Turnover of myosin heavy and light chains in cultured embryonic chick cardiac and skeletal muscle

The relative rates of synthesis and breakdown of myosin heavy and light chains were studied in primary cell cultures of embryonic chick cardiac and skeletal muscle. Measurements were made after 4 days in culture, at which time both skeletal and cardiac cultures were differentiated and contracted spontaneously. Following a 4-hr pulse of radioactive leucine, myosin and its heavy and light chains were extracted to 90% or greater purity and the specific activities of the proteins were determined. In cardiac muscle, myosin heavy chains were synthesized approximately 1.6 times the rate of myosin light chains, and in skeletal muscle, heavy chains were synthesized at approximately 1.4 times the rate of light chains. Relative rates of degradation of muscle proteins were determined using a dual-isotope technique. In general, the soluble and myofibrillar proteins of both types of muscle had decay rates proportional to their molecular weights (larger proteins generally had higher decay rates) based on analyses utilizing sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A notable exception to this general rule was myosin heavy chains, which had decay rates only slightly higher than the myosin light chains. Direct measurements on purified proteins indicated that the heavy chains of myosin were turning over at a slightly greater rate (approximately 20%) than the myosin light chains in both cardiac and skeletal muscle. The reasons for the apparent discrepancy between these measurements of myosin heavy and light chain synthesis and degradation are discussed.     

Inhibition of contraction of cultured muscle fibers results in increased turnover of myofibrillar proteins but not of intermediate- filament proteins

Muscle fibers are maintained in culture in a fully contractile state and are relaxed by the addition of 10(-7) M tetrodotoxin (TTX). This toxin binds to muscle membrane Na+- channels, abolishes spontaneous contractions and causes failure of the fiber to accumulate myosin heavy chains. These effects are reversible on removal of TTX. Synthesis and accumulation kinetics have been obtained for myofibrillar and for cytoplasmic filament proteins in normal, active muscle and in TTX- relaxed muscle fibers in culture. In relaxed fibers the synthesis of most proteins remained normal or slightly elevated. However, the accumulation of all myofibrillar proteins examined was markedly inhibited in TTX-treated cultures, whereas the accumulation of cytoplasmic filament proteins was normal or slightly elevated. Myofibrillar proteins examined were alpha-actin, troponin-C, myosin fast light chain 1, myosin fast light chain 2, alpha, beta-tropomyosins and the phosphorylated forms of tropomyosin and fast light chain 2. Cytoplasmic filament proteins studied were vimentin, alpha, beta-desmin and beta, alpha-actin. We also examined the synthesis and accumulation of six unidentified muscle-specific proteins and nine unidentified nonmuscle-specific proteins. Most of these proteins showed a normal accumulation pattern in TTX-relaxed fibers. We concluded that muscle fibers made inactive by TTX display an increased instability of all myofibrillar proteins while cytoplasmic filament proteins and cytoplasmic proteins in general are relatively unaffected. We suggest that TTX interferes, in a manner as yet unidentified, with assembly and normal stability of myofibrils. Decreased assembly and/or increased instability of myofibrils would lead to increased rates of myofibrillar protein degradation.     

Regulation of muscle gene expression: The accumulation of messenger RNAs coding for muscle-specific proteins during myogenesis in a mouse cell line

The expression of RNA sequences coding for myofibrillar proteins has been followed during terminal differentiation in a mouse skeletal muscle cell line. Cloned complementary DNA probes hybridizing with the actins, skeletal muscle α-actin, myosin heavy chain and the myosin alkali light chains were employed in Northern blotting experiments with total cellular poly (A)-containing RNA extracted from the cultures at different times after plating. At the same times, parallel cultures were pulse-labelled with [³⁵S]methionine and the pattern of newly synthesized proteins was analysed by two-dimensional gel electrophoresis. Synthesis of skeletal muscle α-actin and of the myosin alkali light chains (LC_lemb, LC₁, LC₃) was not detectable in dividing myoblast cultures. From the onset of cell fusion, the synthesis of myosin heavy chain, LC_lemb and α-actin increases with similar kinetics. Synthesis of LC₃ (and trace amounts of LC_1F) is detectable and subsequently increases at later stages of myotube formation. The corresponding messenger RNAs coding for myosin heavy chain and skeletal muscle α-actin are first detectable immediately before the initiation of myofibrillar protein synthesis. mRNAs coding for the non-muscle actins are accumulated in myoblasts and diminish after cell fusion. Comparisons between muscle mRNAs depend on the relative sensitivities of the different probes, reflecting mainly their homology with the isoform of the actin or myosin multigene family expressed. Quantitative analysis of Northern blots gives an estimated increase in skeletal muscle α-actin mRNA, with an homologous probe, of at least 130-fold with a minimum level of detection of 40 to 80 molecules per cell. Accumulation of this species and of the myosin heavy chain mRNA follows similar kinetics. mRNA coding for LC₃, the principal myosin light chain detected with the probe, appears to accumulate to a lesser extent initially, paralleling synthesis of the corresponding protein. These results using cloned probes demonstrate a close temporal correlation between muscle mRNA accumulation and protein synthesis during terminal myogenesis in this muscle line.     

Heterogeneity of myofibrillar proteins in lobster fast and slow muscles: variants of troponin, paramyosin, and myosin light chains comprise four distinct protein assemblages

Fast and slow muscles from the claws and abdomen of the American lobster Homarus americanus were examined for adenosine triphosphatase (ATPase) activity and for differences in myofibrillar proteins. Both myosin and actomyosin ATPase were correlated with fiber composition and contractile speed. Four distinct patterns of myofibrillar proteins observed in sodium dodecyl sulfate-polyacrylamide gels were distinguished by different assemblages of regulatory and contractile protein variants. A total of three species of troponin-T, five species of troponin-I, and three species of troponin-C were observed. Lobster myosins contained two groups of light chains (LC), termed "alpha" and "beta." There were three alpha-LC variants and two beta-LC variants. There were no apparent differences in myosin heavy chain, actin, and tropomyosin. Only paramyosin showed a pattern completely consistent with muscle fiber type: slow fibers contained a species (105 kD) slightly smaller than the principle variant (110 kD) in fast fibers. It is proposed that the type of paramyosin present could provide a biochemical marker to identify the fiber composition of muscles that have not been fully characterized. The diversity of troponin and myosin LC variants suggests that subtle differences in physiological performance exist within the broader categories of fast- and slow-twitch muscles.     

Phosphorylation of vertebrate nonmuscle and smooth muscle myosin heavy chains and light chains

In this article we review the various amino acids present in vertebrate nonmuscle and smooth muscle myosin that can undergo phosphorylation. The sites for phosphorylation in the 20 kD myosin light chain include serine-19 and threonine-18 which are substrates for myosin light chain kinase and serine-1 and/or-2 and threonine-9 which are substrates for protein kinase C. The sites in vertebrate smooth muscle and nonmuscle myosin heavy chains that can be phosphorylated by protein kinase C and casein kinase II are also summarized.Original data indicating that treatment of human T-lymphocytes (Jurkat cell line) with phorbol 12-myristate 13-acetate results in phosphorylation of both the 20 kD myosin light chain as well as the 200 kD myosin heavy chain is presented. We identified the amino acids phosphorylated in the human T-lymphocytes myosin light chains as serine-1 or serine-2 and in the myosin heavy chains as serine-1917 by 1-dimensional isoelectric focusing of tryptic phosphopeptides. Untreated T-lymphocytes contain phosphate in the serine-19 residue of teh myosin light chain and in a residue tentatively identified as serine-1944 in the myosin heavy chain.Abbreviations MLC myosin light chain - MHC myosin heavy chain - Tris tris(hydroxymethyl)aminomethane - EGTA [ethylenebis(oxyethylenenitrilo)]tetraacetic acid - EDTA ethylenediaminetetraacetate - TPCK N-tosyl-L-phenylalanine chloromethyl ketone - PMA phorbol 12-myristate 13-acetate     

调节轻链在平滑肌肌球蛋白分子上的定位

 应用凝胶电泳覆盖技术和放射自显影法研究了32~P-标记的平滑肌肌球蛋白调节轻链在肌球蛋白分子上的定位。实验结果表明调节轻链(LC_(20))可重新结合于平滑肌肌球蛋白重链(200kD),重酶解肌球蛋白(130kD)及其62kD和26kD肽段上。这提示调节轻链的结合点位于平滑肌肌球蛋白亚段-1羧基端的26kD肽段上。     

Diazepam induces relaxation of chick embryo muscle fibers in vitro and inhibits myosin synthesis

Diazepam (Valium/Roche) causes an immediate cessation of spontaneous contraction in chick embryo skeletal muscle fibers growing in vitro. Between 24–48 h later in the presence of 100 μM diazepam the relaxed muscle fibers no longer accumulate myosin as measured by the total amount of myosin heavy-chain peptide extracted from the cell cultures and identified by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. The myosin heavy chain assay procedure was standardized by quantitative precipitation of myosin with antibody to column purified chicken skeletal muscle myosin. Failure to accumulate myosin is related to a progressive inhibition of myosin synthesis. Diazepam-treated cultures showed an 80% inhibition of myosin heavy-chain synthesis over a period of 4 days. At the same time the rate of myosin heavy-chain degradation increases in diazepam-treated cultures relative to matched control cultures. Total protein synthesis was only marginally affected suggesting that diazepam may differentially inhibit myofibrillar protein synthesis. All of the observed effects of diazepam were reversible if drug exposure was limited to 48 h. The apparent specificity and reversibility of diazepam suggests that the drug will be useful in probing the mechanisms of terminal skeletal muscle cell differentiation and the hypotrophic relationship between chronic relaxation and inhibition of accumulation of myosin and perhaps other myofibrillar proteins.     

Further studies on single fibres of bovine muscles.

Young & Davey (1981) (Biochem. J. 195, 317-327) identified numbers of polymorphs of myofibrillar proteins by sodium dodecyl sulphate/polyacrylamide gel electrophoresis of single muscle fibres isolated from three bovine muscles. Fibres were classed according to the distribution of polymorphs. The study has now been extended to eight diverse bovine muscles. The previous distinction made between fast and slow fibres is valid without exception in the extended study. Within these classes, variations in myofibrillar expression are examined within and between fibres, muscles and animals. Two slow muscles are contrasted; masseter is homogeneous in fibre type, whereas diaphragm is subtly heterogeneous, possibly arising from greater physiological demands. Of the myofibrillar polymorphs, attention is concentrated on two variants of fast-muscle myosin heavy chain. Both are present in all fast and mixed muscles examined, except sternomandibularis, and each is respectively associated with certain unidentified proteins. Within a muscle the fast-muscle myosin light-chain expression is the same irrespective of the heavy-chain variant. Histochemical techniques demonstrated that the variants are respectively associated with types IIA and IIB as defined by other investigators.     

Analysis of myosin light and heavy chain types in single human skeletal muscle fibers

In this study, myosin types in human skeletal muscle fibers were investigated with electrophoretic techniques. Single fibers were dissected out of lyophilized surgical biopsies and typed by staining for myofibrillar ATPase after preincubation in acid or alkaline buffers. After 14C-labelling of the fiber proteins in vitro by reductive methylation, the myosin light chain pattern was analysed on two-dimensional gels and the myosin heavy chains were investigated by one-dimensional peptide mapping. Surprisingly, human type I fibers, which contained only the slow heavy chain, were found to contain variable amounts of fast myosin light chains in addition to the two slow light chains LC1s and LC2s. The majority of the type I fibers in normal human muscle showed the pattern LC1s, LC2s and LC1f. Further evidence for the existence in human muscle of a hybrid myosin composed of a slow heavy chain with fast and slow light chains comes from the analysis of purified human myosin in the native state by pyrophosphate gel electrophoresis. With this method, a single band corresponding to slow myosin was obtained; this slow myosin had the light chain composition LC1s, LC2s and LC1f. Type IIA and IIB fibers, on the other hand, revealed identical light chain patterns consisting of only the fast light chains LC1f, LC2f and LC3f but were found to have different myosin havy chains. On the basis of the results presented, we suggest that the histochemical ATPase normally used for fibre typing is determined by the myosin heavy chain type (and not by the light chains). Thus, in normal human muscle a number of 'hybrid' myosins were found to occur, namely two extreme forms of fast myosins which have the same light chains but different heavy chains (IIA and IIB) and a continuum of slow forms consisting of the same heavy chain and slow light chains with a variable fast light chain composition. This is consistent with the different physiological roles these fibers are thought to have in muscle contraction.     

Disproportionate reduction of actin synthesis in hearts of starved rats

We examined the synthesis of proteins in rat myocardium after starvation. Rates of total protein synthesis in myofibrillar and nonmyofibrillar fractions of myocardium of starved animals were reduced similarly (to 70-80% of the rates in hearts of fed animals, p less than 0.002), but rates of synthesis of some individual proteins were affected discoordinately. Radiolabeled proteins from atrial and ventricular explants, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, revealed that starvation for 2 days reduced the rate of cardiac actin synthesis to 26-38% of control levels, while the rate of myosin heavy chain synthesis in the same hearts was only moderately reduced (74-80% of control levels). This starvation-induced reduction in actin synthesis could be accounted for at least in part by disproportionately decreased levels of actin mRNA in starved hearts, as revealed by Northern blot hybridization and by in vitro translation analysis. The dramatic decrease in cardiac actin synthesis was rapidly reversible, and actin synthesis returned to normal after a single day of refeeding. The selective reduction of actin synthesis after starvation was specific for the heart: rates of myosin heavy chain and actin synthesis in skeletal muscles (soleus and extensor digitorum longus) were coordinately reduced in response to starvation. To our knowledge, this is the first example of such dramatic discoordinate regulation of myofibrillar protein synthesis in response to a physiological stimulus.     

Effect of glucocorticoids on the turnover rate of actin and myosin heavy and light chains on different types of skeletal muscle fibres

The catabolic action of glucocorticoids on the molecular level of the two main muscular proteins, myosin and actin, was found to depend on the type of muscle fibres. The synthesis rate of actin and myosin heavy chain was decreased in all types of muscle fibres, and in myosin light chain only in the slow-twitch red fibres. The turnover rate of actin and myosin heavy chain was also found decreased in all types of muscle fibres. The myosin light chains turned over more rapidly in dexamethasone-treated than in the control rats in all types of muscle fibres except in the case of the slow-twitch red ones as was shown by single and double isotope methods. Dexamethasone treatment enhanced the urinary 3-methylhistidine excretion in rats by 60%.     

Regulation of myofibrillar accumulation in chick muscle cultures: evidence for the involvement of calcium and lysosomes in non-uniform turnover of contractile proteins

The effect of calcium on myofibrillar turnover in primary chick leg skeletal muscle cultures was examined. Addition of the calcium ionophore A23187 at subcontraction threshold levels (0.38 microM) increased significantly rates of efflux of preloaded 45Ca+2 but had no effect on total protein accumulation. However, A23187 as well as ionomycin caused decreased accumulation of the myofibrillar proteins, myosin heavy chain (MHC), myosin light chain 1f (LC1f), 2f (LC2f), alpha-actin (Ac), and tropomyosin (TM). A23187 increased the degradation rate of LC1f, LC2f, and TM after 24 h. In contrast, the calcium ionophore caused decreased degradation of Ac and troponin-C and had no effect on the degradation of MHC, troponin-T, troponin-I, or alpha, beta-desmin (Dm). In addition, A23187 did not alter degradation of total myotube protein. The ionophore had little or no effect on the synthesis of total myotube proteins, but caused a marked decrease in the synthesis of MHC, LC1f, LC2f, Ac, TM, and Dm after 48 h. The mechanisms involved in calcium-stimulated degradation of the myofibrillar proteins were also investigated. Increased proteolysis appeared to involve a lysosomal pathway, since the effect of the Ca++ ionophore could be blocked by the protease inhibitor leupeptin and the lysosomotropic agents methylamine and chloroquine. The effects of A23187 occur in the presence of serum, a condition in which no lysosomal component of overall protein degradation is detected. The differential effect of A23187 on the degradative rates of the myofibrillar proteins suggests a dynamic structure for the contractile apparatus.     

Easily releasable myofilaments from skeletal and cardiac muscles maintained in vitro. Role in myofibrillar assembly and turnover

Gentle treatment with an ATP-containing relaxing solution of isolated myofibrils from rat diaphragm, soleus, extensor digitorum longus, and left atria maintained in vitro releases a small amount of myofilaments constituting less than 5% of total myofibrillar protein. Successive extraction of myofibrils produced little further filament release. Releasable myofilaments lack alpha-actinin (Mr = 95,000), certain very high molecular weight proteins (greater than 200,000), and possibly M-line protein but contain other myofibrillar proteins. After pulse-labeling with [3H]leucine for 8 min, specific activity of the myosin heavy chain in the easily releasable myofilaments is 3-6 times higher than the specific activity of myosin heavy chain in the residual myofibrils, although 85-90% of total label is in the myofibrillar myosin. In the absence of protein synthesis, releasable filament specific activity decreases, with a half-time of 60-90 min, to that of the myofibrillar myosin. This labeling pattern appears inconsistent with a simple precursor-product relationship between releasable filaments and myofibrils suggesting that the filaments originate largely from myofibrils. Preincubation of muscles with several factors known to decrease proteolysis, i.e. passive stretch, leupeptin, colchicine, and cycloheximide, reduced the size of the releasable filament fraction. Treatment of muscles with the calcium ionophore A23187, which accelerates proteolysis, and pretreatment of myofibrils with either trypsin or calcium-dependent protease increased filament release. Therefore, the releasable filament fraction may contain intermediates in the breakdown of myofibrils. The labeling kinetics may indicate a mixing of myofilaments within myofibrils which functions in the movement of contractile protein to its possible site of degradation, i.e. the myofibrillar surface.     

Contents of myofibrillar proteins in cardiac, skeletal, and smooth muscles

The in situ contents of myosin, actin, alpha-actinin, tropomyosin, troponin, desmin were estimated in dog cardiac, rabbit skeletal, and chicken smooth muscles. Whole muscle tissues were dissolved with 8 M guanidine hydrochloride and subjected to two-dimensional gel electrophoresis, which is a nonequilibrium pH gradient electrophoresis (Murakami, U. & Uchida, K. (1984) J. Biochem. 95, 1577-1584) with some modification. The amount of protein in a spot on a slab gel was determined by quantification of the extracted dye. Dye binding capacity of individual myofibrillar proteins was determined by using the purified protein. Myosin contents were 82 +/- 7 pmol/mg wet weight in cardiac muscle, 105 +/- 10 pmol/mg wet weight in skeletal muscle, and 45 +/- 4 pmol/mg wet weight in smooth muscle. Actin contents were 339 +/- 15 pmol/mg wet weight in cardiac muscle, 625 +/- 27 pmol/mg wet weight in skeletal muscle, and 742 +/- 13 pmol/mg wet weight in smooth muscle. The subunit stoichiometry of myosin in the three types of muscles was two heavy chains and four light chains, and there was one light chain 2 for every heavy chain. The molar ratio of actin to tropomyosin was 7/1 in the three types of muscles. Striking differences were seen in the molar ratio of myosin to actin: 1.0/4.1 in cardiac muscle, 1.0/6.0 in skeletal muscle, and 1.0/16.5 in smooth muscle.     

Myofibrillar protein oxidation and contractile dysfunction in hyperthyroid rat diaphragm.

The purpose of the present study was to test the hypothesis that administration of thyroid hormone [3,5,3'-triiodo-L-thyronine (T(3))] could result in oxidation of myofibrillar proteins and, in turn, induce alterations in respiratory muscle function. Daily injection of T(3) for 21 days depressed isometric forces of diaphragm fiber bundles across a range of stimulus frequencies (1, 10, 20, 40, 75, and 100 Hz) (P < 0.05). These reductions in force production were accompanied by a remarkable increment (104%; P < 0.05) in carbonyl groups of myofibrillar proteins. In contrast, T(3) treatment has no effects on the carbonyl content in myosin heavy chain. In additional experiments, we have also tested the efficacy of carvedilol, a nonselective beta(1)- beta(2)-blocker that possesses antioxidative properties. Treatment with carvedilol dramatically improved isometric tetanic force production at stimulus frequencies from 40 to 100 Hz (P < 0.05). Carvedilol also prevented T(3)-induced contractile protein oxidation (P < 0.05). These data suggest that the oxidative modification of myofibrillar proteins may account, at least in part, for an impairment of diaphragm in hyperthyroidism.     

Age-related changes in the incorporation of [14-C] leucine into myofibrillar and sarcoplasmic proteins of red and white muscles of chicks.

Studies on the incorporation of DL-[1- 14-C] leucine into myosin, total myofibrillar protein and total sarcoplasmic protein have shown age-dependent alterations in the rate of synthesis of these protiens in red and white skeletal muscles of chicks. During the early phase of ex ovo development white muscle synthesizes significantly higher amounts of myofibrillar proteins, especially myosin, in comparison with red muscle. The rate of sarcoplasmic protein synthesis in red and white muscles one day after hatching is almost identical. The red muscle shows a markedly higher rate of sarcoplasmic protein synthesis from 10 days after hatching. The incorporation of amino acid into various protein fractions of both the muscle types decreases with advancing age. In adult chicks red muscle displays a higher ability to synthesize sarcoplasmic and myofibrillar proteins.     

Electrophoretic analysis of proteins from single bovine muscle fibres.

A number of single fibres were isolated by dissection of four bovine masseter (ma) muscles, three rectus abdominis (ra) muscles and eight sternomandibularis (sm) muscles. By histochemical criteria these muscles contain respectively, solely slow fibres (often called type I), predominantly fast fibres (type II), and a mixture of fast and slow. The fibres were analysed by conventional sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and the gels stained with Coomassie Blue. Irrespective of the muscle, every fibre could be classed into one of two broad groups based on the mobility of proteins in the range 135000-170000 daltons. When zones containing myosin heavy chain were cut from the single-fibre gel tracks and 'mapped' [Cleveland, Fischer, Kirschner & Laemmli (1977) J. Biol. Chem. 252, 1102-1106] with Staphylococcus proteinase, it was found that one group always contained fast myosin heavy chain, whereas the second group always contained the slow form. Moreover, a relatively fast-migrating alpha-tropomyosin was associated with the fast myosin group and a slow-migrating form with the slow myosin group. All fibres also contained beta-tropomyosin; the coexistence of alpha- and beta-tropomyosin is at variance with evidence that alpha-tropomyosin is restricted to fast fibres [Dhoot & Perry (1979) Nature (London) 278, 714-718]. Fast fibres containing the expected fast light chains and troponins I and C fast were identified in the three ra muscles, but in only four sm muscles. In three other sm muscles, all the fast fibres contained two troponins I and an additional myosin light chain that was more typical of myosin light chain 1 slow. The remaining sm muscle contained a fast fibre type that was similar to the first type, except that its myosin light chain 1 was more typical of the slow polymorph. Troponin T was bimorphic in all fast fibres from a ra muscles and in at least some fast fibres from one sm muscle. Peptide 'mapping' revealed two forms of fast myosin heavy chain distributed among fast fibres. Each form was associated with certain other proteins. Slow myosin heavy chain was unvarying in three slow fibre types identified. Troponin I polymorphs were the principal indicator of slow fibre types. The myofibrillar polymorphs identified presumably contribute to contraction properties, but beyond cud chewing involving ma muscle, nothing is known of the conditions that gave rise to the variable fibre composites in sm and ra muscles.     

Biochemical investigations into the absence of rigor mortis in the Norway lobster Nephrops norvegicus

It was found that the striated muscle of the Norway lobster (Nephrops norvegicus) does not exhibit the rigor mortis state otherwise typical for this type of muscle. This absence of rigor was investigated, concentrating on changes in the structure, ultrastructure and post-mortem biochemistry of the muscle. Samples were initially fixed for light and electron microscopy at the time of death and at different times post-mortem (3, 6, 12 and 24 h). Protein extracts were obtained in parallel to compare the banding patterns of the myofibrillar proteins using SDS-PAGE. A Western blot was applied to elucidate if myosin - a representative major myofibrillar protein - was degraded post-mortem. And finally, ATP levels in the muscle were analyzed using HPLC. Using TEM imaging it was found that between 12 and 24 h post-mortem at a storage temperature of 10 °C, when rigor mortis should set in (according to the muscular ATP concentrations), an extensive, but rather specific breakdown of myofibrillar proteins occurred. The Z-disks were degraded and the myofibrillar structure was lost. SDS-PAGE and Western blot clearly demonstrated the post-mortem breakdown of myosin. The nature of the observed protein breakdown seems to impede rigor mortis in some way by the activation of at least one of the several proteolytic systems (cathepsins, calpains and others) found in vertebrates and invertebrates. It is speculated that the proteolysis simply overtakes the rigor-inducing post-mortem changes.     

During muscle atrophy,thick, but not thin,filament components are degraded by MuRF1-dependent ubiquitylation

Loss of myofibrillar proteins is a hallmark of atrophying muscle. Expression of muscle RING-finger 1 (MuRF1), a ubiquitin ligase, is markedly induced during atrophy, and MuRF1 deletion attenuates muscle wasting. We generated mice expressing a Ring-deletion mutant MuRF1, which binds but cannot ubiquitylate substrates. Mass spectrometry of the bound proteins in denervated muscle identified many myofibrillar components. Upon denervation or fasting, atrophying muscles show a loss of myosin-binding protein C (MyBP-C) and myosin light chains 1 and 2 (MyLC1 and MyLC2) from the myofibril, before any measurable decrease in myosin heavy chain (MyHC). Their selective loss requires MuRF1. MyHC is protected from ubiquitylation in myofibrils by associated proteins, but eventually undergoes MuRF1-dependent degradation. In contrast, MuRF1 ubiquitylates MyBP-C, MyLC1, and MyLC2, even in myofibrils. Because these proteins stabilize the thick filament, their selective ubiquitylation may facilitate thick filament disassembly. However, the thin filament components decreased by a mechanism not requiring MuRF1.