Further characterization of the structural and functional properties of the cross-linked complex between F-actin and myosin S-1

Design and functional analysis of actomyosin motor domain chimera proteins

To gain more structural and functional information on the actomyosin complexes, we have engineered chimera proteins carrying the entire Dictyostelium actin in the loop 2 sequence of the motor domain of Dictyostelium myosin II. Although the chimera proteins were unable to polymerize by themselves, addition of skeletal actin promoted polymerization. Electron microscopic observation demonstrated that the chimera proteins were incorporated into actin filaments, when copolymerized with skeletal actin. Copolymerization with skeletal actin greatly enhanced the MgATPase, while the chimera proteins without added skeletal actin hydrolyzed ATP at a very low rate. These results indicate that the actin part and the motor domain part of the chimera proteins are correctly folded, but the chimera proteins are structurally stressed so that efficient polymerization is inhibited.

Characterization of three regulatory states of the striated muscle thin filament

The troponin-tropomyosin-linked regulation of striated muscle contraction occurs through allosteric control by both Ca(2+) and myosin. The thin filament fluctuates between two extreme states: the inactive "off" state and the active "on" state. Intermediate states have been proposed from structural studies and transient kinetic measurements. However, in contrast to the well-characterised, on and off states, the mechanochemical properties of the intermediate states are much less well understood because of the instability of those states. In the present study, we have characterized a myosin-induced intermediate that is stabilized by cross-linking myosin motor domains (S1) to actin filaments (with a maximum of one S1 molecule for 50 actin monomers). A single S1 molecule is known to interact with two adjacent actin monomers. A detailed analysis revealed that thin filaments containing S1 molecules cross-linked to just one actin monomer (actin(1)-S1 complexes) are regulated with a 79% inhibition of the ATPase in the absence of Ca(2+). In contrast, filaments containing S1 molecules cross-linked at two positions, to two adjacent actin monomers (actin(2)-S1 complexes) totally lose their regulation in a highly cooperative manner. This loss of regulation was due both to an enhancement of the ATPase activity without calcium and an inhibition of the ATPase with calcium. Filaments containing actin(2)-S1 complexes, with significant ATPase activity in the absence of calcium (about 50%), did not move on a myosin-coated surface unless calcium was present. This partial uncoupling between the ATPase activity and in vitro motility in the absence of calcium demonstrates that the mechanical steps require actin-myosin contacts, which take place only in the on state and not in the off or intermediate states. These data provide new insights concerning the difference in cooperativity of Ca(2+) regulation that exists between the biochemical and mechanical cycles of the actin-myosin motor.

Interaction of F-actin with synthetic peptides spanning the loop region of human cardiac beta-myosin heavy chain containing Arg403

The atomic model of the F-actin-myosin subfragment 1 complex (acto-S-1) from skeletal muscle suggests that the transition of the complex from a weakly to a strongly binding state, generating mechanical force during the contractile cycle, may involve the attachment of the upper 50-kDa subdomain of myosin subfragment 1 (S-1) to the interface between subdomains 1 and 3 of actin. For the human cardiac myosin, this putative interaction would take place at the ordered loop including Arg403 of the beta-heavy chain sequence, a residue whose mutation into Gln is known to elicit a severe hypertrophic cardiomyopathy caused by a decrease of the rate of the actomyosin ATPase activity. Moreover, in several nonmuscle myosins the replacement of a Glu residue within the homolog loop by Ser or Thr also results in the reduction of the actomyosin ATPase rate that is alleviated by phosphorylation. As an approach to the characterization of the unknown interaction properties of F-actin with this particular S-1 loop region, we have synthesized four 17-residue peptides corresponding to the sequence Gly398-Gly414 of the human beta-cardiac myosin. Three peptides included Arg403 (GG17) or Gln403 (GG17Q) or Ser409 (GG17S) and the fourth peptide (GG17sc) was a scrambled version of the normal GG17 sequence. Using fluorescence polarization, cosedimentation analyses and photocross-linking, we show that the three former peptides, but not the scrambled sequence, directly associate in solution to F-actin, at a nearly physiological ionic strength, with almost identical affinities (Kd approximately 40 microM). The binding strength of the F-actin-GG17 peptide complex was increased fivefold (Kd = 8 microM) in the presence of subsaturating concentrations of added skeletal S-1 relative to actin, without apparent competition between the peptide and S-1. Each of the three actin-binding peptides inhibited the steady-state actin-activated MgATPase of skeletal S-1 by specifically decreasing about twofold the Vmax of the reaction without changing the actin affinity for the S-1-ATP intermediate. Cosedimentation assays indicated the binding of about 0.65 mol peptide/mol actin under conditions inducing 70% inhibition. Collectively, the data point to a specific and stoichiometric interaction of the peptides with F-actin that uncouples its binding to S-1 from ATP hydrolysis, probably by interfering with the proper attachment of the S-1 loop segment to the interdomain connection of actin.

New insights into actin filament dynamics

Great progress has been made in advancing an atomic-level model for F-actin. A growing body of data shows, however, that any picture of F-actin must take into account allosteric interactions within subunits, long-range cooperative effects that occur between subunits, and the fact that several conformations of the filament can exist.

The mechanism of inhibition of the actin-activated myosin MgATPase by calponin

Calponin inhibits the actin-activated ATPase of smooth muscle myosin and thus has been proposed as a thin filament-based regulatory component in smooth muscle. To obtain information on the mechanism of inhibition by calponin we have used chemical modification of actin and cross-linking of actin and subfragment 1. Modification of Lys 61 of actin had no effect on the inhibition by calponin of acto-heavy meromyosin ATPase, i.e. different from tropomyosin-troponin. In addition, modification of the acidic N-terminal region of actin did not impair the ability of calponin to bind to F-actin. Finally, calponin was effective in inhibiting ATPase activity of cross-linked acto-subfragment 1. Therefore the mechanism of inhibition by calponin is distinct from troponin-tropomyosin and caldesmon in that it does not involve either the N-terminal acidic region of actin nor the area around Lys 61 and does not fit a simple steric blocking model.

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.

Characterization of an actin-myosin head interface in the 40-113 region of actin using specific antibodies as probes

Evidence for the participation of the 1-7 and 18-28 N-terminal sequences of actin at different steps of actin-myosin interaction process is well documented in the literature. Cross-linking of the rigor complex between filamentous actin and skeletal-muscle myosin subfragment 1 was accomplished by the carboxy-group-directed zero-length protein cross-linker, 1-ethyl-3-[3-(dimethylamino)propyl]carbodi-imide. After chaotropic depolymerization and thrombin digestion, which cleaves only actin, the covalent complex with Mr 100,000 was characterized by PAGE. The linkage was identified as being between myosin subfragment 1 (S-1) heavy chain and actin-(1-28)-peptide. The purified complex retained in toto its ability to combine reversibly with fresh filamentous actin, but showed a decrease in the Vmax. of actin-dependent Mg2(+)-ATPase. By using e.l.i.s.a., S-1 was observed to bind to coated monomeric actin or its 1-226 N-terminal peptide. This interaction strongly interfered with the binding of antibodies directed against the 95-113 actin sequence. Moreover, S-1 was able to bind with coated purified actin-(40-113)-peptide. Finally, antibodies directed against the 18-28 and 95-113 actin sequence, which strongly interfered with S1 binding, were unable to compete with each other. These results suggest that two topologically independent regions are involved in the actin-myosin interface: one located in the conserved 18-28 sequence and the other near residues 95-113, including the variable residue at position 89. Other experiments support the 'multisite interface model', where the two actin sites could modulate each other during S-1 interaction.

Covalent crosslinking of myosin subfragment-1 and heavy meromyosin to actin at various molar ratios: different correlations between ATPase activity and crosslinking extent

This paper describes a systematic study of crosslinking of skeletal muscle myosin subfragment-1 (S1) and heavy meromyosin (HMM) to F-actin in the rigor state with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC). We followed the time courses of S1 or HMM head crosslinking at various actin:S1 or actin:HMM head molar ratios and the resulting superactivation of ATPase activity. The ATPase activity of the covalent complexes was measured at 0.5 M KCl, where the covalent complexes retain superactivated ATPase activity but the activity of uncrosslinked myosin heads is not activated by actin. S1 crosslinking was slowest at the actin:S1 molar ratio of 1:1, but faster at larger molar ratios, where more than 80% of added S1 could be crosslinked to actin. In spite of the dependence of crosslinking rate on actin:S1 ratio, there were two linear correlations between ATPase activity and the extent of S1 crosslinking to actin: one for S1 crosslinked to actin at actin:S1 molar ratios more than 2.7:1 and the other for S1 crosslinked at a molar ratio of 1:1. Extrapolation of the former correlation line to 100% crosslinked S1 gave an ATPase activity of 39 s-1 for actin-S1 covalent complex at 25 degrees C, whereas that of the other correlation line gave 21 s-1. The latter smaller activity suggests that the interface between actin and S1 in their rigor complexes at a molar ratio of 1:1 is different from that at molar ratios of more than 2.7:1. The acto-HMM crosslinking rate depended on the ratio of actin to HMM head, like that of S1 crosslinking to actin. The ATPase activity of crosslinked actin-HMM was, unlike that of actin-S1 covalent complexes, bell-shaped as a function of the crosslinked heads, but chymotryptic conversion of HMM to S1 in the covalent complexes made the bell-shaped characteristics disappear and increased the activity close to that of actin-S1 covalent complexes. These results indicate that some physical constraint imposed on myosin heads suppresses the actin-activated ATPase activity of HMM crosslinked to actin.

Resonance energy transfer between points in a reconstituted skeletal muscle thin filament. A conformational change of the thin filament in response to a change in Ca2+ concentration

The spatial relationships between Lys-61, Cys-374 on actin or SH1 on myosin subfragment-1 (S1) and Cys-190 on tropomyosin or Cys-133 on troponin-I (TnI) in a reconstituted thin filament were studied by fluorescence resonance energy transfer. 5-(2-Iodoacetylaminoethyl)aminonaphthalene 1-sulfonic acid (IAEDANS) attached to Lys-190 on tropomyosin or to Cys-133 on TnI was used as a donor. Fluorescein 5-isothiocyanate (FITC) attached to Lys-61 or 5-(iodoacetoamido)fluorescein (IAF) attached to Cys-374 on actin and 4-dimethylaminophenyl-azophenyl 4'-maleimide (DABMI) attached to SH1 on S1 were used as an acceptor. The transfer efficiency between AEDANS attached to Cys-190 on tropomyosin and FITC attached to Lys-61 on actin was 0.42 in the absence of troponin, 0.46 in the presence of troponin and Ca2+ and 0.55 in the presence of troponin and absence of Ca2+. The corresponding distances between the probes were calculated to be 4.7 nm, 4.6 nm and 4.3 nm respectively, assuming a random orientation factor K2 = 2/3. A large difference in the transfer efficiency from AEDANS attached to Cys-133 on TnI to FITC attached to Lys-61 on actin was observed between in the presence (0.52) and absence (0.70) of Ca2+. The corresponding distances between the probes were calculated to be 4.5 nm in the presence of Ca2+ and 3.9 nm in the absence of Ca2+. The distance between Cys-190 on tropomyosin and Cys-374 on actin was measured to be 5.1 nm and the transfer efficiency (0.35) did not change upon addition of troponin whether Ca2+ is present or not, in agreement with the previous report [Tao, T., Lamkin, M. & Lehrer, S. S. (1983) Biochemistry 22, 3059-3064]. The distance between Cys-133 on TnI and Cys-374 on actin was measured to be 4.4 nm. No detectable change in transfer efficiency (0.58) was observed between values in the presence and absence of Ca2+. These results suggest that a relative movement of the two domains of actin monomer in a reconstituted thin filament occurs in response to a change in Ca2+ concentration. The transfer efficiencies between DABMI attached to SH1 on S1 and AEDANS attached to Cys-190 on tropomyosin or Cys-133 on TnI were too small (less than 2%) for an accurate estimation of the distances, suggesting the distances are longer than 7.3 nm.

ATPase activity of the cross-linked complex between cardiac myosin subfragment 1 and actin in several models of chronic overloading. A new approach to the biochemistry of contractility

Myosin ATPase activity is usually considered to reflect the contractile capacity of a given muscle since it correlates with the maximum initial speed of shortening of the unloaded muscle (Vmax). There are several exceptions to this scheme, and it was the goal of this study to determine if the Mg2+-ATPase activity of the covalently bound actomyosin S1 is a more physiological index of contractility. On polyacrylamide gels, the complex obtained after condensation of fast skeletal myosin S1 to skeletal actin is identical to that obtained with myosin S1 from the ventricles of different species, including rat, guinea pig, and human, cross-linked to cardiac or skeletal actin. In every condition, the ATPase activity of the complex is 700-fold higher than that of myosin S1. It correlates linearly with the Vmax both in phylogeny and in conditions in which an isomyosin shift has been reported, such as hypothyroidism and chronic cardiac overload. Such a relation indicates that, in species that already have a low Vmax, a small change in myosin ATPase may induce dramatic consequences in the shortening velocity. Cardiac hypertrophy in humans, where the drop in Vmax is not associated with a myosin change, does not fit into this scheme. The enzymatic activity of the complex is also unmodified in this condition, which shows that, in humans, the myosin ATPase is not a determinant of Vmax and suggests that other mechanisms may be involved. Measurement of this type of ATPase activity provides a new tool to explore contractility biochemically, which is more reproducible and, from a technical point of view, easier to perform than a kinetic assay. It also correlates better with mechanical data obtained with skinned fibers than with those measured on fresh papillary muscles.

Cross-linking of the skeletal myosin subfragment 1 heavy chain to the N-terminal actin segment of residues 40-113

Glutaraldehyde (GA) and N-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline (EEDQ), a hydrophobic, carboxyl group directed, zero-length protein cross-linker, were employed for the chemical cross-linking of the rigor complex between F-actin and the skeletal myosin S-1. The enzymatic properties and structure of the new covalent complexes obtained with both reagents were determined and compared to those known for the EDC-acto-S-1 complex. The GA- or EEDQ-catalyzed covalent attachment of F-actin to the S-1 heavy chain induced an elevated Mg2+-ATPase activity. The turnover rates of the isolated cross-linked complexes were similar to those for EDC-acto-S-1 (30 s-1). The solution stability of the new complexes is also comparable to that exhibited by EDC-acto-S-1. The proteolytic digestion of the isolated AEDANS-labeled covalent complexes and direct cross-linking experiments between actin and various preformed proteolytic S-1 derivatives indicated that, as observed with EDC, the COOH-terminal 20K and the central 50K heavy chain fragments are involved in the cross-linking reactions of GA and EEDQ. KI-depolymerized acto-S-1 complexes cross-linked by EDC, GA, or EEDQ were digested by thrombin which cuts only actin, releasing S-1 heavy chain-actin peptide cross-linked complexes migrating on acrylamide gels with Mr 100K (EDC), 110K and 105K (GA), and 102K (EEDQ); these were fluorescent only when fluorescent S-1 was used. They were identified by immunostaining with specific antibodies directed against selected parts of he NH2-terminal actin segment of residues 1-113.(ABSTRACT TRUNCATED AT 250 WORDS)

The interaction of troponin-I with the N-terminal region of actin

The interaction between troponin-I and actin that underlies thin-filament regulation in striated muscle has been studied using proton magnetic resonance spectroscopy. A restricted portion of skeletal muscle troponin-I (residues 96-116) has previously been shown to be capable of inhibiting the MgATPase activity of actomyosin in a manner enhanced by tropomyosin [Syska et al. (1976) Biochem. J. 153, 375-387]. On the basis of homologous spectral effects for signals of specific groups observed in different complexes formed using the native proteins and a variety of defined peptides, it is concluded that the segment of troponin-I which has inhibitory activity interacts with the N-terminal region of actin. The surface of contact of the inhibitory segment of troponin-I with actin involves two regions of the N-terminal of actin. These are located between residues 1-7 and 19-44. The data are discussed in the context of a structural mechanism for the inhibition of myosin ATPase activation.

Anti-actin antibodies. An immunological approach to the myosin-actin and the tropomyosin-actin interfaces

The topography of the rigor complex between subfragment-1 (S-1) of myosin and actin was investigated by using several specific antibodies directed to well-located sequences in actin. A major contact area for S-1 was characterized in the hydrophilic 18-28 constant sequence, and the variable 1-7 sequence was only found to be in close proximity to the interface. The C-terminal extremity of actin situated around Cys-374 appeared to be included in a region close to the S-1 heavy chain and the N-terminal part of actin. The interaction between tropomyosin and actin was also studied. Neither of the terminal parts of actin were involved in this interaction. Thus, the regions involved in the interactions of S-1 and tropomyosin with actin do not overlap.

Interaction of phalloidin with chemically modified actin

Modification of Tyr-69 with tetranitromethane impairs the polymerizability of actin in accordance with the previous report [Lehrer, S. S. and Elzinga, M. (1972) Fed. Proc. 31, 502]. Phalloidin induces this chemically modified actin to form the same characteristic helical thread-like structure as normal F-actin. The filaments bind myosin heads and activate the myosin ATPase activity as effectively as normal F-actin. When a dansyl group is introduced at the same point [Chantler, P. D. and Gratzer, W. B. (1975) Eur. J. Biochem. 60, 67-72], phalloidin still induces the polymerization. The filaments bind myosin heads and activate the myosin ATPase activity. These results indicate that Tyr-69 is not directly involved in either an actin-actin binding site or the myosin binding site on actin. Moreover, the results suggest that phalloidin binds to actin monomer in the presence of salt and its binding induces a conformational change in actin which is essential for polymerization, or that actin monomer fluctuates between in unpolymerizable and polymerizable form while phalloidin binds to actin only in the polymerizable form and its binding locks the conformation which causes the irreversible polymerization of actin. Modification of Tyr-53 with 5-diazonium-(1H)tetrazole blocks actin polymerization [Bender, N., Fasold, H., Kenmoku, A., Middelhoff, G. and Volk, K. E. (1976) Eur. J. Biochem. 64, 215-218]. Phalloidin is unable to induce the polymerization of this modified actin nor does it bind to it. Phalloidin does not induce the polymerization of the trypsin-digested actin core. These results indicate that the site at which phalloidin binds is involved in polymerization and the probable conformational change involved in polymerization may be modulated through this site.