Interaction of C-protein with myosin, myosin rod and light meromyosin

Immunochemical analysis of porcine cardiac C-protein

C-protein has been isolated from pig heart and its immunochemical properties studied. It is extracted with myosin, and separated from the myosin on a DEAE-Sephadex column. The amount of C-protein recovered from crude myosin is approx. 3.5%. The molecular weight of C-protein is 150,000. Anti-C-protein serum reacts with crude myosin and purified C-protein but not with purified myosin in immunodiffusion plates. Cardiac C-protein does not react with anti-skeletal white muscle C-protein serum. Immunoblotting experiments show that anti-cardiac C-protein serum reacts with a Mr = 150,000 component in myofibrils or crude myosin. C-protein is located in the A-band, except the M-line region, of the myofibrils. These results indicate that C-protein is an intrinsic component of the thick filaments in pig heart myofibrils.

Localization of binding sites of F-protein (phosphofructokinase) on the myosin molecule

To determine the location of F-protein binding sites on myosin, the interaction of F-protein with myosin and its proteolytic fragments in 0.1 M KCl, 10 mM potassium phosphate, pH 6.5, has been investigated using sedimentation, electron microscopy and optical diffraction methods. Sedimentation experiments show that F-protein can bind to myosin and myosin rod rather than to light meromyosin or subfragment-1. The F-protein binding to myosin and rod is of a similar character. The calculated values of the constants of F-protein binding to myosin and rod are 2.6 X 10(5) M-1 and 2.1 X 10(5) M-1, respectively. The binding sites are probably located on the subfragment-2 portion of the myosin molecule. The number of the F-protein binding sites calculated per chain weight of 80,000 is 5 +/- 1. Electron microscopic observations confirm the sedimentation results. F-protein does not bind to light meromyosin paracrystals, but decorates myosin and rod filaments with the interval of 14.3 nm regardless of whether F-protein is added prior to or after filamentogenesis. The comparison of optical diffraction patterns obtained from myosin and rod filaments with those from decorated ones reveals the marked enhancement of meridional reflection at (14.3 nm)-1 in the latter case. Neither the increase in ionic strength from 0.1 to 0.15 and pH from 6.5 to 7.3 nor substitution of potassium phosphate buffer by imidazole-HCl buffer, or Tris-HCl influences F-protein binding to myosin and rod filaments as visualized by electron microscopy. The possible significance of F-protein location in the thick filament structure is discussed.

The binding of skeletal muscle C-protein to regulated actin

The binding of C-protein, a component of thick filament of myofibrils, to regulated actin filaments in the presence or absence of Ca2+ was studied. The amount of C-protein bound to regulated actin filaments in the presence of Ca2+ was higher than those in the absence of Ca2+. The addition of C-protein to regulated actin caused an increase in turbidity, especially in the presence of Ca2+, and this was found to result from side-by-side association of actin filaments into bundles. In the absence of Ca2+, no actin filament bundles were formed.

Electron microscopy of C-protein molecules from chicken skeletal muscle

C-protein from chicken pectoralis muscle has been purified by sequential DEAE-Sephadex and hydroxyapatite chromatography and examined by transmission electron microscopy after spraying in glycerol onto mica and replicating by rotary shadowing with platinum. The most frequently observed particles were of three forms: rod-shaped, U-shaped and V-shaped. Within a size range of 15-40 nm these three groups accounted for 70% of over 800 particles categorized and measured. The remaining particles could not be classified. Since the relative abundance of each of these three forms was well in excess of any of the contaminating proteins detectable by SDS-polyacrylamide gel electrophoresis, we conclude that these variant forms represent C-protein molecules in differing conformations and/or deformations. Particles were observed which were intermediate between rod-shaped and tightly curved U-shaped forms, and between rod and acutely angled V-shaped forms. These results are compatible with a molecular model of a 32 nm X 3 nm flexible, rod-shaped C-protein monomer similar to one previously proposed from hydrodynamic studies and extend recent observations on the ultrastructure of cardiac C-protein. Infrequently, a discontinuously larger V-shaped form was seen, possibly representing a C-protein dimer.

Novel thick filament protein of chicken pectoralis muscle: the 86 kd protein. I. Purification and characterization

A new thick-filament-associated protein, the 86 kd protein, of chicken pectoralis major muscle was isolated from a crude C-protein preparation by a method similar to that used to purify H-protein from rabbit skeletal muscle. However, the protein with an apparent Mr of 86,000 and 370,000 as estimated by gel electrophoresis and gel permeation, respectively, is not related to C-protein and differs from rabbit H-protein by its elution behaviour from hydroxyapatite columns, by its molecular weight, ultraviolet light spectrum, amino acid composition and localization, and by its amount present in myofibrils. The amino acid composition reveals a high content of proline and gel permeation indicates an either highly asymmetric or polymeric structure of the molecule. Antibodies raised in rabbits against the 86 kd protein were demonstrated by double immunodiffusion and immunoblotting experiments to be specific for this protein. They show no cross-reactivity with any other myofibrillar protein of chicken pectoralis muscle, e.g. myosin, M-band proteins, titin or C-protein, nor did they exhibit a significant cross-reactivity with H-protein from rabbit. The 86 kd protein, which has been purified also by antibody affinity chromatography from a freshly prepared Guba-Straub extract of washed myofibrils, is a specific myofibrillar component located within each half of the A-band.

Effects of phosphorylated and unphosphorylated C-protein on cardiac actomyosin ATPase

C-protein, a component of the thick filaments of striated muscles, is reversibly phosphorylated and dephosphorylated in heart. It has been hypothesized that C-protein may be involved in regulating contraction, because the extent of C-protein phosphorylation correlates with the rate of cardiac relaxation. To test this hypothesis, the effects of phosphorylated and unphosphorylated C-protein on the actin-activated ATPase activity of myosin filaments prepared from DEAE-Sephadex-purified myosin were examined. Unphosphorylated C-protein (0.1 microM to 1.5 microM) stimulated actin-activated myosin ATPase activity in a dose-dependent manner. With a myosin: C-protein molar ratio of approximately 1, actin-activated myosin ATPase activity was elevated up to 3.2 times that of the control. Phosphorylated C-protein (2.5 mol PO4/mol C-protein) stimulated the activity somewhat less (2.5 times that of control). The stimulation of ATPase activity by C-protein was due to an increase in the Vmax value (from 0.25/second to 0.62/second) and a decrease in the Km value (from 11.9 microM to 6.7 microM). The addition of C-protein to actomyosin solutions produced an increase in the light-scattering of the actomyosin solution and a distinct precipitation of the actomyosin with time. Phosphorylated C-protein had a smaller effect on light-scattering than dephosphorylated C-protein. C-protein had a negligible effect on Ca-ATPase, EDTA-K-ATPase, or Mg-ATPase activities in the absence of actin. C-protein had only small effects on the actin-activated ATPase of heavy meromyosin. These results suggest that C-protein stimulates actin-activated myosin ATPase activity by enhancing the formation of stable aggregates between actin and myosin filaments.

The isoforms of C protein and their distribution in mammalian skeletal muscle

A monoclonal antibody that is specific for the slow skeletal muscle isoform of C protein of rabbit muscle has been prepared by immunizing mice with a crude preparation of human myosin. It reacted with the X protein fraction of rabbit skeletal muscle and stained all type I cells in this tissue. It also stained a fraction of the type II cells with varying intensities. The type II cells staining with antibody to slow C protein also stained with a polyclonal antibody prepared against rabbit fast muscle C protein. The type II cells not staining with antibody to slow C protein stained strongly with antibody to fast C protein. In the human skeletal muscle antibody to slow C protein stained all cells whereas antibody to fast C stained only type II cells. It is concluded that the distribution of the isoforms of C protein in adult vertebrate skeletal muscle is more complex than is the case with proteins such as components of the troponin complex.

The structure of C-protein and X-protein molecules and a polymer of X-protein

C-protein and X-protein are components of the thick filaments in vertebrate skeletal muscles and occupy similar locations in different fibre types. We find that the molecules are both rods about 30 to 40 A wide, but they differ significantly in their lengths, the X-protein molecule being about 350 A long and the C-protein molecule about 280 A. This suggests they are not isoforms. The short length of the C-protein molecule implies that it cannot act in the thick filament as a length-determining agent by a simple vernier mechanism. X-protein associates at low ionic strength (KCl concentration less than 0.07 M) but, unlike C-protein, forms long ordered polymers. These have been examined by electron microscopy to gain information on the molecular shape and on how the molecules interact. The polymers are helically twisted ribbons with a repeat distance along the axis of 660 A. The cross-section of the ribbon is approximately elliptical with major and minor axes of 405 A and 166 A, respectively. From an analysis of the micrographs by optical diffraction, we deduce that the molecules run across the face of the ribbon at an angle of about 15 degrees to the diameter and lie on a two-stranded helix. Models for the polymer are discussed in which the molecules are slightly bowed outwards and bind to each other only at their ends. We suggest that interactions similar to those in the polymer might occur in the thick filaments of muscle, and propose that at each axial position where X-protein attaches along the myosin filament, three X-protein molecules might form an approximately triangular ring around the filament backbone. The appearance of the X-protein polymers is similar to that of the twisted structures called paired helical filaments that make up the neurofibrillary tangles associated with dementia of the Alzheimer type.

MgATP specifically controls in vitro self-assembly of vertebrate skeletal myosin in the physiological pH range

The appearances in the electron microscope of rat and rabbit skeletal muscle myosin filaments and rod aggregates, formed in the presence of variable amounts of MgATP, were compared at different pH values. It is shown that small amounts of MgATP, similar to those sufficient to trigger the dissociation of the actomyosin complex, were able to modify the geometry of myosin filaments profoundly in the physiological pH range, whereas the conformation of rod aggregates remained unchanged even in the presence of high concentrations of MgATP. Myosin filaments formed in the absence of MgATP displayed the classical spindle-shaped conformation and variable diameters at all pH values, whereas myosin filaments formed in the presence of MgATP in the physiological pH range had constant diameters, similar to those of natural thick filaments. These filaments of constant diameter frayed, rapidly and reversibly, into two types of subfilaments with respective diameters of 4 to 5 nm and 9 to 10 nm, when the pH of the medium was raised above 7.2. Spindle-shaped myosin filaments and rod aggregates remained unchanged by such small changes in pH. It was possible to change the conformation of preformed spindle-shaped filaments by simply adding MgATP to the medium, but this reaction was slow and took several hours to be completed. Relatively high concentrations of MgATP, similar to those in the living cell, increased the solubility of both myosin filaments and rod aggregates in the alkaline pH range (pH greater than or equal to 7.0). Low pH values (less than or equal to 6.5) and excess free Mg2+ (greater than or equal to 6 to 7 mM) abolished both the specific effect of MgATP on myosin filament conformation and its solubilizing effect on both myosin filaments and rod aggregates. The degree of purity of the myosin preparations and the level of phosphorylation of the LC-2 light chains did not influence filament behaviour noticeably and rat and rabbit myosins behaved similarly.

Structure of C protein purified from cardiac muscle

C protein is a component of the thick filament of striated muscles. Although the function of C protein remains unknown, a variety of evidence suggests that C protein may regulate actin-myosin interaction or be involved in structural support or elasticity of the sarcomere. We have previously proposed (Hartzell, H. C., 1984, J. Gen. Physiol., 83:563-588) that C protein is involved in regulating twitch relaxation in cardiac muscle. To gain further insight into the function of C protein, we have studied the structure of C protein purified from chicken heart. C protein was purified from extracts of detergent-washed myofibrils by sequential hydroxylapatite and DEAE-Sephacel chromatography. C protein was judged greater than 95% pure by SDS PAGE. The polypeptide subunit had a molecular weight of 155,000 and the native molecule sedimented on linear sucrose or glycerol gradients at 4-5S. For electron microscopy, purified C protein was dialyzed and diluted into a volatile buffer in 50% glycerol, aspirated onto mica, dried under vacuum, and rotary platinum-shadowed. Replicas revealed particles of relatively homogeneous overall dimensions. Over half of the particles were V-shaped. The "arm" lengths of the V-shaped particles were 22 +/- 4.5 nm (SD). Gel filtration on Sephacryl S-300 demonstrated that purified C protein had a Stokes' radius of 5.07 nm. Measurements of viscosity gave an intrinsic viscosity of 16.5 cm3/g. These data are consistent with the electron microscopic data and suggest that C protein in heart muscle is asymmetric. The C protein molecule is large enough to extend from the surface of a thick filament to adjacent thin or thick filaments.

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.

Phosphorylation of C-protein in intact amphibian cardiac muscle. Correlation between 32P incorporation and twitch relaxation

The molecular mechanisms by which neurotransmitters modulate the force of contraction of cardiac muscle are incompletely understood. Hartzell and Titus (1982. J. Biol. Chem. 257:2111-2120) have recently reported that C-protein, an integral component of the thick filament, is reversibly phosphorylated in response to ionotropic agents. In this communication, C-protein phosphorylation (as measured by isotopic labeling with 32P) is correlated with changes in the rate of relaxation of twitch tension. On the average, isoproterenol simultaneously increases peak systolic tension twofold, decreases twitch relaxation time from a control value of approximately 450 to approximately 300 ms, and increases C-protein phosphorylation two- to threefold, with a maximum effect occurring less than 60 s after addition of 1 microM isoproterenol. Carbamylcholine, in contrast, decreases peak systolic tension more rapidly than it affects relaxation or C-protein phosphorylation. The maximum decrease in peak tension (60%) occurs within 1 min of addition of 0.5 microM carbamylcholine, but relaxation time increases slowly to 800 ms over approximately 6 min. The increase in relaxation time correlates well with the decrease in 32P incorporation into C-protein (r = 0.94). Changing beat frequency between 0.2 and 1/s has no effect on C-protein phosphorylation but does alter relaxation time (relaxation time decreases approximately 100 ms when beat frequency is changed from 0.5 to 1/s) and thus alters the quantitative relationship between C-protein phosphorylation and relaxation rate. These results suggest that two separate processes affect relaxation. It is proposed that the level of C-protein phosphorylation sets the boundaries over which relaxation is regulated by a second process that is dependent upon beat frequency and probably involves changes in intracellular Ca.

Localization of C-protein isoforms in chicken skeletal muscle: ultrastructural detection using monoclonal antibodies

Monoclonal antibodies (McAbs) specific for the fast (MF-1) and slow (ALD-66) isoforms of C-protein from chicken skeletal muscle have been produced and characterized. Using these antibodies it was possible to demonstrate that skeletal muscles of varying fiber type express different isoforms of this protein and that in the posterior latissimus dorsi muscle both isoforms are co-expressed in the same myofiber (17, 18). Since we had shown that both isoforms were present in all sarcomeres, it was feasible to test whether the two isoforms co- distributed in the same 43-nm repeat within the A-band, thereby establishing a minimum number of C-proteins per repeat in the thick filaments. Here we describe the ultrastructural localization of C- protein in myofibers from three muscle types of the chicken using these same McAbs. We observed that although C-protein was present in a 43-nm repeat along the filaments in all three muscles, there were marked differences in the absolute number and position occupied by the different isoforms. Since McAbs MF-1 and ALD-66 decorated the same 43- nm repeats in the A-bands of the posterior latissimus dorsal muscle, we suggest that at least two C-proteins can co-localize at binding sites 43 nm apart along thick filaments of this muscle.

H-protein and X-protein. Two new components of the thick filaments of vertebrate skeletal muscle

With a view to obtaining a more complete view of the composition and structure of the thick filaments of vertebrate skeletal muscle, we have isolated and characterized two new myofibrillar components, H-protein and X-protein. These were purified by hydroxyapatite column chromatography of an impure C-protein preparation itself made from impure myosin extracted from rabbit back and leg muscles. H-protein is the protein responsible for band H on sodium dodecyl sulphate/polyacrylamide gel electrophoresis of crude myosin. X-protein, although present in such preparations in significant quantities, was not detected previously since it is difficult to resolve from C-protein by sodium dodecyl sulphate/polyacrylamide gel electrophoresis. Physical-chemical parameters have been determined for the new proteins and compared with those of C-protein. The apparent chain weight of H-protein estimated by sodium dodecyl sulphate/polyacrylamide gel electrophoresis is 69,000, whereas that of X-protein (152,000) is only slightly greater than that of C-protein (140,000). The molecular weights of H- and X-proteins determined by sedimentation equilibrium centrifugation show that the molecules contain only a single polypeptide chain. The circular dichroism spectra indicate that the proteins have low alpha-helical contents. Both proteins, particularly H-protein, have a high proline content. Although X-protein is of similar chain weight to C-protein, the two show distinct differences in other properties. The sedimentation coefficient of X-protein is markedly lower than that of C-protein, suggesting X-protein is a more asymmetrical molecule. The amino acid compositions, although broadly similar, also show clear differences. Antibodies to H-protein, X-protein and C-protein have been raised in goats and shown not to cross-react.

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.

Cardiac function and phosphorylation of contractile proteins

The primary regulation of cardiac contractility is probably through changes in the level of cytoplasmic free Ca 2+ . In the stimulation of contraction by catecholamines, secondary controls may be present at the level of the contractile proteins. Troponin-I, a subunit of the troponin complex of the thin filament, and C-protein, a thick filament component, are both phosphorylated in perfused hearts in response to catecholamines over time courses similar to that for the increase in contraction. Both proteins are also phosphorylated rapidly in vitro by cyclic-AMP-dependent protein kinase. Phosphorylation of troponin-I causes a decrease in the sensitivity of both cardiac myofibrillar ATPase and tension development of skinned fibre preparations to Ca 2+ , and also an increase in the rate of dissociation of Ca 2+ from isolated troponin. These results support the hypothesis that the role of phosphorylation of cardiac troponin-I is to contribute to the increased rate of relaxation of the heart that is observed with catecholamines. C-protein is phosphorylated to a maximum of 4-5 mol phosphate per mole protein both in vivo and in vitro . At present, however, the functions of both C-protein itself and its phosphorylation are unknown. Dephosphorylation of these contractile proteins after catecholamine stimulation is slow in perfused heart, although the rate can be increased by cholinergic agents. Phosphorylase, in contrast, is rapidly dephosphorylated under these circumstances. Phosphoprotein phosphatases relatively specific for phosphorylase have been identified in rat heart, whereas troponin-I appears to be dephosphorylated by general phosphatases. These observations account for the different rates of dephosphorylation of phosphorylase and the contractile proteins, but do not explain the slow dephosphorylation of the latter. In control perfused hearts myosin P-light chain was 50 % phosphorylated, and this was not changed by perfusion with positive inotropic agents or by short-term ischaemia. It was also unchanged during long-term hormonal modifications. Perfusions with 32 P 1 in rat heart give a half-time for the turnover of phosphate bound to the P-light chain of 2-4 min, showing that the myosin light chain kinase and phosphatase are active in the heart. It is hypothesized that under control conditions the kinase is already fully active, and that an increase in cytoplasmic Ca 2+ cannot therefore cause further activation of the enzyme.

Characterization of the C-protein from posterior latissimus dorsi muscle of the adult chicken: heterogeneity within a single sarcomere

Specific isoforms of myofibrillar proteins are expressed in different muscles and in various fiber types within a single muscle. We have isolated and characterized monoclonal antibodies against C-proteins from slow tonic (anterior latissimus dorsi, ALD) and fast twitch (pectoralis major) muscles of the chicken. Although the antibody against "fast" C-protein (MF-1) did not bind to the "slow" isoform and the antibody to the "slow" C-protein (ALD-66) did not bind to the "fast" isoform, we observed that both antibodies bound C-protein from the posterior latissimus dorsi (PLD) muscle. Here we demonstrate that in the PLD muscle the binding sites of these two antibodies reside in two different C-protein isoforms which have different molecular weights and can be separated by hydroxylapatite column chromatography. Since we have shown previously that both these antibodies stain all myofibers and myofibrils derived from PLD muscle, we conclude that all myofibers in this muscle contain both isoforms with all sarcomeres.

Effects of taxol and Colcemid on myofibrillogenesis

To determine the relationship between thin filaments, Z-bands, microtubules, intermediate filaments (IFs), T-tubules, and sarcoplasmic reticulum (SR) during myofibrillogenesis, myotubes were selectively depleted of their myofibrils with 12-tetradecanoylphorbol 13-acetate (TPA) and then were allowed to regenerate in (i) normal medium, (ii) taxol, and (iii) Colcemid. Myofibrils assembled in normal medium formed typical A-, I-, Z-, M-, and H-bands and associated IFs, T-tubules, and SR. Myofibrils assembled in taxol formed "A-bands" of aligned thick filaments interdigitating with long microtubules and "I-bands" consisting only of microtubules. These unprecedented sarcomeres lacked thin filaments, Z-bands, and associated IFs and SR. "Solitary A-bands," consisting exclusively of laterally aligned bipolar thick filaments 1.6 microM in length without either thin filaments or microtubules, were observed. Myofibrils assembled in Colcemid formed all myofibrillar components in the absence of microtubules but these did not achieve rigorous lateral alignment. Colcemid and taxol induced the formation of patchy Z-bands that invariably served as insertion sites for thin filaments, irrespective of the presence or absence of adjacent thick filaments. Z-bands may function as actin-organizing centers for each sarcomere.

Structural studies of isolated native thick filaments from rabbit psoas muscle with AMP deaminase decoration

AMP deaminase (adenylate deaminase; AMP aminohydrolase, EC 3.5.4.6), a large flat tetrameric enzyme found in skeletal muscle, binds strongly and specifically to the subfragment-2 region of rabbit skeletal muscle myosin. This allows its use as a structural probe in myosin and myosin rod aggregation studies. When mixed with a preparation of isolated native thick filaments, AMP deaminase decorates the entire filament backbone except for the central bare zone. Binding is particularly ordered in the banded region, where 11 stripes of about 43-nm spacing on either side of the bare zone have been observed in studies of isolated A-bands. No systematic absence of deaminase is seen here, indicating that the presence of the C-protein and H-protein bands does not make the binding site inaccessible to the tetramer. Optical diffraction patterns of the decorated filaments show the expected 42.9-nm periodicities and a reflection indexing at 28.6 nm. Because of the bulkiness of the tetramer relative to the number of available binding sites at each 14.3-nm interval along the filament shaft, the helix net is being sampled at a lower frequency than is the underlying myosin organization. As a result, reflections on layer lines other than orders of 42.9 nm are also observed (e.g., 57.2); these reflections strongly indicate a structure based on a 12/1 primitive helix. The results appear to eliminate the symmetric double two-fold and three-fold models of thick filament structure but are consistent with an asymmetric four-fold structure.

Quasi-elastic light scattering studies of rabbit skeletal myosin solutions

Homodyne measurements of the laser light spectrum scattered from solutions of rabbit skeletal muscle myosin in high ionic-strength media manifested a characteristic D value dependence on myosin concentrations. Using the typical D versus myosin concentration curves obtained in the presence of 0.5 M phosphate and 0.2 M phosphate respectively as references, it has been shown that: (1) the observed phenomena are completely reversible; (2) minor components such as C- and F-protein do not significantly influence the measured D values; and (3) the effect of preparation procedures on these dynamic light-scattering measurements is negligible. A common argument (irreversible aggregation) against a monomer-dimer equilibrium is ruled out; on the other hand, some doubt still remains with regard to the existence and physiological significance of a reversible dimerization.

The aggregation characteristics of column-purified rabbit skeletal myosin in the presence and absence of C-protein at pH 7.0

The aggregation properties of column-purified rabbit skeletal myosin at pH 7.0 were investigated as functions of ionic strength, protein concentration, and time. Filaments prepared by dialysis exhibited the same average length and population distribution at 0.10 and 0.15 M KCl at protein concentrations greater than 0.10 mg/ml; similar results were obtained at .0.20 M KCl, although average filament length was approximately 0.5 micrometer shorter. Once formed, these length distributions remained virtually unchanged over an 8-d period. At and below 0.10 mg/ml, average filament length decreased as a function of protein concentration; filaments prepared from an initial concentration of 0.02 mg/ml were half the length of those prepared at 0.2 mg/ml. Filaments prepared by dilution exhibited a sharp increase in average length as the time-course increased up to 40 s, then altered only slightly over a further period of 4 min. Addition of C-protein in a molar ratio of 1-3.3 myosin molecules affected most of these results. Average filament length was affected neither by ionic strength nor by initial protein concentration down to 0.04 mg/ml or over an 8-d period. Filaments formed by dilution in the presence of C-protein exhibited a constant average length and hypersharp length distribution over variable time courses up to 7 min. It is possible that C-protein acts to stabilize the antiparallel intermediate during filamentogenesis, and may also affect subunit addition to this nucleus.

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.

Fluorescence microscope study of the binding of added C protein to skeletal muscle myofibrils

The binding of extra C protein to rabbit skeletal muscle myofibrils has been investigated by fluorescence microscopy with fluorescein-labeled C protein or unmodified C protein plus fluorescein-labeled anti-C protein. Added C protein binds strongly to the I bands, which is consistent with its binding to F actin in solution (Moos, C., C. M. Mason, J. M. Besterman, I. M. Feng, and J. H. Dubin. 1978. J. Mol. Biol. 124:571-586). Of particular interest, the binding to the I band is calcium regulated: it requires a free calcium ion concentration comparable to that which activates the myofibrillar ATPase. This increases the likelihood that C protein-actin interaction might be physiologically significant. When I band binding is suppressed, binding in the A band becomes evident. It appears to occur particularly near the M line, and possibly at the edges of the A band as well, suggesting that those parts of the thick filaments that lack C protein in vivo may nevertheless be capable of binding added C protein.

C-protein from rabbit soleus (red) muscle

A new form of skeletal-muscle C-protein has been isolated from rabbit soleus (red) muscle. This new form of C-protein has been purified to homogeneity by a procedure similar to that used to purify C-protein from white skeletal muscle. In soleus muscle, only this new form of C-protein could be detected, whereas in psoas (white) muscle, only the previously identified form of C-protein was detected. The content of C-protein in rabbit soleus muscle is comparable with that found in psoas muscle. Other rabbit skeletal muscles composed of a mixture of fibre types contained at least two forms of C-protein. C-Protein derived from red skeletal muscle bound to myosin isolated from either red or white tissue, with maximum binding occurring at a ratio of approximately 13 microgram of red C-protein/100 microgram of myosin. Polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate indicated that C-protein isolated from red skeletal muscle has a molecular weight approx. 7% greater than that of C-protein isolated from white skeletal muscle. The amino acid content of both forms of C-protein was similar but major differences in the mol % of isoleucine and threonine were found. Antiserum against C-protein from white rabbit skeletal muscle formed a single precipitin line with rabbit C-protein on double in agar. This antiserum did not form a precipitin line when diffused against red C-protein from rabbit skeletal muscle. Also, this antiserum bound specifically to the A-band region of myofibrils isolated from psoas (white) muscle, but it did not bind to myofibrils prepared from soleus (red) muscle.

Adenylate deaminase binding to synthetic thick filaments of myosin

Adenylate deaminase (AMP deaminase; AMP aminohydrolase, EC 3.5.4.6), a tetrameric enzyme found at particularly high concentrations in skeletal muscle, has previously been shown to bind strongly to the subfragment-2-portion of myosin in vitro and to the ends of the A band in vivo. It is shown here that when adenylate deaminase is dialyzed with skeletal myosin during formation of synthetic filaments at pH 7.0 it decorates the filament at 14.3-nm intervals, presumably in the region of exposed backbone between crossbridge levels. Optical diffraction of the aggregates reveals both enhancement of reflections arising from underlying myosin organization and other reflections arising from adenylate deaminase arrangement on the filament surface. Adenylate deaminase can thus be used as a specific label in the study of myosin presence and organization.

Effect of C-protein on actomyosin ATPase

The effect of C-protein on the actin-activated ATPase of column-purified skeletal muscle myosin has been investigated at varied ionic strength. At ionic strengths below about 0.1, C-protein is a potent inhibitor. The inhibition is not reversed by increasing the actin concentration, showing that it is caused by C-protein bound to the myosin filaments. When the ionic strength is raised above about 0.12, on the other hand, the inhibition vanishes and C-protein becomes a mild activator of the actomyosin ATPase. Both effects appear rapidly upon addition of C-protein to pre-formed myosin filaments, so C-protein probably acts by binding to the surface of the filaments.

The interaction of C-protein with heavy meromyosin and subfragment-2

C-protein has previously been shown to bind to the light-meromyosin region of the myosin tail. Examination of mixtures of C-protein with heavy meromyosin or subfragment-2 or subfragment-1 in the analytical ultracentrifuge shows that there is also a binding site for C-protein in the subfragment-2 region of the tail.

Myosin and paramyosin of Caenorhabditis elegans: biochemical and structural properties of wild-type and mutant proteins

Myosin and paramyosin have been purified from the nematode, Caenorhabditis elegans. The properties of the myosin in general resemble those of other myosins. The native molecule is a dimer of heavy (210,000 dalton) polypeptide chains and contains 18,000 and 16,000 dalton light chains. When rapidly precipitated from solution, it forms small, bipolar aggregates, about 150 nm long, consistent with the expected molecular structure of a rigid rod with a globular head region at one end. Its ATPase activity is stimulated by Ca2+ and EDTA. The myosin binds to F actin in a polar and ATP-sensitive manner, and the Mg2+-ATPase is activated by either F actin or nematode thin filaments. Dialysis of myosin to low ionic strength produces very long filaments. When a myosin-paramyosin mixture is dialyzed under the same condtions, co-filaments form which consist of a myosin cortex, surrounding a paramyosin core. Some properties of myosin from the mutants E675 and E190, which have functionally and structurally altered body wall muscles, are compared with those of wild-type myosin. These myosins of these results are discussed in terms of the myosin heavy chain composition.