A protein that binds specifically to the M-line of skeletal muscle is identified as the muscle form of creatine kinase

Distinct myofibrillar sub-proteomic profiles are associated with the instrumental texture of aged pork loin

Fresh pork tenderness contributes to consumer satisfaction with the eating experience. Postmortem proteolysis of proteins within and between myofibrils has been closely linked with pork tenderness development. A clear understanding of the molecular features associated with pork tenderness development will provide additional targets and open the door to new solutions to improve and make pork tenderness development more consistent. Therefore, the objective was to utilize liquid chromatography and mass spectrometry with tandem mass tag (TMT) multiplexing to evaluate myofibrillar sub-proteome differences between pork chops of different instrumental star probe values. Pork loins (N = 120) were collected from a commercial harvest facility at 24 h postmortem. Quality and sensory attributes were evaluated at 24 h postmortem and after ~2 weeks of postmortem aging. Pork chops were grouped into 4 groups based on instrumental star probe value (group A,x¯ = 4.23 kg, 3.43 to 4.55 kg; group B,x¯ = 4.79 kg, 4.66 to 5.00 kg; group C,x¯ = 5.43 kg, 5.20 to 5.64 kg; group D,x¯ = 6.21 kg, 5.70 to 7.41 kg; n = 25 per group). Myofibrillar proteins from the samples aged ~2 wk were fractionated, washed, and solubilized in 8.3 M urea, 2 M thiourea, and 1% dithiothreitol. Proteins were digested with trypsin, labeled with 11-plex isobaric TMT reagents, and identified and quantified using a Q-Exactive Mass Spectrometer. Between groups A and D, 54 protein groups were differentially abundant (adjusted P < 0.05). Group A had a greater abundance of proteins related to the thick and thin filament and a lesser abundance of Z-line-associated proteins and metabolic enzymes than group D chops. These data highlight that distinct myofibrillar sub-proteomes are associated with pork chops of different tenderness values. Future research should evaluate changes immediately and earlier postmortem to further elucidate myofibrillar sub-proteome differences over the postmortem aging period.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

The advantage of channeling nucleotides for very processive functions

Nucleoside triphosphate (NTP)s, like ATP (adenosine 5'-triphosphate) and GTP (guanosine 5'-triphosphate), have long been considered sufficiently concentrated and diffusible to fuel all cellular ATPases (adenosine triphosphatases) and GTPases (guanosine triphosphatases) in an energetically healthy cell without becoming limiting for function. However, increasing evidence for the importance of local ATP and GTP pools, synthesised in close proximity to ATP- or GTP-consuming reactions, has fundamentally challenged our view of energy metabolism. It has become evident that cellular energy metabolism occurs in many specialised 'microcompartments', where energy in the form of NTPs is transferred preferentially from NTP-generating modules directly to NTP-consuming modules. Such energy channeling occurs when diffusion through the cytosol is limited, where these modules are physically close and, in particular, if the NTP-consuming reaction has a very high turnover, i.e. is very processive. Here, we summarise the evidence for these conclusions and describe new insights into the physiological importance and molecular mechanisms of energy channeling gained from recent studies. In particular, we describe the role of glycolytic enzymes for axonal vesicle transport and nucleoside diphosphate kinases for the functions of dynamins and dynamin-related GTPases.

The advantage of channeling nucleotides for very processive functions

Nucleoside triphosphate (NTP)s, like ATP (adenosine 5'-triphosphate) and GTP (guanosine 5'-triphosphate), have long been considered sufficiently concentrated and diffusible to fuel all cellular ATPases (adenosine triphosphatases) and GTPases (guanosine triphosphatases) in an energetically healthy cell without becoming limiting for function. However, increasing evidence for the importance of local ATP and GTP pools, synthesised in close proximity to ATP- or GTP-consuming reactions, has fundamentally challenged our view of energy metabolism. It has become evident that cellular energy metabolism occurs in many specialised 'microcompartments', where energy in the form of NTPs is transferred preferentially from NTP-generating modules directly to NTP-consuming modules. Such energy channeling occurs when diffusion through the cytosol is limited, where these modules are physically close and, in particular, if the NTP-consuming reaction has a very high turnover, i.e. is very processive. Here, we summarise the evidence for these conclusions and describe new insights into the physiological importance and molecular mechanisms of energy channeling gained from recent studies. In particular, we describe the role of glycolytic enzymes for axonal vesicle transport and nucleoside diphosphate kinases for the functions of dynamins and dynamin-related GTPases.

Creatine kinase in cell cycle regulation and cancer

The phosphocreatine-creatine kinase (CK) shuttle system is increasingly recognized as a fundamental mechanism for ATP homeostasis in both excitable and non-excitable cells. Many intracellular processes are ATP dependent. Cell division is a process requiring a rapid rate of energy turnover. Cell cycle regulation is also a key point to understanding the mechanisms underlying cancer progression. It has been known for about 40 years that aberrant CK levels are associated with various cancers and for over 30 years that CK is involved in mitosis regulation. However, the underlying molecular mechanisms have not been investigated sufficiently until recently. By maintaining ATP at sites of high-energy demand, CK can regulate cell cycle progression by affecting the intracellular energy status as well as by influencing signaling pathways that are essential to activate cell division and cytoskeleton reorganization. Aberrant CK levels may impair cell viability under normal or stressed conditions and induce cell death. The involvement of CK in cell cycle regulation and cellular energy metabolism makes it a potential diagnostic biomarker and therapeutic target in cancer. To understand the multiple physiological/pathological functions of CK, it is necessary to identify CK-binding partners and regulators including proteins, non-coding RNAs and participating endogenous small molecular weight chemical compounds. This review will focus on molecular mechanisms of CK in cell cycle regulation and cancer progression. It will also discuss the implications of recent mechanistic studies, the emerging problems and future challenges of the multifunctional enzyme CK.

Creatine for women: a review of the relationship between creatine and the reproductive cycle and female-specific benefits of creatine therapy

The creatine/phosphocreatine/creatine kinase circuit is instrumental in regulating high-energy phosphate metabolism, and the maintenance of cellular energy turnover. The mechanisms by which creatine is able to buffer and regulate cellular energy balance, maintain acid-base balance, and reduce the effects of oxidative stress have led to a large number of studies into the use of creatine supplementation in exercise performance and to treat diseases associated with cellular energy depletion. Some of these studies have identified sex-specific responses to creatine supplementation, as such; there is the perception, that females might be less receptive to the benefits of creatine supplementation and therapy, compared to males. This review will describe the differences in male and female physique and physiology that may account for such differences, and discuss the apparent endocrine modulation of creatine metabolism in females. Hormone-driven changes to endogenous creatine synthesis, creatine transport and creatine kinase expression suggest that significant changes in this cellular energy circuit occur during specific stages of a female's reproductive life, including pregnancy and menopause. Recent studies suggest that creatine supplementation may be highly beneficial for women under certain conditions, such as depression. A greater understanding of these pathways, and the consequences of alterations to creatine bioavailability in females are needed to ensure that creatine is used to full advantage as a dietary supplement to optimize and enhance health outcomes for women.

Fibre typing of intrafusal fibres

The first descriptions of muscle spindles with intrafusal fibres containing striated myofibrils and nervous elements were given approximately 150 years ago. It took, however, another 100 years to establish the presence of two types of intrafusal muscle fibres: nuclear bag and nuclear chain fibres. The present paper highlights primarily the contribution of Robert Banks in fibre typing of intrafusal fibres: the confirmation of the principle of two types of nuclear bag fibres in mammalian spindles and the variation in occurrence of a dense M-band along the fibres. Furthermore, this paper summarizes how studies from the Umeå University group (Laboratory of Muscle Biology in the Department of Integrative Medical Biology) on fibre typing and the structure and composition of M-bands have contributed to the current understanding of muscle spindle complexity in adult humans as well as to muscle spindle development and effects of ageing. The variable molecular composition of the intrafusal sarcomeres with respect to myosin heavy chains and M-band proteins gives new perspectives on the role of the intrafusal myofibrils as stretch-activated sensors influencing tension/stiffness and signalling to nuclei.

Serum proteomic profiling reveals fragments of MYOM3 as potential biomarkers for monitoring the outcome of therapeutic interventions in muscular dystrophies

Therapy-responsive biomarkers are an important and unmet need in the muscular dystrophy field where new treatments are currently in clinical trials. By using a comprehensive high-resolution mass spectrometry approach and western blot validation, we found that two fragments of the myofibrillar structural protein myomesin-3 (MYOM3) are abnormally present in sera of Duchenne muscular dystrophy (DMD) patients, limb-girdle muscular dystrophy type 2D (LGMD2D) and their respective animal models. Levels of MYOM3 fragments were assayed in therapeutic model systems: (1) restoration of dystrophin expression by antisense oligonucleotide-mediated exon-skipping in mdx mice and (2) stable restoration of α-sarcoglycan expression in KO-SGCA mice by systemic injection of a viral vector. Following administration of the therapeutic agents MYOM3 was restored toward wild-type levels. In the LGMD model, where different doses of vector were used, MYOM3 restoration was dose-dependent. MYOM3 fragments showed lower inter-individual variability compared with the commonly used creatine kinase assay, and correlated better with the restoration of the dystrophin-associated protein complex and muscle force. These data suggest that the MYOM3 fragments hold promise for minimally invasive assessment of experimental therapies for DMD and other neuromuscular disorders.

A pilot study of muscle plasma protein changes after exercise

INTRODUCTION

Creatine kinase (CK) and myoglobin (Mb) do not possess all good qualities as biomarkers of skeletal muscle damage. We investigated the utility of troponin I (TnI) and telethonin (Tcap) as markers and examined their temporal profiles after skeletal muscle damage.

METHODS

Plasma profiles were measured before and after exercise in 3 groups: subjects affected by either Becker muscular dystrophy or McArdle disease, and healthy subjects.

RESULTS

Mb and TnI appeared early in the blood, and the increase of TnI was only observed in patients with muscle disease. The CK increase was more delayed in plasma. Tcap was not detectable at any time.

CONCLUSIONS

Our results suggest that TnI is a marker of more severe damage signifying sarcomeric damage, and it could therefore be an important supplement to CK and Mb in clinical practice. Tcap is not useful as a marker for skeletal muscle damage.

Unchanged mitochondrial organization and compartmentation of high-energy phosphates in creatine-deficient GAMT-/- mouse hearts

Disruption of the creatine kinase (CK) system in hearts of CK-deficient mice leads to changes in the ultrastructure and regulation of mitochondrial respiration. We expected to see similar changes in creatine-deficient mice, which lack the enzyme guanidinoacetate methyltransferase (GAMT) to produce creatine. The aim of this study was to characterize the changes in cardiomyocyte mitochondrial organization, regulation of respiration, and intracellular compartmentation associated with GAMT deficiency. Three-dimensional mitochondrial organization was assessed by confocal microscopy. On populations of permeabilized cardiomyocytes, we recorded ADP and ATP kinetics of respiration, competition between mitochondria and pyruvate kinase for ADP produced by ATPases, ADP kinetics of endogenous pyruvate kinase, and ATP kinetics of ATPases. These data were analyzed by mathematical models to estimate intracellular compartmentation. Quantitative analysis of morphological and kinetic data as well as derived model fits showed no difference between GAMT-deficient and wild-type mice. We conclude that inactivation of the CK system by GAMT deficiency does not alter mitochondrial organization and intracellular compartmentation in relaxed cardiomyocytes. Thus, our results suggest that the healthy heart is able to preserve cardiac function at a basal level in the absence of CK-facilitated energy transfer without compromising intracellular organization and the regulation of mitochondrial energy homeostasis. This raises questions on the importance of the CK system as a spatial energy buffer in unstressed cardiomyocytes.

Hydrogen peroxide targets the cysteine at the active site and irreversibly inactivates creatine kinase

In our study, we showed that at a relatively low concentration, H(2)O(2) can irreversibly inactivate the human brain type of creatine kinase (HBCK) and that HBCK is inactivated in an H(2)O(2) concentration-dependent manner. HBCK is completely inactivated when incubated with 2mM H(2)O(2) for 1h (pH 8.0, 25°C). Inactivation of HBCK is a two-stage process with a fast stage (k(1)=0.050 ± 0.002 min(-1)) and a slow (k(2)=0.022 ± 0.003 min(-1)) stage. HBCK inactivation by H(2)O(2) was affected by pH and therefore we determined the pH profile of HBCK inactivation by H(2)O(2). H(2)O(2)-induced inactivation could not be recovered by reducing agents such as dl-dithiothreitol, N-acetyl-L-cysteine, and l-glutathione reduced. When HBCK was treated with DTNB, an enzyme substrate that reacts specifically with active site cysteines, the enzyme became resistant to H(2)O(2). HBCK binding to Mg(2+)ATP and creatine can also prevent H(2)O(2) inactivation. Intrinsic and 1-anilinonaphthalene-8-sulfonate-binding fluorescence data showed no tertiary structure changes after H(2)O(2) treatment. The thiol group content of H(2)O(2)-treated HBCK was reduced by 13% (approximately 1 thiol group per HBCK dimer, theoretically). For further insight, we performed a simulation of HBCK and H(2)O(2) docking that suggested the CYS283 residue could interact with H(2)O(2). Considering these results and the asymmetrical structure of HBCK, we propose that H(2)O(2) specifically targets the active site cysteine of HBCK to inactivate HBCK, but that substrate-bound HBCK is resistant to H(2)O(2). Our findings suggest the existence of a previously unknown negative form of regulation of HBCK via reactive oxygen species.

Characterization of caged compounds binding to proteins by NMR spectroscopy

Photolysable caged ligands are used to investigate protein function and activity. Here, we investigate the binding properties of caged nucleotides and their photo released products to well established but evolutionary and structurally unrelated nucleotide-binding proteins, rabbit muscle creatine kinase (RMCK) and human annexin A6 (hAnxA6), using saturation transfer difference NMR spectroscopy. We detect the binding of the caged nucleotides and discuss the general implications on interpreting data collected with photolysable caged ligands using different techniques. Strategies to avoid non-specific binding of caged compound to certain proteins are also suggested.

Muscle RING-finger protein-1 (MuRF1) as a connector of muscle energy metabolism and protein synthesis

During pathophysiological muscle wasting, a family of ubiquitin ligases, including muscle RING-finger protein-1 (MuRF1), has been proposed to trigger muscle protein degradation via ubiquitination. Here, we characterized skeletal muscles from wild-type (WT) and MuRF1 knockout (KO) mice under amino acid (AA) deprivation as a model for physiological protein degradation, where skeletal muscles altruistically waste themselves to provide AAs to other organs. When WT and MuRF1 KO mice were fed a diet lacking AA, MuRF1 KO mice were less susceptible to muscle wasting, for both myocardium and skeletal muscles. Under AA depletion, WT mice had reduced muscle protein synthesis, while MuRF1 KO mice maintained nonphysiologically elevated levels of skeletal muscle protein de novo synthesis. Consistent with a role of MuRF1 for muscle protein turnover during starvation, the concentrations of essential AAs, especially branched-chain AAs, in the blood plasma significantly decreased in MuRF1 KO mice under AA deprivation. To clarify the molecular roles of MuRF1 for muscle metabolism during wasting, we searched for MuRF1-associated proteins using pull-down assays and mass spectrometry. Muscle-type creatine kinase (M-CK), an essential enzyme for energy metabolism, was identified among the interacting proteins. Coexpression studies revealed that M-CK interacts with the central regions of MuRF1 including its B-box domain and that MuRF1 ubiquitinates M-CK, which triggers the degradation of M-CK via proteasomes. Consistent with MuRF1's role of adjusting CK activities in skeletal muscles by regulating its turnover in vivo, we found that CK levels were significantly higher in the MuRF1 KO mice than in WT mice. Glucocorticoid modulatory element binding protein-1 and 3-hydroxyisobutyrate dehydrogenase, previously identified as potential MuRF1-interacting proteins, were also ubiquitinated MuRF1-dependently. Taken together, these data suggest that, in a multifaceted manner, MuRF1 participates in the regulation of AA metabolism, including the control of free AAs and their supply to other organs under catabolic conditions, and in the regulation of ATP synthesis under metabolic-stress conditions where MuRF1 expression is induced.

Regulation of skeletal muscle creatine kinase from a hibernating mammal

Control over skeletal muscle energetics is critical in hibernation to sustain viability over weeks of cold torpor and to support shivering thermogenesis during arousal. Creatine kinase (CK) has a key role in muscle energetics and this study analyzes muscle CK from ground squirrels, Spermophilus richardsonii. CK activity was approximately 20% lower during hibernation than in euthermia, as was CK protein whereas CK mRNA was reduced by approximately 70%. Hibernator CK showed reduced affinity for ATP and creatine, compared with euthermic CK. Incubations that promoted endogenous protein kinase or phosphatase action, coupled with ion exchange chromatography to separate high and low phosphate forms, showed that soluble CK from euthermic squirrels was a mix of phosphorylated and dephosphorylated forms whereas only phospho-CK was detected in hibernating animals. High and low phosphate CK forms had different affinities for ATP and creatine substrates but did not differ in stability to urea denaturation. About 20-25% of CK was bound to the insoluble fraction of muscle and bound CK showed different kinetic responses to kinase and phosphatase treatments.

Muscle-specific creatine kinase gene polymorphism and running economy responses to an 18-week 5000-m training programme

OBJECTIVE

To investigate the association between muscle-specific creatine kinase (CKMM) gene polymorphism and the effects of endurance training on running economy.

METHODS

102 biologically unrelated male volunteers from northern China performed a 5000-m running programme, with an intensity of 95-105% ventilatory threshold. The protocol was undertaken three times per week and lasted for 18 weeks. Running economy indexes were determined by making the participants run on a treadmill before and after the protocol, and the A/G polymorphism in the 3' untranslated region of CKMM was detected by polymerase chain reaction-restricted fragment length polymorphism (NcoI restriction enzyme).

RESULTS

Three expected genotypes for CKMM-NcoI (AA, AG and GG) were observed in the participants. After training, all running economy indexes declined markedly. Change in steady-state consumption of oxygen, change in steady-state consumption of oxygen by mean body weight, change in steady-state consumption of oxygen by mean lean body weight and change in ventilatory volume in AG groups were larger than those in AA and GG groups.

CONCLUSIONS

The findings indicate that the CKMM gene polymorphism may contribute to individual running economy responses to endurance training.

Transgenesis and cardiac energetics: new insights into cardiac metabolism

Transgenesis in the mouse heart has provided new and important insights into many aspects of ATP synthesis, supply and utilization. Cardiac energetics has also been useful in assessing the consequences of manipulating proteins in the mouse heart. Here, four topics are reviewed. Part 1 presents a description of the role of "energy circuits" in addressing these questions: how is ATP made in the mitochondria supplied to spatially separated ATPases rapidly enough to support variable and abrupt increases in work? Given the barriers to rapid diffusion of ADP, how is a high chemical driving force maintained at the various sites of ATP hydrolysis; i.e. how is [ADP] maintained low throughout the cell? What are the metabolic sensors matching ATP synthesis and utilization? How are they monitored, delivered to the appropriate sensors and translated to accomplish a constant ATP supply? In Part 2, the consequences of manipulating glucose supply to the heart and regulation of the synthesis of enzymes in glycolysis and fatty acid oxidation are discussed. The questions are: what are the signals that lead to long-term molecular reprogramming of metabolic pathways for ATP synthesis and utilization? How is this accomplished? In Part 3, the focus is on sarcomeric proteins addressing the question: what changes in sarcomeric proteins determine the cost of contraction? Finally, in Part 4, examples are given of how energetics has been used to define the consequences of transgenic manipulations.

Elements of the major myofibrillar binding peptide motif are present in the earliest of true muscle type creatine kinases

Most vertebrates possess two genes for cytoplasmic creatine kinase (CK) coding for muscle (M-CK) and brain (B-CK) isoforms which assemble into homo-dimeric (MM, BB) and hetero-dimeric (MB) active enzymes. In mammals and birds, a significant fraction of MM-CK is bound to the myofibrillar M-line where it is thought to facilitate energy buffering and transport. Myofibrillar binding is mediated by major and minor lysine charge clamp motifs (K104/K115 [major] and K8/K24 [minor] in chicken M-CK) located in the N-terminal region [J. Cell Biol. 149 (2000) 1225]. We have obtained the cDNA and deduced amino acid sequences for cytoplasmic CKs from two hagfish, Myxine glutinosa and Eptatretus stoutii, non-vertebrate craniates, and the sequences for two cytoplasmic CKs from the lamprey Lampetra japonica, a jawless true vertebrate. All four cDNAs code for CKs consisting of approximately 380 residues. Phylogenetic analyses showed that the hagfish and lamprey CKs are coded for by genes which are clearly muscle type (M) creatine kinases. Two of these four M-CKs have the K104/K115-equivalent residues of the major myofibrillar binding region while the other two have the K115 equivalent but lack the corresponding K104 residue. All four M-CKs lack the K8/K24 equivalent elements of the minor myofibrillar binding region. Comparison of these sequences to corresponding sequences of cytoplasmic CKs from two protochordates (tunicate, amphioxus) and M- and B-CKs from true fish and above reveal a pattern of acquisition (and loss) of key lysine residues consistent with the physiological context in which these enzymes operate.

Muscle-type creatine kinase interacts with central domains of the M-band proteins myomesin and M-protein

Muscle-type creatine kinase (MM-CK) is a member of the CK isoenzyme family with key functions in cellular energetics. MM-CK interacts in an isoform-specific manner with the M-band of sarcomeric muscle, where it serves as an efficient intramyofibrillar ATP-regenerating system for the actin-activated myosin ATPase located nearby on both sides of the M-band. Four MM-CK-specific and highly conserved lysine residues are thought to be responsible for the interaction of MM-CK with the M-band. A yeast two-hybrid screen led to the identification of MM-CK as a binding partner of a central portion of myomesin (My7-8). An interaction was observed with domains six to eight of the closely related M-protein but not with several other Ig-like domains, including an M-band domain, of titin. The observed interactions were corroborated and characterised in detail by surface plasmon resonance spectroscopy (BiaCore). In both cases, they were CK isoform-specific and the MM-CK-specific lysine residues (K8. K24, K104 and K115) are involved in this interaction. At pH 6.8, the dissociation constants for the myomesin/MM-CK and the M-protein/MM-CK binding were in the range of 50-100 nM and around 1 microM, respectively. The binding showed pronounced pH-dependence and indicates a dynamic association/dissociation behaviour, which most likely depends on the energy state of the muscle. Our data propose a simple model for the regulation of this dynamic interaction.

Postnatal suppression of myomesin, muscle creatine kinase and the M-line in rat extraocular muscle

The M-line and its associated creatine kinase (CK) M-isoform (CK-M) are ubiquitous features of skeletal and cardiac muscle. The M-line maintains myosin myofilaments in register, links the contractile apparatus to the cytoskeleton for external force transfer and localizes CK-based energy storage and transfer to the site of highest ATP demand. We establish here that the muscle group responsible for movements of the eye, extraocular muscle (EOM), is divergent from other striated muscles in lacking both an M-line and its associated CK-M. Although an M-line forms during myogenesis, both in vivo and in vitro, it is actively repressed after birth. Transcripts of the major M-line structural proteins, myomesin 1 and myomesin 2, follow the same pattern of postnatal downregulation, while the embryonic heart-specific EH-myomesin 1 transcript is expressed early and retained in adult eye muscle. By immunocytochemistry, myomesin protein is absent from adult EOM sarcomeres. M-line suppression does not occur in organotypic co-culture with oculomotor motoneurons, suggesting that the mechanism for suppression may lie in muscle group-specific activation or workload patterns experienced only in vivo. The M-line is, however, still lost in dark-reared rats, despite the developmental delay this paradigm produces in the visuomotor system and EOMs. EOM was low in all CK isoform transcripts except for the sarcomeric mitochondrial (Ckmt2) isoform. Total CK enzyme activity of EOM was one-third that of hindlimb muscle. These findings are singularly unique among fast-twitch skeletal muscles. Since EOM exhibits isoform diversity for other sarcomeric proteins, the M-line/CK-M divergence probably represents a key physiological adaptation for the unique energetics and functional demands placed on this muscle group in voluntary and reflexive eye movements.

Paradoxical absence of M lines and downregulation of creatine kinase in mouse extraocular muscle

The M lines are structural landmarks in striated muscles, necessary for sarcomeric stability and as anchoring sites for the M isoform of creatine kinase (CK-M). These structures, especially prominent in fast skeletal muscles, are missing in rodent extraocular muscle, a particularly fast and active muscle group. In this study, we tested the hypotheses that 1). myomesin and M protein (cytoskeletal components of the M lines) and CK-M are downregulated in mouse extraocular muscle compared with the leg muscles, gastrocnemius and soleus; and 2). the expression of other cytosolic and mitochondrial CK isoforms is correspondingly increased. As expected, mouse extraocular muscles expressed lower levels of myomesin, M protein, and CK-M mRNA than the leg muscles. Immunocytochemically, myomesin and M protein were not detected in the banding pattern typically seen in other skeletal muscles. Surprisingly, message abundance for the other known CK isoforms was also lower in the extraocular muscles. Moreover, total CK activity was significantly decreased compared with that in the leg muscles. Based on these data, we reject our second hypothesis and propose that other energy-buffering systems may be more important in the extraocular muscles. The downregulation of major structural and metabolic elements and relative overexpression of two adenylate kinase isoforms suggest that the extraocular muscle group copes with its functional requirements by using strategies not seen in typical skeletal muscles.

Effect of creatine manipulation on fast-twitch skeletal muscle of the mouse

1. The effect of short-term, reversible alteration of muscle total creatine content (Crtot) on force development was sought in fast-twitch extensor digitorum longus (EDL) muscles of female mice. 2. Three in vivo interventions were investigated: 1% creatine-supplementation, treatment with the creatine-uptake inhibitor beta-guanidino propionic acid (beta-GPA; 1%) or beta-GPA treatment followed by creatine supplementation for 5 days. 3. The Crtot of isolated muscles, determined using reverse-phase high-performance liquid chromatography, was 133 +/- 38 mmol/kg dry in 11 EDL control muscles and was not significantly affected by dietary creatine-supplementation (152 +/- 25 mmol/kg dry; n = 8). Significant creatine depletion was observed in the beta-GPA-fed group (65 +/- 6 mmol/kg dry; n = 9) and this was reversed by 5 days of creatine supplementation (133 +/- 21 mmol/kg dry; n = 10). 4. Creatine depletion did not affect maximum tetanic stress. However, when muscle creatine was restored by creatine supplementation, a substantial increase in tetanic stress was observed. Significant correlations were observed between Crtot and maximum tetanic stress (r = 0.56) and relaxation rate (r = 0.43). The enhancement of force was not due to effects of creatine on muscle fibre type because neither mechanical tests of fibre characteristics nor the fibre types of the muscles were affected. 5. We conclude that, in muscles that contain large numbers of fast-twitch fibres, maximum tetanic stress is determined, in part, by muscle creatine stores.

Mutation of conserved active-site threonine residues in creatine kinase affects autophosphorylation and enzyme kinetics

Muscle-type creatine kinase (MM-CK) is a member of an isoenzyme family with key functions in cellular energetics. It has become a matter of debate whether the enzyme is autophosphorylated, as reported earlier [Hemmer, Furter-Graves, Frank, Wallimann and Furter (1995) Biochim. Biophys. Acta 1251, 81-90], or exclusively nucleotidylated. In the present paper, we demonstrate unambiguously that CK is indeed autophosphorylated. However, this autophosphorylation is not solely responsible for the observed microheterogeneity of MM-CK on two-dimensional isoelectric focusing gels. Using phosphoamino-acid analysis of (32)P-labelled CK isoforms, phosphothreonine (P-Thr) residues were identified as the only product of autophosphorylation for all CK isoenzymes. The phosphorylated residues in chicken MM-CK were allocated to a region in the vicinity of the active site, where five putative phosphorylation sites were identified. Site-directed threonine-valine-replacement mutants reveal that autophosphorylation is not specific for one particular residue but occurs at all examined threonine residues. The enzyme kinetic parameters indicate that the autophosphorylation of CK exerts a modulatory effect on substrate binding and the equilibrium constant, rather than on the catalytic mechanism itself.

Role of creatine kinase isoenzymes on muscular and cardiorespiratory endurance: genetic and molecular evidence

The ability to perform well in activities that require muscular and cardiorespiratory endurance is a trait influenced, in a considerable part, by the genetic make-up of individuals. Early studies of performance and recent scans of the human genome have pointed at various candidate genes responsible for the heterogeneity of these phenotypes within the population. Among these are the genes for the various creatine kinase (CK) isoenzyme subunits. CK and phosphocreatine (PCr) form an important metabolic system for temporal and spatial energy buffering in cells with large variations in energy demand. The different CK isoenzyme subunits (CK-M and CK-B) are differentially expressed in the tissues of the body. Although CK-M is the predominant form in both skeletal and cardiac muscle, CK-B is expressed to a greater extent in heart than in skeletal muscle. Studies in humans and mice have shown that the expression of CK-B messenger RNA (mRNA) and the abundance and activity of the CK-MB dimer increase in response to cardiorespiratory endurance training. Increases in muscle tissue CK-B content can be energetically favourable because of its lower Michaelis constant (Km) for ADP. The activity of the mitochondrial isoform of CK (Scmit-CK) has also been significantly and positively correlated to oxidative capacity and to CK-MB activity in muscle. In mice where the CK-M gene has been knocked out, significant increases in fatigue resistance together with cellular adaptations increasing aerobic capacity have been observed. These observations have led to the notion that this enzyme may be responsible for fatigue under normal circumstances, most likely because of the local cell compartment increase in inorganic phosphate concentration. Studies where the Scmit-CK gene was knocked out have helped demonstrate that this isoenzyme is very important for the stimulation of aerobic respiration. Human studies of CK-M gene sequence variation have shown a significant association between a polymorphism, distinguished by the NcoI restriction enzyme, and an increase in cardiorespiratory endurance as indexed by maximal oxygen uptake following 20 weeks of training. In conclusion, there is now evidence at the tissue, cell and molecular level indicating that the CK-PCr system plays an important role in determining the phenotypes of muscular and cardiorespiratory endurance. It is envisioned that newer technologies will help determine how the genetic variability of these genes (and many others) impact on performance and health-related phenotypes.

Gene duplication events producing muscle (M) and brain (B) isoforms of cytoplasmic creatine kinase: cDNA and deduced amino acid sequences from two lower chordates

Creatine kinase (CK) is coded for by at least four loci in higher vertebrates--two cytoplasmic isoforms, muscle (M) and brain (B), and two mitochondrial isoforms, sarcomeric and ubiquitous. M is expressed primarily in skeletal muscle, while B is expressed in a variety of cells, including cardiac and smooth muscle fibers, neurons, transport epithelia, and photoreceptors. M and B subunits form very stable homodimers (MM [M-CK], BB [B-CK]) and heterodimers (MB). M-CK is capable of binding to the M line of the myofibril, thereby creating an energy transfer microcompartment; BB and MB CKs are not. M- and B-like CKs are present in all vertebrates yet examined, including fish. Cytoplasmic, dimeric CKs are widely distributed in the invertebrates. The only available amino acid sequence for an invertebrate dimeric CK, that of the protostome polychaete Chaetopterus variopedatus, is just as similar to the vertebrate M isoform as to the B isoform. Echinoderms lack dimeric, cytoplasmic CKs, which appear to be replaced by a dimeric arginine kinase which evolved secondarily from CK. Thus, it is likely that the gene duplication event producing the M and B isoforms occurred after the divergence of the chordates from echinoderms. To narrow down the timing of this duplication event, we obtained the cDNA and deduced amino acid sequences of dimeric CKs from the tunicate Ciona intestinalis (subphylum Urochordata) and the lancelet Branchiostoma floridae (subphylum Cephalochordata). Our results show that these CKs are strikingly similar to both invertebrate and vertebrate CKs. However, phylogenetic analyses by neighbor-joining and parsimony show that these two enzymes appeared to have diverged before the point of divergence of the M and B isoforms. Thus, the gene duplication event for formation of the muscle and brain isoforms of CK most likely occurred during the radiation of the fish, a time noted for gene duplication events at a variety of other loci.

Evolution and physiological roles of phosphagen systems

Phosphagens are phosphorylated guanidino compounds that are linked to energy state and ATP hydrolysis by corresponding phosphagen kinase reactions: phosphagen + MgADP + H(+) <--> guanidine acceptor + MgATP. Eight different phosphagens (and corresponding phosphagen kinases) are found in the animal kingdom distributed along distinct phylogenetic lines. By far, the creatine phosphate/creatine kinase (CP/CK) system, which is found in the vertebrates and is widely distributed throughout the lower chordates and invertebrates, is the most extensively studied phosphagen system. Phosphagen kinase reactions function in temporal ATP buffering, in regulating inorganic phosphate (Pi) levels, which impacts glycogenolysis and proton buffering, and in intracellular energy transport. Phosphagen kinase reactions show differences in thermodynamic poise, and the phosphagens themselves differ in terms of certain physical properties including intrinsic diffusivity. This review evaluates the distribution of phosphagen systems and tissue-specific expression of certain phosphagens in an evolutionary and functional context. The role of phosphagens in regulation of intracellular Pi levels likely evolved early. Thermodynamic poise of the phosphagen kinase reaction profoundly impacts this capacity. Furthermore, it is hypothesized that the capacity for intracellular targeting of CK evolved early as a means of facilitating energy transport in highly polarized cells and was subsequently exploited for temporal ATP buffering and dynamic roles in metabolic regulation in cells displaying high and variable rates of aerobic energy production.

Coupling of creatine kinase to glycolytic enzymes at the sarcomeric I-band of skeletal muscle: a biochemical study in situ

The specific interaction of muscle type creatine-kinase (MM-CK) with the myofibrillar M-line was demonstrated by exchanging endogenous MM-CK with an excess of fluorescently labeled MM-CK in situ, using chemically skinned skeletal muscle fibers and confocal microscopy. No binding of labeled MM-CK was noticed at the I-band of skinned fibers, where the enzyme is additionally located in vivo, as shown earlier by immunofluorescence staining of cryosections of intact muscle. However, when rhodamine-labeled MM-CK was diffused into skinned fibers that had been preincubated with phosphofructokinase (PFK), a glycolytic enzyme known to bind to actin, a striking in vivo-like interaction of Rh-MM-CK with the I-band was found, presumably mediated by binding of Rh-MM-CK to the glycolytic enzyme. Aldolase, another actin-binding glycolytic enzyme was also able to bind Rh-MM-CK to the I-band, but formation of the complex occurred preferably at long sarcomere length (> 3.0 microm). Neither pyruvate kinase, although known for its binding to actin, nor phosphoglycerate kinase (PGK), not directly interacting with the I-band itself, did mediate I-band targeting of MM-CK. Anchoring of MM-CK to the I-band via PFK, but not so via aldolase, was strongly pH-dependent and occurred below pH 7.0. Labeling performed at different sarcomere length indicated that the PFK/MM-CK complex bound to thin filaments of the I-band, but not within the actomyosin overlap zones. The physiological consequences of the structural interaction of MM-CK with PFK at the I-band is discussed with respect to functional coupling of MM-CK to glycolysis, metabolic regulation and channeling in multi-enzyme complexes. The in situ binding assay with skinned skeletal muscle fibers described here represents a useful method for further studies of specific protein-protein interactions in a structurally intact contractile system under various precisely controlled conditions.

Differential assembly of alpha- and gamma-filagenins into thick filaments in Caenorhabditis elegans

Muscle thick filaments are highly organized supramolecular assemblies of myosin and associated proteins with lengths, diameters and flexural rigidities characteristic of their source. The cores of body wall muscle thick filaments of the nematode Caenorhabditis elegans are tubular structures of paramyosin sub-filaments coupled by filagenins and have been proposed to serve as templates for the assembly of native thick filaments. We have characterized alpha- and gamma-filagenins, two novel proteins of the cores with calculated molecular masses of 30,043 and 19,601 and isoelectric points of 10.52 and 11.49, respectively. Western blot and immunoelectron microscopy using affinity-purified antibodies confirmed that the two proteins are core components. Immunoelectron microscopy of the cores revealed that they assemble with different periodicities. Immunofluorescence microscopy showed that alpha-filagenin is localized in the medial regions of the A-bands of body wall muscle cells whereas gamma-filagenin is localized in the flanking regions, and that alpha-filagenin is expressed in 1.5-twofold embryos while gamma-filagenin becomes detectable only in late vermiform embryos. The expression of both proteins continues throughout later stages of development. C. elegans body wall muscle thick filaments of these developmental stages have distinct lengths. Our results suggest that the differential assembly of alpha- and gamma-filagenins into thick filaments of distinct lengths may be developmentally regulated.

Isoenzyme-specific interaction of muscle-type creatine kinase with the sarcomeric M-line is mediated by NH(2)-terminal lysine charge-clamps

Creatine kinase (CK) is located in an isoenzyme-specific manner at subcellular sites of energy production and consumption. In muscle cells, the muscle-type CK isoform (MM-CK) specifically interacts with the sarcomeric M-line, while the highly homologous brain-type CK isoform (BB-CK) does not share this property. Sequence comparison revealed two pairs of lysine residues that are highly conserved in M-CK but are not present in B-CK. The role of these lysines in mediating M-line interaction was tested with a set of M-CK and B-CK point mutants and chimeras. We found that all four lysine residues are involved in the isoenzyme-specific M-line interaction, acting pair-wise as strong (K104/K115) and weak interaction sites (K8/K24). An exchange of these lysines in MM-CK led to a loss of M-line binding, whereas the introduction of the very same lysines into BB-CK led to a gain of function by transforming BB-CK into a fully competent M-line-binding protein. The role of the four lysines in MM-CK is discussed within the context of the recently solved x-ray structures of MM-CK and BB-CK.

Structural and functional implications of the amino acid sequences of dimeric, cytoplasmic and octameric mitochondrial creatine kinases from a protostome invertebrate

The cDNA and deduced amino-acid sequences for dimeric and octameric isoforms of creatine kinase (CK) from a protostome, the polychaete Chaetopterus variopedatus, were elucidated and then analysed in the context of available vertebrate CK sequences and the recently determined crystal structure of chicken sarcomeric mitochondrial CK (MiCK). As protostomes last shared a common ancestor with vertebrates roughly 700 million years ago, observed conserved residues may serve to confirm or reject contemporary hypotheses about the roles of particular amino acids in functional/structural processes such as dimer/octamer formation and membrane binding. The isolated cDNA from the dimeric CK consisted of 1463 nucleotides with an open reading frame of 1116 nucleotides encoding a 372-amino-acid protein having a calculated molecular mass of 41.85 kDa. The percentage identity of C. variopedatus dimeric CK to vertebrate CK is as high as 69%. The octameric MiCK cDNA is composed of 1703 nucleotides with an open reading frame of 1227 nucleotides. The first 102 nucleotides of the open reading frame encode a 34-amino-acid leader peptide whereas the mature protein is composed of 375 amino acids with a calculated molecular mass of 42.17 kDa. The percentage identity of C. variopedatus MiCK to vertebrate CK is as high as 71%. This similarity is also evident in residues purported to be important in the structure and function of dimeric and octameric CK: (a) presence of seven basic amino acids in the C-terminal end thought to be important in binding of MiCK to membranes; (b) presence of a lysine residue (Lys110 in chicken MiCK) also thought to be involved in membrane binding; and (c) presence of a conserved tryptophan thought to be important in dimer stabilization which is present in all dimeric and octameric guanidino kinases. However, C. variopedatus MiCK lacks the N-terminal heptapeptide present in chicken MiCK, which is thought to mediate octamer stabilization. In contrast with vertebrate MiCK, polychaete octamers are very stable indicating that dimer binding into octamers may be mediated by additional and/or other residues. Phylogenetic analyses showed that both octamer and dimer evolved very early in the CK lineage, well before the divergence of deuterostomes and protostomes. These results indicate that the octamer is a primitive feature of CK rather than being a derived and advanced character.

Different domains of the M-band protein myomesin are involved in myosin binding and M-band targeting

Myomesin is a 185-kDa protein located in the M-band of striated muscle where it interacts with myosin and titin, possibly connecting thick filaments with the third filament system. By using expression of epitope-tagged myomesin fragments in cultured cardiomyocytes and biochemical binding assays, we could demonstrate that the M-band targeting activity and the myosin-binding site are located in different domains of the molecule. An N-terminal immunoglobulin-like domain is sufficient for targeting to the M-band, but solid-phase overlay assays between individual N-terminal domains and the thick filament protein myosin revealed that the unique head domain contains the myosin-binding site. When expressed in cardiomyocytes, the head domains of rat and chicken myomesin showed species-specific differences in their incorporation pattern. The head domain of rat myomesin localized to a central area within the A-band, whereas the head domain of chicken myomesin was diffusely distributed in the cytoplasm. We therefore conclude that the head domain of myomesin binds to myosin but that this affinity is not sufficient for the restriction of the domain to the M-band in vivo. Instead, the neighboring immunoglobulin-like domain is essential for the precise incorporation of myomesin into the M-band, possibly because of interaction with a yet unknown protein of the sarcomere.

The isoenzyme-diagnostic regions of muscle-type creatine kinase, the M-260 and M-300 box, are not responsible for its binding to the myofibrillar M-band

Muscle-type creatine kinase is known for its unique interaction with the myofibrillar M-band, but the molecular origin for this structural relationship is not well understood. A systematic sequence comparison between the highly homologous cytosolic isoforms, muscle-type and brain-type creatine kinase, yielded two isoenzyme-specific regions in the muscle-type creatine kinases, the M-260 box (residues 258-270) and the M-300 box (residues 300-315). These particular regions were conspicuous for the specific interaction of this CK isoenzyme, but not of brain-type creatine kinase, with the sarcomeric M-band. In situ diffusion assays with fluorescently labeled native, as well as mutated muscle-type creatine kinase variants, were used to study by laser confocal microscopy their association with the M-band of chemically skinned muscle fibers. Neither a set of charge mutants of the M-260 box and/or the M-300 box nor a hybrid construct of both isoforms with the entire C-terminal region derived from the brain-type isoform showed any significant alteration in the in situ M-band-binding properties when compared to the wild-type form of muscle-type creatine kinase. This indicates that in the intact protein of muscle type creatine kinase, these C-terminal isoenzyme-specific regions are not important for M-band interaction and that the actual M-band interaction domain(s) lay mostly within the N-terminal half of the molecule. The highly conserved motives (M-260 box and M-300 box) may serve an isoenzyme-specific purpose yet to be identified.

Functional aspects of the X-ray structure of mitochondrial creatine kinase: a molecular physiology approach

Mitochondrial creatine kinase (Mi-CK) is a central enzyme in energy metabolism of tissues with high and fluctuating energy requirements. In this review, recent progress in the functional and structural characterization of Mi-CK is summarized with special emphasis on the solved X-ray structure of chicken Mib-CK octamer (Fritz-Wolf et al., Nature 381, 341-345, 1996). The new results are discussed in a historical context and related to the characteristics of CK isoforms as known from a large number of biophysical and biochemical studies. Finally, two hypothetical functional aspects of the Mi-CK structure are proposed: (i) putative membrane binding motifs at the top and bottom faces of the octamer and (ii) a possible functional role of the central 20 A channel.

Cytoarchitectural and metabolic adaptations in muscles with mitochondrial and cytosolic creatine kinase deficiencies

We have blocked creatine kinase (CK) mediated phosphocreatine (PCr) <==> ATP transphosphorylation in mitochondria and cytosol of skeletal muscle by knocking out the genes for the mitochondrial (ScCKmit) and the cytosolic (M-CK) CK isoforms in mice. Animals which carry single or double mutations, if kept and tested under standard laboratory conditions, have surprisingly mild changes in muscle physiology. Strenuous ex vivo conditions were necessary to reveal that MM-CK absence in single and double mutants leads to a partial loss of tetanic force output. Single ScCKmit deficiency has no noticeable effects but in combination the mutations cause slowing of the relaxation rate. Importantly, our studies revealed that there is metabolic and cytoarchitectural adaptation to CK defects in energy metabolism. The effects involve mutation type-dependent alterations in the levels of AMP, IMP, glycogen and phosphomonoesters, changes in activity of metabolic enzymes like AMP-deaminase, alterations in mitochondrial volume and contractile protein (MHC isoform) profiles, and a hyperproliferation of the terminal cysternae of the SR (in tubular aggregates). This suggests that there is a compensatory resiliency of loss-of-function and redirection of flux distributions in the metabolic network for cellular energy in our mutants.

Myofibrillar interaction of cytosolic creatine kinase (CK) isoenzymes: allocation of N-terminal binding epitope in MM-CK and BB-CK

The molecular origin of the isoenzyme-specific interaction of cytosolic creatine kinase isoenzymes, muscle-type creatine kinase and brain-type creatine kinase, with myofibrillar structures has been studied by confocal microscopy in an functional in situ binding assay with chemically skinned, unfixed skeletal muscle fibers using wild-type and chimeric creatine kinase isoproteins. The specific interaction of both wild-type isoforms with the sarcomeric structure resulted in a stable, isoform-characteristic labeling pattern with muscle-type creatine kinase bound exclusively and tightly to the sarcomeric M-band while brain-type creatine kinase was confined to the I-band region. Chimeric proteins of both muscle-type and brain-type creatine kinases were constructed to localize the corresponding binding domain(s). Exchanged domains included the N-terminal part (residues 1–234), the region containing an isoenzyme ‘diagnostic box’ (residues 235–285) and the C-terminal part (residues 286–380). The purified recombinant proteins were all fully intact and enzymatically active. All chimeric proteins containing the N-terminal region (amino acid 1–234) of muscle-type or brain-type creatine kinase were always specifically targeted to the sarcomeric M-band or I-band, respectively. We therefore propose that the relevant epitope(s), determining the isoenzyme-specific targeting in skeletal muscle, are entirely located within the N-terminal regions of both cytosolic creatine kinase isoforms.

Muscle-specific creatine kinase gene polymorphism and VO2max in the HERITAGE Family Study

This study examined the association between a DNA polymorphism in the muscle-specific creatine kinase (CKMM) gene and VO2max in the sedentary state, as well as its response (deltaVO2max) to a standardized 20-wk endurance training program. The subjects were 160 biologically unrelated Caucasian parents (80 women, 80 men) and 80 biologically unrelated adult offspring of the HERITAGE Family Study. The CKMM polymorphism was detected by PCR and digestion with the NcoI restriction enzyme. VO2max was measured during maximal cycle ergometer tests. VO2max was 2119 +/- 45 mL x min(-1) (mean +/- SE) or 26 +/- 0.4 mL x kg(-1) x min(-1). Both sexes had a significant (P < 0.05) increase in the deltaVO2max (women = 283 +/- 20 mL x min[-1] and men = 363 +/- 25 mL x min[-1]). Allele and genotype frequencies were not significantly different (P > 0.05) between sexes. Age and sex adjusted VO2max was significantly (P = 0.007) associated with the CKMM genotype in the parents, whereas no association (P > 0.05) was observed in the offspring. DeltaVO2max values adjusted for age, sex, VO2max, and body mass were characterized by genotype differences in both parents (P = 0.0004) and offspring (P = 0.0025). A significantly (P < 0.05) lower deltaVO2max to endurance training was detected in both parents and offspring homozygotes for the rare allele. The genotype accounted for at least 9% of the variance in deltaVO2max. These results indicate that the NcoI polymorphism in the 3' untranslated region of the muscle-specific creatine kinase gene is associated with the deltaVO2max to endurance training.

Flight muscle function in Drosophila requires colocalization of glycolytic enzymes

Structural relationships between the myofibrillar contractile apparatus and the enzymes that generate ATP for muscle contraction are not well understood. We explored whether glycolytic enzymes are localized in Drosophila flight muscle and whether localization is required for function. We find that glycerol-3-phosphate dehydrogenase (GPDH) is localized at Z-discs and M-lines. The glycolytic enzymes aldolase and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) are also localized along the sarcomere with a periodic pattern that is indistinguishable from that of GPDH localization. Furthermore, localization of aldolase and GAPDH requires simultaneous localization of GPDH, because aldolase and GAPDH are not localized along the sarcomere in muscles of strains that carry Gpdh null alleles. In an attempt to understand the process of glycolytic enzyme colocalization, we have explored in more detail the mechanism of GPDH localization. In flight muscle, there is only one GPDH isoform, GPDH-1, which is distinguished from isoforms found in other tissues by having three C-terminal amino acids: glutamine, asparagine, and leucine. Transgenic flies that can produce only GPDH-1 display enzyme colocalization similar to wild-type flies. However, transgenic flies that synthesize only GPDH-3, lacking the C-terminal tripeptide, do not show the periodic banding pattern of localization at Z-discs and M-lines for GPDH. In addition, neither GAPDH nor aldolase colocalize at Z-discs and M-lines in the sarcomeres of muscles from GPDH-3 transgenic flies. Failure of the glycolytic enzymes to colocalize in the sarcomere results in the inability to fly, even though the full complement of active glycolytic enzymes is present in flight muscles. Therefore, the presence of active enzymes in the cell is not sufficient for muscle function; colocalization of the enzymes is required. These results indicate that the mechanisms by which ATP is supplied to the myosin ATPase, for muscle contraction, requires a highly organized cellular system.

Isoform-specific interaction of the myosin-binding proteins (MyBPs) with skeletal and cardiac myosin is a property of the C-terminal immunoglobulin domain

Full-length cDNAs encoding chicken and human skeletal MyBP-H and MyBP-C have been isolated and sequenced (1-5). All are members of a protein family with repetitive immunoglobulin C2 and fibronectin type III motifs. The myosin binding domain was mapped to a single immunoglobulin motif in cardiac MyBP-C and skeletal MyBP-H. Limited alpha-chymotryptic digestion of cardiac MyBP-C generated three peptides, similar in relative mobility to those of skeletal MyBP-C: approximately 100, 40, and 15 kDa. Tryptic digestion of MyBP-H yielded two peptides: approximately 50 and 14 kDa. Partial amino acid sequences proved that the 15- and 14-kDa fragments are located at the C termini of cardiac MyBP-C and skeletal MyBP-H, respectively. Only the 14- and 15-kDa peptides bound to myosin. Thus, the myosin binding site in all three proteins resides within an homologous, C-terminal immunoglobulin domain. Binding reactions (2) between the skeletal and cardiac MyBPs and corresponding myosin isoforms demonstrated saturable binding of the MyBP proteins and their C-terminal peptides to myosin, but there are higher limiting stoichiometries with the homologous isoform partners. Evidence is presented indicating that MyBP-H and -C compete for binding to a discrete number of sites in myosin filaments.

Functional equivalence of creatine kinase isoforms in mouse skeletal muscle

Creatine kinase (CK) is a highly conserved enzyme abundant in skeletal muscle that has a key role in high energy phosphate metabolism. The localization of the muscle isoenzyme of CK (MM-CK) to the M line and the sarcoplasmic reticulum of myofibrils has been suggested to be important for proper force development in skeletal muscle. The importance of this subcellular compartmentation has not been directly tested in vivo. To test the role of myofibrilar localization of CK, the consequences of a complete CK isoform switch from MM-CK to the brain (BB-CK) isoform, which does not localize to the M line, was studied in transgenic mouse skeletal muscle. In MM-CK knockout mice there are large contractile defects. When MM-CK was replaced by BB-CK, the aberrant contractile phenotypes seen in MM-CK knockout mice were returned to normal despite the lack of myofibrillar localization. These results indicate that CK compartmentation to the myofibril of skeletal muscle is not essential for contractile function and that there is functional equivalence of creatine kinase isoforms in supporting cellular energy metabolism.

Tissue-specific isoforms of chicken myomesin are generated by alternative splicing

Myomesin is a high molecular weight protein that is present in the M-band of all fiber types of cross-striated skeletal muscle and heart. We have isolated two cDNAs encoding tissue-specific isoforms of chicken myomesin with calculated molecular masses of 174 kDa in skeletal muscle and 182 kDa in heart. Distinct sequences are found at the 3'-end of the two cDNAs, giving rise to different C-terminal domains. Partial analysis of the gene structure has shown that in chicken, both isoforms are generated by alternative splicing of a composite exon. Amino acid sequences show that the main body of myomesin consists of five fibronectin type III (class I motifs) and seven immunoglobulin-like domains (class II motifs). An identical structure was found in M-protein and human 190K protein (the human counterpart of chicken myomesin), and a comparable domain arrangement occurs in the M-band-associated protein skelemin. We postulate that myomesin, M-protein, and skelemin belong to the same subfamily of high molecular weight M-band-associated proteins of the immunoglobulin superfamily and that they probably have the same ancestor in evolution.

Contractile and metabolic effects of increased creatine kinase activity in mouse skeletal muscle

The effects of increased expression of creatine kinase (CK) in skeletal muscle were studied in control and transgenic animals homozygous for expression of the B subunit of CK. CK activity was 47% higher in transgenic gastrocnemius muscle. The CK activity was distributed as follows: 45 +/- 1% MM dinner, 31 +/- 4% MB dimer, and 22 +/- 5% BB dimer. No significant differences in metabolic or contractile proteins were detected except for a 22% decrease in lactate dehydrogenase activity and a 9% decrease in adenylate kinase activity. The only significant effect in contractile activity was that the rise time of a 5-s isometric contraction was 28% faster in the transgenic muscle. 31P nuclear magnetic resonance (NMR) spectra were obtained from control and transgenic muscles during mechanical activation, and there were no NMR measurable differences detected. These results indicate that a 50% increase in CK activity due to expression of the B subunit does not have large effects on skeletal muscle metabolism or contractile function. Therefore, control muscle has sufficient CK activity to keep up with changes in cellular high-energy phosphates except during the early phase of intense contractile activity.