chain domain orientations shifts toward the one associated with strong actin binding. This shift in distribution is linearly related to the increase in force, suggesting that rotation of the light-chain domain is coupled to strong actin binding. However, when nucleotide analogues are used to trap myosin in the pre- and posthydrolysis states of its ATPase cycle in relaxed muscle, there is no change in the distribution between light-chain domain orientations, showing that the rotation of the light-chain domain is not directly coupled to the ATP hydrolysis step. Instead, it is likely that in relaxed muscle the myosin thick filament stabilizes two light-chain domain orientations that are independent of the nucleotide analogue bound at the active site. We conclude that a large and distinct rotation of the light-chain domain of myosin is responsible for force generation and is coupled to strong actin binding but is not coupled to a specific step in the myosin ATPase reaction.

Microsecond rotational dynamics of spin-labeled myosin regulatory light chain induced by relaxation and contraction of scallop muscle

We have used saturation transfer electron paramagnetic resonance (ST-EPR) to study the rotational dynamics of spin-labeled regulatory light chain (RLC) in scallop (Placopecten magellanicus) muscle fibers. The single cysteine (Cys 51) in isolated clam (Mercenaria) RLC was labeled with an indanedione spin label (InVSL). RLC was completely and specifically extracted from scallop striated muscle fibers, eliminating the Ca sensitivity of ATPase activity and isometric force, which were both completely restored by stoichiometric incorporation of labeled RLC. The EPR spectrum of the isolated RLC revealed nanosecond rotational motions within the RLC, which were completely eliminated when the labeled RLC was bound to myosin heads in myofibrils or fibers in rigor. This is the most strongly immobilized RLC-bound probe reported to date and thus offers the most reliable detection of the overall rotational motion of the LC domain. Conventional EPR spectra of oriented fibers indicated essentially complete probe disorder, independent of ATP and Ca, eliminating orientational dependence and thus making this probe ideal for unambiguous measurement of microsecond rotational motions of the LC domain by ST-EPR. ST-EPR spectra of fibers in rigor indicated an effective rotational correlation time (taureff) of 140 +/- 5 microseconds, similar to that observed for the same spin label bound to the catalytic domain. Relaxation by ATP induced microsecond rotational motion (taureff = 70 +/- 4 microseconds), and this motion was slightly slower upon Ca activation of isometric contraction (taureff = 100 +/- 5 microseconds). These motions in relaxation and contraction are similar to, but slower than, the motions previously reported for the same spin label bound to the catalytic domain. These results support a model for force generation involving rotational motion of the LC domain relative to the catalytic domain and dynamic disorder-to-order transitions in both domains.

X-ray structures of the MgADP, MgATPgammaS, and MgAMPPNP complexes of the Dictyostelium discoideum myosin motor domain

The three-dimensional structures of the truncated myosin head from Dictyostelium discoideum myosin II (S1dC) complexed with MgAMPPNP, MgATPgammaS, and MgADP are reported at 2.1, 1.9, and 2.1 A resolution, respectively. Crystals were obtained by cocrystallization and were isomorphous with respect to those of S1dC. MgADP.BeFx [Fisher, A. J., et al. (1995) Biochemistry 34, 8960-8972]. In all three structures, the electron density for the entire nucleotide was clearly discernible. The overall structures of all three complexes are very similar to that of the beryllium fluoride complex which suggests that the differences in the physiological effects of ATPgammaS and AMPPNP are due to the changes in the equilibrium between the actin-bound and actin-free states of myosin caused by the lower affinity of AMPPNP for myosin. In S1dC.MgAMPPNP, the presence of the bridging nitrogen prompts the side chain of Asn233 to rotate which disrupts the hydrogen bonding pattern in the nucleotide binding pocket and alters the water structure surrounding the ribose hydroxyl groups. It appears that this change is responsible for the reduced affinity of AMPPNP for myosin relative to ATPgammaS. In contrast to the G-proteins, there is no major change in the conformation of the ligands that coordinate the nucleotide in S1dC.MgADP. This is due to three water molecules that adopt the approximate positions of the three oxygens on the gamma-phosphate and maintain the interactions with the Mg2+ ion and protein molecule. Interestingly, the thiophosphate group is evident in S1dC.MgATPgammaS even though it is slowly hydrolyzed by myosin. This suggests that the conformation observed here and in chicken skeletal myosin subfragment-1 [Rayment, I., et al. (1993) Science 261, 50-58] is unable to hydrolyze ATP and represents the structure of the prehydrolysis weak binding state of myosin.