Equatorial Reflections Research Articles

Numerical investigation of the late-time tails of the solutions of the Fackerell–Ipser equation

The late-time behaviour of the solutions of the Fackerell–Ipser equation (which is a wave equation for the spin-zero component of the electromagnetic field strength tensor) on the closure of the domain of outer communication of sub-extremal Kerr spacetime is studied numerically. Within the Kerr family, the case of Schwarzschild background is also considered. Horizon-penetrating compactified hyperboloidal coordinates are used, which allow the behaviour of the solutions to be observed at the event horizon and at future null infinity as well. For the initial data, pure multipole configurations that have compact support and are either stationary or non-stationary are taken. It is found that with such initial data the solutions of the Fackerell–Ipser equation converge at late times either to a known static solution (up to a constant factor) or to zero. As the limit is approached, the solutions exhibit a quasinormal ringdown and finally a power-law decay. The exponents characterizing the power-law decay of the spherical harmonic components of the field variable are extracted from the numerical data for various values of the parameters of the initial data, and based on the results a proposal for a Price’s law relevant to the Fackerell–Ipser equation is made. Certain conserved energy and angular momentum currents are used to verify the numerical implementation of the underlying mathematical model. In the construction of these currents a discrete symmetry of the Fackerell–Ipser equation, which is the product of an equatorial reflection and a complex conjugation, is also taken into account.

Observational features of reflection asymmetric black holes

The Kerr spacetime is symmetric with respect to a well-defined equatorial plane. When testing the equatorial reflection symmetry of an isolated black hole, one is at the same time testing the Kerr hypothesis in General Relativity. In this work, we investigate the possible observational features when a Keplerian disk is surrounding a rotating black hole without reflection symmetry. When such symmetry is broken, generically, the photon trajectories around the black hole and the Keplerian orbits on the accretion disk are distorted vertically away from the equatorial plane by an amount that depends on their distance to the black hole. In the reflection asymmetric spacetime we are considering, these two kinds of orbits are distorted in opposite directions. Interestingly, while the size and shape of black hole shadows closely resemble those of Kerr black holes, distinct observational characteristics can emerge in the disk image and emission line profiles. When observing the disk edge-on, a pronounced concave shape may appear along its innermost edge on the incoming side. Furthermore, distinctive horn-like features might be observed on the spectral line profile at the blue-shifted side. These special features can serve as compelling indicators of the reflection asymmetry present in rotating black holes.

Activation of the myosin motors in fast-twitch muscle of the mouse is controlled by mechano-sensing in the myosin filaments.

Myosin motors in resting muscle are inactivated by folding against the backbone of the myosin filament in an ordered helical array and must be released from that conformation to engage in force generation. Time‐resolved X‐ray diffraction from single fibres of amphibian muscle showed that myosin filament activation could be inhibited by imposing unloaded shortening at the start of stimulation, suggesting that filaments were activated by mechanical stress. Here we improved the signal‐to‐noise ratio of that approach using whole extensor digitorum longus muscles of the mouse contracting tetanically at 28°C. Changes in X‐ray signals associated with myosin filament activation, including the decrease in the first‐order myosin layer line associated with the helical motor array, increase in the spacing of a myosin‐based reflection associated with packing of myosin tails in the filament backbone, and increase in the ratio of the 1,1 and 1,0 equatorial reflections associated with movement of motors away from the backbone, were delayed by imposing 10‐ms unloaded shortening at the start of stimulation. These results show that myosin filaments are predominantly activated by filament stress, as in amphibian muscle. However, a small component of filament activation at zero load was detected, implying an independent mechanism of partial filament activation. X‐ray interference measurements indicated a switch‐like change in myosin motor conformation at the start of force development, accompanied by transient disordering of motors in the regions of the myosin filament near its midpoint, suggesting that filament zonal dynamics also play a role in its activation. Key points Activation of myosin filaments in extensor digitorum longus muscles of the mouse is delayed by imposing rapid shortening from the start of stimulation.Stress is the major mechanism of myosin filament activation in these muscles, but there is a small component of filament activation during electrical stimulation at zero stress.Myosin motors switch rapidly from the folded inhibited conformation to the actin‐attached force‐generating conformation early in force development.

Abstract P1056: Stretch Reduces Inter-filament Spacing And Triggers Alterations In Thick-filament Structure In Healthy Left-atrial Fibers

Introduction: Left-atrial dilatation is thought to play a key pathological role in facilitating remodeling in both genetic and acquired cardiac myopathies, as well as enabling the onset and maintenance of atrial fibrillation. However, despite its likely role in disease, the structural responses of the atria to stretch remain understudied. The myofilament responses to stretch have been extensively studied in ventricular tissue, where strain-dependent recruitment of reserve, OFF-state, myosin heads from the thick-filament contribute to length dependent activation (LDA, i.e., greater forcer at longer sarcomere lengths). In these experiments, the effects of stretch in both atrial sarcomere and thick-filament structure were evaluated. Methods: Myofilament structure was evaluated via x-ray diffraction at two sarcomere lengths (SL, SHORT: 2.1 μm, and LONG: 2.3 μm) in relaxed left-atrial and left-ventricular myocardial skinned fibers from Yucatan mini-pigs. In these experiments, both equatorial and meridional reflections reflecting sarcomeric structure and ordering were evaluated. In addition, the biomechanical effects of stretch were also evaluated in both skinned porcine fibers and engineered heart tissues (EHT) from atrial and ventricular hIPSc-derived myocytes via a proprietary system (MyoPod; Propria Biosciences). Results: In both atrial skinned fibers and EHTs, stretch triggered moderate increases in force, demonstrating LDA. LA fibers, stretch decreased lattice spacing but failed to alter the I 11 /I 10 equatorial intensity ratio, suggesting negligible length-dependent recruitment of OFF-state myosin heads from the thick-filament. LA fibers, however, tended to have higher I 11 /I 10 values at short SL (vs. LV fibers) suggesting an intrinsically higher population of ON-state myosin heads. In addition, stretch triggered alterations in thick filament ordering in both LA and LV fibers, as reflected by changes in both the 3 rd and 6 th order meridional reflections (M3 and M6), suggesting effects in both myosin heads and thick-filament backbone. Conclusion: Atrial stretch results in marked sarcomere structural alterations, reducing both inter-filament spacing and thick-filament ordering that likely contribute to length dependent activation. The role of these alteration in atrial pathophysiology remains to be studied

Testing black hole equatorial reflection symmetry using Sgr A* shadow images

The recently released images of the supermassive black holes in the M87 galaxy and the galaxy of our own make probing black hole spacetimes and testing general relativity (GR) possible. The violation of equatorial reflection symmetry of black hole spacetimes is clearly a smoking gun of physics beyond GR. In this paper, we place constraints on the violation of black hole reflection symmetry using the bounds on the ring size of the $\mathrm{Sgr}\text{ }{\mathrm{A}}^{*}$ black hole images, which are consistent with the critical curve radius predicted in GR within $\ensuremath{\sim}10%$. Adopting a theory-agnostic framework, we consider a Kerr-like metric in which the violation of reflection symmetry is parametrized by a single parameter, without altering the multipoles of the black hole. The critical curves are always vertically symmetric due to the existence of hidden symmetry associated with the Killing tensor, even though the spacetime is reflection asymmetric. We find that the size of critical curves is sensitive to the amount of reflection symmetry being violated, and place the first constraints on the parameter space of the model using the $\mathrm{Sgr}\text{ }{\mathrm{A}}^{*}$ images.

Curved accretion disks around rotating black holes without reflection symmetry

Rotating black holes without equatorial reflection symmetry can naturally arise in effective low-energy theories of fundamental quantum gravity, in particular, when parity-violating interactions are introduced. Adopting a theory-agnostic approach and considering a recently proposed Kerr-like black hole model, we investigate the structure and properties of accretion disk around a rotating black hole without reflection symmetry. In the absence of reflection symmetry, the accretion disk is in general a curved surface in shape, rather than a flat disk lying on the equatorial plane. Furthermore, the parameter epsilon that controls the reflection asymmetry would shrink the size of the prograde innermost stable circular orbits, and enhance the efficiency of the black hole in converting rest-mass energy to radiation during accretion. The retrograde innermost stable circular orbits are stretched but the effects are substantially suppressed. In addition, we find that spin measurements based on the gravitational redshift observations of the disk, assuming a Kerr geometry, may overestimate the true spin values if the central object is actually a Kerr-like black hole with conspicuous equatorial reflection asymmetry. The qualitative results that the accretion disk becomes curved and the prograde innermost stable circular orbits are shrunk turn out to be generic in our model when the reflection asymmetry is small.

Myofibril orientation as a metric for characterizing heart disease

Myocyte disarray is a hallmark of many cardiac disorders. However, the relationship between alterations in the orientation of individual myofibrils and myofilaments to disease progression has been largely underexplored. This oversight has predominantly been because of a paucity of methods for objective and quantitative analysis. Here, we introduce a novel, less-biased approach to quantify myofibrillar and myofilament orientation in cardiac muscle under near-physiological conditions and demonstrate its superiority as compared with conventional histological assessments. Using small-angle x-ray diffraction, we first investigated changes in myofibrillar orientation at increasing sarcomere lengths in permeabilized, relaxed, wild-type mouse myocardium from the left ventricle by assessing the angular spread of the 1,0 equatorial reflection (angle σ). At a sarcomere length of 1.9 μm, the angle σ was 0.23 ± 0.01 rad, decreased to 0.19 ± 0.01 rad at a sarcomere length of 2.1 μm, and further decreased to 0.15 ± 0.01 rad at a sarcomere length of 2.3 μm (p < 0.0001). Angle σ was significantly larger in R403Q, a MYH7 hypertrophic cardiomyopathy model, porcine myocardium (0.24 ± 0.01 rad) compared with wild-type myocardium (0.14 ± 0.005 rad; p < 0.0001), as well as in human heart failure tissue (0.19 ± 0.006 rad) when compared with nonfailing samples (0.17 ± 0.007 rad; p = 0.01). These data indicate that diseased myocardium suffers from greater myofibrillar disorientation compared with healthy controls. Finally, we showed that conventional, histology-based analysis of disarray can be subject to user bias and/or sampling error and lead to false positives. Our method for directly assessing myofibrillar orientation avoids the artifacts introduced by conventional histological approaches that assess myocyte orientation and only indirectly evaluate myofibrillar orientation, and provides a precise and objective metric for phenotypically characterizing myocardium. The ability to obtain excellent x-ray diffraction patterns from frozen human myocardium provides a new tool for investigating structural anomalies associated with cardiac diseases.

Abstract P350: Myofibril Orientation In Cardiac Muscle And Its Implication For Heart Disease

Rationale: Myocyte disarray is a hallmark of cardiomyopathy. However, the orientation of individual myofibrils and myofilaments and how their alignment may be altered in disease progression have been largely underexplored. This oversight has been predominantly due to a paucity of methods for objective and quantitative analysis. Objective: To introduce a novel, less-biased approach to quantify myofibrillar and myofilament orientation in cardiac muscle under near physiological conditions and demonstrate its superiority versus traditional histological assessments. Methods and Results: Using small-angle X-ray diffraction, we first investigated changes in myofibrillar orientation at increasing sarcomere lengths in skinned, relaxed, wildtype mouse myocardium by assessing the angular spread of the 1,0 equatorial reflection (angle σ). At a sarcomere length (SL) of 1.9 μm, the angle σ was 0.23±0.01 rad, decreased to 0.19±0.01 rad at a SL of 2.1 μm, and further decreased to 0.15±0.01 rad at a SL of 2.3 μm (p&lt;0.0001). Angle σ was significantly larger in R403Q (a MYH7 HCM model) porcine myocardium (0.24±0.01 rad) compared to WT myocardium (0.14±0.005 rad, p&lt;0.0001) as well as in biopsied human heart failure tissue (0.19±0.006 rad) when compared to non-failing samples (0.17±0.007 rad, p=0.01). These data indicate that diseased myocardium suffers from myofibrillar disorientation compared to healthy controls. Finally, using control samples, we showed that traditional, histological-based analysis of disarray can be subject to user bias and/or sampling error and lead to false positives. Conclusions: Our method for assessing myofibrillar orientation limits the artifacts introduced by traditional histological processing and provides a precise and objective metric for phenotypically characterizing myocardium. The ability to obtain excellent X-ray diffraction patterns from frozen, biopsied human myocardium opens up new avenues of inquiry regarding the relation of myofibrillar structure to function in health and disease.

Effect of Active Lengthening and Shortening on Small-Angle X-ray Reflections in Skinned Skeletal Muscle Fibres.

Our purpose was to use small-angle X-ray diffraction to investigate the structural changes within sarcomeres at steady-state isometric contraction following active lengthening and shortening, compared to purely isometric contractions performed at the same final lengths. We examined force, stiffness, and the 1,0 and 1,1 equatorial and M3 and M6 meridional reflections in skinned rabbit psoas bundles, at steady-state isometric contraction following active lengthening to a sarcomere length of 3.0 µm (15.4% initial bundle length at 7.7% bundle length/s), and active shortening to a sarcomere length of 2.6 µm (15.4% bundle length at 7.7% bundle length/s), and during purely isometric reference contractions at the corresponding sarcomere lengths. Compared to the reference contraction, the isometric contraction after active lengthening was associated with an increase in force (i.e., residual force enhancement) and M3 spacing, no change in stiffness and the intensity ratio I1,1/I1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I1,1/I1,0, M3 and M6 spacings, and M3 intensity. This suggests that residual force enhancement is achieved without an increase in the proportion of attached cross-bridges, and that force depression is accompanied by a decrease in the proportion of attached cross-bridges. Furthermore, the steady-state isometric contraction following active lengthening and shortening is accompanied by an increase in cross-bridge dispersion and/or a change in the cross-bridge conformation compared to the reference contractions.

Insights into strain-induced solid mesophases in melt-spun polymer fibers

This article provides an overview of strain-induced solid mesophases in polymer fibers, melt-spun from amorphous (polycarbonate, cyclo-olefin polymer, copolyamide, polyethylene terephthalate glycol) and semi-crystalline materials (polyethylene terephthalate, poly-3-hydroxybutyrate). The discussed mesophases are made of stretched, conformationally disordered macromolecules that are highly oriented along the fiber axis, and that reveal variations in their lateral spacing. Fitting algorithms for respective wide-angle x-ray diffraction (WAXD) patterns are put forward, and are directly tested on fibers melt-drawn from amorphous as well as semi-crystalline polymers. Equatorial, meridional, off-axis and azimuthal profiles are extracted from WAXD patterns and are simultaneously fitted. Best fit parameters contain detailed structural information, and lead to simulated 2D WAXD patterns, which in turn can be used to quantify the phases that are present in the fibers. We postulate that mesophases, which lead to broad equatorial reflections in WAXD patterns, can be found in all types of drawn melt-spun polymer fibers.

Possible connection between the reflection symmetry and existence of equatorial circular orbit

We study a viable connection between the circular-equatorial orbits and reflection symmetry across the equatorial plane of a vacuum stationary axis-symmetric spacetime in general relativity. The behavior of the circular equatorial orbits in the direction perpendicular to the equatorial plane is studied, and different outcomes in the presence and in the absence of the reflection symmetry are discussed. We conclude that in the absence of the equatorial reflection symmetry neither stable nor unstable circular orbit can exist on the equatorial plane. Moreover, to address the observational aspects, we provide two possible examples relating gravitational wave astronomy and the thin accretion disk which can put constraints on the symmetry breaking parameters.

Brill Transition in Nylons: The Structural Scenario()

Pleated and rippled sheet crystal structures of even–even nylons that feature twisted amide group–aliphatic part links were introduced in the previous contribution (Macromolecules, 2021, 54, DOI: 10.1021/acs.macromol.0c02404). Pleated sheets are made of isochiral conformational isomeric stems, and rippled sheets are racemic stereocomplexes of enantiomeric stem conformations. These models are used to reanalyze the various and puzzling manifestations of the Brill transition first reported in 1942. The transition is generally considered to result from the generation of gauche bonds in the aliphatic parts of the nylon α phase which travel to the junctions with the amide groups. Recognition of the pleated/rippled sheet structures suggests a different scenario that takes into account, but challenges, this analysis. The transition takes place in pleated/rippled sheets with twisted chain conformations. It is a dynamic interconversion between the two possible conformers (mirror images) of the stems in the crystal lattice. The interconversion timescale is in the ≈100 picosecond range, as determined by NMR. During the interconversion, the aliphatic part C–C–C plane flips by ≈120° (from + to −60°). The intrasheet H bonds are preserved, but transitory intersheet H bonds may be formed, which accounts for the frequent but not systematic pseudo-hexagonal cell of the Brill structure. This scenario is fully consistent (and more so than the earlier models) with the wide body of experimental data available on the Brill transition. The interconversion between conformers is generic for all types of nylons (even, even–even, etc.) and is therefore a valid scenario for their Brill transitions as well. On this basis, the Brill transition should be defined by a molecular process (the conformation interconversion) rather than by an experimental criterion that lacks general validity (the merging of equatorial reflections).

Note on equatorial geodesics in circular spacetimes

General results on equatorial geodesics are exposed in the case of circular spacetimes featuring an equatorial reflection symmetry. The way the geodesic equation equivalently rewrites in terms of an effective potential is explicitly recalled for circular and non-circular equatorial geodesics. This provides a practical tool to discuss their stability. Statements are illustrated in Kerr spacetime.

Tensile study of melt-spun poly(3-hydroxybutyrate) P3HB fibers: Reversible transformation of a highly oriented phase

Poly-3-hydroxybutyrate (P3HB) typically crystallizes into the orthorhombic α-form. The additional intense and rather broad equatorial reflection in wide-angle X-ray diffraction (WAXD) patterns seen in our melt-spun P3HB fibers cannot be attributed to the α-form crystals. We propose that a non-crystalline mesophase, which consists of disordered but highly oriented and stretched molecules located between α-crystals, leads to the observed broad equatorial reflection. We show that the transformation of this mesophase from and into the α-form phase is partially reversible under cyclic tensile loading. Structure factor calculations, which are based on atomic positions from molecular dynamics simulations of a set of stretched P3HB molecules, support this model. The WAXD patterns were analyzed with azimuthal, radial, equatorial and meridional scans and the changes in the crystal orientation, changes in the percentages of individual phases and changes in the α-crystal lattice spacings were analyzed as a function of the applied tension. Changes in long spacings, crystal sizes and coherence lengths under cyclic loading were determined with small-angle X-ray scattering (SAXS) measurements. The long spacing between α-crystals increases when tensile stress is applied and it snaps back to the original spacing when the tensile stress is released.

Amyloid-Like Fibrillary Morphology Originated by Tyrosine-Containing Aromatic Hexapeptides.

Phenylalanine-based nanostructures have attracted the attention of the material science community for their functional properties. These properties strongly depend on the hierarchic organization of the nanostructure that in turn can be finely tuned by punctual chemical modifications of the building blocks. Herein, we investigate how the partial or the complete replacement of the Phe residues in PEG8 -(Phe)6 (PEG8 -F6) with tyrosines to generate PEG8 -(Phe-Tyr)3 (PEG8 -(FY)3) or PEG8 -(Tyr)6 (PEG8 -Y6) affects the structural/functional properties of the nanomaterial formed by the parental compound. Moreover, the effect of the PEG derivatization was evaluated through the characterization of the peptides without the PEG moiety (Tyr)6 (Y6) and (Phe-Tyr)3 ((FY)3). Both PEG8 -Y6 and PEG8 -(FY)3 can self-assemble in water at micromolar concentrations in β-sheet-rich nanostructures. However, WAXS diffraction patterns of these compounds present significant differences. PEG8 -(FY)3 shows a 2D WAXS oriented fiber diffraction profile characterized by the concomitant presence of a 4.7 Å meridional and a 12.5 Å equatorial reflection that are generally associated with cross-β structure. On the other hand, the pattern of PEG8 -Y6 is characterized by the presence of circles typically observed in the presence of PEG crystallization. Molecular modeling and dynamics provide an atomic structural model of the peptide spine of these compounds that is in good agreement with WAXS experimental data. Gelation phenomenon was only detected for PEG8 -(FY)3 above a concentration of 1.0 wt % as confirmed by storage (G'≈100 Pa) and loss (G''≈28 Pa) moduli in rheological studies. The cell viability on CHO cells of this soft hydrogel was certified to be 90 % after 24 hours of incubation.

Effects of Myosin Inhibitors on the X-Ray Diffraction Patterns of Relaxed and Calcium-Activated Rabbit Skeletal Muscle Fibers

Two potent myosin inhibitors, N-benzyl-p-toluenesulfonamide (BTS) and blebbistatin, suppress the contractile force of skeletal muscle fibers at micromolar concentrations. To gain insights into the mechanism of inhibition by these chemicals, we studied their effects on the X-ray diffraction patterns from rabbit skeletal muscle fibers under relaxing and activating conditions. At 100 µM, both inhibitors almost completely suppress the contractile force, but still substantial changes were observed in the diffraction patterns upon calcium activation. (1) The 2nd actin layer line reflection was enhanced normally, indicating that calcium binding to troponin and the subsequent movement of tropomyosin are not inhibited, (2) the myosin layer line reflections became much weaker, and (3) the 1,1/1,0 intensity ratio of the equatorial reflections was increased. The observations (2) and (3) indicate that, even in the presence of the inhibitors at a high concentration, myosin heads leave the helix on the myosin filaments and approach the actin filaments. Interestingly, the d1,0 spacing of the filament lattice remains unchanged upon activation of inhibited fibers, in contrast to the case of normal activation in which the spacing is decreased. This suggests that the normal activated myosin heads exert a pull in both axial and radial directions, but in the presence of the inhibitors, the pull is suppressed, and as a result, the heads simply bind to actin without exerting any force. The results support the idea that the inhibitors do not block the myosin binding to actin, but block the step of force-producing transition of the bound actomyosin complex.

The Structure of the Infectious Prion Protein Constrains Potential Prion Replication Mechanisms

X-ray fiber diffraction and cryo electron microscopy revealed that the infectious prion protein, PrPSc, contains a four-rung beta-solenoid structure as a key element of its self-propagating conformation. In particular, amyloid fibrils with their regular and repetitive nature allowed important insights into the structure of PrPSc. X-ray fiber diffraction revealed a height of 19.2 Å per molecule of PrPSc along the fibril axis. Meridional diffraction signals at 4.8, 6.4, and 9.6 Å indicated four rungs of beta structure (4 x 4.8 Å = 19.2 Å), while the lack of a ∼10 Å equatorial reflection implied a beta-solenoid configuration. Unprocessed cryo EM micrographs of PrPSc fibrils displayed 4.8 Å spacings, confirming the cross-beta structure. 3D reconstructions of individual PrPSc fibrils indicated a molecular height of ∼17.7 Å, in good agreement with the X-ray fiber diffraction results. Moreover, single particle averaging of short fibril segments revealed periodicities of ∼2 and ∼4 nm. The first measurement is consistent with the molecular height of PrPSc, while the latter suggests a higher order such as a head-to-head dimerization. The four-rung beta-solenoid structure provides insights into the PrPC to PrPSc conversion mechanism. The folding properties of regular beta-solenoid proteins revealed a nearly universal occurrence of “capping structures” that prevent the propagation of the beta-solenoid conformation. These caps consist of short alpha-helices, loops, or other structures and their removal resulted in aggregation and amyloid formation. The absence of capping structures in PrPSc allows the propagation of its beta-solenoid conformation to neighboring PrPC molecules. The putative head-to-head arrangement of the PrPSc dimers implies a role for matching sequences in the conversion mechanism. The fidelity of the templating process, which is a hallmark of the PrPSc propagation, can thus be attributed to molecular properties of the PrPSc structure.

Effects of myosin inhibitors on the X-ray diffraction patterns of relaxed and calcium-activated rabbit skeletal muscle fibers.

We studied the effect of myosin inhibitors, N-benzyl-p-toluenesulfonamide (BTS), blebbistatin, and butanedione monoxime (BDM) on X-ray diffraction patterns from rabbit psoas fibers under relaxing and contracting conditions. The first two inhibitors suppressed the contractile force almost completely at a 100 μM concentration, and a similar effect was obtained at 50 mM for BDM. However, still substantial changes were observed in the diffraction patterns upon calcium-activation of inhibited muscle fibers. (1) The 2nd actin layer-line reflection was enhanced normally, indicating that calcium binding to troponin and the subsequent movement of tropomyosin are not inhibited, (2) the myosin layer-line reflections became much weaker, and (3) the 1,1/1,0 intensity ratio of the equatorial reflections was increased. The observations (2) and (3) indicate that, even in the presence of the inhibitors at a saturating concentration, myosin heads leave the helix on the thick filaments and approach the thin filaments. Interestingly, the d1,0 spacing of the filament lattice remained unchanged upon activation of inhibited fibers, in contrast to the case of normal activation in which the spacing is decreased. This suggests that the normal activated myosin heads exert a pull in both axial and radial directions, but in the presence of the inhibitors, the pull is suppressed, and as a result, the heads simply bind to actin without exerting any force. The results support the idea that the inhibitors do not block the myosin binding to actin, but block the step of force-producing transition of the bound actomyosin complex.

The tymbal muscle of cicada has flight muscle-type sarcomeric architecture and protein expression

BackgroundThe structural and biochemical features of the tymbal (sound-producing) muscle of cicadas were studied by X-ray diffraction and immunochemistry, and compared with those of flight muscles from the same species.ResultsThe X-ray diffraction pattern of the tymbal muscle was very similar to that of the dorsal longitudinal flight muscle: In both muscles, the 2,0 equatorial reflection is much more intense than the 1,1, indicating that both muscles have a flight muscle-type myofilament lattice. In rigor, the first myosin/actin layer line reflection was finely lattice-sampled, indicating that the contractile proteins are arranged with a crystalline regularity as in asynchronous flight muscles. In contrast, the diffraction pattern from the tensor muscle, which modulates the sound by stressing the tymbal, did not show signs of such high regularity or flight muscle-type filament lattice. Electrophoretic patterns of myofibrillar proteins were also very similar in the tymbal muscle and flight muscles, but distinct from those from the tensor or leg muscles. The antibody raised against the flight muscle-specific troponin-I isoform reacted with an 80-kDa band from both tymbal and flight muscles, but with none of the bands from the tensor or leg muscles.ConclusionThe close similarities of the structural and biochemical profiles between the tymbal and the flight muscles suggest the possibility that a set of flight muscle-specific proteins is diverted to the tymbal muscle to meet its demand for fast, repetitive contractions.

The A2V mutation as a new tool for hindering A\u03b2 aggregation: A neutron and x-ray diffraction study

We have described a novel C-to-T mutation in the APP gene that corresponds to an alanine to valine substitution at position 673 in APP (A673V), or position 2 of the amyloid-β (Aβ) sequence. This mutation is associated with the early onset of AD-type dementia in homozygous individuals, whereas it has a protective effect in the heterozygous state. Correspondingly, we observed differences in the aggregation properties of the wild-type and mutated Aβ peptides and their mixture. We have carried out neutron diffraction (ND) and x-ray diffraction (XRD) experiments on magnetically-oriented fibers of Aβ1-28WT and its variant Aβ1-28A2V. The orientation propensity was higher for Aβ1-28A2V suggesting that it promotes the formation of fibrillar assemblies. The diffraction patterns by Aβ1-28WT and Aβ1-28A2V assemblies differed in shape and position of the equatorial reflections, suggesting that the two peptides adopt distinct lateral packing of the diffracting units. The diffraction patterns from a mixture of the two peptides differed from those of the single components, indicating the presence of structural interference during assembly and orientation. The lowest orientation propensity was observed for a mixture of Aβ1-28WT and a short N-terminal fragment, Aβ1-6A2V, which supports a role of Aβ’s N-terminal domain in amyloid fibril formation.