Functional sequences of the myosin head

Stepwise modulation of ATPase activity, nucleotide trapping, and sliding motility of myosin S1 by modification of the thiol region with residues of increasing size

Rabbit muscle myosin S1 was modified either at SH1 alone or at both SH1 and SH2, using a series of alkylthiolating reagents of increasing size, designed for correlating gradually changing structural disturbances in the thiol region with functional impairments in the myosin head. The reagents were of the type H(CH(2))(n)()-S-NTB, (NTB = 2-nitro-5-thiobenzoate) (n = 1, 2, 5, 8, 9, 10, 11, and 12). Modification of only SH1 led to the expected activation of the Ca(2+)-ATPase, but only with small reagents, while reagents with n > or = 10 caused inhibition of the Ca(2+)-ATPase. Modification of both SH1 and SH2 showed the expected inhibition of Ca(2+)-ATPase but likewise allowed considerable residual Ca(2+)-ATPase activity if the residues were small. Trapping of the nucleotide, known to occur with cross-linking reagents, was seen also with monovalent reagents, provided their length exceeded n = 9 or 10. All S1 derivatives prepared in this study possessed an affinity for actin comparable to native S1 but lacked sliding motility in in vitro motility assays. The biochemical data of this study can be related to existing models of myosin S1 and recent structural data [Houdusse, A., Kalabokis, V. N., Himmel, D., Szent-Györgyi, A. G., and Cohen, C. (1999) Cell 97, 459-470] by making the assumptions that modification at SH1 prevents the formation of the SH1 helix mandatory for the transmission of conformational energy and that mobility of the thiol region is a prerequisite for ATPase activity. Immobilization of the thiol region by residues of increasing size apparently leads to lower enzyme activity and, finally, to inhibition of nucleotide exchange.

Myosin II folding is mediated by a molecular chaperonin

The folding pathway of the heavy meromyosin subfragment (HMM) of a skeletal muscle myosin has been investigated by in vitro synthesis of the myosin heavy and light chains in a coupled transcription and translation assay. Analysis of the nascent translation products for folding intermediates has identified a major intermediate that contains all three myosin subunits in a complex with the eukaryotic cytosolic chaperonin. Partially folded HMM is released from this complex in an ATP-dependent manner. However, biochemical and functional assays reveal incomplete folding of the myosin motor domain. Dimerization of myosin heavy chains and association of heavy and light chains are accomplished early in the folding pathway. To test for other factors necessary for the complete folding of myosin, a cytoplasmic extract was prepared from myotubes produced by a mouse myogenic cell line. This extract dramatically enhanced the folding of HMM, suggesting a role for muscle-specific factors in the folding pathway. We conclude that the molecular assembly of myosin is mediated by the eukaryotic cytosolic chaperonin with folding of the motor domain as the slow step in the pathway.

Trinitrophenylated reactive lysine residue in myosin detects lever arm movement during the consecutive steps of ATP hydrolysis

Trinitrophenylation of the reactive lysine (Lys84) in skeletal myosin subfragment 1 (S1) introduces a chiral probe (TNP) into an interface of the catalytic and lever arm domains of S1 [Muhlrad (1977) Biochim. Biophys. Acta 493, 154-166]. Characteristics of the TNP absorption and circular dichroism (CD) spectra in TNP-modified S1 (TNP-Lys84-S1), and the Lys84 trinitrophenylation rate in native S1, indicate a one-to-one correspondence between ATPase transients and trapped phosphate analogues. Phosphate analogue-induced structures of TNP-Lys84-S1 were modeled using the crystallographic coordinates of S1 [Rayment et al. (1993) Science 261, 50-58] with swivels at Gly699 and Gly710 to approximate conformational changes during ATPase. The CD and absorption spectral characteristics of the model structures were compared to those observed for analogue-induced structures. The model calculations, first tested on a trinitrophenylated hexapeptide with known structure, were applied to TNP-Lys84-S1. They showed that ATP binding initiates swiveling at Gly699 and that swiveling at both Gly710 and Gly699 accompanied ATP splitting just prior to product release. The computed lever arm trajectory during ATPase suggests (i) a plausible mechanism for the nucleotide-induced inhibition of Lys84 trinitrophenylation, and (ii) trinitrophenylation-induced changes in S1 Mg2+- and K+-EDTA ATPase are from collision of the lever arm with TNP at Lys84. TNP is a site-specific structural perturbant of S1 and a chiral reporter group for the effect of Lys84 modification on dynamic S1 structure. As such, TNP-Lys84-S1 is equivalent to a genetically engineered mutant with intrinsic sensitivity to structure local to the modified residue.

Cryostabilization mechanism of fish muscle proteins by maltodextrins

Maltodextrins of varying mean molecular weights (MW) were evaluated for cryoprotective ability in Alaska pollock surimi (leached mince) versus sucrose or a sucrose-sorbitol mixture. Treatments were stored either isothermally at -8, -14, or -20 degrees C for 3 months or freeze-thaw (F/T) cycled six times to induce freeze-related protein denaturation, measured as a decrease in myosin Ca+2 ATPase activity and change in heat-induced gel-forming ability. Results indicated good cryoprotection by all maltodextrins at -20 degrees C isothermal storage irrespective of MW, but poor cryoprotection by higher MW maltodextrins at higher isothermal storage temperatures or after F/T cycling. These observations, and surface tension measurements of maltodextrin solutions, indicated that lower MW maltodextrins likely cryoprotect by a preferential solute exclusion mechanism, similar to sucrose and sorbitol. Higher MW maltodextrins presumably cryoprotect at lower storage temperatures via a reduced water mobility mechanism. As the MW of maltodextrins increased the gelling ability of the surimi was increasingly impaired, such that evidence of cryoprotection from gelation data was obscured. Copyright 1999 Academic Press.

Point mutations in human beta cardiac myosin heavy chain have differential effects on sarcomeric structure and assembly: an ATP binding site change disrupts both thick and thin filaments, whereas hypertrophic cardiomyopathy mutations display normal assembly

Hypertrophic cardiomyopathy is a human heart disease characterized by increased ventricular mass, focal areas of fibrosis, myocyte, and myofibrillar disorganization. This genetically dominant disease can be caused by mutations in any one of several contractile proteins, including beta cardiac myosin heavy chain (beta MHC). To determine whether point mutations in human beta MHC have direct effects on interfering with filament assembly and sarcomeric structure, full-length wild-type and mutant human beta MHC cDNAs were cloned and expressed in primary cultures of neonatal rat ventricular cardiomyocytes (NRC) under conditions that promote myofibrillogenesis. A lysine to arginine change at amino acid 184 in the consensus ATP binding sequence of human beta MHC resulted in abnormal subcellular localization and disrupted both thick and thin filament structure in transfected NRC. Diffuse beta MHC K184R protein appeared to colocalize with actin throughout the myocyte, suggesting a tight interaction of these two proteins. Human beta MHC with S472V mutation assembled normally into thick filaments and did not affect sarcomeric structure. Two mutant myosins previously described as causing human hypertrophic cardiomyopathy, R249Q and R403Q, were competent to assemble into thick filaments producing myofibrils with well defined I bands, A bands, and H zones. Coexpression and detection of wild-type beta MHC and either R249Q or R403Q proteins in the same myocyte showed these proteins are equally able to assemble into the sarcomere and provided no discernible differences in subcellular localization. Thus, human beta MHC R249Q and R403Q mutant proteins were readily incorporated into NRC sarcomeres and did not disrupt myofilament formation. This study indicates that the phenotype of myofibrillar disarray seen in HCM patients which harbor either of these two mutations may not be directly due to the failure of the mutant myosin heavy chain protein to assemble and form normal sarcomeres, but may rather be a secondary effect possibly resulting from the chronic stress of decreased beta MHC function.

Telokin (kinase-related protein) modulates the oligomeric state of smooth-muscle myosin light-chain kinase and its interaction with myosin filaments

Telokin, an abundant gizzard protein, inhibited phosphorylation of regulatory light chain when filamentous myosin was used as the substrate but no inhibition was observed with myosin subfragment 1. At physiological telokin-to-myosin molar ratio (1:1), the inhibition amounted to a 3.5-fold reduction in the initial phosphorylation rate whereas at high molar excess of telokin over myosin, we observed an up to 20-fold decrease in this rate. In agreement with previous observations [Shirinsky, Vorotnikow, Birukov, Nanaev, Collinge, Lukas, Sellers and Watterson (1993) J. Biol. Chem. 268, 16578-16583], telokin did not inhibit phosphorylation of the isolated regulatory light chain of myosin and only moderately (35%) inhibited that of heavy meromyosin. To gain a better understanding of the mechanism of this inhibition, we investigated the effects of telokin on the recently described [Babiychuk, Babiychuk and Sobieszek (1995) Biochemistry 34, 6366-6372] oligomeric properties of smooth-muscle myosin light-chain kinase (MLCK). We showed, on the one hand, that telokin rapidly solubilized the large kinase oligomers formed at low ionic strength. With soluble kinase, on the other hand, telokin acted to increase the relative concentration of MLCK dimers and to decrease that of the hexamers and octamers. This, in turn, resulted in a reduction in the amount of MLCK bound to myosin because filamentous myosin appeared to exhibit a higher affinity for the hexamers than for the dimers. Telokin by itself was also shown to dimerize and oligomerize in solution and this oligomerization was greatly enhanced in the presence of MLCK. We suggest that telokin affects myosin phosphorylation by modulation of the oligomeric state of MLCK and its interaction with myosin filaments.

Structure-function analysis of the motor domain of myosin

Motor proteins perform a wide variety of functions in all eukaryotic cells. Recent advances in the structural and mutagenic analysis of the myosin motor has led to insights into how these motors transduce chemical energy into mechanical work. This review focuses on the analysis of the effects of myosin mutations from a variety of organisms on the in vivo and in vitro properties of this ubiquitous motor and illustrates the positions of these mutations on the high-resolution three-dimensional structure of the myosin motor domain.

Enzymatic characterization and functional domain mapping of brain myosin-V

The actin binding and ATPase properties, as well as the functional domain structure of chick brain myosin-V, a two-headed, unconventional myosin, is reported here. Compared to conventional myosin from skeletal muscle, brain myosin-V exhibits low K-EDTA- and Ca-ATPase activities (1.8 and 0.8 ATP/s per head). The physiologically relevant Mg-ATPase is also low (approximately 0.3 ATP/s), unless activated by the presence of both F-actin and Ca2+ (Vmax of 27 ATP/s). Ca2+ stimulates the actin-activated Mg-ATPase over a narrow concentration range between 1 and 3 microM. In the presence of saturating Ca2+ and 75 mM KCl, surprisingly low concentrations of F-actin activate the Mg-ATPase in a hyperbolic manner (KATPase of 1.3 microM). Brain myosin-V also binds with relatively high affinity (compared to other known myosins) to F-actin in the presence of ATP, as assayed by cosedimentation. Digestion of brain myosin-V with calpain yielded a 65-kDa head domain fragment that cosediments with actin in an ATP-sensitive manner and a 80-kDa tail fragment that does not interact with F-actin. The 80-kDa fragment results from cleavage one residue beyond the proline-, glutamate-, serine-, threonine-rich region. Our data indicate that the Mg-ATPase cycle of brain myosin-V is tightly regulated by Ca2+, probably via direct binding to the calmodulin light chains in the neck domain, which like brush border myosin-I, results in partial (approximately 30%) dissociation of the calmodulin associated with brain myosin-V. The effect of Ca2+ binding, which appears to relieve suppression by the neck domain, can be mimicked by calpain cleavage near the head/neck junction.

A minimal motor domain from chicken skeletal muscle myosin

The myosin head (S1) consists of a wide, globular region that contains the actin- and nucleotide-binding sites and an alpha-helical, extended region that is stabilized by the presence of two classes of light chains. The essential light chain abuts the globular domain, whereas the regulatory light chain lies near the head-rod junction of myosin. Removal of the essential light chain by a mild denaturant exposes the underlying heavy chain to proteolysis by chymotrypsin. The cleaved fragment, or "motor domain" (MD), migrates as a single band on SDS-polyacrylamide gel electrophoresis, with a slightly greater mobility than S1 prepared by papain or chymotrypsin. Three-dimensional image analysis of actin filaments decorated with MD reveals a structure similar to S1, but shorter by an amount consistent with the absence of a light chain-binding domain. The actin-activated MgATPase activity of MD is similar to that of S1 in Vmax and Km. But the ability of MD to move actin filaments in a motility assay is considerably reduced relative to S1. We conclude that the globular, active site region of the myosin head is a stable, independently folded domain with intrinsic motor activity, but the coupling efficiency between ATP hydrolysis and movement declines markedly as the light chain binding region is truncated.

Sudden cardiac death in patients with hypertrophic cardiomyopathy: from bench to bedside with an emphasis on genetic markers

Hypertrophic cardiomyopathy (HCM) is the most common cause of death in the young, particularly in young competitive athletes. Death often occurs suddenly in asymptomatic, apparently healthy individuals. Several clinical parameters as well as genetic factors have been characterized that can identify those HCM patients who are at high risk for sudden cardiac death (SCD). The clinical parameters that have some predictive values for SCD in HCM patients are the following: a prior history of SCD, a family history of SCD, history of syncope, symptomatic ventricular tachycardia on Holter monitoring, inducible ventricular tachycardia during electrophysiologic studies, and myocardial ischemia in children with HCM. Recent identification of mutations in the beta myosin heavy chain gene and genotype-phenotype correlation in HCM patients have shown that the beta myosin heavy chain mutations are also prognosticators in HCM families. Several mutations such as Arg403Gln and Arg719Gln are associated with a high incidence of SCD, while Leu908Val mutation is associated with a benign course and a low incidence of SCD in HCM families. Additional genetic factors such as a polymorphism in angiotensin-converting enzyme I gene may also contribute to a high incidence of SCD in HCM families. Identification and characterization of HCM patients at high risk for SCD provide the opportunity to render prophylactic therapeutic interventions, such as implantation of defibrillators, in these individuals.

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].

Ca(2+)-calmodulin-dependent modification of smooth-muscle myosin light-chain kinase leading to its co-operative activation by calmodulin

It has recently been shown that at relatively high molar ratios of myosin light-chain kinase (MLCKase) to calmodulin (CM) almost complete inhibition of the kinase activity occurs [Sobieszek (1991) J. Mol. Biol. 220, 947-957]. This inhibition resulted in a highly co-operative activation of MLCKase by CM, whereas the opposite activation (of CM by kinase) was hyperbolic, as expected (unco-operative). This difference in activation was observed only for kinase preparations preincubated with sub-stoichiometric amounts of CM, and only when micromolar concentrations of Ca2+ were present. The inhibitory effect was variable and depended not only on the concentration ratio of kinase to CM but also on the MLCKase preparation. For most of the preparations full inhibition required 5-15 min of preincubation at 25 degrees C and a 3-6-fold molar excess of kinase over CM. The inhibition was reversible, since full activity could be obtained after saturation of the kinase by additional CM. The inhibitory effect did not require ATP (excluding phosphorylation-type modifications of the kinase), and dephosphorylation of the kinase was not involved, since inhibition of an endogenous MLCK phosphatase by microcystin-LR did not decrease the inhibitory effect. Since the co-operative activation by CM was observed for cross-linked MLCKase preparations enriched in kinase dimers, but was absent for the analogous preparations enriched in the oligomers, we concluded that Ca(2+)-CM-dependent changes in the oligomeric state of the kinase were responsible for the modification observed. The exact nature of these modifications remains to be established.

Transient kinetics of the interaction of actin with myosin subfragment-1 in the absence of nucleotide

The kinetics of the association of actin with myosin subfragment-1 (S1) has been studied by using S1 labeled at the sulfhydryl group SH1 with 5-(iodoacetamido)fluorescein (S1-AF). Upon rapid mixing in a stopped-flow apparatus, the fluorescence intensity of the fluorescein moiety increased by 50%, followed by a slower increase that was well resolved. This slow phase of the fluorescence change could not be fitted to either a monoexponential or a biexponential function, but it could be fitted to a sum of three exponential terms yielding three observed first-order rate constants (lambda i). The dissociation of acto.-(S1-AF) was studied by displacement of S1-AF from the complex with native S1. The dissociation kinetics was characterized by a single rate constant (approximately 0.012 s-1 at 20 degrees C), and this constant was independent of S1 concentration. Together with previous equilibrium data that were obtained under identified conditions for formation of acto-subfragment-1 (Lin, S.-H., and H. C. Cheung. 1991. Biochemistry. 30:4317-4323), a six-state two-pathway model is proposed as a minimum kinetic scheme for formation of rigor acto.S1. In this model, unbound subfragment-1 exists in two conformational states (S1' and S1) which are in equilibrium with each other, one corresponding to the previously established low-temperature state and the other to the high-temperature state. Each subfragment-1 state can interact with actin to form a collision complex, followed by two isomerizations to form two acto-subfragment-1 states (A.S1' and A.S1). Both isomerizations were visible in stopped-flow experiments. Two special cases of the model were considered: 1) a rapid pre-equilibration of the initial collision complex with actin and S1, and 2) trace accumulation of the collision complex. The first case required that the three combinations of the three observed rate constants be independent of actin concentration. The data were incompatible with this approximation. The other special case required that the sum of the lambda i vary linearly with actin concentration and the other two combinations of lambda i vary with actin concentration in a quadratic fashion. The present data were in agreement with the second case. At 20 degrees C and in 60 mM KCl, 2 mM MgCl2, 30 mM 2-([-hydroxy-1,1-bis(hydroxymethyl)ethyl]amino)ethanesulfonic acid, and pH 7.5, the biomolecular association rate constants for the interaction of actin with S1' and S1 were 8.58 x 10(5) and 1.11 x 10(6) M-1 s-1, respectively.

Cell signalling and motility in Entamoeba histolytica

Entamoeba histolytica crawls as a polarized cell following external stimuli, with the translocation of signals modifying extracellular matrix interactions and the amoeba cytoskeleton. Nancy Guillén here describes how the gliding of E. histolytica cells requires the activity of the actomyosin complex, and how actomyosin functions related to motility are necessary for pathogenesis and for amoebal escape from the host immune response.

Localization of two myosin-subfragment-1 binding contacts in the 96-132 region of actin subdomain-1

Many direct observations and indirect experimental approaches have pin-pointed two segments (sequences 1-28 and 360-372) in actin subdomain-1 which bind to myosin subfragment-1. In a previous investigation [Labbé, J. P., Méjean, C., Benyamin, Y. & Roustan, C. (1990) Biochem. J. 271, 407-413], we have observed competition between myosin subfragment-1 and anti-actin antibodies specific to epitopes including Thr103. A multisite interface model has also been proposed to take into account myosin-head binding to the N-terminal and C-terminal regions and to more central 40-113 sequence of actin. In the present study, two limited actin segments encompassing residues 96-103 and 112-125 were identified as myosin-head-binding sites. Myosin subfragment-1 competed for monomeric actin with the antibodies directed against sequences 96-105 and 114-120 and its binding to the tryptic 96-113 and synthetic 112-125 actin peptides was prevented by magnesium pyrophosphate but not by calcium pyrophosphate. In the presence of ATP-Mg2+, myosin subfragment-1 was dissociated by filamin from its complex with monomeric actin or with peptide 105-120. Contact points of filamin on actin were previously located in the 105-120 and 360-372 actin sequences [Méjean, C., Lebart, M. C., Boyer, M., Roustan, C. & Benyamin, Y. (1992) Eur. J. Biochem. 209, 555-562]. The in vitro inhibitory effect of filamin on actin-activated Mg2+-ATPase would thus be explained by this competition. Furthermore, the (27-kDa-50-kDa-20-kDa) trypsin-split myosin subfragment-1 which could no longer be activated by actin, did not bind at all to the two sites located in the 96-125 region, but it still interacted with the 360-372 segment. Our results regarding the position of the myosin head on actin monomers in rigor conditions provide evidence on the presence of two topologically independent contact points in the myosin-head/actin interface. One group exposed residues in the 1-7, 21-29, 77-95 and 96-103 actin segment, another, on the opposite side of subdomain-1, included residues from 112-125 and 360-372 sequences.

Identification and characterization of a myosin heavy chain gene (mhcA) from the human parasitic pathogen Entamoeba histolytica

The mhcA gene from the human pathogen Entamoeba histolytica was identified using the polymerase chain reaction. It is a single copy gene expressed as a 6.4-kb mRNA. The deduced MhcA protein sequence is highly similar to myosin II from both Dictyostelium discoideum and Acanthamoeba castellanii. The globular head domain of MhcA contains the specific regions involved in ATP binding, actin binding, and interaction with myosin light chain. The tail domain is organized in an alpha-helical coiled coil structure, which suggests that MhcA is an alpha-fibrous protein. The coiled coil is interrupted by two prolines indicating that like other myosins, either from smooth muscle or from non-muscle cells, the tail of MhcA folds twice on itself. In addition, MhcA presents sequence similarities with the heavy chain phosphorylation sites of smooth and non-muscle vertebrate myosins.

Identification, characterization and cloning of myr 1, a mammalian myosin-I

We have identified, characterized and cloned a novel mammalian myosin-I motor-molecule, called myr 1 (myosin-I from rat). Myr 1 exists in three alternative splice forms: myr 1a, myr 1b, and myr 1c. These splice forms differ in their numbers of putative calmodulin/light chain binding sites. Myr 1a-c were selectively released by ATP, bound in a nucleotide-dependent manner to F-actin and exhibited amino acid sequences characteristic of myosin-I motor domains. In addition to the motor domain, they contained a regulatory domain with up to six putative calmodulin/light chain binding sites and a tail domain. The tail domain exhibited 47% amino acid sequence identity to the brush border myosin-I tail domain, demonstrating that myr 1 is related to the only other mammalian myosin-I motor molecule that has been characterized so far. In contrast to brush border myosin-I which is expressed in mature enterocytes, myr 1 splice forms were differentially expressed in all tested tissues. Therefore, myr 1 is the first mammalian myosin-I motor molecule with a widespread tissue distribution in neonatal and adult tissues. The myr 1a splice form was preferentially expressed in neuronal tissues. Its expression was developmentally regulated during rat forebrain ontogeny and subcellular fractionation revealed an enrichment in purified growth cone particles, data consistent with a role for myr 1a in neuronal development.

Localization of myosin heavy chain A in the human pathogen Entamoeba histolytica

To recognize myosin II in trophozoites of the human pathogen Entamoeba histolytica, a specific antimyosin polyclonal serum was raised against a fusion protein consisting of a 146-amino-acid fragment of the myosin II heavy chain A of E. histolytica (MhcA) fused with beta-galactosidase. The hybrid protein was encoded by a chimera gene formed by a DNA fragment, from the mhcA gene, amplified by polymerase chain reaction and fused with the lacZ gene of Escherichia coli. Polymerase chain reaction-amplified DNA is located within the region encoding the tail domain of myosin. This antibody recognized a 250-kDa protein in extracts of E. histolytica trophozoites. Confocal microscope analysis of antibody-labelled trophozoites indicated that MhcA localizes at the posterior pole of locomoting cells and concentrates within the uroid. These results might indicate that MhcA is involved in movement and in the uroid formation which help amoebas to escape the host immune response. These data are the first evidence indicating that myosin exists in E. histolytica. In addition, two other peptides were found in myosin-enriched extracts of amoebas, indicating that other myosins may be present in this parasite.

Structural and developmental analysis of two linked myosin heavy chain genes

We have identified at the molecular level two murine myosin heavy chain (MHC) genes which encode adult skeletal isoforms. As is the case for the murine cardiac MHC genes, the genes are closely linked, head to tail. To identify the isoform encoded by each gene, we located and sequenced the 3' termini and assessed the genes' temporal and tissue-specific expression patterns using probes specific for the 3' untranslated regions. These analyses indicate that the myosin encoded by the gene located 5' in the linked pair is the murine 2A isoform. The isoform encoded by the 3'-most gene of the linked pair appears to encode an additional isoform that is expressed in a number of adult skeletal muscles. The pattern of expression of the 3'-most gene indicates that it is tightly controlled developmentally. Transcripts from this gene, when compared to those from the MHC2A and beta-MHC genes, are the predominant MHC transcripts found in the diaphragm, tongue, soleus, and masseter, indicating that it encodes a major skeletal MHC isoform.

Inactivation, subunit dissociation, aggregation, and unfolding of myosin subfragment 1 during guanidine denaturation

The effect of guanidine hydrochloride on ATPase activity, gel filtration, turbidity, exposure of thiol groups, far-UV circular dichroism, and the fluorescence emission intensity of myosin subfragment 1 (S-1) was studied under equilibrium conditions. It was found that the denaturation process involves several intermediate states. The enzymatic activity of S-1 is at first lost at very low concentrations of GdnHCl (lower than 0.5 M). At a slightly higher GdnHCl concentration (about 0.5 M), the light chains dissociate and this dissociation is closely followed by the formation of aggregates between the naked heavy chains of S-1 molecules in the guanidine hydrochloride range of concentrations 0.5-1 M. At GdnHCl concentrations above 1 M, aggregates gradually disappear and S-1 loses its secondary and tertiary structures. These phenomena are partly reversible, and ATPase activity is only partially recovered under highly limited conditions. These results are discussed in relation to the nature of myosin subunit assembly. The head fragment of 20 kDa is thus suggested to be implicated in the binding of light chain to heavy chain and in the self-association of free heavy chains.

Regulatory light chain influences alterations of myosin head induced by actin

The effect of magnesium-for-calcium exchange and phosphorylation of regulatory light chain (LC2) on structural organization of rabbit skeletal myosin head was studied by limited tryptic digestion. In the presence of actin, exchange of magnesium bound to LC2 by calcium in dephosphorylated myosin accelerates the digestion of myosin and heavy meromyosin heavy chain and increases the accumulation of a 50 kDa fragment. This effect is significantly diminished in the case of phosphorylated myosin. Thus, both phosphorylation and cation exchange influences the effect of actin binding on the structural organization of myosin head.

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.

Effects of sulphydryl modification on skinned rat skeletal muscle fibres using 5,5'-dithiobis(2-nitrobenzoic acid)

1. The sulphydryl groups of skinned skeletal muscle fibres have been reacted with 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) in order to determine whether the effects of modifications to the contractile proteins are reflected in changes in the physiological properties of the contractile apparatus and Ca(2+)-regulatory system. 2. Results obtained from fast-twitch and slow-twitch rat fibres which were treated with DTNB (10 mM, pH 8.6, 5 degrees C) for various periods of time under relaxing conditions showed that a major effect of the modification was to reduce the level of maximally Ca(2+)-activated force and fibre stiffness. Force and fibre stiffness were found to decline in proportion. Treatment with DTNB under these conditions did not cause a rise in force or fibre stiffness in relaxed fibres of either type. 3. The effects induced by DTNB under relaxing conditions were substantially reversed by exposure to the reducing agent dithiothreitol (DTT) (10 mM, pH 7.1, 23 degrees C). Force abolished by 30-35 s treatment with DTNB recovered after subsequent DTT treatment to 67 +/- 3% (mean +/- S.E.M., n = 4) in fast-twitch fibres and to 91 +/- 2% (n = 7) in slow-twitch fibres. These results were significantly different (t test, P less than 0.001) indicating that the level of force recovery depended upon the fibre type. 4. DTNB was found to affect not only the maximal Ca(2+)-activated force, but also the force-pCa (pCa = -log10[Ca2+]) relationships of the fibres in a complex, fibre-type specific way. DTT treatment partially reversed these DTNB effects. 5. The skinned fibre preparations reacted differently with DTNB under rigor conditions than under relaxing conditions, indicating that rigor modifies the reactivity of the functional sulphydryl groups to the thiol-targeted agents. 6. When superprecipitation assays (an in vitro analogue of fibre contraction) were carried out with recombined myofibrillar proteins which had been previously reacted with DTNB it was found that modification of myosin, but not modification of thin filament proteins, led to changes in the superprecipitation reaction. 7. Both the skinned fibre results and the superprecipitation results indicate that the effects of DTNB upon the fibre characteristics are primarily due to modifications of the sulphydryl groups of myosin. Therefore, these results show that myosin is not only involved in determining the ability of the contractile apparatus to develop force but also in determining the Ca(2+)-regulatory characteristics of the muscle fibre.

The myosin head can bind two actin monomers

Force impulse is thought to be generated in muscle when myosin head (S-1), while weakly bound to actin filament, undergoes orientational change to form a strong (rigor) bond with actin. There is ample evidence that this bond involves interaction of 1 myosin head with 1 actin monomer. However, X-ray diffraction data of muscle decorated with S-1, as well as recently proposed model of the thin filaments, suggested that each S-1 molecule interacted with two actin monomers. We reinvestigated this controversy and found that the stoichiometry of acto-S-1 bond depended on the relative amounts of actin and myosin present during titrations: when increasing amounts of actin were added to a fixed amount of S-1 (i.e. when myosin heads were initially in excess over actin), the saturating stoichiometry was 1 mol of S-1 per 1 mol of actin. However, when increasing amounts of S-1 were added slowly to a fixed amount of F-actin (i.e. when actin was initially in excess over S-1), the stoichiometry at saturation was 1 mol of S-1 per 2 mols of actin. The ability of S-1 to bind either one or two actin monomers suggests a way that force could be generated during muscle contraction.

Cryoenzymic studies on actomyosin ATPase: kinetic evidence for communication between the actin and ATP sites on myosin

The post-ATP binding steps of myosin subfragment 1 (S1) and actomyosin subfragment 1 (actoS1) ATPases were studied at -15 degrees C with 40% ethylene glycol as antifreeze. The cleavage and release of Pi steps were studied by the rapid-flow quench method and the interaction of actin with S1 plus ATP by light scattering in a stopped-flow apparatus. At -15 degrees C, the interaction of actin with S1 remains tight, and the Km for the activation of S1 ATPase is very small (0.3 microM). The chemical data were interpreted by E + ATP----E*.ATP----E**.ADP.Pi----E*.ADP----products, where E is S1 or actoS1. In Pi burst experiments with S1, there was a large Pi burst of free Pi, but E**.ADP.Pi could not be detected. Here the predominant complex in the seconds time range is E*.ATP and in the steady-state E*.ADP. With actoS1, there was a small Pi burst of E**.ADP.Pi, evidence that the cleavage steps for S1 and actoS1 are different. From the stopped-flow experiments, the dissociation of actoS1 by ATP was complete, even at actin concentrations 60X its Km. Further, no interaction of actin with the key intermediate M*.ATP could be detected. Therefore, at -15 degrees C, actoS1 ATPase occurs by a dissociative pathway; in particular, the cleavage step appears to occur in the absence of actin.(ABSTRACT TRUNCATED AT 250 WORDS)

The yeast type II myosin heavy chain: analysis of its predicted polypeptide sequence

We have completed the nucleotide sequence of the yeast MYO1 gene and deduced its amino acid sequence. The gene is 5553 bp long and contains no introns. Analysis of the sequence, as well as its comparison with other myosins, demonstrate that the yeast protein is a type II myosin heavy chain with characteristic head and tail regions. The latter domain contains six proline residues in two clusters of three, at approximately two thirds from the start of the gene.

Molecular structure of F-actin and location of surface binding sites

Comparisons of three-dimensional maps of vertebrate muscle thin filaments obtained by cryo-electron microscopy and image analysis, reveal the molecular structure of F-actin, the location of the C terminus of the monomer and the positions of the binding sites of tropomyosin, the myosin head and the N-terminal portion of the myosin A1 light chain on the filament. These data provide strong constraints for evaluating models built from the atomic structure of the monomer and the subsequent identification of molecular contacts.

The complete sequence of the human beta-myosin heavy chain gene and a comparative analysis of its product

We have isolated and sequenced the gene and the cDNA coding for the human cardiac beta-myosin heavy chain (designated MYH7). The gene is 22,883 bp long. The 1935 amino acids of this protein (Mr223,111) are encoded by 38 exons. The 5' untranslated region (86 bp) is split by two introns. The 3' untranslated region is 114 bp long. Three Alu repeats were identified within the gene and a fourth one in the 3' flanking intergenic region. The molecular organization of this gene reflects the conservative pattern with respect to size, coding ratio, and number or position of introns characteristic of vertebrate sarcomeric myosin heavy chain genes. The protein sequence of the human beta-heavy chain was compared with corresponding (homologous) sequences of rabbit, rat, and hamster as well as with the (heterologous) embryonic heavy chain sequences of rat, chicken, and man. The results show that protein subregions responsible for basic functions of myosin heavy chains (nucleotide binding and actin binding) are very similar in homologous and heterologous heavy chains. Regions that differ in their primary sequences in heterologous heavy chains appear to be highly conserved within mammalian beta-myosin heavy chains. Constant and variable subregions of heavy chains are discussed in terms of functional significance and evolutionary relatedness.

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.

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.

Atomic structure of the actin:DNase I complex

The atomic models of the complex between rabbit skeletal muscle actin and bovine pancreatic deoxyribonuclease I both in the ATP and ADP forms have been determined by X-ray analysis at an effective resolution of 2.8 A and 3A, respectively. The two structures are very similar. The actin molecule consists of two domains which can be further subdivided into two subdomains. ADP or ATP is located in the cleft between the domains with a calcium ion bound to the beta- or beta- and gamma-phosphates, respectively. The motif of a five-stranded beta sheet consisting of a beta meander and a right handed beta alpha beta unit appears in each domain suggesting that gene duplication might have occurred. These sheets have the same topology as that found in hexokinase.

MgADP-induced changes in the structure of myosin S1 near the ATPase-related thiol SH1 probed by cross-linking

The structural consequences of MgADP binding at the vicinity of the ATPase-related thiol SH1 (Cys-707) have been examined by subjecting myosin subfragment 1, premodified at SH2 (Cys-697) with N-ethylmaleimide (NEM), to reaction with the bifunctional reagent p-phenylenedimaleimide (pPDM) in the presence and absence of MgADP. By monitoring the changes in the Ca2(+)-ATPase activity as a function of reaction time, it appears that the reagent rapidly modifies SH1 irrespective of whether MgADP is present or not. In the absence of nucleotide, only extremely low levels of cross-linking to the 50-kDa middle segment of S1 can be detected, while in the presence of MgADP substantial cross-linking to this segment is observed. A similar cross-link is also formed if MgADP is added subsequent to the reaction of the SH2-NEM-pre-modified S1 with pPDM in the absence of nucleotide. Isolation of the labeled tryptic peptide from the cross-linked adduct formed with [14C]pPDM, and subsequent partial sequence analyses, indicates that the cross-link is made from SH1 to Cys-522. Moreover, it appears that this cross-link results in the trapping of MgADP in this S1 species. These data suggest that the binding of MgADP results in a change in the structure of S1 in the vicinity of the SH1 thiol relative to the 50-kDa "domain" which enables Cys-522 to adopt the appropriate configuration to enable it to be cross-linked to SH1 by pPDM.

New subfragment 1 of skeletal muscle myosin obtained by thrombin cleavage

The head of the myosin molecule (i.e., subfragment 1 with a heavy chain of 95 kDa) is usually obtained by chymotryptic cleavage in the presence of a divalent cation chelator. In the present work, we used another specific proteolytic enzyme, thrombin, to produce a limited cut within the myosin molecule, resulting in a new species of N-terminal fragment. Treatment of skeletal muscle myosin yielded a 97-kDa split heavy chain associated with intact light chains, corresponding to a single cut. The ATPase activities of this new S-1 derivative were slightly affected by the breakdown. It recognized actin in an ATP-dependent manner, as expected, with an affinity 2-5 times higher than that of the usual chymotryptic S-1 preparation but with a very different electron microscopic pattern. Functional differences are noted, and we involve them more precisely in relation to possible structural aspects of the additional C-terminal segment extending the usual S-1 heavy chain from 95 to 97 kDa.