Modifying preselected sites on proteins: the stretch of residues 633-642 of the myosin heavy chain is part of the actin-binding site

MYH7 in cardiomyopathy and skeletal muscle myopathy

Myosin heavy chain gene 7 (MYH7), a sarcomeric gene encoding the myosin heavy chain (myosin-7), has attracted considerable interest as a result of its fundamental functions in cardiac and skeletal muscle contraction and numerous nucleotide variations of MYH7 are closely related to cardiomyopathy and skeletal muscle myopathy. These disorders display significantly inter- and intra-familial variability, sometimes developing complex phenotypes, including both cardiomyopathy and skeletal myopathy. Here, we review the current understanding on MYH7 with the aim to better clarify how mutations in MYH7 affect the structure and physiologic function of sarcomere, thus resulting in cardiomyopathy and skeletal muscle myopathy. Importantly, the latest advances on diagnosis, research models in vivo and in vitro and therapy for precise clinical application have made great progress and have epoch-making significance. All the great advance is discussed here.

N-terminal polar amino acids of the C2 insert of nonmuscle myosin II-C2 regulate its functional properties

In this study, we investigated the regions in the alternatively spliced C2 insert of nonmuscle myosin (NM) II-C conferring unique functional properties to the protein. We used constructs carrying deletions within different regions of C2 in neuronal cells; namely, the polar N terminus, the proline/serine-rich middle, and the nonpolar C terminus. We compared the wild-type NM II-C2 and deletion mutants with respect to ATPase activity, coassembly with NM II-B, regulation by myosin light-chain kinase (MLCK), and solubility, to determine the C2 region(s) involved in these processes. In addition, we examined the ability of the mutants to rescue the neurite-shortening phenotype upon NM II-C2 knockdown in Neuro-2a cells. Our data highlight the importance of the polar N terminus in NM II-C2 function.

Structural model of weak binding actomyosin in the prepowerstroke state

We present the first in silico model of the weak binding actomyosin in the initial powerstroke state, representing the actin binding-induced major structural changes in myosin. First, we docked an actin trimer to prepowerstroke myosin then relaxed the complex by a 100-ns long unrestrained molecular dynamics. In the first few nanoseconds, actin binding induced an extra primed myosin state, i.e. the further priming of the myosin lever by 18° coupled to a further closure of switch 2 loop. We demonstrated that actin induces the extra primed state of myosin specifically through the actin N terminus-activation loop interaction. The applied in silico methodology was validated by forming rigor structures that perfectly fitted into an experimentally determined EM map of the rigor actomyosin. Our results unveiled the role of actin in the powerstroke by presenting that actin moves the myosin lever to the extra primed state that leads to the effective lever swing.

Differential scanning calorimetric study of myosin subfragment 1 with tryptic cleavage at the N-terminal region of the heavy chain

The thermal unfolding of myosin subfragment 1 (S1) cleaved by trypsin was studied by differential scanning calorimetry. In the absence of nucleotides, trypsin splits the S1 heavy chain into three fragments (25, 50, and 20 kDa). This cleavage has no appreciable influence on the thermal unfolding of S1 examined in the presence of ADP, in the ternary complexes of S1 with ADP and phosphate analogs, such as orthovanadate (Vi) or beryllium fluoride (BeFx), and in the presence of F-actin. In the presence of ATP and in the complexes S1.ADP.Vi or S1.ADP.BeFx, trypsin produces two additional cleavages in the S1 heavy chain: a faster cleavage in the N-terminal region between Arg23 and Ile24, and a slower cleavage at the 50 kDa fragment. It has been shown that the N-terminal cleavage strongly decreases the thermal stability of S1 by shifting the maximum of its thermal transition by about 7 degrees C to a lower temperature, from 50 degrees C to 42.4 degrees C, whereas the cleavage at both these sites causes dramatic destabilization of the S1 molecule leading to total loss of its thermal transition. Our results show that S1 with ATP-induced N-terminal cleavage is able, like uncleaved S1, to undergo global structural changes in forming the stable ternary complexes with ADP and Pi analogs (Vi, BeFx). These changes are reflected in a pronounced increase of S1 thermal stability. However, S1 cleaved by trypsin in the N-terminal region is unable, unlike S1, to undergo structural changes induced by interaction with F-actin that are expressed in a 4-5 degrees C shift of the S1 thermal transition to higher temperature. Thus, the cleavage between Arg23 and Ile24 does not significantly affect nucleotide-induced structural changes in the S1, but it prevents structural changes that occur when S1 is bound to F-actin. The results suggest that the N-terminal region of the S1 heavy chain plays an important role in structural stabilization of the entire motor domain of the myosin head, and a long-distance communication pathway may exist between this region and the actin-binding sites.

Temperature plasticity of contractile proteins in fish muscle

Three myosin heavy chain isoforms with different actin-activated Mg2+-ATPase activities were found in the fast skeletal muscle from carp (Cyprinus carpio) acclimated to 10 and 30°C. The composition of three types of myosin heavy chain was dependent on acclimation temperature,demonstrating the presence of temperature-specific myosin isoforms in carp. Subsequently, the temperature-dependence of the sliding velocity of fluorescent F-actin in myosins isolated from 10°C- and 30°C-acclimated carp was measured. At 8°C, the filament velocity was three times higher for myosin from 10°C- than from 30°C-acclimated fish. Activation energies (Ea) for the sliding velocity of F-actin were 63 and 111 kJ mol-1 for myosins from 10°C- and 30°C-acclimated fish, respectively. Activation energy for actin-activated Mg2+-ATPase activity was 0.46 kJ mol-1 in myosin from 10°C-acclimated fish and 0.54 kJ mol-1 in myosin from 30°C-acclimated fish. The inactivation rate constant(KD) of Ca2+-ATPase was 7.5×10-4s-1 at 30°C for myosin from 10°C-acclimated fish, which was approximately twice that for myosin from 30°C-acclimated fish. It is suggested that these differences in thermostability reflect a more flexible structure of the myosin molecule in cold-acclimated carp, which results in a reduced activation enthalpy for contraction and, hence, a higher sliding velocity at low temperatures. Structural analysis of cDNAs encoding the carp myosin heavy chain demonstrated striking differences in two surface loops of myosin subfragment-1 (S1), loops 1 and 2, between the 10°C and 30°C types, which were predominantly expressed in carp acclimated to 10°C and 30°C, respectively. Chimeric myosins composed of Dictyostelium discoideum myosin backbones with loop sequences of carp S1 heavy chain isoforms demonstrated that the diversity of the loop 2 sequence of carp S1 affected the Vmax of actin-activated Mg2+-ATPase activity.

Two conserved lysines at the 50/20-kDa junction of myosin are necessary for triggering actin activation

Actin stimulates myosin's activity by inducing structural alterations that correlate with the transition from a weakly to a strongly bound state, during which time inorganic phosphate (P(i)) is released from myosin's active site. The surface loop at the 50/20-kDa junction of myosin (loop 2) is part of the actin interface. Here we demonstrate that elimination of two highly conserved lysines at the C-terminal end of loop 2 specifically blocks the ability of heavy meromyosin to undergo a weak to strong binding transition with actin in the presence of ATP. Removal of these lysines has no effect on strong binding in the absence of nucleotide, on the rate of ADP binding or release, or on the basal ATPase activity. We further show that the 16 amino acids of loop 2 preceding the lysine-rich region are not essential for actin activation, although they do modulate myosin's affinity for actin in the presence of ATP. We conclude that interaction of the conserved lysines with acidic residues in subdomain 1 of actin either triggers a structural change or stabilizes a conformation that is necessary for actin-activated release of P(i) and completion of the ATPase cycle.

Changes in the thermal unfolding of p-phenylenedimaleimide-modified myosin subfragment 1 induced by its 'weak' binding to F-actin

Differential scanning calorimetry (DSC) was used to analyze the thermal unfolding of myosin subfragment 1 (S1) with the SH1 (Cys-707) and SH2 (Cys-697) groups cross-linked by N,N'-p-phenylenedimaleimide (pPDM-S1). It has been shown that F-actin affects the thermal unfolding of pPDM-S1 only at very low ionic strength, when some part of pPDM-S1 binds weakly to F-actin, but not at higher ionic strength (200 mM KCl). The weak binding of pPDM-S1 to F-actin shifted the thermal transition of pPDM-S1 by about 5 degrees C to a higher temperature. This actin-induced increase in thermal stability of pPDM-S1 was similar to that observed with 'strong' binding of unmodified S1 to F-actin. Our results show that actin-induced structural changes revealed by DSC in the myosin head occur not only upon strong binding but also on weak binding of the head to F-actin, thus suggesting that these changes may occur before the power-stroke and play an important role in the motor function of the head.

Functional characterization of vertebrate nonmuscle myosin IIB isoforms using Dictyostelium chimeric myosin II

The alternatively spliced isoform of nonmuscle myosin II heavy chain B (MHC-IIB) with an insert of 21 amino acids in the actin-binding surface loop (loop 2), MHC-IIB(B2), is expressed specifically in the central nervous system of vertebrates. To examine the role of the B2 insert in the motor activity of the myosin II molecule, we expressed chimeric myosin heavy chain molecules using the Dictyostelium myosin II heavy chain as the backbone. We replaced the Dictyostelium native loop 2 with either the noninserted form of loop 2 from human MHC-IIB or the B2-inserted form of loop 2 from human MHC-IIB(B2). The transformant Dictyostelium cells expressing only the B2-inserted chimeric myosin formed unusual fruiting bodies. We then assessed the function of chimeric proteins, using an in vitro motility assay and by measuring ATPase activities and binding to F-actin. We demonstrate that the insertion of the B2 sequence reduces the motor activity of Dictyostelium myosin II, with reduction of the maximal actin-activated ATPase activity and a decrease in the affinity for actin. In addition, we demonstrate that the native loop 2 sequence of Dictyostelium myosin II is required for the regulation of the actin-activated ATPase activity by phosphorylation of the regulatory light chain.

Variable surface loops and myosin activity: accessories to a motor

The catalytic head of myosin is a globular structure that has historically been divided into three segments of 25, 50, and 20 kDa. The solvent-exposed, proteolytically-sensitive surface loops of myosin that join these three segments are highly variable in their sequences. While surface loops have not traditionally been thought to affect enzymatic activities, these loops lie near the ATP and actin-binding sites and have been implicated in the modulation of myosin's kinetic activities. In this work we review the wealth of data regarding the loops that has accumulated over the years and discuss the roles of the loops in contributing to the different activities displayed by different myosin isoforms.

Unconventional myosins and the genetics of hearing loss

Mutations of the unconventional myosins genes encoding myosin VI, myosin VIIA and myosin XV cause hearing loss and thus these motor proteins perform fundamental functions in the auditory system. A null mutation in myosin VI in the congenitally deaf Snell's waltzer mice (Myo6(sv)) results in fusion of stereocilia and subsequent progressive loss of hair cells, beginning soon after birth, thus reinforcing the vital role of cytoskeletal proteins in inner ear hair cells. To date, there are no human families segregating hereditary hearing loss that show linkage to MYO6 on chromosome 6q13. The discovery that the mouse shaker1 (Myo7(ash1)) locus encodes myosin VIIA led immediately to the identification of mutations in this gene in Usher syndrome type 1B; subsequently, mutations in this gene were also found associated with recessive and dominant nonsyndromic hearing loss (DFNB2 and DFNA11). Stereocilla of sh1 mice are severely disorganized, and eventually degenerate as well. Myosin VIIA has been implicated in membrane trafficking and/or endocytosis in the inner ear. Mutant alleles of a third unconventional myosin, myosin XV, are associated with nonsyndromic, recessive, congenital deafness DFNB3 on human chromosome 17p11.2 and deafness in shaker2 (Myo15(sh2)) mice. In outer and inner hair cells, myosin XV protein is detectable in the cell body and stereocilia. Hair cells are present in homozygous sh2 mutant mice, but the stereocilia are approximately 1/10 of the normal length. This review focuses on what we know about the molecular genetics and biochemistry of myosins VI, VIIA and XV as relates to hereditary hearing loss. Am. J. Med. Genet. (Semin. Med. Genet.) 89:147-157, 1999. Published 2000 Wiley-Liss, Inc.

Characterization of the carp myosin heavy chain multigene family

We isolated partial coding sequences for 29 carp myosin heavy chain genes (MyoHCs) and determined the nucleotide sequences around the region encoding the loop 2 of the myosin molecule. The predicted amino acid sequences from the isolated genes all showed very high similarity to those of skeletal and cardiac muscles from higher vertebrates, but not to those of smooth and non-muscle counterparts. Among all clones isolated, carp MyoHC10, MyoHCI-1-3 and MyoHC30 showed exon-nucleotide sequences identical to those of cDNAs encoding the loop 2 region of the 10 degrees C-, intermediate- and 30 degrees C-type fast skeletal isoforms [Hirayama and Watabe, Euro. J. Biochem. 246 (1997) 380-387]. The loop 2 of 28 types of carp MyoHCs was encoded by two exons separated by an intron corresponding to that of the 16th in higher vertebrate MyoHCs, whilst this intron was not found in carp MyoHC30. Although carp MyoHC30 had a gene organization different from those of higher vertebrates and other carp MyoHCs, its predicted amino acid sequence for loop 2 showed the highest homology to those of higher vertebrates among carp MyoHCs. In the 28 carp MyoHCs containing the intron, a combination of different nucleotide sequences for the two resulted in 14 distinct series for the combined coding sequence. These different nucleotide sequences encoded nine distinct amino acid sequences. Phylogenetic analysis for the present loop 2 and light meromyosin previously reported for carp MyoHCs [Imai et al., J. Exp. Biol. 200 (1997) 27-34] revealed that carp MyoHCs have recently diverged and are more closely related to each other than to MyoHCs from other species.

Differences in the ionic interaction of actin with the motor domains of nonmuscle and muscle myosin II

Changes in the actin-myosin interface are thought to play an important role in microfilament-linked cellular movements. In this study, we compared the actin binding properties of the motor domain of Dictyostelium discoideum (M765) and rabbit skeletal muscle myosin subfragment-1 (S1). The Dictyostelium motor domain resembles S1(A2) (S1 carrying the A2 light chain) in its interaction with G-actin. Similar to S1(A2), none of the Dictyostelium motor domain constructs induced G-actin polymerization. The affinity of monomeric actin (G-actin) was 20-fold lower for M765 than for S1(A2) but increasing the number of positive charges in the loop 2 region of the D. discoideum motor domain (residues 613-623) resulted in equivalent affinities of G-actin for M765 and for S1. Proteolytic cleavage and cross-linking approaches were used to show that M765, like S1, interacts via the loop 2 region with filamentous actin (F-actin). For both types of myosin, F-actin prevents trypsin cleavage in the loop 2 region and F-actin segment 1-28 can be cross-linked to loop 2 residues by a carbodiimide-induced reaction. In contrast with the S1, loop residues 559-565 of D. discoideum myosin was not cross-linked to F-actin, probably due to the lower number of positive charges. These results confirm the importance of the loop 2 region of myosin for the interaction with both G-actin and F-actin, regardless of the source of myosin. The differences observed in the way in which M765 and S1 interact with actin may be linked to more general differences in the structure of the actomyosin interface of muscle and nonmuscle myosins.

Modulation of actin affinity and actomyosin adenosine triphosphatase by charge changes in the myosin motor domain

The effects of mutations in an actin-binding surface loop of myosin (loop 2) are described. Part of loop 2, the segment between myosin residues 618 and 622, was replaced with sequences enlarged by the introduction of positively charged GKK or neutral GNN motifs. Constructs with loops carrying up to 20 additional amino acids and charge variations from -1 to +12 were produced. Steady-state and transient kinetics were used to characterize the enzymatic behavior of the mutant motor domains. Binding of nucleotide was not affected by any of the alterations in loop 2. In regard to their interaction with actin, constructs with moderate charge changes (-1 to +2) displayed wild-type-like behavior. Introduction of more than one GKK motif led to stronger coupling between the actin- and nucleotide-binding sites of myosin and an up to 1000-fold increased affinity for actin in the absence of ATP and at zero ionic strength. In comparison to the wild-type construct M765, constructs with 4-12 extra charges displayed an increased dependence on ionic strength in their interaction with actin, a 2-3-fold increase in kcat, a more than 10-fold reduction in Kapp for actin, and a 34-70-fold increase in catalytic efficiency.

Myosin heavy chain transitions during development. Functional implications for the respiratory musculature

The myosin heavy chain (MHC) exists as multiple isoforms that are encoded for by a family of genes. The respiratory musculature demonstrates muscle-specific and temporally-dependent changes in MHC isoform expression during maturation. Developmental expression of MHC isoforms correlate well with postnatal changes in actomyosin ATPase activity, specific force generation (P0/CSA), maximum unloaded velocity of shortening (V0) and and fatigue resistance. More specifically, as the expression of MHCneonatal declines and MHC2A, MHC2X, and MHC2B increase, actomyosin ATPase activity, P0/CSA, V0, and muscle fatigability increase. The increase in actomyosin ATPase activity with maturation is partially offset by a postnatal increase in oxidative capacity; however, as fatigue resistance declines with development it is apparent that the energy costs of contraction are not fully matched by an increase in energy production. Developmental transitions in smooth muscle MHC phenotype also occur although their functional importance remains unclear.

Transglutaminase-induced cross-linking between subdomain 2 of G-actin and the 636-642 lysine-rich loop of myosin subfragment 1

G-actin was covalently cross-linked with S1 in a bacterial transglutaminase-catalyzed reaction. The cross-linking sites were identified with the help of fluorescent probes and limited proteolysis as the Gln-41 on the DNase I binding loop of subdomain 2 in G-actin and a lysine-rich loop (residues 636-642) on the S1 heavy chain. The same lysine-rich loop was cross-linked to another region of G-actin in a former study (Combeau, C., D. Didry, and M-F. Carlier. 1992. J. Biol. Chem. 267:14038-14046). This indicates the existence of more than one G-actin-S1 complex. In contrast to G-actin, no cross-linking was induced between F-actin and S1 by the transglutaminase reaction. This shows that in F-actin the inner part of the DNase I binding loop, where Gln-41 is located, is not accessible for S1. The cross-linked G-actin-S1 polymerized upon addition of 2 mM MgCl2 as indicated by electron microscopy and sedimentation experiments. The filaments obtained from the polymerization of cross-linked actin and S1 were much shorter than the control actin filaments. The ATPase activity of the cross-linked S1 was not activated by actin, whereas the K+ (EDTA)-activated ATPase activity of S1 was unaffected by the cross-linking. The cross-linking between G-actin and S1 was not influenced by the exchange of the tightly bound calcium to magnesium; however, it was inhibited by the exchange of the actin-bound ATP to ADP. This finding supports the view that the structure of the DNase binding loop in ADP-G-actin is somewhere between the structures of ATP-G-actin and F-actin.

Hypertrophic obstructive cardiomyopathy due to a novel T-to-A transition at codon 624 in the beta-myosin heavy chain (beta-MHC) gene possibly related to the sudden death

Many missense mutations in the beta-myosin heavy chain have been reported in patients with hypertrophic obstructive cardiomyopathy (HOCM). However, the controversy is present whether the mutation accompanying the change of electric charge is related with poorer prognosis. The proband, a 48-year-old female, of the family was diagnosed clinically as HOCM, and a structural analysis of the cardiac beta-MHC gene showed that the proband and her junior daughter had a novel mutation with T to A transition in codon 624 replacing tyrosine with asparagine, which was not present in her husband, elder daughter and son. The proband's husband, son and two daughters were healthy except that the ECG of junior daughter (15-year-old) showed complete right bundle branch block. Proband's mother died suddenly after the delivery of the proband and the proband also collapsed suddenly. The occurrence of sudden death in proband and her mother suggested that HOCM with this novel mutation might be associated with a high risk of sudden death irrespective of the absence of charge alteration.

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.

The structure of the acto-myosin subfragment 1 complex: results of searches using data from electron microscopy and x-ray crystallography

Surmises of how myosin subfragment 1 (S1) interacts with actin filaments in muscle contraction rest upon knowing the relative arrangement of the two proteins. Although there exist crystallographic structures for both S1 and actin, as well as electron microscopy data for the acto-S1 complex (AS1), modeling of this arrangement has so far only been done "by eye." Here we report fitted AS1 structures obtained using a quantitative method that is both more objective and makes more complete use of the data. Using undistorted crystallographic results, the best-fit AS1 structure shows significant differences from that obtained by visual fitting. The best fit is produced using the F-actin model of Holmes et al. [Holmes, K. C., Popp, D., Gebhard, W. & Kabsch, W. (1990) Nature (London) 347, 44-49]. S1 residues at the AS1 interface are now found at a higher radius as well as being translated axially and rotated azimuthally. Fits using S1 plus loops missing from the crystal structure were achieved using a homology search method to predict loop structures. These improved fits favor an arrangement in which the loop at the 50- to 20-kDa domain junction of S1 is located near the N terminus of actin. Rigid-body movements of the lower 50-kDa domain, which further improve the fit, produce closure of the large 50-kDa domain cleft and bring conserved residues in the lower 50-kDa domain into an apparently appropriate orientation for close interaction with actin. This finding supports the idea that binding of ATP to AS1 at the end of the ATPase cycle disrupts the actin binding site by changing the conformation of the 50-kDa cleft of S1.

Structural differences in the crossbridge head of temperature-associated myosin subfragment-1 isoforms from carp fast skeletal muscle

We determined the primary structures of the three acclimation-temperature-associated isoforms of myosin subfragment-1 heavy chain from fast skeletal muscle of thermally acclimated carp. These isoforms were cloned by extending 5'-regions of cDNAs that encode the rod part of myosin heavy chain specifically expressed in 10 degrees C- and 30 degrees C-acclimated carp, together with the region that encodes an intermediate structure [Imai, J., Hirayama, Y., Kikuchi, K., Kakinuma, M. & Watabe, S. (1997) J. Exp. Biol. 200, 27-34]. These three isoforms generally resembled each other in primary structure, showing 94.8, 90.9, and 92% similarity between the 10 degrees C- and intermediate-type, between the 10 degrees C- and 30 degrees C-type, and between the intermediate- and 30 degrees C-type myosin heavy chains, respectively. However, isoform-specific differences were clearly observed between the 10 degrees C- and 30 degrees C-type heavy chains in the first 60 amino acid residues from the N-terminus, where the intermediate-type showed an intermediate feature in its sequence compared to the 10 degrees C- and 30 degrees C-type isoforms. Other striking differences were observed in two surface loops between the 10 degrees C- and 30 degrees C-type isoform. Five amino acid residues out of sixteen were different in loop 1 near the ATP-binding pocket, and six out of twenty were different in loop 2 on the actin-binding site. The loops connecting beta-sheets that are known to surround the ATP-binding pocket were highly conserved in primary structure for the three types. In northern blot analysis, the accumulated mRNA levels of the 10 degrees C- and intermediate-type isoforms were significantly higher in carp acclimated to 10 degrees C and 20 degrees C than carp acclimated to 30 degrees C, whereas the level of the 30 degrees C-type isoform was significantly higher in carp acclimated to 30 degrees C than those acclimated to 10 degrees C and 20 degrees C.

Small segmental rearrangements in the myosin head can explain force generation in muscle

Poisson-Boltzmann calculations of the distribution of electrostatic potentials around an actin filament in physiological-strength solutions show that negative isopotential surfaces protrude into the solvent. Each protrusion follows the actin two-start helix and is located on the sites implicated in the formation of the actomyosin complex. Molecular dynamic calculations on the S1 portion of the myosin molecule indicate that in the presence of ATP the crystallographically invisible loops (comprising residues 624-649 and 564-579) remain on the surface, whereas in the absence of ATP they can move toward the actin-binding sites and experience electrostatic forces that range from 1 to 10 pN. The molecular dynamics calculations also suggest that during the ATP cycle there exist at least three states of electrostatic interactions between the loops and actin. Every time a new interaction is formed, the strain in the myosin head increases and the energy of the complex decreases by 2kT to 5kT. This can explain muscular contraction in terms of a Huxley-Simmons-type mechanism, while requiring only rearrangements of small mobile S1 segments rather than the large shape changes in the myosin molecule postulated by the conventional tilting head model.

The role of surface loops (residues 204-216 and 627-646) in the motor function of the myosin head

A characteristic feature of all myosins is the presence of two sequences which despite considerable variations in length and composition can be aligned with loops 1 (residues 204-216) and 2 (residues 627-646) in the chicken myosin-head heavy chain sequence. Recently, an intriguing hypothesis has been put forth suggesting that diverse performances of myosin motors are achieved through variations in the sequences of loops 1 and 2 [Spudich, J. (1994) Nature (London) 372, 515-518]. Here, we report on the study of the effects of tryptic digestion of these loops on the motor and enzymatic functions of myosin. Tryptic digestions of myosin, which produced heavy meromyosin (HMM) with different percentages of molecules cleaved at both loop 1 and loop 2, resulted in the consistent decrease in the sliding velocity of actin filaments over HMM in the in vitro motility assays, did not affect the Vmax, and increased the Km values for actin-activated ATPase of HMM. Selective cleavage of loop 2 on HMM decreased its affinity for actin but did not change the sliding velocity of actin in the in vitro motility assays. The cleavage of loop 1 and HMM decreased the mean sliding velocity of actin in such assays by almost 50% but did not alter its affinity for HMM. To test for a possible kinetic determinant of the change in motility, 1-N6-ethenoadenosine diphosphate (epsilon-ADP) release from cleaved and uncleaved myosin subfragment 1 (S1) was examined. Tryptic digestion of loop 1 slightly accelerated the release of epsilon-ADP from S1 but did not affect the rate of epsilon-ADP release from acto-S1 complex. Overall, the results of this work support the hypothesis that loop 1 can modulate the motor function of myosin and suggest that such modulation involves a mechanism other than regulation of ADP release from myosin.

Cross-bridge binding to actin and force generation in skinned fibers of the rabbit psoas muscle in the presence of antibody fragments against the N-terminus of actin

To assess the significance of the NH2-terminus of actin for cross-bridge action in muscle, skinned fibers of rabbit psoas muscle were equilibrated with Fab fragments of antibodies directed against the first seven N-terminal residues of actin. With the antibody fragment, active force is more inhibited than relaxed fiber stiffness, or stiffness in rigor or in the presence of magnesium pyrophosphate. Inhibition of stiffness in rigor or with magnesium pyrophosphate does not necessarily indicate involvement of the NH2-terminus of actin in strong cross bridge binding to actin but may simply result from the large size of the Fab. At high Fab concentrations, active force is essentially abolished, whereas stiffness is still detectible under all conditions. Thus, complete inhibition of active force apparently is not due to interference with cross-bridge binding to actin but may result from the Fab-mimicking inhibition of the thin filament by Troponin-1 binding to the NH2-terminus of actin at low Ca2+. However, although Troponin-1 is released from the NH2-terminus at high Ca2+, the Fab is not, thus disallowing force generation upon increase in Ca2+. These data are consistent with involvement of the NH2-terminus of actin in both weak cross-bridge binding to actin and Ca2+ regulation of the thin filament.

The putative actin-binding role of hydrophobic residues Trp546 and Phe547 in chicken gizzard heavy meromyosin

In the course of myosin-catalyzed ATP hydrolysis, certain amino acid residues in myosin interact with counterparts in actin to produce the relational changes that underlie muscle contraction; some of these interactions are ionic, but the stronger interactions are hydrophobic. In an effort to identify myosin residues participating in hydrophobic interactions, myosin (from smooth muscle) fragments with mutations at suspected sites were engineered and compared with wild-type fragments. It was found that the ATPase of doubly mutated (Trp546Ser and Phe547His) fragments was minimally activated by actin and did not decorate actin well to form the regular arrowhead pattern characteristic of myosin binding to actin filaments. Thus, we suggest that Trp546 and Phe547 are important participants in the hydrophobic actin-myosin interaction.

Immunochemical probing of the functional role of the 238-246 and 567-574 sequences of myosin heavy chain

Polyclonal site-directed peptide antibodies were raised against the 567-574 and 238-246 sequences of the rabbit skeletal muscle myosin heavy chain. These sequences, which are located in the subfragment 1 (S1) segment of myosin, have been implicated by former studies in actin and nucleotide binding of the molecule and in the communication between the two binding sites. The antibodies obtained from rabbit sera were found to be conformation-sensitive since they specifically reacted with S1 in solid-phase binding assay but not in Western blot. The binding of both antibodies to S1 was strongly inhibited by actin. The antibody against the 567-574 sequence, Ab567-574, moderately decreased the binding of S1 to actin filaments in rigor but not in the weakly-attached state, while Ab238-246 did not influence the binding of S1 to actin under either conditions. Both antibodies inhibited the actin activation of the MgATPase of S1 but did not affect MgATPase without actin or the Ca- and K(EDTA)-activated ATPase activities of S1. The sliding velocity of actin filaments in the in vitro motility assays were also reduced in the presence of the antibodies. Ab567-574 had especially strong inhibitory effect on the movement of actin filaments. The results indicate that the binding of antibodies may induce conformational changes, which propagate in the S1 structure, perturb the coupling between the binding sites and impair the motor function of myosin.

Mapping of the actomyosin interfaces

Recombinant DNA methods were used to obtain soluble, undenatured fragments of the heavy chain of myosin subfragment 1 (S-1). These fragments were of preselected lengths and could include protease-sensitive segments that are destroyed when other preparation methods are used. Actin binding by each of the three contiguous segments (residues 1-248, 249-524, and 518-722, essentially spanning the entire S-1 heavy chain) was demonstrated. ATP binding, comparable to that of native S-1, was obtained only with a segment consisting of residues 1-524. Competition among the various fragments for actin was also studied. The data are discussed in relation to the recently reported resolved structure of S-1 [Rayment, I., Rypnieski, R. W., Schmidt-Bäse, K., Smith, R., Tomchick, D. R., Benning, M. M., Winkelmann, D. A., Wesenberg, G. & Holden, H. M. (1993) Science 261, 50-58].

Structural organization of the human cardiac alpha-myosin heavy chain gene (MYH6)

The human myocardium expresses two cardiac myosin heavy chain (MyHC) isoforms, alpha and beta, that exist in tandem array on chromosome 14q12. We have previously sequenced the entire human cardiac beta-MyHC gene and now report the complete nucleotide sequence of the human cardiac alpha-MyHC, encompassing 26,159 bp as well as the entire 4484-bp 5'-flanking intergenic region. The gene (MYH6) consists of 39 exons, 37 of which contain coding information. The 5'-untranslated region is split into 3 exons, with the third exon containing the AUG translation initiation codon. With the exception of the 13th intron of the human cardiac beta-MyHC, which is not present within the alpha-isogene, all exon/intron boundaries are conserved. Conspicuous sequence motifs contained within the alpha-MyHC gene include four Alu repeats, a single (GT)n element, and a homopurine-homopyrimidine tract containing 23 GAA repeating units followed by 10 GAG repeating units. Comparison of the encoded amino acid sequence with a previously reported human alpha-MyHC cDNA sequence reveals several potential polymorphisms.

Cross-linking myosin subfragment 1 Cys-697 and Cys-707 modifies ATP and actin binding site interactions

Skeletal muscle myosin is an enzyme that interacts allosterically with MgATP and actin to transduce the chemical energy from ATP hydrolysis into work. By modifying myosin structure, one can change this allosteric interaction and gain insight into its mechanism. Chemical cross-linking with N,N'-p-phenylenedimaleimide (pPDM) of Cys-697 to Cys-707 of the myosin-ADP complex eliminates activity and produces a species that resembles myosin with ATP bound (Burke et al., 1976). Nucleotide-free pPDM-modified myosin subfragment 1 (S1) was prepared, and its structural and allosteric properties were investigated by comparing the nucleotide and actin interactions of S1 to those of pPDM-S1. The structural properties of the nucleotide-free pPDM-S1 are different from those of S1 in several respects. pPDM-S1 intrinsic tryptophan fluorescence intensity is reduced 28%, indicating a large increase of an internal quenching reaction (the fluorescence intensity of the related vanadate complex of S1, S1-MgADP-Vi, is reduced by a similar degree). Tryptophan fluorescence anisotropy increases from 0.168 for S1 to 0.192 for pPDM-S1, indicating that the unquenched tryptophan population in pPDM-S1 has reduced local freedom of motion. The actin affinity of pPDM-S1 is over 6,000-fold lower than that of S1, and the absolute value of the product of the net effective electric charges at the acto-S1 interface is reduced from 8.1 esu2 for S1 to 1.6 esu2 for pPDM-S1. In spite of these changes, the structural response of pPDM-S1 to nucleotide and the allosteric communication between its ATP and actin sites remain intact. Compared to pPDM-S1, the fluorescence intensity of pPDM-S1 *MgADP is increased 50%(compared to 8 and 31% increases, respectively, for MgADP and MgATP binding to S1). Compared to acto-pPDM-S1, the absolute value of the product of the net effective electric charge at the actin binding interface of acto-pPDM-S1 *MgADP increases 7.3 esu2 (compared to a 0.9 esu2 decrease and an 11.0 esu2 increase, respectively, for MgADP and MgATP binding to acto-Sl).The interaction free energy for the ligands MgADP and actin, is -2.0 kcal/mol for pPDM-S1, compared to -1.2 kcal/mol for unmodified S1.

Effect of actin on the tryptic digestion of myosin subfragment 1 in the weakly attached state

The structure of myosin subfragment 1 (S1) in the weakly attached complex with actin was studied at three specific sites, at the 50-kDa/20-kDa and 27-kDa/50-kDa junctions, and at the N-terminal region, using tryptic digestion as a structure-exploring tool. The structure of S1 at the vicinity of the 50-kDa/20-kDa junction is pH dependent in the weakly attached state because the tryptic cleavage at this site was fully protected by actin at pH 6.2, but the protection was only partial at pH 8.0. Since the actin protection is complete in rigor at both pH values, the results indicate that the structure of S1 at the 50-kDa/20-kDa junction differs in the two states at pH 8.0, but not at pH 6.2. Actin restores the ADP-suppressed tryptic cleavage after Lys213 at the 27-kDa/50-kDa junction in the strongly attached state, but not in the weakly attached state, which indicates structural difference between the two states at this site. ATP and ADP open a new site for tryptic cleavage in the N-terminal region of the S1 heavy chain between Arg23 and Ile24. Actin was found to suppress this cleavage in both weakly and strongly attached states, which shows that, in the vicinity of this site, the structure of S1 is similar in both states. The results indicate that the binding of S1 to actin induces localized changes in the S1 structure, and the extent of these changes is different in the various actin-S1 complexes.

Myosin heavy chain expression in respiratory muscles of the rat

Myosin heavy chain (MHC) isoforms of hind limb adult rat muscles and muscles with a range of respiratory activities were analyzed by a sodium dodecyl sulfate polyacrylamide gel electrophoresis technique that allowed electrophoretic separation of the three fast and one slow MHC isoform found in typical rat muscle. Costal and crural diaphragm muscle samples expressed a mixture of MHC beta/slow, MHC2A, and MHC2X but little MHC2B. In contrast, MHC2B was the dominant MHC isoform in the genioglossus, intercostal, and three abdominal muscles, all of which exhibited minimal expression of MHC beta/slow. The amount of MHC2X (relative to total MHC composition) was similar in the diaphragm, genioglossus, and transversus abdominis muscles, while considerably less was detected in the rectus abdominis and external oblique muscles. These results indicate that MHC2X is broadly and variably distributed among respiratory muscles. Furthermore, these data suggest that a large portion of 2X fibers (containing MHC2X), which cannot be detected by standard histochemical analysis, may be present in the genioglossus and transversus abdominis muscles as has been demonstrated for the diaphragm muscle. We speculate that an association exists between the level of MHC2X expression and frequency of respiratory recruitment.

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

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

Complete sequence of human cardiac alpha-myosin heavy chain gene and amino acid comparison to other myosins based on structural and functional differences

We have obtained the 5820 nucleotide sequence encoding all 1939 amino acids of the human cardiac alpha-myosin heavy chain (alpha-MHC), as established by dideoxy sequencing of cloned cDNA, genomic DNA and polymerase chain reaction (PCR) amplification products. This sequence represents overlapping fragments of the entire coding sequence. Amino acid sequence comparison of the human cardiac alpha-MHC with the published human cardiac beta-MHC have demonstrated that there are, at least, 7 isoform-specific divergent regions, including functionally important binding protein-related sites such as ATP, actin and myosin light chain. It has been reported that in the rat, there are 8 isoform-specific divergent regions. The 7th divergent area (residue area 1633-1657, which is thought to mediate thick filament formation) in the light meromyosin region in the rat is not apparent in the human. The amino acid compositions of cardiac alpha- and beta-MHCs in the human and the rat, and human embryonic skeletal muscle and chicken gizzard smooth muscles were compared. Amino acid sequences in cardiac alpha- and beta-MHCs in the human and the rat are well conserved. In the head portion, the amino acid composition divergence of human cardiac alpha-MHC is ranked between rat cardiac alpha-MHC and human cardiac beta- or rat cardiac beta-MHC; human skeletal muscle MHC is the most divergent of the myosin isoform examined. These data predict that human cardiac alpha-MHC may have undergone evolutionary changes toward obtaining the biochemical and physiological properties of cardiac beta-MHC.

Nucleotide-induced changes in the interaction of myosin subfragment 1 with actin: detection by antibodies against the N-terminal segment of actin

The binding of myosin subfragment 1 (S-1) to actin in the presence and absence of nucleotides was determined under conditions of partial saturation of actin, up to 80%, by Fab(1-7), the antibodies against the first seven N-terminal residues on actin. In the absence of nucleotides, the binding constant of S-1 to actin (2 x 10(7) M-1) was decreased by 1 order of magnitude by Fab(1-7). The binding of S-1 to actin caused only limited displacement of Fab, and between 30 and 50% of actin appeared to bind both proteins. In the presence of MgAMP.PNP, MgADP, and MgPPi and at low S-1 concentrations, the same antibodies caused a large decrease in the binding of S-1 to actin. However, the binding of S-1.nucleotide to actin in the presence of Fab(1-7) increased cooperatively with the increase in S-1 concentration. Also, in contrast to rigor conditions, there was no indication for the binding of Fab(1-7) and S-1.nucleotide to the same actin molecules. These results show a nucleotide-induced transition in the actomyosin interface, most likely related to the different roles of the N-terminal segment of actin in the binding of S-1 and S-1.nucleotide. The possible implications of these findings to the regulation of actomyosin interactions are discussed.

Effects of ions on vanadate-induced photocleavage of myosin subfragment 1

Myosin subfragment 1 (S1) is cleaved by near-ultraviolet irradiation in the presence of vanadate at three sites located at 23, 31 and 74 kDa from the N-terminus. Since vanadate is considered to be a good structural analogue of phosphate, it is assumed that the cleavage sites participate in forming the phosphate-binding site(s) of S1. In this work, the effect of various ions on the vanadate-induced photocleavage of S1 was studied. Monovalent anions were found to inhibit photocleavage in the 50-200 mM range. The inhibition is more expressed at a site 74 kDa from the N-terminus than at the 23-kDa and 31-kDa sites. The inhibitory effect of the monovalent anions increases in the order acetate = F- less than Cl- less than Br- less than I- = SCN-. The order of the inhibitory effect is identical to the protein-structure-damaging effect of monovalent anions in the von Hippel series [von Hipel, P. H. & Wong, K. Y. (1964) Science 145, 577-581]. Therefore, it is assumed that decreased photocleavage is due to local perturbations of structure, especially at the 74-kDa site, in addition to increased ionic strength. Divalent anions, sulfate and thiosulfate, strongly inhibit photocleavage at 2 mM. The inhibition is very pronounced at the 23-kDa and 31-kDa sites, while the 74-kDa site is hardly affected. Since photocleavage at the 23-kDa and 31-kDa sites is regulated jointly and independently from cleavage at the 74-kDa site, it is assumed that S1 has two distinct phosphate-binding sites: the regions of the 23-kDa and 31-kDa cleavage sites, which are proximal to each other in the spatial structure, participate in forming the first phosphate-binding site, while the 74-kDa site is part of the second binding site. Sulfate was also found to inhibit the trapping of vanadate and to facilitate its release from the S1-MgADP-Vi (Vi, inorganic vanadate) complex. Photocleavage of S1 takes place at all three sites, both in the presence or absence of divalent cations, indicating that these, including Mg2+, are not essential for cleavage.

Molecular charge dominates the inhibition of actomyosin in skinned muscle fibers by SH1 peptides

It is not definitively known whether the highly conserved region of myosin heavy chain around SH1 (Cys 707) is part of the actin-binding site. We tested this possibility by assaying for competitive inhibition of maximum Ca-activated force production of skinned muscle fibers by synthetic peptides which had sequences derived from the SH1 region of myosin. Force was inhibited by a heptapeptide (IRICRKG) with an apparent K0.5 of about 4 mM. Unloaded shortening velocity of fibers, determined by the slack test, and maximum Ca-activated myofibrillar MgATPase activity were also inhibited by this peptide, but both required higher concentrations. We found that other cationic peptides also inhibited force in a manner that depended on the charge of the peptide; increasing the net positive charge of the peptide increased its efficacy. The inhibition was not significantly affected by altering solution ionic strength (100-200 mM). Disulfide bond formation was not involved in the inhibitory mechanism because a peptide with Thr substituted for Cys was inhibitory in the presence or absence of DTT. Our data demonstrate that the net charge was the predominant molecular characteristic correlated with the ability of peptides from this region of myosin heavy chain to inhibit force production. Thus, the hypothesis that the SH1 region of myosin is an essential part of the force-producing interaction with actin during the cross-bridge cycle (Eto, M., R. Suzuki, F. Morita, H. Kuwayama, N. Nishi, and S. Tokura., 1990, J. Biochem. 108:499-504; Keane et al., 1990, Nature (Lond.). 344:265-268) is not supported.

Electrostatic contributions to the binding of myosin and myosin-MgADP to F-actin in solution

The ionic strength dependence of skeletal myosin subfragment 1 (S1) binding to unregulated F-actin was measured in solutions containing from 0 to 0.50 M added lithium acetate (LiOAc) in the absence and presence of MgADP. The data were analyzed by using a theory based on an ion interaction model that is rigorous for high ionic strength solutions [Pitzer, K. S. (1973) J. Phys. Chem. 77, 268-277] in order to obtain values for K, the equilibrium association constant when the ionic strength is zero, and for [zMzA[, the absolute value of the product of the net electric charges of the actin binding site on myosin (zM) and the myosin binding site on actin (zA). The presence of MgADP reduced K by a factor of 10, as expected, and reduced [zMzA[ by about 1 esu2. Because the presence of MgADP is not likely to change the net charge of the myosin binding site on actin, these data are consistent with a model in which MgADP binding to S1 reduces its affinity for actin by a mechanism that reduces the net electric charge of the acting binding site on S1. The value of [zMzA[ in the absence of ADP was 8.1 +/- 0.9 esu2, which, if one uses integer values, suggests that zM and zA are in the 8+ to 1+ esu and 1- to 8- esu ranges, respectively. ADP binding then reduces zM to the 7+ to 0.88+ esu range.

Expression of human beta-myosin heavy chain fragments in Escherichia coli; localization of actin interfaces on cardiac myosin

A cDNA clone coding for an internal fragment of slow-cardiac beta-myosin heavy chain was isolated from a lambda gt10 human skeletal muscle library. Six overlapping cDNA subclones, which span myosin heavy chain subregions and presumably interact with actin, were derived from this clone, fused to a beta-galactosidase vector and expressed in Escherichia coli. Three of the subclones were obtained by PCR (polymerase chain reaction) which enables gene or cDNA fragments to be amplified independently of preexisting restriction sites. Initially, various experiments were carried out using a long MHC (myosin heavy chain) fusion protein containing the 50 kDa-20 kDa connecting region, the whole 20 kDa region and the short subfragment 2 region. This MHC fusion protein was chemically or proteolytically cleaved in the same conditions as the native myosin molecule. Whole and truncated forms of the MHC fusion protein were separated on polyacrylamide gels, electroblotted on nitrocellulose sheets and renatured. They were then assayed in overlay experiments with F-actin and/or myosin light chains in solution. Specific antibodies were used to detect interactions between heavy chain fragments and F-actin or light chains. We thus observed that one long heavy chain fragment synthesized by E. coli behaved like proteolytic or chemical MHC preparations made from native myosin molecules. Two chymotryptic fragments of the MHC fusion protein, which are soluble at low ionic strength, cosedimented with F-actin in solution. Our results demonstrate that, in actin overlay experiments with whole fusion proteins, interactions seem to be due to the heavy chain fragment, not to the bacterial component. All interactions were non ATP-sensitive. We further investigated the possible participation of the six recombinant MHC fragments in contributing to the actomyosin interfaces on the 50 kDa-20 kDa regions of the human cardiac beta-MHC. The present procedure, which enables the synthesis of any MHC fragment independent of any protease site, is a powerful new tool for studying structure-function relationships within the myosin molecule family.

51V NMR study of vanadate binding to myosin and its subfragment 1

The binding of various forms of vanadate to myosin and myosin subfragment 1 (S-1) was studied by 51V NMR at increasing vanadate concentrations between 0.06 and 1.0 mM. The distribution of the various forms of vanadate in the solution depended on the total concentration of vanadate. At low concentrations, the predominant vanadate form was monomeric, while at high concentration, it was tetrameric. The presence of myosin or S-1 in the solution produced a significant broadening of the signal of each form of vanadate, indicating that all of them bind to the protein. Addition of ATP, which does not affect the 51V NMR spectra in the absence of proteins, causes their significant alteration in the presence of myosin or S-1. The changes, which include the broadening of the signal of the monomeric and the narrowing of the signal of the oligomeric vanadate forms, indicate that more monomeric and less oligomeric vanadate binds to the proteins in the presence than in the absence of ATP. Irradiation by near-UV light in the presence of vanadate cleaves S-1 at three specific sites--at 23, 31, and 74 kDa from the N-terminus. The cleavages at 23 and 31 kDa are specifically inhibited by the addition of ATP. The vanadate-associated photocleavage of S-1 also depends on the total concentration of vanadate; it is observed only when the concentration of vanadate is at least 0.2 mM. This was also the lowest concentration at which oligomeric vanadate was detected in the 51V NMR spectra. From the parallel concentration dependence of the photocleavage and the appearance of the tetrameric vanadate, it is concluded that photocleavage occurs only when tetrameric vanadate binds to S-1.

Conformational transitions within the head and at the head-rod junction in smooth muscle myosin studied with a limited proteolysis method

It was previously shown that tryptic digestion of subfragment 1 (S1) of skeletal muscle myosins at 0 degree C results in cleavage of the heavy chain at a specific site located 5 kDa from the NH2-terminus. This cleavage is enhanced by nucleotides and suppressed by actin and does not occur at 25 degrees C, except in the presence of nucleotide. Here we show a similar temperature sensitivity and protection by actin of an analogous chymotryptic cleavage site in the heavy chain of gizzard S1. The results support the view that the myosin head, in general, can exist in two different conformational states even in the absence of nucleotides and actin, and indicate that the heavy chain region 5 kDa from the NH2-terminus is involved in the communication between the sites of nucleotide and actin binding. We also show here for the first time that the S1-S2 junction in gizzard myosin can be cleaved by chymotrypsin and that this cleavage (observed in papain-produced S1 devoid of the regulatory light chain) is also temperature-dependent but insensitive to nucleotides and actin. It is suggested that the temperature-dependent alteration in the flexibility of the head-rod junction, which is apparent from these and similar observations on skeletal muscle myosin [Miller, L. & Reisler, E. (1985) J. Mol. Biol. 182, 271-279; Redowicz, M.J. & Strzelecka-Gołaszewska, H. (1988) Eur. J. Biochem. 177, 615-624], may contribute to the temperature dependence of some steps in the cross-bridge cycle.

Interactions between G-actin and myosin subfragment 1: immunochemical probing of the NH2-terminal segment on actin

The role of the N-terminal segment of actin in myosin-induced polymerization of G-actin was studied by using peptide antibodies directed against the first seven N-terminal residues of alpha-skeletal actin. Light scattering, fluorescence, and analytical ultracentrifugation experiments showed that the Fab fragments of these antibodies inhibited the polymerization of G-actin by myosin subfragment 1 (S-1) by inhibiting the binding of these proteins to each other. Fluorescence measurements using actin labeled with pyrenyliodoacetamide revealed that Fab inhibited the initial step in the binding of S-1 to G-actin. It is deduced from these results and from other literature data that the initial contact between G-actin and S-1 involves residues 1-7 on actin and residues 633-642 on the S-1 heavy chain. This interaction appears to be of major importance for the binding of S-1 and G-actin. The presence of additional myosin contact sites on G-actin was indicated by concentration-dependent recovery of S-1 binding to G-actin without displacement of Fab. The reduced Fab inhibition of S-1 binding to polymerizing and polymerized actin is consistent with the tightening of acto-S-1 binding at these sites or the creation of new sites upon formation of F-actin.

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

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

Isolation and characterization of the G-actin-myosin head complex

The two main proteins involved in muscular contraction and cell motility, myosin and actin, possess the intrinsic property of being able to form filamentous structures. This property poses a serious impediment to the study of their structures and interactions, and a considerable effort has thus been made to isolate their functional domains. The globular part of myosin, subfragment-1 (S1), which possesses ATPase and actin-binding sites as well as supporting the movement of actin filaments during in vitro assays, has been isolated. But because S1 is efficient in inducing actin polymerization, as is myosin, it has not been possible to prepare and characterize a complex of S1 with monomeric actin (G-actin). We have now used chromatographically purified proteins to show that only the S1 isoenzyme carrying the A1 light-chain subunit promotes actin polymerization. The other isoenzyme, S1 (A2), carrying the A2 light-chain subunit, binds to actin, forming a tight complex of G-actin and S1 in a 1:1 ratio. This new functional difference between myosin isoforms directly implicates the A1 light-chain in myosin-induced actin polymerization. Additionally, this finding should lead to the purification of the stable G-actin-S1 complex needed to resolve the structure and to understand the molecular dynamics of the actin-myosin system.

On the origin and transmission of force in actomyosin subfragment 1

A proximity map showing the three-dimensional arrangement of 12 chemically defined points in actomyosin subfragment 1 is developed and roughly correlated with published electron microscope reconstruction of others. Several additional points and topological relationships in the primary polypeptide chain folding are assimilated into this model. Certain crosslinkings and distance change observations are interpreted as indicators of transmission of force/displacement between the nucleotide-binding and an actin-binding site--i.e., as indications of how energy is transduced in this system.