Isolation and characterization of the 165 000 dalton protein component of the M-line of rabbit skeletal muscle and its interaction with creatine kinase

Ultrastructural distribution of the M form of creatine phosphokinase in human muscle by immunogold labeling

Creatine phosphokinase regenerates ATP from ADP using creatine phosphate. Isoenzymes of creatine phosphokinase are bound to certain cellular structures or are compartmentalized in areas of the cell, and this has been used as a basis for defining the role of these isoenzymes in energy metabolism. The M isoenzyme of creatine phosphokinase has been morphologically associated with the M-line of striated muscle in many species. In this present study the ultrastructural distribution and the relative concentration of the M form of creatine phosphokinase in human muscle tissue was determined using immunogold and electron microscopy. The M-line of the sarcomere, comprising only 3-4% of the sarcomere area, was found to contain over 20% of the total M isoenzyme signal of the entire sarcomere. This technique represents a quantitative, ultrastructural method to study the subcellular distribution of this isoenzyme. These data suggest that localized concentrations of M-CPK may be important for normal energy metabolism, and may also serve as a foundation for a better understanding of the relationship between abnormal creatine metabolism and the pathogenesis of neuromuscular disease.

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 Z-band 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.

Functionality of muscle proteins in gelation mechanisms of structured meat products

Recent advances in muscle biology concerning the discoveries of a large variety of proteins have been described in this review. The existence of polymorphism in several muscle proteins is now well established. Various isoforms of myosin not only account for the difference in physiological functions and biochemical activity of different fiber types or muscles, but also seem to differ in functional properties in food systems. The functionality of various muscle proteins, especially myosin and actin in the gelation process in modal systems which simulate structured meat products, is discussed at length. Besides, the role of different subunits and subfragments of myosin molecule in the gelation mechanism, and the various factors affecting heat-induced gelation of actomyosin in modal systems are also highlighted. Finally, the areas which need further investigation in this discipline have been suggested.

An in vitro study of the interactions of skeletal muscle M-protein and creatine kinase with myosin and its subfragments

Two proteins reported to be located in the M-band of skeletal muscle are M-protein (Mr 160,000) and creatine kinase (Mr 83,000). We have isolated and purified these proteins from adult chicken pectoralis muscle, and have studied their in vitro interactions with myosin, heavy meromyosin, light meromyosin and subfragment-2 in order to obtain a fuller understanding of the role these proteins play in the M-band of skeletal muscle. Experiments using the techniques of analytical ultracentrifugation, affinity chromatography and electron microscopy were carried out near physiological pH and ionic strength, under which conditions the M-band proteins are known to be firmly bound to the myofibril in situ. The results of our studies indicate that such interactions are either weak or absent in vitro. Discrepancies between our results and those from several other studies are discussed. We conclude that additional components may be required in order to observe interactions in vitro which are similar to those present in the intact myofibril.

Novel staining pattern of skeletal muscle M-lines upon incubation with antibodies against MM-creatine kinase

Incubation of chicken skeletal muscle fibers with an excess of anti-M-creatine kinase (CK) immunoglobulin G and an excess of anti-M-CK Fab fragments leads to heavy decoration of the M-line (Wallimann, T., D.C. Turner, and H.M. Eppenberger, 1977, J. Cell Biol. 75:297-317) and to removal of the electron-dense M-line structure (Walliman, T., G. W. Pelloni, D.C. Turner, and H.M. Eppenberger, 1978, Proc. Natl. Acad. Sci. USA., 75:4296-4300), respectively. On the other hand, incubation with low concentrations of monovalent anti-M-CK Fab did not extract but rather decorated the M-line, giving rise to a distinct two-line staining pattern. A similar double-line staining pattern, although less pronounced, was also observed within the M-line of paraformaldehyde-prefixed myogenic cells, which after permeabilization were incubated with low concentrations of divalent anti-M-CK antibody. In both cases, the two decorated lines appearing in the middle of the A-band were spaced axially 42-44 nm apart and correspond most likely to the two M4 and M4' m-bridge rows described by Sjöström and Squire (1977, J. Mol. Biol., 109:49-68; 1977, J. Microscopy., 111:239-278). It is concluded that the muscle-specific form of creatine kinase, MM-CK, contributes mainly to the electron density of these M4 and M4' m-bridges within the M-line structure. This specific labeling pattern is a further demonstration that CK is an integral part of the M-line.

Ultrastructural localization of M-band proteins in chicken breast muscle as revealed by combined immunocytochemistry and ultramicrotomy

Cryo-ultramicrotomy and "conventional" plastic sectioning have been used in combination with extraction and immunolabeling techniques to determine the location of the two M-band proteins characterized to date, MM-creatine kinase (MM-CK: Mr, 80,000) and M-protein "myomesin" (Mr, 165,000) within the M-region of chicken pectoralis muscle. The following main results were obtained. (1) The M-band in chicken pectoralis muscle contains five major striations (M1, M4 and M4', M6 and M6' in the terminology of Sjöström & Squire, 1977a). (2) Extraction of the bulk of the electron-dense M-band with low ionic strength removes the M-striations M1, M4 and M4' while M6 and M6' are retained. Cross-sections through the M-region of such muscles lack primary M-bridges connecting the thick myosin filaments. (3) Labeling with antibodies against MM-CK enhances the M-striations M4 and M4'; sometimes the whole region between M4 and M4' is labeled. (4) Incubation with antibodies against myomesin results in the labeling of the whole M-band from M6 to M6'; no label is found in the rest of the bare zone outside M6 and M6'. (5) Incubation of low ionic strength extracted muscle fibers with antibodies against myomesin leads to an "incomplete" labeling of the M-band between M6 and M6'; lines M6 and M6' are sometimes seen to be enhanced presumably due to antibody labeling. From these results it is concluded that MM-CK is the major protein of the M4 and M4' (and possibly also of the M1) M-bridges. Myomesin is bound within the M-band along the thick filaments from M6 to M6'. Two hypothetical models for the possible location of myomesin are discussed. According to these models myomesin would either make up the M-filaments or be directly attached to and along the central bare zone of thick myosin filaments.

Dynamic aspects of structural proteins in vertebrate skeletal muscle

In this review, our current knowledge on the structural proteins of vertebrate skeletal muscle is briefly outlined. Structural proteins include the contractile proteins (actin and myosin), the major regulatory proteins (troponin and tropomyosin), the minor regulatory proteins (M-protein, C-protein, F-protein, I-protein, and actinins), and the scaffold proteins (connectin, desmin, and Z-protein). In addition, the relative turnover rates of the muscle proteins (M-protein greater than or equal to troponin greater than soluble protein as a whole greater than tropomyosin not equal to alpha-actinin greater than myosin greater than 10S-actinin greater than actin) are discussed. The changes in the turnover of muscle proteins are compared in denervated and dystrophic muscles. The properties of the various proteases in muscle, including alkaline protease, calcium-activated neutral protease (CANP), and acidic protease (cathepsins), and the structural alterations of myofibrils by these proteases are also described. Finally, the role of proteases and their inhibitors in diseased muscle is summarized, with focus on CANP and its inhibitors, leupeptin and E-64.

Postnatal development of the M-band in rat cardiac myofibrils

Cardiac muscle fibers in rats at 1 and 5 days after birth showed little evidence of M-bands. An ultrastructural analysis of myofibrils failed to demonstrate dense M-band material in longitudinal sections or M-bridges in transverse sections of sarcomeres. M-bands began to increase in number after 5 days of postnatal life and were present in 60% of all sarcomeres at 11 days of age. Polypeptides with molecular weights of 190,000 (Ma) and 175,000 (Mb) were obtained by polyacrylamide-sodium dodecyl sulfate (SDS) gel electrophoresis of myofibril preparations. These two proteins were found in both 1- and 11-day-old rats and were considered to be specific components of the M-band. The densitometric analysis demonstrated that Ma and mb polypeptides increased approximately 2-fold during the interval from 1 to 11 days after birth. These structural and biochemical changes of the M-band material in myocytes appear to be related to the maturation of contractile function in the young heart.

Interaction studies of the 165 000 dalton protein component of the M-line with the S2 subfragment of myosin

The M-line protein component of molecular weight 165 000 was isolated and purified from rabbit skeletal muscle using ion exchange chromatography. Sodium dodecyl sulphate electrophoresis revealed that the protein was homogeneous. Circular dichroism measurements indicated that the protein interacts with myosin and heavy meromyosin subfragment 2 (S2). There was an increase in negative ellipticity at 221 nm upon interaction, relative to the calculated values assuming no interprotein interaction. The net increaes in negative ellipticity at 221 nm as a result of interaction of M-protein with myosin and subfragment 2 were 600 degrees and 800 degrees respectively. When the protein was mixed with subfragment 2 in a 1 : 1 mol ratio in 0.5 M KCl/25 mM Tris buffer at pH 8.0, low speed sedimentation equilibrium studies gave a molecular weight of 235 000 +/- 10 000 for the complex, indicative of an interaction of the two components. On a Bio gel A 0.5 m column, M-protein and S2 when applied in 1 : 1 mol ratio, were eluted as a single symmetrical peak and a molecular weight of 230 000 was obtained for the complex from the observed elution volume. Both circular dichroism and sedimentation equilibrium studies indicated no interaction of M-line protein with light meromyosin and subfragment 1. Interaction of the 165 000 component with the flexible hinge region of myosin may have special significance in terms of the mechanism accounting for the reversible expansion of the interfilament distance which occurs during contraction.