Identification of a region susceptible to proteolysis in myosin subfragment-2

Tetrasodium pyrophosphate promotes light meromyosin crosslinking by microbial transglutaminase

Phosphates are commonly included in meat processing, where oxidation is inevitable, to improve water binding. This present study attempted to reveal the interactive roles of protein oxidation and tetrasodium pyrophosphate (TSPP) on the crosslinking pattern of myosin mediated by transglutaminase (TGase). Mild oxidation at 1 mM H₂O₂ facilitated the TGase-initiated crosslinking, with the dominate crosslinking site shifted from S1 (in nonoxidized myosin) to Rod. The introduction of TSPP alleviated the oxidation stress on proteins, and was conductive to the crosslinking reaction notably at the LMM domain. The crosslinking sites in untreated myosin were identified as Gln-613 (S1) and Gln-1498 (LMM) by amino-acid sequence analysis, while strongly oxidation resulted in the loss of Gln-1498. Contrastively, four new reactive crosslinking sites were generated by TSPP, one (Gln-558/Gln-567) located on S1 and three (Gln-1362, Gln-1374, and Gln-1423/Gln-1426) on LMM. Yet, Gln-1362 was eliminated under strong oxidation at 50 mM H₂O₂.

Heavy meromyosin molecules extending more than 50 nm above adsorbing electronegative surfaces

In the in vitro motility assay, actin filaments are propelled by surface-adsorbed myosin motors, or rather, myosin motor fragments such as heavy meromyosin (HMM). Recently, efforts have been made to develop actomyosin powered nanodevices on the basis of this assay but such developments are hampered by limited understanding of the HMM adsorption geometry. Therefore, we here investigate the HMM adsorption geometries on trimethylchlorosilane- [TMCS-] derivatized hydrophobic surfaces and on hydrophilic negatively charged surfaces (SiO(2)). The TMCS surface is of great relevance in fundamental studies of actomyosin and both surface substrates are important for the development of motor powered nanodevices. Whereas both the TMCS and SiO(2) surfaces were nearly saturated with HMM (incubation at 120 microg mL(-1)) there was little actin binding on SiO(2) in the absence of ATP and no filament sliding in the presence of ATP. This contrasts with excellent actin-binding and motility on TMCS. Quartz crystal microbalance with dissipation (QCM-D) studies demonstrate a HMM layer with substantial protein mass up to 40 nm above the TMCS surface, considerably more than observed for myosin subfragment 1 (S1; 6 nm). Together with the excellent actin transportation on TMCS, this strongly suggests that HMM adsorbs to TMCS mainly via its most C-terminal tail part. Consistent with this idea, fluorescence interference contrast (FLIC) microscopy showed that actin filaments are held by HMM 38 +/- 2 nm above the TMCS-surface with the catalytic site, on average, 20-30 nm above the surface. Viewed in a context with FLIC, QCM-D and TIRF results, the lack of actin motility and the limited actin binding on SiO(2) shows that HMM adsorbs largely via the actin-binding region on this surface with the C-terminal coiled-coil tails extending >50 nm into solution. The results and new insights from this study are of value, not only for the development of motor powered nanodevices but also for the interpretation of fundamental biophysical studies of actomyosin function and for the understanding of surface-protein interactions in general.

Cross-bridge movement in muscle and the conformation of the myosin hinge

The force-generating mechanism in muscle is discussed and it is shown that a helix-coil transition in the S-2 link of the cycling cross-bridge is compatible with the physical and chemical properties of this region of the myosin molecule. Thermal melting and temperature-jump experiments are described demonstrating that the light meromyosin-heavy meromyosin (LMM-HMM) hinge domain of S-2 is a segment of low thermal stability. This region can undergo alpha-helix-random coil transitions on a time-scale comparable to the quick-recovery tension transient observed when isometrically contracting muscle is abruptly shortened or stretched. Cross-linking and enzyme probe studies of glycerinated muscle fibres and myofibrils in resting, rigor and activating solvents suggest that the polypeptide chains within the hinge region of S-2 undergo a conformational transition to a more open, proteolytically sensitive structure when the S-2 link is released from the thick filament surface.

Passive stiffness in Drosophila indirect flight muscle reduced by disrupting paramyosin phosphorylation, but not by embryonic myosin S2 hinge substitution

High passive stiffness is one of the characteristic properties of the asynchronous indirect flight muscle (IFM) found in many insects like Drosophila. To evaluate the effects of two thick filament protein domains on passive sarcomeric stiffness, and to investigate their correlation with IFM function, we used microfabricated cantilevers and a high resolution imaging system to study the passive IFM myofibril stiffness of two groups of transgenic Drosophila lines. One group (hinge-switch mutants) had a portion of the endogenous S2 hinge region replaced by an embryonic version; the other group (paramyosin mutants) had one or more putative phosphorylation sites near the N-terminus of paramyosin disabled. Both transgenic groups showed severely compromised flight ability. In this study, we found no difference (compared to the control) in passive elastic modulus in the hinge-switch group, but a 15% reduction in the paramyosin mutants. All results were corroborated by muscle fiber mechanics experiments performed on the same lines. The fact that myofibril elasticity is unaffected by hinge switching implies alternative S2 hinges do not critically affect passive sarcomere stiffness. In contrast, the mechanical defects observed upon disrupting paramyosin phosphorylation sites in Drosophila suggests that paramyosin phosphorylation is important for maintaining high passive stiffness in IFM myofibrils, probably by affecting paramyosin's interaction with other sarcomeric proteins.

Solution structure of heavy meromyosin by small-angle scattering

Elucidation of x-ray crystal structures for the S1 subfragment of myosin afforded atomic resolution of the nucleotide and actin binding sites of the enzyme. The structures have led to more detailed hypotheses regarding the mechanisms by which force generation is coupled to ATP hydrolysis. However, the three-dimensional structure of double-headed myosin consisting of two S1 subfragments has not yet been solved. Therefore, to investigate the overall shape and relative orientations of the two heads of myosin, we performed small-angle x-ray and neutron scattering measurements of heavy meromyosin containing all three light chains (LC(1-3)) in solution. The resulting small-angle scattering intensity profiles were best fit by models of the heavy meromyosin head-tail junction in which the angular separation between heads was less than 180 degrees. The S1 heads of the best fit models are not related by an axis of symmetry, and one of the two S1 heads is bent back along the rod. These results provide new information on the structure of the head-tail junction of myosin and indicate that combining scattering measurements with high resolution structural modeling is a feasible approach for investigating myosin head-head interactions in solution.

Mutations in beta-myosin S2 that cause familial hypertrophic cardiomyopathy (FHC) abolish the interaction with the regulatory domain of myosin-binding protein-C

The myosin filaments of striated muscle contain a family of enigmatic myosin-binding proteins (MyBP), MyBP-C and MyBP-H. These modular proteins of the intracellular immunoglobulin superfamily contain unique domains near their N termini. The N-terminal domain of cardiac MyBP-C, the MyBP-C motif, contains additional phosphorylation sites and may regulate contraction in a phosphorylation dependent way. In contrast to the C terminus, which binds to the light meromyosin portion of the myosin rod, the interactions of this domain are unknown. We demonstrate that fragments of MyBP-C containing the MyBP-C motif localise to the sarcomeric A-band in cardiomyocytes and isolated myofibrils, without affecting sarcomere structure. The binding site for the MyBP-C motif resides in the N-terminal 126 residues of the S2 segment of the myosin rod. In this region, several mutations in beta-myosin are associated with FHC; however, their molecular implications remained unclear. We show that two representative FHC mutations in beta-myosin S2, R870H and E924K, drastically reduce MyBP-C binding (Kd approximately 60 microM for R870H compared with a Kd of approximately 5 microM for the wild-type) down to undetectable levels (E924K). These mutations do not affect the coiled-coil structure of myosin. We suggest that the regulatory function of MyBP-C is mediated by the interaction with S2, and that mutations in beta-myosin S2 may act by altering the interactions with MyBP-C.

Location of the head-tail junction of myosin

The tails of double-headed myosin molecules consist of an alpha-helical/coiled-coil structure composed of two identical polypeptides with a heptad repeat of hydrophobic amino acids that starts immediately after a conserved proline near position 847. Both muscle and nonmuscle myosins have this heptad repeat and it has been assumed that proline 847 is physically located at the head-tail junction. We present two lines of evidence that this assumption is incorrect. First, we localized the binding sites of several monoclonal antibodies on Acanthamoeba myosin-II both physically, by electron microscopy, and chemically, with a series of truncated myosin-II peptides produced in bacteria. These data indicate that the head-tail junction is located near residue 900. Second, we compared the lengths of two truncated recombinant myosin-II tails with native myosin-II. The distances from the NH2 termini to the tips of these short tails confirms the rise per residue (0.148 nm/residue) and establishes that the 86-nm tail of myosin-II must start near residue 900. We propose that the first 53 residues of heptad repeat of Acanthamoeba myosin-II and other myosins are located in the heads and the proteolytic separation of S-1 from rod occurs within the heads.

Possible role of helix-coil transitions in the microscopic mechanism of muscle contraction

Local helix-coil transitions in the coiled coil portion of myosin have long been implicated as a possible origin of tension generation in muscle. From a statistical mechanical theory of conformational transitions in coiled coils, the free energy required to form a randomly coiled bubble in the hinge region of myosin of the type conjectured by Harrington (Harrington, W. F., 1979, Proc. Natl. Acad. Sci. USA, 76:5066-5070) is estimated to be approximately 25 kcal/mol. Unfortunately this is far more than the free energy available from ATP hydrolysis if the crossbridges operate independently. Thus, in solution such bubbles are predicted to be absent, and the theory requires that the rod portion of myosin be a hingeless, continuously deforming rod. While such bubble formation in vivo cannot be entirely ruled out, it appears to be unlikely. We further conjecture that in solution the swivel located between myosin subfragments 1 and 2 (S-2 and S-1) is due to a locally random conformation of the chains caused by the presence of a proline residue at the point that physically separates the coiled coil from the globular portion of myosin. On attachment of S-1 to actin in the strong binding state, the configurational entropy of the random coil in the swivel region is greatly reduced relative to the case where the ends are free. This produces a spontaneous coil-to-helix transition in the swivel region that causes rotation of S-1 and the translation of actin. Thus, the model predicts that the actin filaments are pushed rather than pulled past the thick filaments by the crossbridges. The specific mechanism of force generation is examined in detail, and a simple statistical mechanical realization of the model is proposed. We find that the model gives a substantial number of qualitative and at times quantitative predictions in accord with experiment, and is particularly appealing in that it provides a simple means of free energy transduction--the well known fact that topological constraints shift the equilibrium between helical and random coil states.

Two chymotrypsin-susceptible sites of myosin rod from chicken gizzard

The rod prepared from chicken gizzard myosin has been found to have two sites sensitive to limited digestion with chymotrypsin; these sites were located at a subfragment 2/light meromyosin junction (site 1), and at a site 10 kDa remote from either C-terminal or N-terminal of light meromyosin (site 2). The site 1 was more sensitive to the digestion than the site 2. The cleavage at site 2 of the light meromyosin yielded a 74-kDa fragment that was soluble in a low ionic strength solution, contrary to the insolubility of the parent light meromyosin in the same solution. Studies on the effects of MgCl2, ATP and pH on the susceptibilities of these sites to chymotrypsin have given following results. (a) Millimolar concentrations of MgCl2 protected site 1 and site 2 from the chymotryptic cleavage. (b) The cleavage at site 1 of myosin rod in the low salt solution free of Mg2+ at pH 7.0 and pH 8.5, was not affected by the presence of 5 mM ATP. However, MgCl2-induced protection of site 1 was relieved by addition of ATP. On the other hand, the cleavage at site 2 was stimulated by addition of ATP, irrespective of the presence or absence of MgCl2. (c) The alkaline condition of pH 8.5 was more favorable for the chymotryptic cleavages at both site 1 and site 2 than the neutral condition of pH 7.0. These results suggest that myosin rod contains two flexible regions, the structures of which are influenced by such an ambient factor as MgCl2, ATP or pH.

Localization of a new proteolytic site accessible in oxidized myosin rod

We have compared the proteolysis pattern of reduced and oxidized myosin rods in which the five pairs of SH-groups were interchain crosslinked by employing CuCl2 or 5,5'-dithiobis-2-nitrobenzoate. In the tryptic digest of oxidized rod three new fragments appeared on SDS-polyacrylamide gel electrophoresis (chain masses of 100, 45, and 25 kDa). Based on the N-terminal sequences of the isolated peptides, it is concluded that oxidation creates a new cleavage site 102 residues away from the N-terminus of the rod, in the vicinity of one of the modified SH-groups (Cys-108). This observation indicates that S-S crosslinking of myosin rod leads to a local unfolding of the coiled-coil structure.

Protein structural domains in the Caenorhabditis elegans unc-54 myosin heavy chain gene are not separated by introns

The 1,966-amino acid unc-54 myosin heavy chain sequence was determined from DNA sequence studies of the cloned gene. The gene is split by eight short introns, 48-561 base pairs long, and appears to lack a "TATA" box at its promoter. The physical map of the gene was aligned with the genetic map by locating two point mutations and three internal deletions: 0.01 map units correspond to approximately 5 kilobases. Comparison of the unc-54 protein sequence with the sequence of a second myosin heavy chain from nematode, indicates that the globular head sequence S-1 is more highly conserved than the alpha-helical coiled-coil rod. Major sites of proteolysis in S-1 are associated with variable sequences that have the characteristics of surface loops. In both genes there is no correlation between the positions of introns and the major protein structural domains.

A bent monomeric conformation of myosin from smooth muscle

Smooth muscle myosin filaments formed in 0.15 M KCl are depolymerized by MgATP to a 10S component, rather than to the 6S component typical of myosin monomer in high salt concentrations. This 10S species is also monomeric as determined by sedimentation equilibrium and calculated from the diffusion and sedimentation coefficients. The conformation of 10S myosin is, however, very different from that of 6S myosin, which has a flexible but extended rod. The Stokes radius and the viscosity of 10S myosin are less than those of 6S myosin, consistent with a structure in which the rod is bent. Electron microscopy of rotary-shadowed preparations confirmed that the light meromyosin region of the rod is bent back on subfragment 2, that region of the rod adjacent to the two globular heads. MgATP and dephosphorylation of the 20,000 molecular weight light chain increase the amount of 10S myosin present in 0.15 M KCl; addition of salt converts 10S myosin back to the typical 6S conformation. We conclude that smooth muscle myosin preferentially forms a bent or folded conformation instead of the extended shape usually associated with skeletal muscle myosin, provided that the salt concentration is kept sufficiently low.

Periodic charge distributions in the myosin rod amino acid sequence match cross-bridge spacings in muscle

The amino acid sequence of the rod portion of nematode myosin, deduced for the sequence of the unc-54 heavy chain gene of Caenorhabditis elegans, is highly repetitive and has the characteristics of an alpha-helical coiled coil. The molecular surface contains alternate clusters of positive and negative charge. Interactions between charge clusters on adjacent molecules could account for the observed spacing of the myosin cross-bridges in muscle. Calculations also suggest that the N-terminal third of the rod is only loosely associated with the thick filament backbone. Bending of the rod near the end of this region could allow the N-terminal section to act as a hinged arm during muscle contraction.