Three-dimensional structure of vertebrate cardiac muscle myosin filaments

Three-dimensional reconstruction of tarantula myosin filaments suggests how phosphorylation may regulate myosin activity

Muscle contraction involves the interaction of the myosin heads of the thick filaments with actin subunits of the thin filaments. Relaxation occurs when this interaction is blocked by molecular switches on these filaments. In many muscles, myosin-linked regulation involves phosphorylation of the myosin regulatory light chains (RLCs). Electron microscopy of vertebrate smooth muscle myosin molecules (regulated by phosphorylation) has provided insight into the relaxed structure, revealing that myosin is switched off by intramolecular interactions between its two heads, the free head and the blocked head. Three-dimensional reconstruction of frozen-hydrated specimens revealed that this asymmetric head interaction is also present in native thick filaments of tarantula striated muscle. Our goal in this study was to elucidate the structural features of the tarantula filament involved in phosphorylation-based regulation. A new reconstruction revealed intra- and intermolecular myosin interactions in addition to those seen previously. To help interpret the interactions, we sequenced the tarantula RLC and fitted an atomic model of the myosin head that included the predicted RLC atomic structure and an S2 (subfragment 2) crystal structure to the reconstruction. The fitting suggests one intramolecular interaction, between the cardiomyopathy loop of the free head and its own S2, and two intermolecular interactions, between the cardiac loop of the free head and the essential light chain of the blocked head and between the Leu305-Gln327 interaction loop of the free head and the N-terminal fragment of the RLC of the blocked head. These interactions, added to those previously described, would help switch off the thick filament. Molecular dynamics simulations suggest how phosphorylation could increase the helical content of the RLC N-terminus, weakening these interactions, thus releasing both heads and activating the thick filament.

Myosin binding protein C positioned to play a key role in regulation of muscle contraction: structure and interactions of domain C1

Myosin binding protein C (MyBP-C) is a thick filament protein involved in the regulation of muscle contraction. Mutations in the gene for MyBP-C are the second most frequent cause of hypertrophic cardiomyopathy. MyBP-C binds to myosin with two binding sites, one at its C-terminus and another at its N-terminus. The N-terminal binding site, consisting of immunoglobulin domains C1 and C2 connected by a flexible linker, interacts with the S2 segment of myosin in a phosphorylation-regulated manner. It is assumed that the function of MyBP-C is to act as a tether that fixes the S1 heads in a resting position and that phosphorylation releases the S1 heads into an active state. Here, we report the structure and binding properties of domain C1. Using a combination of site-directed mutagenesis and NMR interaction experiments, we identified the binding site of domain C1 in the immediate vicinity of the S1–S2 hinge, very close to the light chains. In addition, we identified a zinc binding site on domain C1 in close proximity to the S2 binding site. Its zinc binding affinity (K_d of approximately 10–20 μM) might not be sufficient for a physiological effect. However, the familial hypertrophic cardiomyopathy-related mutation of one of the zinc ligands, glutamine 210 to histidine, will significantly increase the binding affinity, suggesting that this mutation may affect S2 binding. The close proximity of the C1 binding site to the hinge, the light chains and the S1 heads also provides an explanation for recent observations that (a) shorter fragments of MyBP-C unable to act as a tether still have an effect on the actomyosin ATPase and (b) as to why the myosin head positions in phosphorylated wild-type mice and MyBP-C knockout mice are so different: Domain C1 bound to the S1–S2 hinge is able to manipulate S1 head positions, thus influencing force generation without tether. The potentially extensive extra interactions of C1 are expected to keep it in place, while phosphorylation dislodges the C1–C2 linker and domain C2. As a result, the myosin heads would always be attached to a tether that has phosphorylation-dependent length regulation.

Blebbistatin stabilizes the helical order of myosin filaments by promoting the switch 2 closed state

Blebbistatin is a small-molecule, high-affinity, noncompetitive inhibitor of myosin II. We have used negative staining electron microscopy to study the effects of blebbistatin on the organization of the myosin heads on muscle thick filaments. Loss of ADP and Pi from the heads causes thick filaments to lose their helical ordering. In the presence of 100 microM blebbistatin, disordering was at least 10 times slower. In the M.ADP state, myosin heads are also disordered. When blebbistatin was added to M.ADP thick filaments, helical ordering was restored. However, blebbistatin did not improve the order of thick filaments lacking bound nucleotide. Addition of calcium to relaxed muscle homogenates induced thick-thin filament interaction and filament sliding. In the presence of blebbistatin, filament interaction was inhibited. These structural observations support the conclusion, based on biochemical studies, that blebbistatin inhibits myosin ATPase and actin interaction by stabilizing the closed switch 2 structure of the myosin head. These properties make blebbistatin a useful tool in structural and functional studies of cell motility and muscle contraction.

Evidence for the oligomeric state of 'elastic' titin in muscle sarcomeres

The giant protein titin has important roles in muscle sarcomere integrity, elasticity and contractile activity. The key role in elasticity was highlighted in recent years by single-molecule mechanical studies, which showed a direct relationship between the non-uniform structure of titin and the hierarchical mechanism of its force-extension behavior. Further advances in understanding mechanisms controlling sarcomere structure and elasticity require detailed knowledge of titin arrangement and interactions in situ. Here we present data on the structure and self-interactive properties of an approximately 290 kDa ( approximately 100 nm long) tryptic fragment from the I-band part of titin that is extensible in situ. The fragment includes the conserved 'distal' tandem Ig segment of the molecule and forms side-by-side oligomers with distinctive 4 nm cross-striations. Comparisons between these oligomers and the end filaments seen at the tips of native thick filaments indicate identical structure. This shows that end-filaments are formed by the elastic parts of six titin molecules connecting each end of the thick filament to the Z-line. Self-association of elastic titin into stiff end-filaments adds a further hierarchical level in the mechanism of titin extensibility in muscle cells. Self-association of this part of titin may be required to prevent interference of the individual flexible molecules with myosin cross-bridges interacting with actin.

Understanding the organisation and role of myosin binding protein C in normal striated muscle by comparison with MyBP-C knockout cardiac muscle

Myosin binding protein C (MyBP-C) is a component of the thick filament of striated muscle. The importance of this protein is revealed by recent evidence that mutations in the cardiac gene are a major cause of familial hypertrophic cardiomyopathy. Here we investigate the distribution of MyBP-C in the A-bands of cardiac and skeletal muscles and compare this to the A-band structure in cardiac muscle of MyBP-C-deficient mice. We have used a novel averaging technique to obtain the axial density distribution of A-bands in electron micrographs of well-preserved specimens. We show that cardiac and skeletal A-bands are very similar, with a length of 1.58 ± 0.01 μm. In normal cardiac and skeletal muscle, the distributions are very similar, showing clearly the series of 11 prominent accessory protein stripes in each half of the A-band spaced axially at 43-nm intervals and starting at the edge of the bare zone. We show by antibody labelling that in cardiac muscle the distal nine stripes are the location of MyBP-C. These stripes are considerably suppressed in the knockout mouse hearts as expected. Myosin heads on the surface of the thick filament in relaxed muscle are thought to be arranged in a three-stranded quasi-helix with a mean 14.3-nm axial cross bridge spacing and a 43 nm helix repeat. Extra “forbidden” meridional reflections, at orders of 43 nm, in X-ray diffraction patterns of muscle have been interpreted as due to an axial perturbation of some levels of myosin heads. However, in the MyBP-C-deficient hearts these extra meridional reflections are weak or absent, suggesting that they are due to MyBP-C itself or to MyBP-C in combination with a head perturbation brought about by the presence of MyBP-C.

Protein kinase A-mediated phosphorylation of cMyBP-C increases proximity of myosin heads to actin in resting myocardium

Protein kinase A-mediated (PKA) phosphorylation of cardiac myosin binding protein C (cMyBP-C) accelerates the kinetics of cross-bridge cycling and may relieve the tether-like constraint of myosin heads imposed by cMyBP-C. We favor a mechanism in which cMyBP-C modulates cross-bridge cycling kinetics by regulating the proximity and interaction of myosin and actin. To test this idea, we used synchrotron low-angle x-ray diffraction to measure interthick filament lattice spacing and the equatorial intensity ratio, I(11)/I(10), in skinned trabeculae isolated from wild-type and cMyBP-C null (cMyBP-C(-/-)) mice. In wild-type myocardium, PKA treatment appeared to result in radial or azimuthal displacement of cross-bridges away from the thick filaments as indicated by an increase (approximately 50%) in I(11)/I(10) (0.22+/-0.03 versus 0.33+/-0.03). Conversely, PKA treatment did not affect cross-bridge disposition in mice lacking cMyBP-C, because there was no difference in I(11)/I(10) between untreated and PKA-treated cMyBP-C(-/-) myocardium (0.40+/-0.06 versus 0.42+/-0.05). Although lattice spacing did not change after treatment in wild-type (45.68+/-0.84 nm versus 45.64+/-0.64 nm), treatment of cMyBP-C(-/-) myocardium increased lattice spacing (46.80+/-0.92 nm versus 49.61+/-0.59 nm). This result is consistent with the idea that the myofilament lattice expands after PKA phosphorylation of cardiac troponin I, and when present, cMyBP-C, may stabilize the lattice. These data support our hypothesis that tethering of cross-bridges by cMyBP-C is relieved by phosphorylation of PKA sites in cMyBP-C, thereby increasing the proximity of cross-bridges to actin and increasing the probability of interaction with actin on contraction.

Orientation of the essential light chain region of myosin in relaxed, active, and rigor muscle

The orientation of the ELC region of myosin in skeletal muscle was determined by polarized fluorescence from ELC mutants in which pairs of introduced cysteines were cross-linked by BSR. The purified ELC-BSRs were exchanged for native ELC in demembranated fibers from rabbit psoas muscle using a trifluoperazine-based protocol that preserved fiber function. In the absence of MgATP (in rigor) the ELC orientation distribution was narrow; in terms of crystallographic structures of the myosin head, the LCD long axis linking heavy-chain residues 707 and 843 makes an angle (beta) of 120-125 degrees with the filament axis. This is approximately 30 degrees larger than the broader distribution determined previously from RLC probes, suggesting that, relative to crystallographic structures, the LCD is bent between its ELC and RLC regions in rigor muscle. The ELC orientation distribution in relaxed muscle had two broad peaks with beta approximately 70 degrees and approximately 110 degrees, which may correspond to the two head regions of each myosin molecule, in contrast with the single broad distribution of the RLC region in relaxed muscle. During isometric contraction the ELC orientation distribution peaked at beta approximately 105 degrees , similar to that determined previously for the RLC region.

Cardiac myosin binding protein-C modulates actomyosin binding and kinetics in the in vitro motility assay

The modulatory role of whole cardiac myosin binding protein-C (cMyBP-C) on myosin force and motion generation was assessed in an in vitro motility assay. The presence of cMyBP-C at an approximate molar ratio of cMyBP-C to whole myosin of 1:2, resulted in a 25% reduction in thin filament velocity (P<0.002) with no effect on relative isometric force under maximally activated conditions (pCa 5). Cardiac MyBP-C was capable of inhibiting actin filament velocity in a concentration-dependent manner using either whole myosin, HMM or S1, indicating that the cMyBP-C does not have to bind to myosin LMM or S2 subdomains to exert its effect. The reduction in velocity by cMyBP-C was independent of changes in ionic strength or excess inorganic phosphate. Co-sedimentation experiments demonstrated S1 binding to actin is reduced as a function of cMyBP-C concentration in the presence of ATP. In contrast, S1 avidly bound to actin in the absence of ATP and limited cMyBP-C binding, indicating that cMyBP-C and S1 compete for actin binding in an ATP-dependent fashion. However, based on the relationship between thin filament velocity and filament length, the cMyBP-C induced reduction in velocity was independent of the number of cross-bridges interacting with the thin filament. In conclusion, the effects of cMyBP-C on velocity and force at both maximal and submaximal activation demonstrate that cMyBP-C does not solely act as a tether between the myosin S2 and LMM subdomains but likely affects both the kinetics and recruitment of myosin cross-bridges through its direct interaction with actin and/or myosin head.

Head-head and head-tail interaction: a general mechanism for switching off myosin II activity in cells

Intramolecular interaction between myosin heads, blocking key sites involved in actin-binding and ATPase activity, appears to be a critical mechanism for switching off vertebrate smooth-muscle myosin molecules, leading to relaxation. We have tested the hypothesis that this interaction is a general mechanism for switching off myosin II-based motile activity in both muscle and nonmuscle cells. Electron microscopic images of negatively stained myosin II molecules were analyzed by single particle image processing. Molecules from invertebrate striated muscles with phosphorylation-dependent regulation showed head-head interactions in the off-state similar to those in vertebrate smooth muscle. A similar structure was observed in nonmuscle myosin II (also phosphorylation-regulated). Surprisingly, myosins from vertebrate skeletal and cardiac muscle, which are not intrinsically regulated, undergo similar head-head interactions in relaxing conditions. In all of these myosins, we also observe conserved interactions between the 'blocked' myosin head and the myosin tail, which may contribute to the switched-off state. These results suggest that intramolecular head-head and head-tail interactions are a general mechanism both for inducing muscle relaxation and for switching off myosin II-based motile activity in nonmuscle cells. These interactions are broken when myosin is activated.

The structure of isolated cardiac Myosin thick filaments from cardiac Myosin binding protein-C knockout mice

Mutations in the thick filament associated protein cardiac myosin binding protein-C (cMyBP-C) are a major cause of familial hypertrophic cardiomyopathy. Although cMyBP-C is thought to play both a structural and a regulatory role in the contraction of cardiac muscle, detailed information about the role of this protein in stability of the thick filament and maintenance of the ordered helical arrangement of the myosin cross-bridges is limited. To address these questions, the structure of myosin thick filaments isolated from the hearts of wild-type mice containing cMyBP-C (cMyBP-C(+/+)) were compared to those of cMyBP-C knockout mice lacking this protein (cMyBp-C(-/-)). The filaments from the knockout mice hearts lacking cMyBP-C are stable and similar in length and appearance to filaments from the wild-type mice hearts containing cMyBP-C. Both wild-type and many of the cMyBP-C(-/-) filaments display a distinct 43 nm periodicity. Fourier transforms of electron microscope images typically show helical layer lines to the sixth layer line, confirming the well-ordered arrangement of the cross-bridges in both sets of filaments. However, the "forbidden" meridional reflections, thought to derive from a perturbation from helical symmetry in the wild-type filament, are weaker or absent in the transforms of the cMyBP-C(-/-) myocardial thick filaments. In addition, the cross-bridge array in the absence of cMyBP-C appears more easily disordered.