Reversible dissociation of dog cardiac myosin regulatory light chain 2 and its influence on ATP hydrolysis

Cardiac myosin binding protein C phosphorylation affects cross-bridge cycle's elementary steps in a site-specific manner

Based on our recent finding that cardiac myosin binding protein C (cMyBP-C) phosphorylation affects muscle contractility in a site-specific manner, we further studied the force per cross-bridge and the kinetic constants of the elementary steps in the six-state cross-bridge model in cMyBP-C mutated transgenic mice for better understanding of the influence of cMyBP-C phosphorylation on contractile functions. Papillary muscle fibres were dissected from cMyBP-C mutated mice of ADA (Ala273-Asp282-Ala302), DAD (Asp273-Ala282-Asp302), SAS (Ser273-Ala282-Ser302), and t/t (cMyBP-C null) genotypes, and the results were compared to transgenic mice expressing wide-type (WT) cMyBP-C. Sinusoidal analyses were performed with serial concentrations of ATP, phosphate (Pi), and ADP. Both t/t and DAD mutants significantly reduced active tension, force per cross-bridge, apparent rate constant (2πc), and the rate constant of cross-bridge detachment. In contrast to the weakened ATP binding and enhanced Pi and ADP release steps in t/t mice, DAD mice showed a decreased ADP release without affecting the ATP binding and the Pi release. ADA showed decreased ADP release, and slightly increased ATP binding and cross-bridge detachment steps, whereas SAS diminished the ATP binding step and accelerated the ADP release step. t/t has the broadest effects with changes in most elementary steps of the cross-bridge cycle, DAD mimics t/t to a large extent, and ADA and SAS predominantly affect the nucleotide binding steps. We conclude that the reduced tension production in DAD and t/t is the result of reduced force per cross-bridge, instead of the less number of strongly attached cross-bridges. We further conclude that cMyBP-C is an allosteric activator of myosin to increase cross-bridge force, and its phosphorylation status modulates the force, which is regulated by variety of protein kinases.

Earning stripes: myosin binding protein-C interactions with actin

Myosin binding protein-C (MyBP-C) was first discovered as an impurity during the purification of myosin from skeletal muscle. However, soon after its discovery, MyBP-C was also shown to bind actin. While the unique functional implications for a protein that could cross-link thick and thin filaments together were immediately recognized, most early research nonetheless focused on interactions of MyBP-C with the thick filament. This was in part because interactions of MyBP-C with the thick filament could adequately explain most (but not all) effects of MyBP-C on actomyosin interactions and in part because the specificity of actin binding was uncertain. However, numerous recent studies have now established that MyBP-C can indeed bind to actin through multiple binding sites, some of which are highly specific. Many of these interactions involve critical regulatory domains of MyBP-C that are also reported to interact with myosin. Here we review current evidence supporting MyBP-C interactions with actin and discuss these findings in terms of their ability to account for the functional effects of MyBP-C. We conclude that the influence of MyBP-C on muscle contraction can be explained equally well by interactions with actin as by interactions with myosin. However, because data showing that MyBP-C binds to either myosin or actin has come almost exclusively from in vitro biochemical studies, the challenge for future studies is to define which binding partner(s) MyBP-C interacts with in vivo.

Structure, interactions and function of the N-terminus of cardiac myosin binding protein C (MyBP-C): who does what, with what, and to whom?

The thick filament protein myosin-binding protein-C shows a highly modular architecture, with the C-terminal region responsible for tethering to the myosin and titin backbone of the thick filament. The N-terminal region shows the most significant differences between cardiac and skeletal muscle isogenes: an entire Ig-domain (C0) is added, together with highly regulated phosphorylation sites between Ig domains C1 and C2. These structural and functional differences at the N-terminus reflect important functions in cardiac muscle regulation in health and disease. Alternative interactions of this part of MyBP-C with the head-tail (S1-S2) junction of myosin or to actin filaments have been proposed, but with conflicting experimental evidence. The regulation of myosin or actin interaction by phosphorylation of the cardiac MyBP-C N-terminus may play an additional role in length-dependent contraction regulation. We discuss here the evidence for these proposed interactions, considering the required properties of MyBP-C, the way in which they may be regulated in muscle contraction and the way they might be related to heart disease. We also attempt to shed some light on experimental pitfalls and future strategies.

Myosin binding protein-C: a regulator of actomyosin interaction in striated muscle

Myosin-Binding protein-C (MyBP-C) is a family of accessory proteins of striated muscles that contributes to the assembly and stabilization of thick filaments, and regulates the formation of actomyosin cross-bridges, via direct interactions with both thick myosin and thin actin filaments. Three distinct MyBP-C isoforms have been characterized; cardiac, slow skeletal, and fast skeletal. Numerous mutations in the gene for cardiac MyBP-C (cMyBP-C) have been associated with familial hypertrophic cardiomyopathy (FHC) and have led to increased interest in the regulation and roles of the cardiac isoform. This review will summarize our current knowledge on MyBP-C and its role in modulating contractility, focusing on its interactions with both myosin and actin filaments in cardiac and skeletal muscles.

SPontaneous Oscillatory Contraction (SPOC): auto-oscillations observed in striated muscle at partial activation

Striated muscle is well known to exist in either of two states-contraction or relaxation-under the regulation of Ca2+ concentration. Described here is a less well-known third, intermediate state induced under conditions of partial activation, known as SPOC (SPontaneous Oscillatory Contraction). This state is characterised by auto-oscillation between rapid-lengthening and slow-shortening phases. Notably, SPOC occurs in skinned muscle fibres and is therefore not the result of fluctuating Ca2+ levels, but is rather an intrinsic and fundamental phenomenon of the actomyosin motor. Summarised in this review are the experimental data on SPOC and its fundamental mechanism. SPOC presents a novel technique for studying independent communication and coordination between sarcomeres. In cardiac muscle, this auto-oscillatory property may work in concert with electro-chemical signalling to coordinate the heartbeat. Further, SPOC may represent a new way of demonstrating functional defects of sarcomeres in human heart failure.

Structure and interactions of myosin-binding protein C domain C0: cardiac-specific regulation of myosin at its neck?

Myosin-binding protein C (MyBP-C) is a multidomain protein present in the thick filaments of striated muscles and is involved in both sarcomere formation and contraction regulation. The latter function is believed to be located at the N terminus, which is close to the motor domain of myosin. The cardiac isoform of MyBP-C is linked to hypertrophic cardiomyopathy. Here, we use NMR spectroscopy and biophysical and biochemical assays to study the three-dimensional structure and interactions of the cardiac-specific Ig-like domain C0, a part of cardiac MyBP-C of which little is known. The structure confirmed that C0 is a member of the IgI class of proteins, showing many of the characteristic features of this fold. Moreover, we identify a novel interaction between C0 and the regulatory light chain of myosin, thus placing the N terminus of the protein in proximity to the motor domain of myosin. This novel interaction is disrupted by several cardiomyopathy-linked mutations in the MYBPC3 gene. These results provide new insights into how cardiac MyBP-C incorporates in the sarcomere and how it can contribute to the regulation of muscle contraction.

Effects of cardiac myosin binding protein-C on the regulation of interaction of cardiac myosin with thin filament in an in vitro motility assay

Modulatory role of whole cardiac myosin binding protein-C (сMyBP-C) in regulation of cardiac muscle contractility was studied in the in vitro motility assay with rabbit cardiac myosin as a motor protein. The effects of cMyBP-C on the interaction of cardiac myosin with regulated thin filament were tested in both in vitro motility and ATPase assays. We demonstrate that the addition of cMyBP-C increases calcium regulated Mg-ATPase activity of cardiac myosin at submaximal calcium. The Hill coefficient for 'pCa-velocity' relation in the in vitro motility assay decreased and the calcium sensitivity increased when сMyBP-C was added. Results of our experiments testifies in favor of the hypothesis that сMyBP-C slows down cross-bridge kinetics when binding to actin.

Viral myocarditis induced by Coxsackievirus B3 in A.BY/SnJ mice: analysis of changes in the myocardial proteome

Enteroviral myocarditis displays highly diverse clinical phenotypes ranging from mild dyspnoea or chest pain to cardiogenic shock and death. Despite detailed studies of the virus life cycle in vitro and in vivo, the molecular interplay between host and virus in disease progression is largely unresolved. Murine models of Coxsackievirus B3 (CVB3)-induced myocarditis well mimic the human disease patterns and can thus be explored to study mechanisms leading from acute to chronic myocarditis. Here, we present a 2-D gel-based proteomic survey of the changes in the murine cardiac proteome that occurs following infection with CVB3. In total, 136 distinct proteins were affected. Proteins, which are involved in immunity and defense and protein metabolism/modification displayed pronounced changes in intensity not only during acute but also at later stages of CVB3 myocarditis. Proteins involved in maintenance of cell structure and associated proteins were particularly influenced in the acute phase of myocarditis, whereas reduction of levels of metabolic enzymes was observed in chronic myocarditis. Studies about changes in protein intensities were complemented by an analysis of protein phosphorylation that revealed infection-associated changes in the phosphorylation of myosin binding protein C, atrial and ventricular isoforms of myosin regulatory light chain 2, desmin, and Rab GDP dissociation inhibitor beta-2.

During muscle atrophy, thick, but not thin, filament components are degraded by MuRF1-dependent ubiquitylation

Loss of myofibrillar proteins is a hallmark of atrophying muscle. Expression of muscle RING-finger 1 (MuRF1), a ubiquitin ligase, is markedly induced during atrophy, and MuRF1 deletion attenuates muscle wasting. We generated mice expressing a Ring-deletion mutant MuRF1, which binds but cannot ubiquitylate substrates. Mass spectrometry of the bound proteins in denervated muscle identified many myofibrillar components. Upon denervation or fasting, atrophying muscles show a loss of myosin-binding protein C (MyBP-C) and myosin light chains 1 and 2 (MyLC1 and MyLC2) from the myofibril, before any measurable decrease in myosin heavy chain (MyHC). Their selective loss requires MuRF1. MyHC is protected from ubiquitylation in myofibrils by associated proteins, but eventually undergoes MuRF1-dependent degradation. In contrast, MuRF1 ubiquitylates MyBP-C, MyLC1, and MyLC2, even in myofibrils. Because these proteins stabilize the thick filament, their selective ubiquitylation may facilitate thick filament disassembly. However, the thin filament components decreased by a mechanism not requiring MuRF1.

The myosin-binding protein C motif binds to F-actin in a phosphorylation-sensitive manner

Cardiac myosin-binding protein C (cMyBP-C) is a regulatory protein expressed in cardiac sarcomeres that is known to interact with myosin, titin, and actin. cMyBP-C modulates actomyosin interactions in a phosphorylation-dependent way, but it is unclear whether interactions with myosin, titin, or actin are required for these effects. Here we show using cosedimentation binding assays, that the 4 N-terminal domains of murine cMyBP-C (i.e. C0-C1-m-C2) bind to F-actin with a dissociation constant (K(d)) of approximately 10 microm and a molar binding ratio (B(max)) near 1.0, indicating 1:1 (mol/mol) binding to actin. Electron microscopy and light scattering analyses show that these domains cross-link F-actin filaments, implying multiple sites of interaction with actin. Phosphorylation of the MyBP-C regulatory motif, or m-domain, reduced binding to actin (reduced B(max)) and eliminated actin cross-linking. These results suggest that the N terminus of cMyBP-C interacts with F-actin through multiple distinct binding sites and that binding at one or more sites is reduced by phosphorylation. Reversible interactions with actin could contribute to effects of cMyBP-C to increase cross-bridge cycling.

The R403Q myosin mutation implicated in familial hypertrophic cardiomyopathy causes disorder at the actomyosin interface

Background

Mutations in virtually all of the proteins comprising the cardiac muscle sarcomere have been implicated in causing Familial Hypertrophic Cardiomyopathy (FHC). Mutations in the β-myosin heavy chain (MHC) remain among the most common causes of FHC, with the widely studied R403Q mutation resulting in an especially severe clinical prognosis. In vitro functional studies of cardiac myosin containing the R403Q mutation have revealed significant changes in enzymatic and mechanical properties compared to wild-type myosin. It has been proposed that these molecular changes must trigger events that ultimately lead to the clinical phenotype.

Principal Findings

Here we examine the structural consequences of the R403Q mutation in a recombinant smooth muscle myosin subfragment (S1), whose kinetic features have much in common with slow β-MHC. We obtained three-dimensional reconstructions of wild-type and R403Q smooth muscle S1 bound to actin filaments in the presence (ADP) and absence (apo) of nucleotide by electron cryomicroscopy and image analysis. We observed that the mutant S1 was attached to actin at highly variable angles compared to wild-type reconstructions, suggesting a severe disruption of the actin-myosin interaction at the interface.

Significance

These results provide structural evidence that disarray at the molecular level may be linked to the histopathological myocyte disarray characteristic of the diseased state.

Identification and validation of selective upregulation of ventricular myosin light chain type 2 mRNA in idiopathic dilated cardiomyopathy

BACKGROUND AND AIMS

the etiology of idiopathic dilated cardiomyopathy (IDCM) is unknown, methods such as suppression subtractive hybridization (SSH) and DNA microarray technology can help to identify genes which might be involved in the pathogenesis of this disease.

METHODS AND RESULTS

we used SSH which compared mRNA populations extracted from the left ventricular tissue of IDCM hearts and from the control tissue to identify sequences which correspond to genes up-regulated in IDCM. We identified ventricular myosin light chain type 2 (MLC2V), skeletal alpha-actin, long-chain-acyl-CoA-synthetase and mRNA for the protein KIAA0465 as differentially up-regulated genes. Expression of MLC2V mRNA was determined by RT-PCR in patients with end-stage heart failure caused by IDCM (n=11) or coronary artery disease (CAD, n=9) who underwent heart transplantation as well as the controls (n=6). MLC2V/GAPDH ratios were 2.95+/-0.32, 0.69+/-0.03 and 0.28+/-0.08 (arbitrary unit) for the IDCM group, the CAD group and controls, respectively (P<0.05). DNA microarray analysis confirmed the finding of MLC2V upregulation in IDCM (3.7- and 1.8-fold increase in MLC2V mRNA).

CONCLUSIONS

we have demonstrated that SSH is a useful method to identify differential myocardial upregulation of genes. Upregulation of MLC2V can be judged as a specific IDCM related feature, which might be clinically helpful.

Functional consequences of mutations in the smooth muscle myosin heavy chain at sites implicated in familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) is frequently associated with mutations in the beta-cardiac myosin heavy chain. Many of the implicated residues are located in highly conserved regions of the myosin II class, suggesting that these mutations may impair the basic functions of the molecular motor. To test this hypothesis, we have prepared recombinant smooth muscle heavy meromyosin with mutations at sites homologous to those associated with FHC by using a baculovirus/insect cell expression system. Several of the heavy meromyosin mutants, in particular R403Q, showed an increase in actin filament velocity in a motility assay and an enhanced actin-activated ATPase activity. Single molecule mechanics, using a laser trap, gave unitary displacements and forces for the mutants that were similar to wild type, but the attachment times to actin following a unitary displacement were markedly reduced. These results suggest that the increases in activity are due to a change in kinetics and not due to a change in the intrinsic mechanical properties of the motor. In contrast to earlier reports, we find that mutations in residues implicated in FHC affect motor function by enhancing myosin activity rather than by a loss of function.

Truncation of vertebrate striated muscle myosin light chains disturbs calcium-induced structural transitions in synthetic myosin filaments

Electron microscopy and negative staining techniques have been used to show that the proteolytic removal of 13 amino acids from the N-terminus of essential light chain 1 and 19 amino acids from the N-terminus of the regulatory light chain of rabbit skeletal and cardiac muscle myosins destroys Ca(2+)-induced reversible movement of subfragment-2 (S2) with heads (S1) away from the backbone of synthetic myosin filaments observed for control assemblies of the myosin under near physiological conditions. This is the direct demonstration of the contribution of the S2 movement to the Ca(2+)-sensitive structural behavior of rabbit cardiac and skeletal myosin filaments and of the necessity of intact light chains for this movement. In muscle, such a mobility might play an important role in proper functioning of the myosin filaments. The impairment of the Ca(2+)-dependent structural behavior of S2 with S1 on the surface of the synthetic myosin filaments observed by us may be of direct relevance to some cardiomyopathies, which are accompanied by proteolytic breakdown or dissociation of myosin light chains.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Calcium-induced structural changes in synthetic myosin filaments of vertebrate striated muscles

Using negative staining, freeze-drying, and shadowing techniques in electron microscopy we have for the first time demonstrated Ca-induced reversible structural transitions in the synthetic filaments of dephosphorylated column-purified rabbit skeletal and cardiac muscle myosins formed by dialysis against solutions containing 120 mM KCl, 1 mM MgCl(2), 10 mM imidazole-HCl buffer (pH 7.0), and either 0.1 mM CaCl(2) or 1 mM EGTA. It has been revealed that the compact ordered structure of the filaments with myosin heads and subfragments-2 (S2) disposed close to the filament backbone with an axial periodicity of about 14.5 nm in the absence of Ca(2+) transforms into a spread disordered structure due to the movement of the heads and S2 away from the filament surface in the presence of Ca(2+). Increasing the pH from neutrality to pH 7.8 leads to a spread, disordered structure while decreasing the pH value to 6.5 returns the filaments to their compact, rather ordered state independent of the Ca(2+) concentrations used. The fact that the reversible structural transitions in synthetic filaments of myosin are observed in the absence of actin and actin- and myosin-associated proteins suggests that Ca(2+)-induced S2 movement is an intrinsic property of myosin itself. Ca(2+)-induced S2 mobility may reflect the existence of functionally significant communications between the myosin head domains and the tails of myosin molecules in thick filaments, and its disappearance can be an indicator of the impairment of these communications, for example, in acute ischemia and myocardial infarction.

Cardiac myosin binding protein C

Myosin binding protein C (MyBP-C) is one of a group of myosin binding proteins that are present in the myofibrils of all striated muscle. The protein is found at 43-nm repeats along 7 to 9 transverse lines in a portion of the A band where crossbridges are found (C zone). MyBP-C contains myosin and titin binding sites at the C terminus of the molecule in all 3 of the isoforms (slow skeletal, fast skeletal, and cardiac). The cardiac isoform also includes a series of residues that contain 3 phosphorylatable sites and an additional immunoglobulin module at the N terminus that are not present in skeletal isoforms. The following 2 major functions of MyBP-C have been suggested: (1) a role in the formation of the sarcomeric myofibril as a result of binding to myosin and titin and (2) in the case of the cardiac isoform, regulation of contraction through phosphorylation. The first is supported by the demonstrated effect of MyBP-C on the packing of myosin in the thick filament, the coincidence of appearance of sarcomeres and MyBP-C during myofibrillogenesis, and the defective formation of sarcomeres when the titin and/or myosin binding sites of MyBP-C are missing. The second is supported by the specific phosphorylation sites in cardiac MyBP-C, the presence in the thick filament of an enzyme specific for MyBP-C phosphorylation, the alteration of thick filament structure by MyBP-C phosphorylation, and the accompaniment of MyBP-C phosphorylation with all major physiological mechanisms of modulation of inotropy in the heart.

Cardiac protein abnormalities in dilated cardiomyopathy detected by two-dimensional polyacrylamide gel electrophoresis

The aim of the investigation was to determine whether there are specific global quantitative and qualitative changes in protein expression in heart tissue from patients with dilated cardiomyopathy (DCM) compared with ischaemic heart disease and undiseased tissue. Two-dimensional (2-D) polyacrylamide gel electrophoresis and computer analysis was used to study protein alteration in DCM biopsy material (n=28) compared with donor heart biopsy samples (n=9) and explanted hearts from individuals suffering from ischaemic heart disease (IHD; n = 21). A total of 88 proteins displayed decreased abundance in DCM versus IHD material while five proteins had elevated levels in the DCM group (p<0.01). The most prominent changes occurred in the contractile protein myosin light chain 2 and in a group of proteins identified as desmin. These changes do not appear to be artefactual degradation events occurring during sample processing. These proteins are not apparent in electrophoretic separations of vascular tissue or cultured endothelial cells, mesothelial cells or cardiac fibroblasts, which are clearly distinguishable from the 2-D protein patterns of whole heart and of isolated cardiac myocytes and do not appear to reflect variations in the cellular composition of biopsy samples. The different protein patterns observed in cardiomyopathy showed no obvious relationship with New York Heart Association (NYHA) functional class or haemodynamic parameters. The study has demonstrated significant alterations in quantitative protein expression in the DCM heart which would have serious implications for myocyte function. These changes might be explained by altered protease activity in DCM which could exacerbate contractile dysfunction in the failing heart.

Isometric and dynamic contractile properties of porcine skinned cardiac myocytes after stunning

The purpose of this study was to investigate myofibrillar mechanisms of depressed contractile function associated with myocardial stunning. We first tested whether the degree of stunning was directly related to changes in myofilament Ca2+ sensitivity. Variable degrees and durations of low-flow ischemia were followed by 30 minutes of reperfusion in an open-chest porcine model of regional myocardial stunning (n = 27). Ca2+ sensitivity of isometric tension was measured in skinned myocytes obtained from endocardial biopsies taken during control aerobic flow and after 30 minutes of reperfusion. The degree of stunning, as assessed by percent systolic wall thickening, ranged from -3% to 75% of control but did not correlate (r = .11) with changes in pCa50, ie, pCa for half-maximal tension. Only in the group (n = 10) with the most severe level of ischemia was there a significant decrease in pCa50 (from 5.97 +/- 0.06 in the control condition to 5.86 +/- 0.07 after ischemia, P < .05). Less severe levels of ischemia (n = 17) resulted in significant stunning (percent systolic wall thickening, 38 +/- 4% of control) but no change in pCa50. To investigate the possibility that alterations in myofibrillar cross-bridge kinetics contribute to depressed function in stunning, maximum velocity of shortening (Vo) was measured in postischemic myocytes. Vo in postischemic myocytes was reduced to 56 +/- 4% of Vo in control myocytes and was independent both of the degree of stunning (r = .26) and changes in Ca2+ sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Human cardiac myosin light chains: sequence comparisons between myosin LC1 and LC2 from normal and idiopathic dilated cardiomyopathic hearts

The primary structures of light chains isolated from the human myocardium with idiopathic dilated cardiomyopathy (IDC) were determined and compared with the sequence structures of myosin light chains obtained from control human heart myosin. Sequences were determined by chemical analysis and the identity of N-terminal residues established by mass spectrometry. The N-terminal residues in essential (ELC) and regulatory (RLC) light chains were blocked and were identified to be trimethyl alanine. The amino acid sequences of ELC and RLC from control human myosin revealed a high degree of homology with those purified from rat and chicken cardiac myosin. Comparison with a published partial chemical sequence of the human heart myosin light chains revealed significant variations. However, there was very good agreement with published sequences obtained by molecular biological techniques. Sequences of the light chains from cardiomyopathic myosin revealed no difference in the primary structures when compared with control human heart myosin light chains indicating IDC had no influence on, nor was caused by, altered myosin light chain gene expression.

Functional effects of LC1-reassociation with cardiac papain Mg.S1

The effect of LC1 on cardiac myosin structure and activity was investigated using as a model S1 prepared by papain digestion in the presence of Mg2+. The resulting S1 contained LC2 but a part of the N-terminal region of LC1 was cleaved. Sequencing the N-terminal part of the band migrating below LC1 on SDS gels revealed it to consist of alternating alanyl and prolyl residues thus establishing LC1 as the origin of this band. However, Western blots did not reveal any LC1 while radioimmunoassays indicated it to be present at the 5% level suggesting the anti-LC1 antibody used in these experiments did not recognize the C-terminal portion of LC1 still attached to Mg.S1. Mixing a 10-15 M excess of isolated light chains with Mg.S1 in the presence of 10 mM ATP, 12 mM MgCl2, 4.7 M NH4Cl allowed LC1 to recombine with LC1-deficient Mg.S1. Equilibrium ultracentrifugation analysis revealed a highly heterogeneous LC1-deficient S1 which upon recombination with intact LC1 became monodisperse as indicated by the superimposition of molecular weight averages all across the centrifuge cell. LC1-deficient Mg.S1 had a Vm of 0.4 s-1, Ka of 30 microM and a Kbind of 28 microM. In the presence of intact LC1, Vm rose to 0.8 s-1 while Ka and Kbind were reduced to 7.5 and 12 microM, respectively. The fourfold decrease in Ka strongly indicated an increased affinity for actin by Mg.S1 in the presence of uncleaved LC1. Also, Ca(2+)-regulation of dog heart myofibrils was suppressed when Ca(2+)-activated MgATPase assays, as a function of Ca2+, were performed in the presence of anti-LC1 antibodies. These observations suggest the presence of intact, uncleaved LC1 in S1 is required for the stability of S1 heavy chains and proper Ca(2+)-regulation.

Light chain 2 profile and activity of human ventricular myosin during dilated cardiomyopathy. Identification of a causal agent for impaired myocardial function

BACKGROUND

A number of parameters reflecting the effects of idiopathic dilated cardiomyopathy (IDC) on the structure and function of myosin from the human myocardium were analyzed.

METHODS AND RESULTS

The content of the regulatory light chain, LC2, was reduced in myopathic heart myosin in contrast to the controls in which it was present in stoichiometric amounts relative to the essential light chain, LC1. In IDC hearts, the absence or significant reduction in amount of LC2 was related to the presence of an active protease, which was isolated and purified about 130-fold. The protease exhibited a significant degree of specificity: It cleaved LC2 almost totally (but not the heavy chains) in human control heart myosin but only partially cleaved LC2 in canine heart or in rabbit skeletal muscle myosins. The protease was present at a very low level or was inactive in control heart tissue. When the LC1/LC2 molar ratio was calculated, it was found to be 1:1.0 in control heart myosin and remained constant in various samples analyzed, whereas in myopathic myosin from different individuals, this ratio varied from 1:0.1 to 1:0.69. The rates of ATP binding to control and myopathic myosins were similar, whereas the Vm of actin-activated ATPase of myopathic myosin was about 25% less than that of the control. However, ATP binding and its hydrolysis by control S1, i.e., the myosin head, were faster by a factor of 2 than that of the myopathic S1. In addition, control myosin synthetic thick filament length as well as turbidity in solution, measured by light scattering, were twice as large as those of the myopathic heart myosin. These effects induced by myopathy in both filament assembly and turbidity were reversed upon reassociation of IDC myosin with LC2.

CONCLUSIONS

The changes in myosin structure and function were linked to a protease-mediated cleavage of LC2 in myosin; a possible role for the protease in the degenerative effects of idiopathic dilated cardiomyopathy is thus defined.

Ca2+ regulation of mechanical properties of striated muscle. Mechanistic studies using extraction and replacement of regulatory proteins

Extraction of regulatory proteins from thick and thin filaments of vertebrate striated muscle has proven to be an important approach in elucidating roles of these proteins in regulating contraction and in probing specific mechanisms of activation. For some proteins, such as LC2 and C protein, extraction has been fundamental in demonstrating the importance of these proteins in modulating contraction and the kinetics of cross-bridge interaction. For other proteins, such as TnC and troponin, extraction has provided significant insight into the importance of thin-filament intermolecular cooperativity in modulating Ca2+ sensitivity of the contractile process. A combination of extraction and readdition has provided a means of introducing mutated or derivatized proteins into fibers to accomplish a variety of experimental objectives. The use of this approach is likely to grow with the need to test the functional consequences of site-specific mutations as part of studies directed to mechanisms of regulation or altered regulation in heart and skeletal muscles under normal and pathophysiological conditions. Such studies are likely to include extraction in combination with other probes of function such as flash photolysis of reaction substrates or products within the cross-bridge interaction cycle. Although extraction is a powerful approach and is likely to be extended to proteins not discussed in this review, an essential element of experimental design in studies such as these is that appropriate control experiments be done to verify that observed effects of the extraction protocol are specifically attributable to the protein that is removed.

Electron microscopy of cardiac myosin: its shape and properties as determined by the regulatory light chain

Structural properties of dog cardiac myosin and the influence of the regulatory light chain (LC2) on the shape of myosin heads were investigated by electron microscopy. LC2 was reversibly removed using a neutral protease from myopathic hamsters (Margossian, J. Biol. Chem. 260 (1985) 13747-54). The distribution of myosin head length centred around 17 nm with the mean length being 18.9 nm. Statistical analysis suggested that myosin heads became more globular upon removal of LC2. No extensive aggregation of myosin could be detected after LC2 was dissociated, either by sedimentation velocity or by gels run under non-denaturing conditions. The centre-to-centre distance between heads remained constant at about 21 nm, regardless of the presence or absence of LC2. The distribution of length of the globular region reveals two peaks at 7.5 and 9.5 nm, suggesting an extended and a shorter configuration of this region. The decrease in mass at the head/tail junction upon LC2 removal suggests that it is the binding site for the regulatory light chains. A bend at 57 nm from the head/tail joint was sometimes noticed, corresponding to the myosin hinge region. In high resolution micrographs individual particles revealed invaginations along the contours of the head, possibly delineating the boundaries of structural domains within the head. The conformation of arrowheads in actin decorated with either subfragment 1 (S1) or heavy meromyosin (HMM) was investigated in the presence and absence of LC2.