M-band Structure Research Articles

Influences of citrus aurantium juice immersion and resonant vibration on the instrumental properties of different cuts from yak carcasses

The influences of citrus aurantium juice (CAJ) immersion and resonant vibration assisted by the immersion on the instrumental properties, cooking loss, vibrated weight gain of supraspinatus (SS), quadriceps femoris (QF) and longissimus thoracis (LT) muscles from yak carcasses were investigated, as well as the collagen characteristics, pyridinoline cross-links, myofibrillar MFI. In the circumstance of CAJ concentration of 1% and the ratio of meat to liquid 1:2, the shear force of SS and QF were significantly decreased by 20.3% and 20.2% (P < 0.05), and cooking loss significantly decreased by 17% and 23% (P < 0.05), respectively, compared to the control. At this combination of the concentration and the ratio of meat to liquid, the total collagen contents of SS and QF were significantly decreased by 23% and 18%, the collagen heat solubility 1.4 times and 1.5 times that of the raw meat (P < 0.05), respectively. At the pH value closed to the pI of myofibrillar proteins, the immersion assisted resonance with the amplitude of 2 mm and vibration duration of 1 h made the LT shear force to be decreased by 12%, 7.8% and 10.9% than the control, 0.5 h and 1.5 h of vibration duration (P < 0.05), respectively. Furthermore, the ultrastructure in LT gave rise to obvious alteration due to the CAJ immersion and its assisted resonance. The present study has confirmed again that the resonance does cause meat tenderization by means of destroying the integrity of the M-band structure in sarcomere.

Diurnal Differences in Human Muscle Isometric Force In Vivo Are Associated with Differential Phosphorylation of Sarcomeric M-Band Proteins

We investigated whether diurnal differences in muscle force output are associated with the post-translational state of muscle proteins. Ten physically active men (mean ± SD; age 26.7 ± 3.7 y) performed experimental sessions in the morning (08:00 h) and evening (17:00 h), which were counterbalanced in order of administration and separated by at least 72 h. Knee extensor maximal voluntary isometric contraction (MVIC) force and peak rate of force development (RFD) were measured, and samples of vastus lateralis were collected immediately after exercise. MVIC force was greater in the evening (mean difference of 67 N, 10.2%; p < 0.05). Two-dimensional (2D) gel analysis encompassed 122 proteoforms and discovered 6 significant (p < 0.05; false discovery rate [FDR] = 10%) diurnal differences. Phosphopeptide analysis identified 1693 phosphopeptides and detected 140 phosphopeptides from 104 proteins that were more (p < 0.05, FDR = 22%) phosphorylated in the morning. Myomesin 2, muscle creatine kinase, and the C-terminus of titin exhibited the most robust (FDR < 10%) diurnal differences. Exercise in the morning, compared to the evening, coincided with a greater phosphorylation of M-band-associated proteins in human muscle. These protein modifications may alter the M-band structure and disrupt force transmission, thus potentially explaining the lower force output in the morning.

Inside Cover: Exploring the Conformation-Regulated Function of Titin Kinase by Mechanical Pump and Probe Experiments with Single Molecules (Angew. Chem. Int. Ed. 6/2010)

The molecular-force sensor titin kinase (TK) is embedded in the M-band structure of the sarcomere at an ideal position to sense force imbalances. H. E. Gaub and E. M. Puchner demonstrate in their Communication on page 1147 ff. a new AFM-based single-molecule pump-and-probe protocol to mechanically prepare different conformations of TK and to read out their function. The results show that the binding pocket for the co-substrate ATP is shielded by two sequential barriers in the force-induced activation pathway.

Exploring the Function of Titin Kinase by Mechanical Single-Molecule Pump-And-Probe

Protein function like catalytic activity or molecular recognition is tightly coupled to conformation and its dynamics. Since protein conformation can be controlled by forces, diverse mechanisms evolved allowing biological systems to respond to mechanical strain. Recently, it was shown in combination with single-molecule force spectroscopy, MD-simulations and enzymatics that titin kinase acts as a force sensor regulating muscle gene expression and protein turnover [1]. However, there is no experimental access to investigate which force-induced conformation during the activation pathway is competent for ATP-binding. Here we develop a new AFM-based single-molecule pump-and-probe protocol to mechanically prepare a predefined conformation and to read out afterwards whether ATP bound. We show that ATP only binds to the conformation prepared after barrier two. Therefore, titin kinase exhibits a dual mechanical autoinhibition, which cannot be overcome by thermal fluctuations but by physiological forces acting on the M-band structure of the muscle sarcomere. This single-molecule approach might also become useful for the investigation of other conformation controlled processes such as hidden binding pockets, catch bonds or motor proteins. [1] Puchner, E.M. et al. Mechanoenzymatics of titin kinase.Proc Natl Acad Sci U S A, 2008. 105(36): p. 13385-90.

M-band: a safeguard for sarcomere stability?

The sarcomere of striated muscle is a very efficient machine transforming chemical energy into movement. However, a wrong distribution of the generated forces may lead to self-destruction of the engine itself. A well-known example for this is eccentric contraction (elongation of the sarcomere in the activated state), which damages sarcomeric structure and leads to a reduced muscle performance. The goal of this review is to discuss the involvement of different cytoskeletal systems, in particular the M-band filaments, in the mechanisms that provide stability during sarcomeric contraction. The M-band is the transverse structure in the center of the sarcomeric A-band, which is responsible both for the regular packing of thick filaments and for the uniform distribution of the tension over the myosin filament lattice in the activated sarcomere. Although some proteins from the Ig-superfamily, like myomesin and M-protein, are the major candidates for the role of M-band bridges, the exact molecular organisation of the M-band is not clear. However, the protein composition of the M-band seems to modulate the mechanical characteristics of the thick filament lattice, in particular its stiffness, adjusting it to the specific demands in different muscle types. The special M-band design in slow fibers might be part of structural adaptations, favouring sarcomere stability for a continuous contractile activity over a broad working range. In conclusion, we discuss why the interference with M-band structure might have fatal consequences for the integrity of the working sarcomere.

Optimum design of vector-quantized subband codecs

This article provides an approach for representing an optimum vector quantizer by a scalar nonlinear gain-plus-additive noise model. The validity and accuracy of this analytic model is confirmed by comparing the calculated model quantization errors with actual simulation of the optimum Linde-Buzo-Gray (1980) vector quantizer. Using this model, we form an MSE measure of an M-band filter bank codec in terms of the equivalent scalar quantization model and find the optimum FIR filter coefficients for each channel in the M-band structure for a given bit rate, filter length, and input signal correlation model. Specific design examples are worked out for four-tap filters in the two-band paraunitary case. These theoretical results are confirmed by extensive Monte Carlo simulation.

M-band structure and composition in relation to fibre types

M-band structure and composition in relation to fibre types

Fine structure of the A-band in cryo-sections diversity of M-band structure in chicken breast muscle

Electron micrographs of longitudinal ultrathin cryo-sections and plastic sections of chicken pectoralis muscle together with their average images have been used to study in detail the axial structure of the M-band. It was found that M-band structure could vary markedly in different fibres, even within the white part of the muscle. Strong M-band density (“M-bridges”) could be seen at M4 and M4′ in all fibres. On the other hand the density at M1 or M6 could vary systematically. Some fibres (probably fast) had M1 strong, M6 weak (a “3-line” M-band), and the Z-band was narrow. Other fibres, especially (but not exclusively) in the red part of the muscle and probably slow, had M6 strong, M1 weak (a “4-line” M-band), and the Z0band was broad. However, the majority of fibres ranged in structure between those two extremes and had a more or less “5-line” M-band with M1 and M6 both strong and a Z-band of intermediate width. Since they were such a constant feature, the M4 lines may be the sites of the primarily structural component of the M-band, whereas the different proteins at M1 and M6 may vary in quantity according to the physiological needs of the fibre. Finally, detailed analysis sometimes revealed substructure within the strong M-bridge lines. This substructure may represent additional unknown M-band proteins or may be an indication of the shape of single proteins at these positions.

Structural diversity in muscle fibres of chicken breast.

Chicken breast muscle is usually considered to be a relatively homogeneous white muscle and has therefore been widely used for studies of muscle proteins. In a previous study, however, we have found different M-region structures in different fibres from this muscle. Because of this result, we have now carried out a combined histochemical and ultrastructural survey of this muscle. In particular, we have made use of large transverse cryo-sections that include most of the muscle cross-section. Although the white region is fairly homogeneous in fibre content according to normal histochemical criteria (mATPase), we have found that there is a gradation of fibre structure across the muscle. The bulk of the muscle stains conventionally for Type-II fibres according to mATPase tests (the "white" part) but, in the small "red" part of the muscle, there are also Type-I fibres together with the Type-II fibres. Superimposed on this division into Type-I and Type-II fibres are variations in fibre size, oxidative and glycolytic staining properties, and variations of Z-band width and M-band structure; there is no strict correlation among any of these parameters. The apparently uniform staining across most of the muscle when tested for myofibrillar ATPase may be a misleading indicator of fibre properties.

Myofibrillar M-band structure and composition of physiologically defined rat motor units.

The isometric contraction time of 19 fast and slow rat motor units in the soleus and the anterior tibial muscles were recorded. The motor unit fibers, subsequently distinguished by glycogen depletion, were histochemically differentiated into fiber types and analyzed immunohistochemically for high molecular weight M-band proteins, as well as ultrastructurally for M-band fine structure, Z-disc width, and volume density of mitochondria. All fibers belonging to slow-twitch motor units in both the anterior tibialis and soleus muscles were histochemically classed as type 1. They lacked the Mr 165,000 M-protein, showed ultrastructurally a four-line M-band pattern, and had broad Z-discs, whereas the volume density of the mitochondria varied considerably. Muscle fibers belonging to the fast-twitch motor units were histochemically classed as types 2A and 2B in anterior tibialis and type 2A in soleus. They contained a three- or a five-line M-band pattern and medium-to-thin Z-discs in the anterior tibialis and a five-line M-band pattern and broad Z-discs in the soleus. Furthermore, the volume density of mitochondria showed considerable variation within and in between soleus and anterior tibialis type 2 fibers. As the differences in M-band composition and structure between fiber types overrode the intragroup variability in contraction times of slow and fast units within and between the two muscles, it is concluded that the M-band composition and structure is fundamentally related to whether the fiber is innervated by a slow or fast motor neuron, whereas other parameters such as contraction time, Z-disc width, and mitochondrial content of fibers of fast and slow units are relative and vary between muscles. Thus the M-band appearance can be used as a reliable marker to distinguish between fibers of slow- and fast-twitch motor units in rat leg muscles.

Myofibrillar M-band proteins represent constituents of native thick filaments, frayed filaments and bare zone assemblages.

A-segments, native thick filaments, frayed filaments, bare zone assemblages, as well as completely disassembled and reassembled thick filaments from chicken pectoralis major were investigated for the presence of M-band proteins by the colloidal gold labelling technique. Specific polyclonal antibodies against the three M-band proteins identified to date, MM-creatine kinase, M-protein (165 kDa) and a 185 kDa protein myomesin, were prepared. Incubation with anti-M-protein and anti-myomesin antibodies resulted in heavy labelling of all thick filament types mentioned above, with the exception of the completely disassembled and reassembled thick filaments. In that case no labelling was detected with either antibody. In contrast, MM-creatine kinase which is an integral component of the intact M-band structure was detectable on isolated native thick filaments with lower frequency and to a variable extent. Also, bare zone assemblages were only rarely labelled by anti-MM-creatine kinase antibodies. This study shows that the 'cuff-like' additional material which had previously been observed in the middle of the bare zone of isolated thick filaments represent remnants of all three M-band proteins, whereas the extra material in intact bare zone assemblages mainly consists of myomesin and M-protein, but not of MM-creatine kinase. Myomesin and M-line protein may be important for the assembly and structural maintenance of thick filaments as well as for anchoring of additional M-band proteins, e.g. MM-creatine kinase which is bound less tightly to thick filaments and, in accordance with earlier results, seems to represent within the M-band some of the prominent bridge-forming structures.

Three-dimensional structure of the insect ( Lethocerus) flight muscle M-band

The oval myosin filament profiles in transverse sections through the M-band of Lethocerus flight muscle are arranged in one of three orientations 60 degrees apart and point along the 11 directions of the hexagonal filament lattice. Relative orientations are not systematically related to give a superlattice structure, but neither are the orientations arranged completely randomly. In fact there is a nearly random structure with a slight bias towards adjacent filaments being identically oriented. This form of M-band structure is explained in terms of interactions between quasi-equivalent M-bridges. Its implications with regard to myosin crossbridge arrangement depend on the rotational symmetry of the crossbridge helix. For 6-stranded helices, 60 degrees rotations have no noticeable effect. However, in the case of the more likely 4-stranded structure, our results show that the crossbridge origins in the insect flight muscle A-band would be highly disordered. This disorder must be accounted for in interpreting both the flared-X crossbridge interactions seen in transverse sections of rigor insect flight muscle and the beautiful X-ray diffraction patterns from the same preparation. It is likely that in rigor insect muscle, some flared-Xs have the two heads of single myosin molecules interacting with two different actin filaments, whereas other flared-Xs have both of the myosin heads in one molecule interacting with the same actin filament.

Z- and M-band appearance in different histochemically defined types of human skeletal muscle fibers.

In order to define ultrastructural features, which alone or in combination with other features could be used to identify different types of fibers in human skeletal muscle, frozen biopsy specimens of m. tibialis anterior were serially sectioned. The thawed sections were prepared either for enzyme histochemistry or for electron microscopy. The same fiber was then identified in all serial sections and its ultrastructure examined under the electron microscope. A total of 75 fibers were included in this investigation. Specimens were also conventionally prepared for electron microscopy. Special interest was devoted to the appearance of the sarcomeric Z- and M-bands. In the same fiber, all myofibrils showed the same Z- as well as M-band structure. On the other hand, it was evident that these structures varied from one type of fiber to another in the same muscle and that their appearance were covariant to a great extent. Low level resolution of Type 1 fibers usually showed broad Z- and M-bands with five strong M-bridge lines. In Type 2A fibers intermediate Z-bands were observed. In the middle portion of the M-bands, three strong M-bridge lines were distinct while the two outer lines were relatively weak. Finally, Type 2B fibers usually appeared with narrow Z-bands. The three M-bridge lines in the middle were strong while the two outer ones were very weak, if seen at all. Discriminant analysis showed that about 70% of the fibers should have been correctly allocated on the basis of the Z-band width alone. When two independent observers classified the fibers on the basis of M-band appearance, more than 95% of the fibers were correctly classified. Thus, both the Z- and M-bands, alone or in combination, can be used as fiber type discriminators. However, the M-band structure proved to be more reliable than the Z-band width, and should therefore be used as the fiber type indicator when only one of these parameters is considered.

Ultrastructure studies on the binding of creatine kinase and the 165,000 molecular weight component to the M-band of muscle

Binding of creatine kinase and the 165,000 molecular weight component to the M-band of rabbit skeletal muscle and bovine cardiac muscle, following their removal by low ionic strength extraction, has been accomplished. Examination of the myofibrils in the electron microscope shows that creatine kinase and the 165,000 M r component will bind to the M-band independently of each other. In the skeletal system, rabbit skeletal creatine kinase and the 165,000 M r protein were used, while in the bovine cardiac case, bovine cardiac creatine kinase and rabbit skeletal 165,000 M r protein were employed. The latter conditions gave results that suggest the presence of a bovine analogue of M r = 165,000 in the M-band of bovine cardiac myofibrils. The fact that bovine creatine kinase will bind to the M-band firmly establishes its presence, for the first time, as an integral part of the M-band structure of mammalian cardiac muscle.