Troponin of asynchronous flight muscle

Role of the interaction between troponin T and AMP deaminase by zinc bridge in modulating muscle contraction and ammonia production

The N-terminal region of troponin T (TnT) does not bind any protein of the contractile machinery and the role of its hypervariability remains uncertain. In this review we report the evidence of the interaction between TnT and AMP deaminase (AMPD), a regulated zinc enzyme localized on the myofibril. In periods of intense muscular activity, a decrease in the ATP/ADP ratio, together with a decrease in the tissue pH, is the stimulus for the activation of the enzyme that deaminating AMP to IMP and NH3 displaces the myokinase reaction towards the formation of ATP. In skeletal muscle subjected to strong tetanic contractions, a calpain-like proteolytic activity produces the removal in vivo of a 97-residue N-terminal fragment from the enzyme that becomes desensitized towards the inhibition by ATP, leading to an unrestrained production of NH3. When a 95-residue N-terminal fragment is removed from AMPD by trypsin, simulating in vitro the calpain action, rabbit fast TnT or its phosphorylated 50-residue N-terminal peptide binds AMPD restoring the inhibition by ATP. Taking in consideration that the N-terminus of TnT expressed in human as well as rabbit white muscle contains a zinc-binding motif, we suggest that TnT might mimic the regulatory action of the inhibitory N-terminal domain of AMPD due to the presence of a zinc ion connecting the N-terminal and C-terminal regions of the enzyme, indicating that the two proteins might physiologically associate to modulate muscle contraction and ammonia production in fast-twitching muscle under strenuous conditions.

Nanoscopy reveals the layered organization of the sarcomeric H-zone and I-band complexes

Sarcomeres are extremely highly ordered macromolecular assemblies where structural organization is intimately linked to their functionality as contractile units. Although the structural basis of actin and Myosin interaction is revealed at a quasiatomic resolution, much less is known about the molecular organization of the I-band and H-zone. We report the development of a powerful nanoscopic approach, combined with a structure-averaging algorithm, that allowed us to determine the position of 27 sarcomeric proteins in Drosophila melanogaster flight muscles with a quasimolecular, ∼5- to 10-nm localization precision. With this protein localization atlas and template-based protein structure modeling, we have assembled refined I-band and H-zone models with unparalleled scope and resolution. In addition, we found that actin regulatory proteins of the H-zone are organized into two distinct layers, suggesting that the major place of thin filament assembly is an M-line-centered narrow domain where short actin oligomers can form and subsequently anneal to the pointed end.

Invertebrate troponin: Insights into the evolution and regulation of striated muscle contraction

The troponin complex plays a central role in regulating the contraction and relaxation of striated muscles. Among the three protein subunits of troponin, the calcium receptor subunit, TnC, belongs to the calmodulin family of calcium signaling proteins whereas the inhibitory subunit, TnI, and tropomyosin-binding/thin filament-anchoring subunit, TnT, are striated muscle-specific regulatory proteins. TnI and TnT emerged early in bilateral symmetric invertebrate animals and have co-evolved during the 500-700 million years of muscle evolution. To understand the divergence as well as conservation of the structures of TnI and TnT in invertebrate and vertebrate organisms adds novel insights into the structure-function relationship of troponin and the muscle type isoforms of TnI and TnT. Based on the significant growth of genomic database of multiple species in the past decade, this focused review studied the primary structure features of invertebrate troponin subunits in comparisons with the vertebrate counterparts. The evolutionary data demonstrate valuable information for a better understanding of the thin filament regulation of striated muscle contractility in health and diseases.

RNA Interference Screening for Genes Regulating Drosophila Muscle Morphogenesis

RNA interference (RNAi) is the method of choice to systematically test for gene function in an intact organism. The model organism Drosophila has the advantage that RNAi is cell autonomous, meaning it does not spread from one cell to the next. Hence, RNAi can be performed in a tissue-specific manner by expressing short or long inverted repeat constructs (hairpins) designed to target mRNAs from one specific target gene. This achieves tissue-specific knock-down of a target gene of choice. Here, we detail the methodology to test gene function in Drosophila muscle tissue by expressing hairpins in a muscle-specific manner using the GAL4-UAS system. We further discuss the systematic RNAi resource collections available which also permit large scale screens in a muscle-specific manner. The full power of such screens is revealed by combination of high-throughput assays followed by detailed morphological assays. Together, this chapter should be a practical guide to enable the reader to either test a few candidate genes, or large gene sets for particular functions in Drosophila muscle tissue and provide first insights into the biological process the gene might be important for in muscle.

Identifying Differential Gene Expression in Wing Polymorphism of Adult Males of the Largest Water Strider: De novo Transcriptome Assembly for Gigantometra gigas (Hemiptera: Gerridae)

Wing polymorphism is common in a wide variety of insect species. However, few studies have reported on adaptations in the wing polymorphism of insects at molecular level, in particular for males. Thus, the adaptive mechanisms need to be explored. The remarkable variability in wing morphs of insects is well represented in the water striders (Hemiptera: Gerridae). Within this family, Gigantometra gigas (China, 1925), the largest water strider known worldwide, displays macropterous and apterous males. In the present study, we used de novo transcriptome assembly to obtain gene expression information and compared body and leg-component lengths of adult males in different wing morphs. The analyses in both gene expression and phenotype levels were used for exploring the adaptive mechanism in wing polymorphism of G. gigas. After checking, a series of highly expressed structural genes were found in macropterous morphs, which were related to the maintenance of flight muscles and the enhancement of flight capacity, whereas in the apterous morphs, the imaginal morphogenesis protein-Late 2 (Imp-L2), which might inhibit wing development and increase the body size of insects, was still highly expressed in the adult stage. Moreover, body and leg-component lengths were significantly larger in apterous than in macropterous morphs. The larger size of the apterous morphs and the differences in highly expressed genes between the two wing morphs consistently demonstrate the adaptive significance of wing polymorphism in G. gigas. These results shed light on the future loss-of-function research of wing polymorphism in G. gigas.

The tymbal muscle of cicada has flight muscle-type sarcomeric architecture and protein expression

BACKGROUND

The structural and biochemical features of the tymbal (sound-producing) muscle of cicadas were studied by X-ray diffraction and immunochemistry, and compared with those of flight muscles from the same species.

RESULTS

The X-ray diffraction pattern of the tymbal muscle was very similar to that of the dorsal longitudinal flight muscle: In both muscles, the 2,0 equatorial reflection is much more intense than the 1,1, indicating that both muscles have a flight muscle-type myofilament lattice. In rigor, the first myosin/actin layer line reflection was finely lattice-sampled, indicating that the contractile proteins are arranged with a crystalline regularity as in asynchronous flight muscles. In contrast, the diffraction pattern from the tensor muscle, which modulates the sound by stressing the tymbal, did not show signs of such high regularity or flight muscle-type filament lattice. Electrophoretic patterns of myofibrillar proteins were also very similar in the tymbal muscle and flight muscles, but distinct from those from the tensor or leg muscles. The antibody raised against the flight muscle-specific troponin-I isoform reacted with an 80-kDa band from both tymbal and flight muscles, but with none of the bands from the tensor or leg muscles.

CONCLUSION

The close similarities of the structural and biochemical profiles between the tymbal and the flight muscles suggest the possibility that a set of flight muscle-specific proteins is diverted to the tymbal muscle to meet its demand for fast, repetitive contractions.

A Beetle Flight Muscle Displays Leg Muscle Microstructure

In contrast to major flight muscles in the Mecynorrhina torquata beetle, the third axillary (3Ax) muscle is a minor flight muscle that uniquely displays a powerful mechanical function despite its considerably small volume, ∼1/50 that of a major flight muscle. The 3Ax muscle contracts relatively slowly, and in flight strongly pulls the beating wing to attenuate the stroke amplitude. This attenuation leads to left-right turning in flight or wing folding to cease flying. What enables this small muscle to be so powerful? To explore this question, we examined the microstructure of the 3Ax muscle using synchrotron x-ray diffraction, optical microscopy, and immunoblotting analysis. We found that the 3Ax muscle has long (∼5 μm) myofilaments and that the ratio of thick (myosin) filaments to thin (actin) filaments is 1:5 or 1:6. These characteristics are not observed in the major flight muscles, which have shorter myofilaments (∼3.5 μm) with a smaller ratio (1:3), and instead are more typical of a leg muscle. Furthermore, the flight-muscle-specific troponin isoform, TnH, is not expressed in the 3Ax muscle. Since such a microstructure is suitable for generating large tension, the 3Ax muscle is appropriately designed to pull the wing strongly despite its small volume.

Structure of myosin filaments from relaxed Lethocerus flight muscle by cryo-EM at 6 Å resolution

We describe a cryo-electron microscopy three-dimensional image reconstruction of relaxed myosin II-containing thick filaments from the flight muscle of the giant water bug Lethocerus indicus. The relaxed thick filament structure is a key element of muscle physiology because it facilitates the reextension process following contraction. Conversely, the myosin heads must disrupt their relaxed arrangement to drive contraction. Previous models predicted that Lethocerus myosin was unique in having an intermolecular head-head interaction, as opposed to the intramolecular head-head interaction observed in all other species. In contrast to the predicted model, we find an intramolecular head-head interaction, which is similar to that of other thick filaments but oriented in a distinctly different way. The arrangement of myosin's long α-helical coiled-coil rod domain has been hypothesized as either curved layers or helical subfilaments. Our reconstruction is the first report having sufficient resolution to track the rod α helices in their native environment at resolutions ~5.5 Å, and it shows that the layer arrangement is correct for Lethocerus. Threading separate paths through the forest of myosin coiled coils are four nonmyosin peptides. We suggest that the unusual position of the heads and the rod arrangement separated by nonmyosin peptides are adaptations for mechanical signal transduction whereby applied tension disrupts the myosin heads as a component of stretch activation.

Recent developments in the study of insect flight

Here we review recent contributions to the study of insect flight, in particular those brought about by advances in experimental techniques. We focus particularly on the following areas: wing flexibility and deformation, the physiology and biophysics of asynchronous insect flight muscle, the aerodynamics of flight, and stability and maneuverability. This recent research reveals the importance of wing flexibility to insect flight, provides a detailed model of how asynchronous flight muscle functions and how it may have evolved, synthesizes many recent studies of insect flight aerodynamics into a broad-reaching summary of unsteady flight aerodynamics, and highlights new insights into the sources of flight stability in insects. The focus on experimental techniques and recently developed apparatus shows how these advancements have occurred and point the way towards future experiments.

Myofilament proteins in the synchronous flight muscles of Manduca sexta show both similarities and differences to Drosophila melanogaster

Insect flight muscles have been classified as either synchronous or asynchronous based on the coupling between excitation and contraction. In the moth Manduca sexta, the flight muscles are synchronous and do not display stretch activation, which is a property of asynchronous muscles. We annotated the M. sexta genes encoding the major myofibrillar proteins and analyzed their isoform pattern and expression. Comparison with the homologous genes in Drosophila melanogaster indicates both difference and similarities. For proteins such as myosin heavy chain, tropomyosin, and troponin I the availability and number of potential variants generated by alternative spicing is mostly conserved between the two insects. The exon usage associated with flight muscles indicates that some exon sets are similarly used in the two insects, whereas others diverge. For actin the number of individual genes is different and there is no evidence for a flight muscle specific isoform. In contrast for troponin C, the number of genes is similar, as well as the isoform composition in flight muscles despite the different calcium regulation. Both troponin I and tropomyosin can include COOH-terminal hydrophobic extensions similar to tropomyosinH and troponinH found in D. melanogaster and the honeybee respectively.

Drosophila miR-9a targets the ECM receptor Dystroglycan to canalize myotendinous junction formation

Establishment of intercellular interactions between various cell types of different origin is vital for organism development and tissue maintenance. Therefore, precise timing, expression pattern, and amounts of extracellular matrix (ECM) proteins must be tightly regulated. Particularly, the ECM is important for the development and function of myotendinous junctions (MTJs). We find that precise levels of the ECM receptor Dystroglycan (Dg) are required for MTJ formation in Drosophila and that Dg levels in this process are controlled by miR-9a. In the embryo, Dg is enriched at the termini of the growing muscles facing the tendon matrix and absent from miR-9a-expressing tendons. This gradient of Dg expression is crucial for proper muscle-tendon attachments and is adjusted by miR-9a. In addition to Dg, miR-9a regulates the expression of several other critical muscle genes, and we therefore propose that during embryogenesis, miR-9a specifically controls the expression of mesodermal genes to canalize MTJ morphogenesis.

Flight muscle-specific Pro-Ala-rich extension of troponin is important for maintaining the insect-type myofilament lattice integrity

Insect flight muscle (IFM) can oscillate at frequencies up to 1000Hz, owing to its capability of stretch activation (SA). It is a highly specialized form of cross striated muscles, and its peculiar features include the IFM-specific isoform of troponin-I (troponin-H or TnH) with an unusually long Pro-Ala-rich extension at the C-terminus. Although we have shown that this extension does not directly take part in SA, questions remain as to what its real role is and why it is expressed only in IFM. Here we explored the structural role of the extension, be comparing X-ray diffraction patterns and electron micrographs of bumblebee IFM fibers before and after enzymatic removal of the extension. The removal had a dramatic effect on diffraction patterns: In IFMs in general, the equatorial 2,0 reflection is much stronger than the 1,1 reflection, but after removal, their intensities became almost equal (stronger 1,1 is a feature of vertebrate skeletal muscle). Electron micrographs revealed that a substantial fraction of the thin filaments showed a tendency to move towards the vertebrate position (the trigonal position between three thick filaments), while the rest of the thin filaments remained in their original insect position (midway between two neighboring thick filaments). Therefore, one of the roles of the extension is suggested to keep the filament lattice in the correct configuration for IFM. This insect-type lattice structure is preserved among IFMs from varied insect orders but not in body muscles, suggesting that the maintenance of this lattice structure is important for flight functions.

The long C-terminal extension of insect flight muscle-specific troponin-I isoform is not required for stretch activation

Stretch-induced enhancement of active force (stretch activation, SA) is observed in striated muscles in general, and most conspicuously in insect flight muscle (IFM). It remains unclear whether a common mechanism underlies the SA of all muscle types, or the SA of IFM relies on its highly specialized features. Recent studies suggest that IFM-specific isoforms of thin filament regulatory proteins (troponin and tropomyosin) are implicated in SA. Among others, IFM-specific troponin-I (troponin-H or TnH), with an unusually long Pro-Ala-rich extension at the C-terminus, has been speculated to transmit the mechanical signal of stretch to the troponin complex. To verify this hypothesis, it was removed by a specific endoproteinase in bumblebee IFM, expecting that it would eliminate SA while leaving intact the capacity for Ca(2+)-activated isometric force. Electrophoretic data showed that the extension was almost completely (97%) removed from IFM fibers after treatment. Unexpectedly, SA force was still conspicuous, and its rate of rise was not affected. Therefore, the results preclude the possibility that the extension is a main part of the mechanism of SA. This leaves open the possibility that SAs of IFM and vertebrate striated muscles, which lack the extension, operate under common basic mechanisms.

Regulating the contraction of insect flight muscle

The rapid movement of the wings in small insects is powered by the indirect flight muscles. These muscles are capable of contracting at up to 1,000 Hz because they are activated mechanically by stretching. The mechanism is so efficient that it is also used in larger insects like the waterbug, Lethocerus. The oscillatory activity of the muscles occurs a low concentration of Ca(2+), which stays constant as the muscles contract and relax. Activation by stretch requires particular isoforms of tropomyosin and the troponin complex on the thin filament. We compare the tropomyosin and troponin of Lethocerus and Drosophila with that of vertebrates. The characteristics of the flight muscle regulatory proteins suggest ways in which stretch-activation works. There is evidence for bridges between troponin on thin filaments and myosin crossbridges on the thick filaments. Recent X-ray fibre diffraction results suggest that a pull on the bridges activates the thin filament by shifting tropomyosin from a blocking position on actin. The troponin bridges are likely to contain extended sequences of tropomyosin or troponin I (TnI). Flight muscle has two isoforms of TnC with different Ca(2+)-binding properties: F1 TnC is needed for stretch-activation and F2 TnC for isometric contractions. In this review, we describe the structural changes in both isoforms on binding Ca(2+) and TnI, and discuss how the steric model of muscle regulation can apply to insect flight muscle.

Structure, function and evolution of insect flight muscle

Insects, the largest group of animals on the earth, owe their prosperity to their ability of flight and small body sizes. The ability of flight provided means for rapid translocation. The small body size allowed access to unutilized niches. By acquiring both features, however, insects faced a new problem: They were forced to beat their wings at enormous frequencies. Insects have overcome this problem by inventing asynchronous flight muscle, a highly specialized form of striated muscle capable of oscillating at >1,000 Hz. This article reviews the structure, mechanism, and molecular evolution of this unique invention of nature.

Binding properties of the calcium-activated F2 isoform of Lethocerus troponin C

While in most muscles contraction is triggered by calcium effluxes, insect flight muscles are also activated by mechanical stretch. We are interested in understanding the role that the troponin C protein, usually the calcium sensor, plays in stretch activation. In the flight muscles of Lethocerus, a giant water bug often used as a model system, there are two isoforms of TnC, F1 and F2, present in an approximately 10:1 ratio. F1 TnC is responsible for activating the muscle following a stretch, whereas F2 TnC produces a sustained contraction, the magnitude of which depends on the concentration of Ca²⁺ in the fiber. We have previously shown that F1 TnC binds only one Ca²⁺ ion in its C-terminal domain and that interaction with troponin H, the insect ortholog of troponin I, is insensitive to Ca²⁺. Here, we have studied the effect of Ca²⁺ and Mg²⁺ on the affinities of the interaction of F2 TnC with troponin H peptides. We show that the presence of two Ca²⁺ ions, one in each of the globular domains, increases the affinity for TnH by at least 1 order of magnitude. The N lobe has a lower affinity for Ca²⁺, but it is also sensitive to Mg²⁺. The C lobe is insensitive to Mg²⁺ as previously demonstrated by mutations of the individual EF-hands. The interaction with TnH seems also to have significant structural differences from that observed for the F1 TnC isoform. We discuss how our findings could account for stretch activation.

Tropomyosin is an interaction partner of the Drosophila coiled coil protein yuri gagarin

The Drosophila gene yuri gagarin is a complex locus encoding three protein isoform classes that are ubiquitously expressed in the organism. Mutations to the gene affect processes as diverse as gravitactic behavior and spermatogenesis. The larger Yuri isoforms contain extensive coiled-coil regions. Our previous studies indicate that one of the large isoform classes (Yuri-65) is required for formation of specialized F-actin-containing structures generated during spermatogenesis, including the so-called actin "cones" that mediate spermatid individualization. We used the tandem affinity purification of a tagged version of Yuri-65 (the TAP-tagging technique) to identify proteins associated with Yuri-65 in the intact organism. Tropomyosin, primarily as the 284-residue isoform derived from the ubiquitously expressed Tropomyosin 1 gene was thus identified as a major Yuri interaction partner. Co-immunoprecipitation experiments confirmed this interaction. We have established that the stable F-actin cones of spermatogenesis contain Tropomyosin 1 (Tm1) and that in mutant yuri(F64), failure of F-actin cone formation is associated with failure of Tm1 to accumulate at the cone initiation sites. In investigating possible interactions of Tm1 and Yuri in other tissues, we discovered that Tm1 and Yuri frequently colocalize with the endoplasmic reticulum. Tropomyosin has been implicated in actin-mediated membrane trafficking activity in other systems. Our findings suggest that Yuri-Tm1 complexes participate in related functions.

In vitro motility of native thin filaments from Drosophila indirect flight muscles reveals that the held-up 2 TnI mutation affects calcium activation

A procedure for the isolation of regulated native thin filaments from the indirect flight muscles (IFM) of Drosophila melanogaster is described. These are the first striated invertebrate thin filaments to show Ca-regulated in vitro motility. Regulated native thin filaments from wild type and a troponin I mutant, held-up-2, were compared by in vitro motility assays that showed that the mutant troponin I caused activation of motility at pCa values higher than wild type. The held-up2 mutation, in the sole troponin I gene (wupA) in the Drosophila genome, is known to cause hypercontraction of the IFM and other muscles in vivo leading to their eventual destruction. The mutation causes substitution of alanine by valine at a homologous and completely conserved troponin I residue (A25) in the vertebrate skeletal muscle TnI isoform. The effects of the held-up 2 mutation on calcium activation of thin filament in vitro motility are discussed with respect to its effects on hypercontraction and dysfunction. Previous electron microscopy and 3-dimensional reconstruction studies showed that the tropomyosin of held-up 2 thin filaments occupies positions associated with the so-called 'closed' state, but independently of calcium concentration. This is discussed with respect to calcium dependent regulation of held-up-2 thin filaments in in vitro motility.

Regulation of oscillatory contraction in insect flight muscle by troponin

Insect indirect flight muscle is activated by sinusoidal length change, which enables the muscle to work at high frequencies, and contracts isometrically in response to Ca(2+). Indirect flight muscle has two TnC isoforms: F1 binding a single Ca(2+) in the C-domain, and F2 binding Ca(2+) in the N- and C-domains. Fibres substituted with F1 produce delayed force in response to a single rapid stretch, and those with F2 produce isometric force in response to Ca(2+). We have studied the effect of TnC isoforms on oscillatory work. In native Lethocerus indicus fibres, oscillatory work was superimposed on a level of isometric force that depended on Ca(2+) concentration. Maximum work was produced at pCa 6.1; at higher concentrations, work decreased as isometric force increased. In fibres substituted with F1 alone, work continued to rise as Ca(2+) was increased up to pCa 4.7. Fibres substituted with various F1:F2 ratios produced maximal work at a ratio of 100:1 or 50:1; a higher proportion of F2 increased isometric force at the expense of oscillatory work. The F1:F2 ratio was 9.8:1 in native fibres, as measured by immunofluorescence, using isoform-specific antibodies. The small amount of F2 needed to restore work to levels obtained for the native fibre is likely to be due to the relative affinity of F1 and F2 for TnH, the Lethocerus homologue of TnI. Affinity of TnC isoforms for a TnI fragment of TnH was measured by isothermal titration calorimetry. The K(d) was 1.01 muM for F1 binding and 22.7 nM for F2. The higher affinity of F2 can be attributed to two TnH binding sites on F2 and a single site on F1. Stretch may be sensed by an extended C-terminal domain of TnH, resulting in reversible dissociation of the inhibitory sequence from actin during the oscillatory cycle.

Gene expression profiling implicates OXPHOS complexes in lifespan extension of flies over-expressing a small mitochondrial chaperone, Hsp22

Aging is a complex process accompanied by a decreased capacity to tolerate and respond to various stresses. Heat shock proteins as part of cell defense mechanisms are up-regulated following stress. In Drosophila, the mitochondrial Hsp22 is preferentially up-regulated in aged flies. Its over-expression results in an extension of lifespan and an increased resistance to stress. Hsp22 has chaperone-like activity in vitro, but the mechanism(s) by which it increases lifespan in flies are unknown. Genome-wide analysis was performed on long-lived Hsp22+ and control flies to unveil transcriptional changes brought by Hsp22. Transcriptomes obtained at 45days, 90% and 50% survival were then compared between them to focus more on genes up- or down-regulated in presence of higher levels of hsp22 mRNA. Hsp22+ flies display an up-regulation of genes mainly related to mitochondrial energy production and protein biosynthesis, two functions normally down-regulated during aging. Interestingly, among the 26 genes up-regulated in Hsp22+ flies, 7 genes encode for mitochondrial proteins, 5 of which being involved in OXPHOS complexes. Other genes that could influence aging such as CG5002, dGCC185 and GstS1 also displayed a regulation linked to Hsp22 expression. The up-regulation of genes of the OXPHOS system in Hsp22+ flies suggest that mitochondrial homeostasis is at the center of Hsp22 beneficial effects on lifespan.

Identification and characterization of eleven glutathione S-transferase genes from the aquatic midge Chironomus tentans (Diptera: Chironomidae)

Eleven cDNAs encoding glutathione S-transferases (GSTs) were sequenced and characterized in Chironomus tentans, an ecologically important aquatic midge. Phylogenetic analysis revealed seven GSTs in three different cytosolic classes including 4 in sigma (CtGSTs1, CtGSTs2, CtGSTs3, CtGSTs4), 2 in delta (CtGSTd1, CtGSTd2), and 1 in omega (CtGSTo1). The remaining four GSTs (CtGSTu1, CtGSTu2, CtGSTu3, CtGSTu4) were unclassified due to their low relatedness to currently known classes of insect GSTs. Reverse-transcription (RT)-PCR analysis of the 11 GST genes showed that CtGSTd1, CtGSTu2, CtGSTu4, CtGSTs1, CtGSTs2, CtGSTs3, CtGSTs4 and CtGSTo1 were expressed in all tissues examined, including salivary glands, hemolymph, midgut, Malpighian tubules, fatbodies and carcass, whereas CtGSTd2 and CtGSTu1 were expressed in a limited number of tissues. CtGSTs1 and CtGSTs4 appeared to be the only two genes, of which expressions can be detected in eggs, whereas all the 11 GST genes showed various expression patterns in the four larval instars. However, expressions of CtGSTd2, CtGSTu1 and CtGSTu2 were not detectable in pupal and adult stages. Real-time quantitative PCR confirmed that the herbicide alachlor increased CtGSTd1, CtGSTs2 and CtGSTs3 gene expression by 2.1-, 2.8- and 4.3-fold, respectively, when fourth-instar midges were exposed to alachlor at 1000 microg/L for 72 h. Such increased gene expressions were associated with 2.2- and 1.8-fold decreases of total GST activities in vivo when CDNB and DCNB were used as substrates, respectively. Further studies showed that 65.5 and 73.5% of GST activities were inhibited in vitro by alachlor at 100 and 1000 microg/L, respectively. Because alachlor has been known as an electrophilic substrate that can be conjugated by glutathione (GSH), rapid in vitro inhibition of GST activities by alachlor suggested that decreased GST activities were likely caused by the depletion of GSH. However, alachlor may regulate different GST genes, as found in other organisms, leading to significantly increased transcriptional levels of CtGSTd1, CtGSTs2 and CtGSTs3 in out of 11 GST genes examined in this study.

Weight and nutrition affect pre-mRNA splicing of a muscle gene associated with performance, energetics and life history

A fundamental feature of gene expression in multicellular organisms is the production of distinct transcripts from single genes by alternative splicing (AS), which amplifies protein and functional diversity. In spite of the likely consequences for organismal biology, little is known about how AS varies among individuals or responds to body condition, environmental variation or extracellular signals in general. Here we show that evolutionarily conserved AS of troponin-t in flight muscle of adult moths responds in a quantitative fashion to experimental manipulation of larval nutrition and adult body weight. Troponin-t (Tnt) isoform composition is known to affect muscle force and power output in other animals, and is shown here to be associated with the thorax mass-specific rate of energy consumption during flight. Loading of adults with external weights for 5 days caused an AS response nearly identical to equal increases in actual body weight. In addition, there were effects of larval feeding history on adult Tnt isoform composition that were independent of body weight, with moths from poorer larval feeding regimes producing isoform profiles associated with reduced muscle performance and energy consumption rate. Thus, Tnt isoform composition in striated muscle is responsive to both weight-sensing and nutrition-sensing mechanisms, with consequent effects on function. In free-living butterflies, Tnt isoform composition was also associated with activity level and very strongly with the rate of egg production. Overall, these results show that AS of a muscle gene responds in a quantitative fashion to whole-organism variables, which apparently serves to coordinate muscle strength and energy expenditure with body condition and life history.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

The regulation of myosin binding to actin filaments by Lethocerus troponin

Lethocerus indirect flight muscle has two isoforms of troponin C, TnC-F1 and F2, which are unusual in having only a single C-terminal calcium binding site (site IV, isoform F1) or one C-terminal and one N-terminal site (sites IV and II, isoform F2). We show here that thin filaments assembled from rabbit actin and Lethocerus tropomyosin (Tm) and troponin (Tn) regulate the binding of rabbit myosin to rabbit actin in much the same way as the mammalian regulatory proteins. The removal of calcium reduces the rate constant for S1 binding to regulated actin about threefold, independent of which TmTn is used. This is consistent with calcium removal causing the TmTn to occupy the B or blocked state to about 70% of the total. The mid point pCa for the switch differed for TnC-F1 and F2 (pCa 6.9 and 6.0, respectively) consistent with the reported calcium affinities for the two TnCs. Equilibrium titration of S1 binding to regulated actin filaments confirms calcium regulated binding of S1 to actin and shows that in the absence of calcium the three actin filaments (TnC-F1, TnC-F2 and mammalian control) are almost indistinguishable in terms of occupancy of the B and C states of the filament. In the presence of calcium TnC-F2 is very similar to the control with approximately 80% of the filament in the C-state and 10-15% in the fully on M-State while TnC-F1 has almost 50% in each of the C and M states. This higher occupancy of the M-state for TnC-F1, which occurs above pCa 6.9, is consistent with this isoform being involved in the calcium activation of stretch activation. However, it leaves unanswered how a C-terminal calcium binding site of TnC can activate the thin filament.

The Structure of Lethocerus Troponin C: Insights into the Mechanism of Stretch Activation in Muscles

To gain a molecular description of how muscles can be activated by mechanical stretch, we have solved the structure of the calcium-loaded F1 isoform of troponin C (TnC) from Lethocerus and characterized its interactions with troponin I (TnI). We show that the presence of only one calcium cation in the fourth EF hand motif is sufficient to induce an open conformation in the C-terminal lobe of F1 TnC, in contrast with what is observed in vertebrate muscle. This lobe interacts in a calcium-independent way both with the N terminus of TnI and, with lower affinity, with a region of TnI equivalent to the switch and inhibitory peptides of vertebrate muscles. Using both synthetic peptides and recombinant proteins, we show that the N lobe of F1 TnC is not engaged in interactions with TnI, excluding a regulatory role of this domain. These findings provide insights into mechanically stimulated muscle contraction.

The myosin filament superlattice in the flight muscles of flies: A-band lattice optimisation for stretch-activation?

Low-angle X-ray diffraction patterns from relaxed fruitfly (Drosophila) flight muscle recorded on the BioCat beamline at the Argonne Advanced Photon Source (APS) show many features similar to such patterns from the "classic" insect flight muscle in Lethocerus, the giant water bug, but there is a characteristically different pattern of sampling of the myosin filament layer-lines, which indicates the presence of a superlattice of myosin filaments in the Drosophila A-band. We show from analysis of the structure factor for this lattice that the sampling pattern is exactly as expected if adjacent four-stranded myosin filaments, of repeat 116 nm, are axially shifted in the hexagonal A-band lattice by one-third of the 14.5 nm axial spacing between crowns of myosin heads. In addition, electron micrographs of Drosophila and other flies (e.g. the house fly (Musca) and the flesh fly (Sarcophaga)) combined with image processing confirm that the same A-band superlattice occurs in all of these flies; it may be a general property of the Diptera. The different A-band organisation in flies compared with Lethocerus, which operates at a much lower wing beat frequency (approximately 30 Hz) and requires a warm-up period, may be a way of optimising the myosin and actin filament geometry needed both for stretch activation at the higher wing beat frequencies (50 Hz to 1000 Hz) of flies and their need for a rapid escape response.

The structural role of high molecular weight tropomyosins in dipteran indirect flight muscle and the effect of phosphorylation

In Drosophila melanogaster two high molecular weight tropomyosin isoforms, historically named heavy troponins (TnH-33 and TnH-34), are encoded by the Tm1 tropomyosin gene. They are specifically expressed in the indirect flight muscles (IFM). Their N-termini are conventional and complete tropomyosin sequences, but their C-termini consist of different IFM-specific domains that are rich in proline, alanine, glycine and glutamate. The evidence indicates that in Diptera these IFM-specific isoforms are conserved and are not troponins, but heavy tropomyosins (TmH). We report here that they are post-translationally modified by several phosphorylations in their C-termini in mature flies, but not in recently emerged flies that are incapable of flight. From stoichiometric measurements of thin filament proteins and interactions of the TmH isoforms with the standard Drosophila IFM tropomyosin isoform (protein 129), we propose that the TmH N-termini are integrated into the thin filament structural unit as tropomyosin dimers. The phosphorylated C-termini remain unlocated and may be important in IFM stretch-activation. Comparison of the Tm1 and Tm2 gene sequences shows a complete conservation of gene organisation in other Drosophilidae, such as Drosophila pseudoobscura, while in Anopheles gambiae only one exon encodes a single C-terminal domain, though overall gene organization is maintained. Interestingly, in Apis mellifera (hymenopteran), while most of the Tm1 and Tm2 gene features are conserved, the gene lacks any C-terminal exons. Instead these sequences are found at the 3' end of the troponin I gene. In this insect order, as in Lethocerus (hemipteran), the original designation of troponin H (TnH) should be retained. We discuss whether the insertion of the IFM-specific pro-ala-gly-glu-rich domain into the tropomyosin or troponin I genes in different insect orders may be related to proposals that the IFM stretch activation mechanism has evolved independently several times in higher insects.

Drosophila as a model for the identification of genes causing adult human heart disease

Drosophila melanogaster genetics provides the advantage of molecularly defined P-element insertions and deletions that span the entire genome. Although Drosophila has been extensively used as a model system to study heart development, it has not been used to dissect the genetics of adult human heart disease because of an inability to phenotype the adult fly heart in vivo. Here we report the development of a strategy to measure cardiac function in awake adult Drosophila that opens the field of Drosophila genetics to the study of human dilated cardiomyopathies. Through the application of optical coherence tomography, we accurately distinguish between normal and abnormal cardiac function based on measurements of internal cardiac chamber dimensions in vivo. Normal Drosophila have a fractional shortening of 87 +/- 4%, whereas cardiomyopathic flies that contain a mutation in troponin I or tropomyosin show severe impairment of systolic function. To determine whether the fly can be used as a model system to recapitulate human dilated cardiomyopathy, we generated transgenic Drosophila with inducible cardiac expression of a mutant of human delta-sarcoglycan (deltasg(S151A)), which has previously been associated with familial dilated cardiomyopathy. Compared to transgenic flies overexpressing wild-type deltasg, or the standard laboratory strain w(1118), Drosophila expressing deltasg(S151A) developed marked impairment of systolic function and significantly enlarged cardiac chambers. These data illustrate the utility of Drosophila as a model system to study dilated cardiomyopathy and the applicability of the vast genetic resources available in Drosophila to systematically study the genetic mechanisms responsible for human cardiac disease.

The Coevolution of Insect Muscle TpnT and TpnI Gene Isoforms

In bilaterians, the main regulator of muscle contraction is the troponin (Tpn) complex, comprising three closely interacting subunits (C, T, and I). To understand how evolutionary forces drive molecular change in protein complexes, we have compared the gene structures and expression patterns of Tpn genes in insects. In this class, while TpnC is encoded by multiple genes, TpnT and TpnI are encoded by single genes. Their isoform expression pattern is highly conserved within the Drosophilidae, and single orthologous genes were identified in the sequenced genomes of Drosophila pseudoobscura, Anopheles gambiae, and Apis mellifera. Apis expression patterns also support the equivalence of their exon organization throughout holometabolous insects. All TpnT genes include a previously unidentified indirect flight muscle (IFM)-specific exon (10A) that has evolved an expression pattern similar to that of exon 9 in TpnI. Thus, expression patterns, sequence evolution trends, and structural data indicate that Tpn genes and their isoforms have coevolved, building species- and muscle-specific troponin complexes. Furthermore, a clear case can be made for independent evolution of the IFM-specific isoforms containing alanine/proline-rich sequences. Dipteran genomes contain one tropomyosin gene that encodes one or two high-molecular weight isoforms (TmH) incorporating APPAEGA-rich sequences, specifically expressed in IFM. Corresponding exons do not exist in the Apis tropomyosin gene, but equivalent sequences occur in a high-molecular weight Apis IFM-specific TpnI isoform (TnH). Overall, our approach to comparatively analyze supramolecular complexes reveals coevolutionary trends not only in gene families but in isoforms generated by alternative splicing.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

Ca-activation and stretch-activation in insect flight muscle

Asynchronous insect flight muscle is specialized for myogenic oscillatory work, but can also produce isometric tetanic contraction. In skinned insect flight muscle fibers from Lethocerus, with sarcomere length monitored by a striation follower, we determined the relation between isometric force (F(0)) at serial increments of [Ca(2+)] and the additional active force recruited at each [Ca(2+)] by a stretch of approximately 12 nm per half-sarcomere (F(SA)). The isometric force-pCa relation shows that 1.5-2 units of pCa are necessary to raise isometric force from its threshold (pCa approximately 6.5) to its maximum (F(0,max)). The amplitude of F(SA) depends only on the preceding baseline level of isometric force, which must reach at least 0.05 F(0,max) to enable stretch-activation. F(SA) rises very steeply to its maximum as F(0) reaches approximately 0.2 F(0,max), then decreases as F(0) increases so as to produce a constant sum (F(0) + F(SA)) = F(max). Thus Ca- and stretch-activation are complementary pathways that trigger a common process of cross-bridge attachment and force production. We suggest that stretch-induced distortion of attached cross-bridges relieves the steric blocking by tropomyosin of additional binding sites on actin, thereby enabling maximum force even at low [Ca(2+)].

Drosophila muscle regulation characterized by electron microscopy and three-dimensional reconstruction of thin filament mutants

Wild-type and mutant thin filaments were isolated directly from "myosinless" Drosophila indirect flight muscles to study the structural basis of muscle regulation genetically. Negatively stained filaments showed tropomyosin with periodically arranged troponin complexes in electron micrographs. Three-dimensional helical reconstruction of wild-type filaments indicated that the positions of tropomyosin on actin in the presence and absence of Ca(2+) were indistinguishable from those in vertebrate striated muscle and consistent with a steric mechanism of regulation by troponin-tropomyosin in Drosophila muscles. Thus, the Drosophila model can be used to study steric regulation. Thin filaments from the Drosophila mutant heldup(2), which possesses a single amino acid conversion in troponin I, were similarly analyzed to assess the Drosophila model genetically. The positions of tropomyosin in the mutant filaments, in both the Ca(2+)-free and the Ca(2+)-induced states, were the same, and identical to that of wild-type filaments in the presence of Ca(2+). Thus, cross-bridge cycling would be expected to proceed uninhibited in these fibers, even in relaxing conditions, and this would account for the dramatic hypercontraction characteristic of these mutant muscles. The interaction of mutant troponin I with Drosophila troponin C is discussed, along with functional differences between troponin C from Drosophila and vertebrates.

A troponin switch that regulates muscle contraction by stretch instead of calcium

The flight muscles of many insects have a form of regulation enabling them to contract at high frequencies. The muscles are activated by periodic stretches at low Ca2+ levels. The same muscles also give isometric contractions in response to higher Ca2+. We show that the two activities are controlled by different isoforms of TnC (F1 and F2) within single myofibrils. F1 binds one Ca2+ with high affinity in the C-terminal domain and F2 binds one Ca2+ in the C-terminal domain and one exchangeable Ca2+ in the N-terminal domain. We have characterised the isoforms and determined their effect on the development of stretch-activated and Ca2+-activated tension by replacing endogenous TnC in Lethocerus flight muscle fibres with recombinant isoforms. Fibres with F1 gave stretch-activated tension and minimal isometric tension; those with F2 gave Ca2+-dependent isometric tension and minimal stretch-activated tension. Regulation by a TnC responding to stretch rather than Ca2+ is unprecedented and has resulted in the ability of insect flight muscle to perform oscillatory work at low Ca2+ concentrations, a property to which a large number of flying insects owe their evolutionary success.

Troponin C in different insect muscle types: identification of two isoforms in Lethocerus, Drosophila and Anopheles that are specific to asynchronous flight muscle in the adult insect

The indirect flight muscles (IFMs) of Lethocerus (giant water bug) and Drosophila (fruitfly) are asynchronous: oscillatory contractions are produced by periodic stretches in the presence of a Ca(2+) concentration that does not fully activate the muscle. The troponin complex on thin filaments regulates contraction in striated muscle. The complex in IFM has subunits that are specific to this muscle type, and stretch activation may act through troponin. Lethocerus and Drosophila have an unusual isoform of the Ca(2+)-binding subunit of troponin, troponin C (TnC), with a single Ca(2+)-binding site near the C-terminus (domain IV); this isoform is only in IFMs, together with a minor isoform with an additional Ca(2+)-binding site in the N-terminal region (domain II). Lethocerus has another TnC isoform in leg muscle which also has two Ca(2+)-binding sites. Ca(2+) binds more strongly to domain IV than to domain II in two-site isoforms. There are four isoforms in Drosophila and Anopheles (malarial mosquito), three of which are also in adult Lethocerus. A larval isoform has not been identified in Lethocerus. Different TnC isoforms are expressed in the embryonic, larval, pupal and adult stages of Drosophila; the expression of the two IFM isoforms is increased in the pupal stage. Immunoelectron microscopy shows the distribution of the major IFM isoform with one Ca(2+)-binding site is uniform along Lethocerus thin filaments. We suggest that initial activation of IFM is by Ca(2+) binding to troponin with the two-site TnC, and full activation is through the action of stretch on the complex with the one-site isoform.

Structure of a Drosophila sigma class glutathione S-transferase reveals a novel active site topography suited for lipid peroxidation products

Insect glutathione-S-transferases (GSTs) are grouped in three classes, I, II and recently III; class I (Delta class) enzymes together with class III members are implicated in conferring resistance to insecticides. Class II (Sigma class) GSTs, however, are poorly characterized and their exact biological function remains elusive. Drosophila glutathione S-transferase-2 (GST-2) (DmGSTS1-1) is a class II enzyme previously found associated specifically with the insect indirect flight muscle. It was recently shown that GST-2 exhibits considerable conjugation activity for 4-hydroxynonenal (4-HNE), a lipid peroxidation product, raising the possibility that it has a major anti-oxidant role in the flight muscle. Here, we report the crystal structure of GST-2 at 1.75A resolution. The GST-2 dimer shows the canonical GST fold with glutathione (GSH) ordered in only one of the two binding sites. While the GSH-binding mode is similar to other GST structures, a distinct orientation of helix alpha6 creates a novel electrophilic substrate-binding site (H-site) topography, largely flat and without a prominent hydrophobic-binding pocket, which characterizes the H-sites of other GSTs. The H-site displays directionality in the distribution of charged/polar and hydrophobic residues creating a binding surface that explains the selectivity for amphipolar peroxidation products, with the polar-binding region formed by residues Y208, Y153 and R145 and the hydrophobic-binding region by residues V57, A59, Y211 and the C-terminal V249. A structure-based model of 4-HNE binding is presented. The model suggest that residues Y208, R145 and possibly Y153 may be key residues involved in catalysis.

The location of ubiquitin in Lethocerus arthrin

Arthrin is a ubiquitinated actin that is present in flight muscles of some insects. In addition, it has been found in the malaria parasite Plasmodium falciparum. The role of this monoubiquitylation is not clear, and it does not appear to be associated with proteolytic degradation. The stoichiometry of arthrin to actin in Lethocerus indirect flight muscle, 1:6, suggests that there would be one arthrin molecule for each Tm-Tn (tropomyosin-troponin) complex. The appearance of arthrin after tropomyosin and troponin in Drosophila development is consistent with the Tm-Tn complex determining which actin subunit is targeted for conjugation with ubiquitin. We have used a new approach of three-dimensional reconstruction of helical filaments, the iterative helical real space reconstruction method, to extract segments of homogeneous arthrin out of long filaments where the conformation of the ubiquitin is more heterogeneous. Surprisingly, the location of the ubiquitin is on the face of actin subdomain 1, opposite to where tropomyosin binds in the "off" state, suggesting that there could not be a direct interaction between the ubiquitin and the tropomyosin. It is possible that the troponin complex in the "on" state that is bound to one actin strand makes an unfavorable contact with a ubiquitin molecule attached to the opposite actin strand. This might be the basis for a destabilization of the on state at rest length. Lys118 is the most likely residue to which the ubiquitin is conjugated, based upon fitting atomic structures of actin and ubiquitin into the reconstruction.

Genetics of the Drosophila flight muscle myofibril: a window into the biology of complex systems

This essay reviews the long tradition of experimental genetics of the Drosophila indirect flight muscles (IFM). It discusses how genetics can operate in tandem with multidisciplinary approaches to provide a description, in molecular terms, of the functional properties of the muscle myofibril. In particular, studies at the interface of genetics and proteomics address protein function at the cellular scale and offer an outstanding platform with which to elucidate how the myofibril works. Two generalizations can be enunciated from the studies reviewed. First, the study of mutant IFM proteomes provides insight into how proteins are functionally organized in the myofibril. Second, IFM mutants can give rise to structural and contractile defects that are unrelated, a reflection of the dual function that myofibrillar proteins play as fundamental components of the sarcomeric framework and biochemical "parts" of the contractile "engine".

Kettin, a major source of myofibrillar stiffness in Drosophila indirect flight muscle

Kettin is a high molecular mass protein of insect muscle that in the sarcomeres binds to actin and alpha-actinin. To investigate kettin's functional role, we combined immunolabeling experiments with mechanical and biochemical studies on indirect flight muscle (IFM) myofibrils of Drosophila melanogaster. Micrographs of stretched IFM sarcomeres labeled with kettin antibodies revealed staining of the Z-disc periphery. After extraction of the kettin-associated actin, the A-band edges were also stained. In contrast, the staining pattern of projectin, another IFM-I-band protein, was not altered by actin removal. Force measurements were performed on single IFM myofibrils to establish the passive length-tension relationship and record passive stiffness. Stiffness decreased within seconds during gelsolin incubation and to a similar degree upon kettin digestion with mu-calpain. Immunoblotting demonstrated the presence of kettin isoforms in normal Drosophila IFM myofibrils and in myofibrils from an actin-null mutant. Dotblot analysis revealed binding of COOH-terminal kettin domains to myosin. We conclude that kettin is attached not only to actin but also to the end of the thick filament. Kettin along with projectin may constitute the elastic filament system of insect IFM and determine the muscle's high stiffness necessary for stretch activation. Possibly, the two proteins modulate myofibrillar stiffness by expressing different size isoforms.

Calcium-bindings of wild type and mutant troponin Cs of Caenorhabditis elegans

Apparent Ca(2+)-binding constant (K(app)) of Caenorhabditis elegans troponin C (CeTnC) was determined by a fluorescence titration method. The K(app) of the N-domain Ca(2+)-binding site of CeTnC was 7.9+/-1.6 x 10(5) M(-1) and that of the C-domain site was 1.2+/-0.6 x 10(6) M(-1), respectively. Mg(2+)-dependence of the K(app) showed that both Ca(2+)-binding sites did not bind competitively Mg(2+). The Ca(2+) dissociation rate constant (k(off)) of CeTnC was determined by the fluorescence stopped-flow method. The k(off) of the N-domain Ca(2+)-binding site of CeTnC was 703+/-208 s(-1) and that of the C-domain site was 286+/-33 s(-1), respectively. From these values we could calculate the Ca(2+)-binding rate constant (k(on)) as to be 5.6+/-2.8 x 10(8) M(-1) s(-1) for the N-domain site and 3.4+/-2.1 x 10(8) M(-1) s(-1) for the C-domain site, respectively. These results mean that all Ca(2+)-binding sites of CeTnC are low affinity, fast dissociating and Ca(2+)-specific sites. Evolutional function of TnC between vertebrate and invertebrate and biological functions of wild type and mutant CeTnCs are discussed.

Characterization of several invertebrate muscle cell types: a comparison with vertebrate muscles

Ultrastructural classification of invertebrate muscles is complex and not always clear. The aim of the present paper was to establish some criteria that might be useful for classification of invertebrate muscles and for a better understanding of the differences between them. The procedures used were: (1) immunochemical evaluation of those proteins that differentiated striated from smooth muscle (troponin, caldesmon, and calponin), and (2) calculations of several myofilament parameters to establish differences among muscles. The muscles studied were: striated muscles from the rat, Drosophila, the crab Callinectes, and the snail Helix (heart); obliquely striated muscles from the earthworm Eisenia foetida and Helix (mouth); and smooth muscles from the rat, and Helix (retractor, body wall, and intestinal wall). Immunochemical studies revealed that troponin was only present in the striated muscles and the obliquely striated muscle from Eisenia, whereas caldesmon and calponin were only present in the smooth muscles and the obliquely striated muscle from Helix. The highest thick filament/thin filament volume ratio was found in the striated muscles, followed by the obliquely striated muscles, and the smooth muscles. This suggests the order in which the contraction strength decreases. The myofilament length is inversely related to the contraction speed, which was higher in the striated muscles than in the obliquely striated muscles. In vertebrates, the smooth muscle seems to be less rapid than the striated muscle because their myofilaments are longer. This assertion cannot be generalized for invertebrate smooth muscle, because myofilament lengths vary widely in both striated and smooth muscles. In smooth muscles, the presence of apparently unordered electron-dense bodies instead of ordered Z lines and the absence of true sarcomeres permit a certain overlapping of thin filaments increasing the range of shortening.

Identification of a chromosome 3p14.3-21.1 gene, APPL, encoding an adaptor molecule that interacts with the oncoprotein-serine/threonine kinase AKT2

AKT2 is a serine/threonine kinase implicated in human ovarian and pancreatic cancers. AKT2 is activated by a variety of growth factors and insulin via phosphatidylinositol 3-kinase (PI3K). However, its normal cellular role is not well understood. To gain insight into the function of AKT2, we performed yeast two-hybrid system to screen for interacting proteins. Using this technique, we identified a novel interactor, designated APPL, which contains a pleckstrin homology (PH) domain, a phosphotyrosine binding (PTB) domain and a leucine zipper, classes of motifs defined in signaling molecules as functional interaction domains with specific targets. The PH domain of APPL shows similarity to those found in GTPase-activating proteins such as oligophrenin-1 and Graf, whereas its PTB domain exhibits homology with CED-6, an adaptor protein that promotes engulfment of apoptotic cells, and IB1, a transactivator of the GLUT2 gene. APPL is highly expressed in skeletal muscle, heart, ovary and pancreas, tissues in which AKT2 mRNA is abundant. APPL interacts with the inactive form of AKT2; moreover, APPL binds to the PI3K catalytic subunit, p110alpha. These data suggest that APPL is an adaptor that may tether inactive AKT2 to p110alpha in the cytoplasm and thereby may expedite recruitment of AKT2 and p110alpha to the cell membrane upon mitogenic stimulation. Furthermore, the APPL gene was mapped to human chromosome 3p14.3-p21.1, where deletions and other rearrangements have often been reported in a variety of tumor types. The identification of APPL may facilitate further analysis of the physiological and oncogenic activities of AKT2.

Genomic organization, expression, and analysis of the troponin C gene pat-10 of Caenorhabditis elegans

We have cloned and characterized the troponin C gene, pat-10 of the nematode Caenorhabditis elegans. At the amino acid level nematode troponin C is most similar to troponin C of Drosophila (45% identity) and cardiac troponin C of vertebrates. Expression studies demonstrate that this troponin is expressed in body wall muscle throughout the life of the animal. Later, vulval muscles and anal muscles also express this troponin C isoform. The structural gene for this troponin is pat-10 and mutations in this gene lead to animals that arrest as twofold paralyzed embryos late in development. We have sequenced two of the mutations in pat-10 and both had identical two mutations in the gene; one changes D64 to N and the other changes W153 to a termination site. The missense alteration affects a calcium-binding site and eliminates calcium binding, whereas the second mutation eliminates binding to troponin I. These combined biochemical and in vivo studies of mutant animals demonstrate that this troponin is essential for proper muscle function during development.

Isolation, electron microscopic imaging, and 3-D visualization of native cardiac thin myofilaments

An increasing number of cardiac diseases are currently pinpointed to reside at the level of the thin myofilaments (e.g., cardiomyopathies, reperfusion injury). Hence the aim of our study was to develop a new method for the isolation of mammalian thin myofilaments suitable for subsequent high-resolution electron microscopic imaging. Native cardiac thin myofilaments were extracted from glycerinated porcine myocardial tissue in the presence of protease inhibitors. Separation of thick and thin myofilaments was achieved by addition of ATP and several centrifugation steps. Negative staining and subsequent conventional and scanning transmission electron microscopy (STEM) of thin myofilaments permitted visualization of molecular details; unlike conventional preparations of thin myofilaments, our method reveals the F-actin moiety and allows direct recognition of thin myofilament-associated porcine cardiac troponin complexes. They appear as "bulges" at regular intervals of approximately 36 nm along the actin filaments. Protein analysis using SDS-polyacrylamide gel electrophoresis revealed that only approximately 20% troponin I was lost during the isolation procedure. In a further step, 3-D helical reconstructions were calculated using STEM dark-field images. These 3-D reconstructions will allow further characterization of molecular details, and they will be useful for directly visualizing molecular alterations related to diseased cardiac thin myofilaments (e.g., reperfusion injury, alterations of Ca2+-mediated tropomyosin switch).

Association of kettin with actin in the Z-disc of insect flight muscle

The Z-discs of insect muscle contain kettin, a modular protein of 500-700 kDa. The Drosophila protein is made up of a chain of immunoglobulin (Ig) domains separated by linker sequences. Kettin differs from other modular muscle proteins of the Ig superfamily in binding to thin filaments rather than thick filaments. Kettin isolated from Lethocerus (waterbug) muscle is an elongated molecule 180 nm long, which binds to F-actin with high affinity (Kd=1.2 nM) and a stoichiometry of one Ig domain per actin protomer. Competition between kettin and tropomyosin for binding to actin excludes tropomyosin from the Z-disc. In contrast, kettin and alpha-actinin bind simultaneously to actin, which would reinforce the Z-disc lattice. In vitro, kettin promotes the antiparallel association of actin filaments, and a similar process may occur in the developing sarcomere: actin filaments interdigitate in an antiparallel fashion in the Z-disc with the N terminus of kettin within the Z-disc, and the C terminus some way outside. We propose a model for the association of kettin with actin in which the molecule follows the genetic helix of actin and Ig domains, separated by linker sequences, bind to each actin protomer.

Structure of the insect troponin complex

Isolated troponin-tropomyosin complex from Lethocerus indicus asynchronous flight muscle forms paracrystals on a positively charged lipid monolayer. Single particle analysis was carried out on individual complexes selected from electron micrographs of negatively stained paracrystals. By a combination of correlation and classification techniques, different average projections of the object were obtained. An initial three-dimensional model was calculated by determining the Euler angles for the different views using a common line approach. This starting model was then used as a reference for the further three-dimensional refinement of the model using the original data set. The refined model of the troponin complex has a diameter of approximately 90 A and a volume corresponding with a molecular mass of about 120 kDa for the globular domain. The resolution of the reconstruction was determined to be 32 A using the differential phase residual method and 26 A using the Fourier shell correlation criterion.

Ca2+-dependent muscle dysfunction caused by mutation of the Caenorhabditis elegans troponin T-1 gene

We have investigated the functions of troponin T (CeTnT-1) in Caenorhabditis elegans embryonic body wall muscle. TnT tethers troponin I (TnI) and troponin C (TnC) to the thin filament via tropomyosin (Tm), and TnT/Tm regulates the activation and inhibition of myosin-actin interaction in response to changes in intracellular [Ca2+]. Loss of CeTnT-1 function causes aberrant muscle trembling and tearing of muscle cells from their exoskeletal attachment sites (Myers, C.D., P.-Y. Goh, T. StC. Allen, E.A. Bucher, and T. Bogaert. 1996. J. Cell Biol. 132:1061-1077). We hypothesized that muscle tearing is a consequence of excessive force generation resulting from defective tethering of Tn complex proteins. Biochemical studies suggest that such defective tethering would result in either (a) Ca2+-independent activation, due to lack of Tn complex binding and consequent lack of inhibition, or (b) delayed reestablishment of TnI/TnC binding to the thin filament after Ca2+ activation and consequent abnormal duration of force. Analyses of animals doubly mutant for CeTnT-1 and for genes required for Ca2+ signaling support that CeTnT-1 phenotypes are dependent on Ca2+ signaling, thus supporting the second model and providing new in vivo evidence that full inhibition of thin filaments in low [Ca2+] does not require TnT.