Resonance energy transfer evidence for two attached states of the actomyosin complex

Conformational distributions and proximity relationships in the rigor complex of actin and myosin subfragment-1

Cyclic conformational changes in the myosin head are considered essential for muscle contraction. We hereby show that the extension of the fluorescence resonance energy transfer method described originally by Taylor et al. (Taylor, D. L., Reidler, J., Spudich, J. A., and Stryer, L. (1981) J. Cell Biol. 89, 362-367) allows determination of the position of a labeled point outside the actin filament in supramolecular complexes and also characterization of the conformational heterogeneity of an actin-binding protein while considering donor-acceptor distance distributions. Using this method we analyzed proximity relationships between two labeled points of S1 and the actin filament in the acto-S1 rigor complex. The donor (N-[[(iodoacetyl)amino]ethyl]-5-naphthylamine-1-sulfonate) was attached to either the catalytic domain (Cys-707) or the essential light chain (Cys-177) of S1, whereas the acceptor (5-(iodoacetamido)fluorescein) was attached to the actin filament (Cys-374). In contrast to the narrow positional distribution (assumed as being Gaussian) of Cys-707 (5 +/- 3 A), the positional distribution of Cys-177 was found to be broad (102 +/- 4 A). Such a broad positional distribution of the label on the essential light chain of S1 may be important in accommodating the helically arranged acto-myosin binding relative to the filament axis.

Actin and the actomyosin interface: a review

This review deals with the structure of the actin monomer, its assembly into filaments and the loci on F-actin involved in binding myosin. Two distinctly different arrangements of monomers have been suggested for actin filaments. One model proposed by Holmes et al. is well developed. It places the so-called 'large' domain close to the filament axis and the so-called 'small' domain out near the surface of the filament. A second, less-well developed, model proposed by Schutt et al. locates the 'small' domain close to the filament axis and they rotate the monomer so that 'bottom' of the 'large' domain is at the highest radius. We analyze the available evidence for the models of F-actin derived from X-ray diffraction, reconstructions from electron micrographs, fluorescence resonance energy transfer spectroscopy, chemical cross-linking, antibody probes, limited proteolysis, site-directed and natural mutations, nuclear magnetic resonance spectroscopy and other techniques. The result is an actin-centered view of the loci on actin which are probably involved in its interaction with the myosin 'head'. From these multiple contacts we speculate on the sequence of steps between the initial weak-binding state of S-1 to the actin filament through to the stable strong-binding state seen in the absence of free Mg-ATP, i.e., the rigor state.

Direct visualization by electron microscopy of the weakly bound intermediates in the actomyosin adenosine triphosphatase cycle

We used a novel stopped-flow/rapid-freezing machine to prepare the transient intermediates in the actin-myosin adenosine triphosphatase (ATPase) cycle for direct observation by electron microscopy. We focused on the low affinity complexes of myosin-adenosine triphosphate (ATP) and myosin-adenosine diphosphate (ADP)-Pi with actin filaments since the transition from these states to the high affinity actin-myosin-ADP and actin-myosin states is postulated to generate the molecular motion that drives muscle contraction and other types of cellular movements. After rapid freezing and metal replication of mixtures of myosin subfragment-1, actin filaments, and ATP, the structure of the weakly bound intermediates is indistinguishable from nucleotide-free rigor complexes. In particular, the average angle of attachment of the myosin head to the actin filament is approximately 40 degrees in both cases. At all stages in the ATPase cycle, the configuration of most of the myosin heads bound to actin filaments is similar, and the part of the myosin head preserved in freeze-fracture replicas does not tilt by more than a few degrees during the transition from the low affinity to high affinity states. In contrast, myosin heads chemically cross-linked to actin filaments differ in their attachment angles from ordered at 40 degrees without ATP to nearly random in the presence of ATP when viewed by negative staining (Craig, R., L.E. Greene, and E. Eisenberg. 1985. Proc. Natl. Acad. Sci. USA. 82:3247-3251, and confirmed here), freezing in vitreous ice (Applegate, D., and P. Flicker. 1987. J. Biol. Chem. 262:6856-6863), and in replicas of rapidly frozen samples. This suggests that many of the cross-linked heads in these preparations are dissociated from but tethered to the actin filaments in the presence of ATP. These observations suggest that the molecular motion produced by myosin and actin takes place with the myosin head at a point some distance from the actin binding site or does not involve a large change in the shape of the myosin head.

Gross structural features of myosin head during sliding movement of actin as studied by quick-freeze deep-etch electron microscopy

With quick-freeze deep-etch electron microscopy coupled with mica-flake technique, I showed previously that myosin subfragment-1 (S1) attached to F-actin in the presence of ATP is short and rounded, in contrast to its elongated and tilted appearance under rigor condition [J. Biochem. 106, 751-770 (1989)]. I further indicated that each head of heavy meromyosin (HMM) changes its configuration in a likely manner as above by the addition of various nucleotides, i.e. heads were pear-shaped in the absence of nucleotide, in a ball-on-a-stick appearance when complexed with ADP and strongly kinked to the particular direction in the presence of ATP or ADP.Vi [J. Muscle Res. Cell Motility 12, 313 (1991)]. Such morphological data not only corroborates the independent biophysical evidences suggesting gross conformational changes of myosin head upon binding ATP or ADP.Vi, but also provide strong evidence for the distinct polarity in the structure of each myosin head. Negatively stained image of chemically cross-linked acto-S1 also included cross-bridges sharply kinked to the same direction, confirming the above observation. Attempts were made to examine if such conformational change of myosin cross-bridge occurs during actomyosin superprecipitation. Samples were quick-frozen during rapid turbidity-increasing phase where actin filaments actively slide past myosin heads. The resultant image included actin-attached myosin heads all in a kinked configuration with the same polarity as observed for HMM. Several heads associated with a single actin filament were bent to the same direction suggesting that myosin heads might be in a kinked configuration with distinct polarity during contraction.

Two different acto-S1 complexes

Based on change in anisotropy of fluorescently labelled S1 and on increase in turbidity of acto-S1 complex when S1 bound to F-actin, we reported previously that depending on the molar ratio of S1 to actin two different complexes of actin monomer (A) and myosin subfragment 1 (S1) could be formed: A1*S1 (one actin with one S1) and A2*S1 (two actins with one S1). Here we extend these findings to F-actin labelled with pyrene and cross-linked to S1 with 1-ethyl-3-(3-dimethyl-aminopropyl)carbodiimide (EDC). The fluorescence of pyrene F-actin decreased with increase in S1 concentration and reached saturation at a molar ratio of S1 to actin of either 0.5 or 1.0, depending on whether S1 was added slowly (5 min) or quickly (10-20 s between additions). Incubation of A2*S1 complex in excess of S1 for > 1 h caused a shift in equilibrium towards the A1*S1 complex. The A2*S1 complexes were not formed at high S1 to actin ratios (> 1.0) owing to competition between heads. Crosslinking experiments showed that the formation of EDC crosslinked products, 175-185 kDa doublet and 265 kDa band, depended on the ratio S1 to actin. To assess the relative ratio of S1 and actin in crosslinked products, we labelled S1 and F-actin with different fluorescent probes (5-IAF and IATR). The S1 to actin ratio was proportional to the ratio of intensities of fluorescence of labelled S1 and actin. The S1 to actin ratio in 265 kDa product was two times smaller than in 175-185 kDa doublet (which is believed to be A1*S1 complex) and therefore 265 kDa band corresponded to A2*S1. Transition between two types of binding may be important to understanding how muscle contracts.

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.

Dynamic quenching studies of fluorophore-labelled myosin subfragment 1 and its alkali light chain subunits

The technique of fluorescence quenching by the non-ionic quenchers acrylamide and nicotinamide has been used to probe the accessibility of the environmentally sensitive N-(bromoacetyl)-N'-(1-sulpho-5-naphthyl) ethylenediamine (1,5-Br-AEDANS) fluorophore attached to either Cys-177 of the A1-light chain or the SH1 thiol (Cys-707) of the myosin subfragment (S1) heavy chain. Neither quencher caused any detrimental effects to the ATPase activities of S1 under the conditions of the experiments. It was found that the fluorophore on the isolated light chain was highly exposed to solvent and although this exposure was reduced on hybridization into S1(A1-AEDANS), the probe was still accessible to solvent. This exposure was unaltered by formation of binary complexes with either Mg.ATP or actin or by the formation of a weakly associated acto-S1 complex (in which the Cys-697 and Cys-707 residues of S1 were crosslinked with p-phenylenedimaleimide). The lack of corresponding change in lambda max of emission and quantum yield supported the quenching date and indicated that actin neither binds directly to this region nor induces any significant conformational changes in this locality despite the observation that the A1-Cys-707 moves some 3 nm closer to a point on actin in the weak-binding state (Trayer, H.R. and Trayer, I.P. (1988) Biochemistry, 27, 5718-5727). Parallel experiments with the fluorophore attached to the Cys-707 of the S1 indicated that this region was less accessible to solvent than the light chain thiol despite its ease of labelling. This exposure was not significantly altered by binary complex formation with actin and Mg.ATP, although spectral changes in the absence of quencher support the notion that some conformational change is occurring in this region.

Dictyostelium discoideum essential myosin light chain: gene structure and characterization

We have used a Dictyostelium essential myosin light chain (EMLC) cDNA clone to isolate additional cDNA clones which supply a different 3' sequence from that previously described. The revised cDNA sequence encodes a polypeptide of 150 amino acids. Amino acid residues 147-167 of the previously reported sequence are replaced by new residues 147 to 150. The new cDNA encodes a polypeptide with 66% amino acid sequence identity with the Physarum polycephalum EMLC, and approximately 30% identity with mammalian EMLC sequences. These new cDNA clones were used to isolate two genomic DNA fragments which contain the entire EMLC gene. The Dictyostelium EMLC gene contains a single intron located immediately 3' of the translation initiation codon and encodes a product most similar to MLC3 isoform of vertebrates. Primer extension analysis places the transcription initiation site approximately 90 nucleotides upstream of the translation initiation site. A DNA fragment containing 350 bases of sequence upstream of the putative transcription initiation site is sufficient to drive expression of a reporter gene upon reintroduction into growing Dictyostelium cells. In addition, the CAT reporter mRNA produced by this construct showed a pattern of developmental regulation similar to that previously reported for the endogenous EMLC mRNA. Based on comparison with published EMLC sequences from a variety of sources, the Dictyostelium EMLC shows slightly higher similarity to vertebrate EMLCs from striated muscle sources than nonmuscle sources. While Dictyostelium and human nonmuscle sequences display only 28% identity over their entire sequence, the region from residue 88 to 108 shows much higher identity (67%). The high evolutionary conservation of this region of the EMLC suggests it may play an important role in EMLC function, and as such, represents a good target for future mutagenesis studies.

Visualization of domains in native and nucleotide-trapped myosin heads by negative staining

Electron microscopy of negatively stained vertebrate skeletal muscle myosin molecules has revealed substructure suggestive of globular domains in the head portions of the molecule. This head substructure has been examined after both low and high electron doe. The results suggest it is probably not an artefact of radiation damage. The most common appearance is of one or two stain-filled clefts which run roughly perpendicular to the long axis of the head, giving rise to the appearance of two or three domains in a line. A large domain is located at the end of the head, while two smaller domains are arranged between this and the head-tail junction. The size of the large distal domain (about 10 nm long and about 7 nm wide at its widest point) is similar in heads showing either two or three domains. Stable analogues of M.ATP and M.ADP.Pi, the predominant complexes present during hydrolysis of ATP by myosin, were prepared by crosslinking the two reactive SH groups (SH1 and SH2) in the myosin head heavy chain with N,N'-p-phenylenedimaleimide (pPDM) in the presence of ADP, and by forming a complex with vanadate ion and ADP. At this resolution (approximately 2 nm) the heads of these modified molecules did not appear markedly different from those of the untreated protein, although there was a small increase in the number of straight as opposed to curved heads after cross-linking with pPDM.

Fluorescence resonance energy transfer within the complex formed by actin and myosin subfragment 1. Comparison between weakly and strongly attached states

Fluorescence resonance energy transfer measurements have been made between Cys-374 on actin and Cys-177 on the alkali light chain of myosin subfragment 1 (S1) using several pairs of donor-acceptor chromophores. The labeled light chain was exchanged into subfragment 1 and the resulting fluorescently labeled subfragment 1 isolated by ion-exchange chromatography on SP-Trisacryl. The efficiency of energy transfer was measured by steady-state fluorescence in a strong binding complex of acto-S1 and found to represent a spatial separation between the two probes of 5.6-6.3 nm. The same measurements were then made with weak binding acto-S1 complexes generated in two ways. First, actin was complexed with p-phenylenedimaleimide-S1, a stable analogue of S1-adenosine 5'-triphosphate (ATP), obtained by cross-linking the SH1 and SH2 heavy-chain thiols of subfragment 1 [Greene, L. E., Chalovich, J. M., & Eisenberg, E. (1986) Biochemistry 25, 704-709]. Large increases in transfer efficiency indicated that the two probes had moved closer together by some 3 nm. Second, weak binding complexes were formed between subfragment 1 and actin in the presence of the regulatory proteins troponin and tropomyosin, the absence of calcium, and the presence of ATP [Chalovich, J. M., & Eisenberg, E. (1982) J. Biol. Chem. 257, 2432-2437]. The measured efficiency of energy transfer again indicated that the distance between the two labeled sites had moved closer by about 3 nm. These data support the idea that there is a considerable difference in the structure of the acto-S1 complex between the weakly and strongly bound states.

Fluorescence resonance energy transfer measurements of distances in actin and myosin. A critical evaluation

The contractile proteins actin and myosin are of considerable biological interest. They are essential for muscle contraction and in eukaryotic cells they play a crucial role in most contractile phenomena. Over the years since the first fluorescence resonance energy transfer (FRET) paper appeared, an extensive body of literature has accumulated on this technique using actin, myosin and the actomyosin complex. These papers are reviewed with several aims in mind: we assess the reliability and consistency of intra- and inter-molecular distances measured between the fluorescent probes attached to specific sites on these proteins; we determine whether the measurements can be assembled into an internally consistent model which can be fitted to the known dimensions of the actomyosin complex; several of the FRET distances are consistent with the available structural data from crystallographic and electron microscopic dimensions; the modelled FRET distances suggest that the assumed value of the orientation factor (k2 = 2/3) is reasonable; we conclude that the model has a predictive value, i.e. it suggests that a small number of the published dimensions may be incorrect and predicts the magnitude of a larger number of measurements which have not yet been reported; and finally (vi) we discuss the contribution of FRET determinations to the current debate on the molecular mechanism of contraction.

Evidence that the N-terminal region of A1-light chain of myosin interacts directly with the C-terminal region of actin. A proton magnetic resonance study

Earlier 1H-NMR experiments on the myosin subfragment-1 (S1) light chain isoenzymes from rabbit fast muscle, containing either the A1 or the A2 alkali light chains [S1(A1) or S1(A2)], have shown that the 41-residue N-terminal extension of A1, rich in proline, alanine and lysine residues, is freely mobile in solution but that this mobility is constrained in the acto-S1(A1) complex [Prince et al. (1981) Eur. J. Biochem. 121, 213-219]. It is now established that this N-terminal region of the A1-light chain interacts directly with the C-terminal region of actin in the acto-S1(A1) complex. This was shown by covalently labelling the Cys-374 residue of actin with a spin-label and observing the enhanced relaxation this paramagnetic centre induced in the 1H-NMR spectrum of S1(A1). In particular, the signal arising from the -N+(CH3)3 protons of alpha-N-trimethylalanine (Me3Ala) were monitored as this residue is uniquely sited at the N-terminus of the A1 light chain [Henry et al. (1982) FEBS Lett. 144, 11-15]. Experiments using complexes of actin with either the N-terminal 37-residue peptide of A1, S1(A1) or heavy meromyosin indicate that the N-terminal region of A1 is binding in a similar manner to actin in each case, with the N-terminal Me3Ala residue within 1.5 nm of the spin label introduced to Cys-374 of actin. A similar strategy was adopted to show that the Me3Ala residue can also be found close (less than 1.5 nm) to the fast-reacting SH1 thiol group on the S1 heavy chain. These data, together with published work, have been used to suggest a possible organisation for the polypeptide chains in the myosin head.

ATP-induced dissociation of rabbit skeletal actomyosin subfragment 1. Characterization of an isomerization of the ternary acto-S1-ATP complex

The adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) induced dissociation of actomyosin subfragment 1 (S1) has been investigated by monitoring the light scattering changes that occur on dissociation. We have shown that ATP gamma S dissociates acto-S1 by a mechanism similar to that of ATP but at a rate 10 times slower. The maximum rate of dissociation is limited by an isomerization of the ternary actin-S1-nucleotide complex, which has a rate of 500 s-1 for ATP gamma S and an estimated rate of 5000 s-1 for ATP (20 degrees C, 0.1 M KCl, pH 7.0). The activation energy for the isomerization is the same for ATP and ATP gamma S, and both show a break in the Arrhenius plot at 5 degrees C. The reaction between acto-S1 and ATP was also followed by the fluorescence of a pyrene group covalently attached to Cys-374. We show that the fluorescence of the pyrene group reports the isomerization step and not actin dissociation. The characterization of this isomerization is discussed in relation to force-generating models of the actomyosin cross-bridge cycle.

Structure of the actin-myosin complex produced by crosslinking in the presence of ATP

The structure of the actin-myosin complex during ATP hydrolysis was studied by covalently crosslinking myosin subfragment 1 (S1) to F-actin in the presence of nucleotides (especially ATP) using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The fluorescence energy transfer was measured between N-(iodoacetyl)-N'-(1-sulfo-5-naphthyl)ethylenediamine and 6-(iodoacetamide)fluorescein bound to the SH1 thiol of S1 and the Cys374 thiol of actin. The covalent acto-S1, produced by crosslinking in the absence of nucleotide or in the presence of ADP, showed transfer efficiency of 0.50 to 0.52 and intersite distance of 4.5 to 4.7 nm, which were equal to those obtained with non-crosslinked acto-S1 in the absence of nucleotide. However, the covalent acto-S1, produced by crosslinking in the presence of either 5'-adenylyl imidodiphosphate (AMPPNP) at high ionic strength or ATP, showed a significant decrease in the efficiency to 0.26 to 0.34 and hence an increase in the distance to 5.2 to 5.5 nm. These results suggest that AM-ATP and/or AM-ADP-P (formed during ATP hydrolysis) and AM-AMPPNP have a very different conformation from AM and AM-ADP (in which A is actin and M is myosin).

The mechanism of muscle contraction

Knowledge of the mechanism of contraction has been obtained from studies of the interaction of actin and myosin in solution, from an elucidation of the structure of muscle fibers, and from measurements of the mechanics and energetics of fiber contraction. Many of the states and the transition rates between them have been established for the hydrolysis of ATP by actin and myosin subfragments in solution. A major goal is to now understand how the kinetics of this interaction are altered when it occurs in the organized array of the myofibril. Early work on the structure of muscle suggested that changes in the orientation of myosin cross-bridges were responsible for the generation of force. More recently, fluorescent and paramagnetic probes attached to the cross-bridges have suggested that at least some domains of the cross-bridges do not change orientation during force generation. A number of properties of active cross-bridges have been defined by measurements of steady state contractions of fibers and by the transients which follow step changes in fiber length or tension. Taken together these studies have provided firm evidence that force is generated by a cyclic interaction in which a myosin cross-bridge attaches to actin, exerts force through a "powerstroke" of 12 nm, and is then released by the binding of ATP. The mechanism of this interaction at the molecular level remains unknown.