Expression and function of the Drosophila ACT88F actin isoform is not restricted to the indirect flight muscles

Most higher eukaryotic genomes contain multiple actin genes, yet the sequence differences between isoforms are few. In Drosophila melanogaster it was previously established that one of the six actin genes, Act88F, is expressed only in the indirect flight muscles (IFMs). These muscles are highly specialised for oscillatory contractions to power flight. The implication was that this isoform had tissue-specific properties. In this paper we show using two reporter constructs expressing either beta-galactosidase, Act88F-lacZ, or the green fluorescent protein, Act88F-GFP, that the Act88F promoter is active in a small number of other muscles, including leg (femoral) and uterine muscles. However, the levels of Act88F driven non-IFM expression are much less than in the IFMs. We have confirmed endogenous Act88F gene expression in these other muscles by in situ hybridisation studies. Using null and antimorphic mutants to show decreased walking ability and delayed/reduced oviposition we demonstrated that Act88F expression is functionally important in multiple muscle groups. Since the mutant effects are mild, this supports the expectation that other actin genes are also expressed in these muscles. The Act88F-GFP promoter-reporter also detects Act88F-driven expression in the bristle-forming cells in the pupal wings. The implications of these results for the functions and developmental expression of the Drosophila ACT88F isoform are discussed.

A tropomyosin-2 mutation suppresses a troponin I myopathy in Drosophila

A suppressor mutation, D53, of the held-up(2) allele of the Drosophila melanogaster Troponin I (wupA) gene is described. D53, a missense mutation, S185F, of the tropomyosin-2, Tm2, gene fully suppresses all the phenotypic effects of held-up(2), including the destructive hypercontraction of the indirect flight muscles (IFMs), a lack of jumping, the progressive myopathy of the walking muscles, and reductions in larval crawling and feeding behavior. The suppressor restores normal function of the IFMs, but flight ability decreases with age and correlates with an unusual, progressive structural collapse of the myofibrillar lattice starting at the center. The S185F substitution in Tm2 is close to a troponin T binding site on tropomyosin. Models to explain suppression by D53, derived from current knowledge of the vertebrate troponin-tropomyosin complex structure and functions, are discussed. The effects of S185F are compared with those of two mutations in residues 175 and 180 of human alpha-tropomyosin 1 which cause familial hypertrophic cardiomyopathy (HCM).

Characterization of muscle actin genes in Drosophila virilis reveals significant molecular complexity in skeletal muscle types

Actin is a ubiquitous and highly conserved eukaryotic protein required for cell motility and locomotion. In this manuscript, we characterize the four muscle actin genes of the insect Drosophila virilis and demonstrate strong similarities between the D. virilis genes and their homologues in Drosophila melanogaster; intron locations are conserved, and there are few amino acid differences between homologues. We also found strong conservation in temporal expression patterns of the muscle actin genes--the homologues of the D. melanogaster genes Act57B and Act87E are expressed throughout the life cycle, whereas the other two D. virilis genes, homologous to Act79B and Act88F are specific to pupal and adult stages. In situ hybridization revealed that each D. virilis gene is expressed in a unique pattern in the muscles of the thorax and abdomen. These muscle-specific patterns of actin isoforms suggest a greater physiological diversity for the adult muscles of insects than has been appreciated to date from their categorization into fibrillar, tubular (non-fibrillar) and supercontractile muscle types.

Alternative exon-encoded regions of Drosophila myosin heavy chain modulate ATPase rates and actin sliding velocity

To investigate the molecular functions of the regions encoded by alternative exons from the single Drosophila myosin heavy chain gene, we made the first kinetic measurements of two muscle myosin isoforms that differ in all alternative regions. Myosin was purified from the indirect flight muscles of wild-type and transgenic flies expressing a major embryonic isoform. The in vitro actin sliding velocity on the flight muscle isoform (6.4 microm x s(-1) at 22 degrees C) is among the fastest reported for a type II myosin and was 9-fold faster than with the embryonic isoform. With smooth muscle tropomyosin bound to actin, the actin sliding velocity on the embryonic isoform increased 6-fold, whereas that on the flight muscle myosin slightly decreased. No difference in the step sizes of Drosophila and rabbit skeletal myosins were found using optical tweezers, suggesting that the slower in vitro velocity with the embryonic isoform is due to altered kinetics. Basal ATPase rates for flight muscle myosin are higher than those of embryonic and rabbit myosin. These differences explain why the embryonic myosin cannot functionally substitute in vivo for the native flight muscle isoform, and demonstrate that one or more of the five myosin heavy chain alternative exons must influence Drosophila myosin kinetics.

Actin residue glu(93) is identified as an amino acid affecting myosin binding

Many mutants have been described that affect the function of the actin encoded by the Drosophila melanogaster indirect flight muscle-specific actin gene, Act88F. We describe the development of procedures for purification of this actin from the other isoforms expressed in the fly as well as in vitro motility, single molecule force/displacement measurements, and stop-flow solution kinetic studies of the wild-type actin and that of the E93K mutation of the Act88F gene. We show that this mutation affects in vitro motility of F-actin, in both the presence and absence of methylcellulose, and the ability of the ACT88F actin to bind the S1 fragment of rabbit skeletal myosin. However, optical tweezer measurements of the actomyosin working stroke and the force transmitted from the rabbit heavy meromyosin to and through F-actin are unchanged by the mutation. These results support the proposal (Holmes, K. C. (1995) Biophys J. 68, (suppl.) 2-7) that actin residue Glu(93) is part of the secondary myosin binding site and suggest that myosin binding occurs first at the primary myosin binding site and then at the secondary site.

The motor protein myosin-I produces its working stroke in two steps

Many types of cellular motility, including muscle contraction, are driven by the cyclical interaction of the motor protein myosin with actin filaments, coupled to the breakdown of ATP. It is thought that myosin binds to actin and then produces force and movement as it 'tilts' or 'rocks' into one or more subsequent, stable conformations. Here we use an optical-tweezers transducer to measure the mechanical transitions made by a single myosin head while it is attached to actin. We find that two members of the myosin-I family, rat liver myosin-I of relative molecular mass 130,000 (M(r) 130K) and chick intestinal brush-border myosin-I, produce movement in two distinct steps. The initial movement (of roughly 6 nanometres) is produced within 10 milliseconds of actomyosin binding, and the second step (of roughly 5.5 nanometres) occurs after a variable time delay. The duration of the period following the second step is also variable and depends on the concentration of ATP. At the highest time resolution possible (about 1 millisecond), we cannot detect this second step when studying the single-headed subfragment-1 of fast skeletal muscle myosin II. The slower kinetics of myosin-I have allowed us to observe the separate mechanical states that contribute to its working stroke.

Quantification of single human dermal fibroblast contraction

Contraction forces produced by single, human dermal fibroblasts (HDF), cultured on deformable silicone substrata, were quantified using video microscopy and image analysis. Cell contraction causes deformation of the substrate, which appears as a series of surface wrinkles perpendicular to the long axis of the cell. Local surface deformation was measured from the two-dimensional displacement of small latex beads embedded in the surface layer to which the HDF adhere. A calibrated glass microneedle was used to measure the force required to stretch the surface by a known amount (the surface stiffness). From the motion of the latex beads, the contractile forces of the cells were calculated. In vivo, such forces are thought to cause contraction of the dermis and hence promote wound closure. Normal contraction is vital to prevent infection and water loss. However, aberrant cellular behaviour is thought to be responsible for a variety of wound pathologies, such as hypertrophic and keloid scarring. We have found that contractile forces of 2.65 microN/cell were produced. This is similar to those produced by single smooth muscle cells and approximately 10 times greater than the forces measured for keratocytes and three orders of magnitude greater than previously published values for fibroblasts that had been cultured in a collagen gel. Our goal is to understand the mechanisms that determine the polarity and maximum force of contraction and also to study differences in the behavior of HDF and myofibroblasts.

The stiffness of rabbit skeletal actomyosin cross-bridges determined with an optical tweezers transducer

Muscle contraction is brought about by the cyclical interaction of myosin with actin coupled to the breakdown of ATP. The current view of the mechanism is that the bound actomyosin complex (or "cross-bridge") produces force and movement by a change in conformation. This process is known as the "working stroke." We have measured the stiffness and working stroke of a single cross-bridge (kappa xb, dxb, respectively) with an optical tweezers transducer. Measurements were made with the "three bead" geometry devised by Finer et al. (1994), in which two beads, supported in optical traps, are used to hold an actin filament in the vicinity of a myosin molecule, which is immobilized on the surface of a third bead. The movements and forces produced by actomyosin interactions were measured by detecting the position of both trapped beads. We measured, and corrected for, series compliance in the system, which otherwise introduces large errors. First, we used video image analysis to measure the long-range, force-extension property of the actin-to-bead connection (kappa con), which is the main source of "end compliance." We found that force-extension diagrams were nonlinear and rather variable between preparations, i.e., end compliance depended not only upon the starting tension, but also upon the F-actin-bead pair used. Second, we measured kappa xb and kappa con during a single cross-bridge attachment by driving one optical tweezer with a sinusoidal oscillation while measuring the position of both beads. In this way, the bead held in the driven optical tweezer applied force to the cross-bridge, and the motion of the other bead measured cross-bridge movement. Under our experimental conditions (at approximately 2 pN of pretension), connection stiffness (kappa con) was 0.26 +/- 0.16 pN nm-1. We found that rabbit heavy meromyosin produced a working stroke of 5.5 nm, and cross-bridge stiffness (kappa xb) was 0.69 +/- 0.47 pN nm-1.

Interaction of troponin-H and glutathione S-transferase-2 in the indirect flight muscles of Drosophila melanogaster

Drosophila indirect flight muscles (IFMs) contain a 35 kDa protein which cross-reacts with antibodies to the IFM specific protein troponin-H isoform 34 (TnH-34). Peptide fingerprinting and peptide sequencing showed that this 35 kDa protein is glutathione S-transferase-2 (GST-2). GST-2 is present in the asynchronous indirect flight muscles but not in the synchronous tergal depressor of the trochanter (jump muscle). Genetic dissection of the sarcomere showed that GST-2 is stably associated with the thin filaments but the presence of myosin is required to achieve the correct stoichiometry, suggesting that there is also an interaction with the thick filament. The two Drosophila TnHs (isoforms 33 and 34) are naturally occurring fusion proteins in which a proline-rich extension of approximately 250 amino acids replaces the 27 C-terminal residues of the muscle-specific tropomyosin II isoform. The proteolytic enzyme, Igase, cleaves the hydrophobic C-terminal sequence of TnH-34 at three sites and TnH-33 at one site. This results in the release of GST-2 from the myofibril. The amount of GST-2 stably bound to the myofibril is directly proportional to the total amount of undigested TnH. It is concluded that GST-2 in the thin filament is stabilized there by interaction with TnH. We speculate that the hydrophobic N-terminal region of GST-2 interacts with the hydrophobic C-terminal extension of TnH, and that both are close to a myosin cross-bridge.

Structural comparisons of muscle and nonmuscle actins give insights into the evolution of their functional differences

Actin is a highly conserved protein although many isoforms exist. In vertebrates and insects the different actin isoforms can be grouped by their amino acid sequence and tissue-specific gene expression into muscle and nonmuscle actins, suggesting that the different actins may have a functional significance. We ask here whether atomic models for G- and F-actins may help to explain this functional diversity. Using a molecular graphics program we have mapped the few amino acids that differ between isoactins. A small number of residues specific for muscle actins are buried in internal positions and some present a remarkable organization. Within the molecule, the replacements observed between muscle and nonmuscle actins are often accompanied by compensatory changes. The others are dispersed on the protein surface, except for a cluster located at the N-terminus which protrudes outward. Only a few of these residues specific for muscle actins are present in known ligand binding sites except the N-terminus, which has a sequence specific for each isoactin and is directly implicated in the binding to myosin. When we simulated the replacements of side chains of residues specific for muscle actins to those specific for nonmuscle actins, the N-terminus appears to be less compact and more flexible in nonmuscle actins. This would represent the first conformational grounds for proposing that muscle and nonmuscle actins may be functionally distinguishable. The rest of the molecule is very similar or identical in all the actins, except for a possible higher internal flexibility in muscle actins. We propose that muscle actin genes have evolved from genes of nonmuscle actins by substitutions leading to some conformational changes in the protruding N-terminus and the internal dynamics of the main body of the protein.

Defects in the Drosophila myosin rod permit sarcomere assembly but cause flight muscle degeneration

We have determined the molecular and ultrastructural defects associated with three homozygous-viable myosin heavy chain mutations of Drosophila melanogaster. These mutations cause a dominant flightless phenotype but allow relatively normal assembly of indirect flight muscle myofibrils. As adults age, the contents of the indirect flight muscle myofibers are pulled to one end of the thorax. This apparently results from myofibril "hyper-contraction", and leads to sarcomere rupture and random myofilament orientation. All three mutations cause single amino acid changes in the light meromyosin region of the myosin rod. Two change the same glutamic acid to a lysine residue and the third affects an amino acid five residues away, substituting histidine for arginine. Both affected residues are conserved in muscle myosins, cytoplasmic myosins and paramyosins. The mutations are associated with age-dependent, site-specific degradation of myosin heavy chain and failure to accumulate phosphorylated forms of flightin, an indirect flight muscle-specific protein previously localized to the thick filament. Given the repeating nature of the hydrophobic and charged amino acid residues of the myosin rod and the near-normal assembly of myofibrils in the indirect flight muscle of these mutants, it is remarkable that single amino acid changes in the rod cause such severe defects. It is also interesting that these severe defects are not apparent in other muscles. These phenomena likely arise from the highly organized nature and rigorous performance requirements of indirect flight muscle, and perhaps from the interaction of myosin with flightin, a protein specific to this muscle type.

Actomyosin kinetics and in vitro motility of wild-type Drosophila actin and the effects of two mutations in the Act88F gene

Two missense mutations of the flight muscle-specific actin gene of Drosophila melanogaster, Act88F, assemble into normally structured myofibrils but affect the flight ability of flies and the mechanical kinetics of isolated muscle fibers. We describe the isolation of actin from different homozygous Act88F strains, including wild-type, an Act88F null mutant (KM88), and two Act88F single point mutations (E316K and G368E), their biochemical interactions with rabbit myosin subfragment 1 (S1), and behavior with rabbit myosin and heavy meromyosin in in vitro motility assays. The rabbit and wild-type Drosophila actins have different association rate constants with S1 (2.64 and 1.77 microM-1 s-1, respectively) and in vitro motilities (2.51, 1.60 microns s-1) clearly demonstrating an isoform-specific difference. The G368E mutation shows a reduced affinity for rabbit S1 compared with the wild type (increasing from 0.11 to 0.17 microM) and a reduced velocity in vitro (reduced by 19%). The E316K mutant actin has no change in affinity for myosin S1 or in vitro motility with heavy meromyosin but does have a reduced in vitro motility (15%) with myosin. These results are discussed with respect to the recently published atomic models for the actomyosin structure and our findings that G368E fibers show a reduced rate constant for delayed tension development and increased fiber stiffness. We interpret these results as possibly caused either by effects on A1 myosin light chain binding or conformational changes within the subdomain 1 of actin, which contains the myosin binding site. E316K is discussed with respect to its likely position within the tropomyosin binding site of actin.

Single-molecule mechanics of heavy meromyosin and S1 interacting with rabbit or Drosophila actins using optical tweezers

Single-molecule mechanical interactions between rabbit heavy meromyosin (HMM) or subfragment 1 (S1) and rabbit actin were measured with an optical tweezers piconewton, nanometer transducer. Similar intermittent interactions were observed with HMM and S1. The mean magnitude of the single interaction isotonic displacements was 20 nm for HMM and 15 nm with S1. The mean value of the force of single-molecule interactions was 1.8 pN for HMM and 1.7 pN with S1. The stiffness of myosin S1 was determined by applying a sinusoidal length change to the thin filament and measuring the corresponding force; the mean stiffness was 0.13 pN nm-1. By moving an actin filament over a long distance past an isolated S1 head, we found that cross-bridge attachment occurred preferentially at a periodicity of about 40 nm, similar to that of the actin helical repeat. Rate constants for the probability of detachment of HMM from actin were determined from histograms of the lifetime of the attached state. This gave a value of 8 s-1 or 0.8 x 10(6) M-1 s-1 for binding of ATP to the rigor complex. We conclude (1) that our HMM-actin interactions involve just one head, (2) that compliance of the cross-bridge is not in myosin subfragment 2, although we cannot say to what extent contributions arise from myosin S1 or actin, and (3) that the elemental movement can be caused by a change of shape of the S1 head, but that this would have to be much greater than the movements suggested from structural studies of S1 (Rayment et al., 1993).

Recovery of dominant, autosomal flightless mutants of Drosophila melanogaster and identification of a new gene required for normal muscle structure and function

To identify further mutations affecting muscle function and development in Drosophila melanogaster we recovered 22 autosomal dominant flightless mutations. From these we have isolated eight viable and lethal alleles of the muscle myosin heavy chain gene, and seven viable alleles of the indirect flight muscle (IFM)-specific Act88F actin gene. The Mhc mutations display a variety of phenotypic effects, ranging from reductions in myosin heavy chain content in the indirect flight muscles only, to reductions in the levels of this protein in other muscles. The Act88F mutations range from those which produce no stable actin and have severely abnormal myofibrillar structure, to those which accumulate apparently normal levels of actin in the flight muscles but which still have abnormal myofibrils and fly very poorly. We also recovered two recessive flightless mutants on the third chromosome. The remaining five dominant flightless mutations are all lethal alleles of a gene named lethal(3)Laker. The Laker alleles have been characterized and the gene located in polytene bands 62A10,B1-62B2,4. Laker is a previously unidentified locus which is haplo-insufficient for flight. In addition, adult wild-type heterozygotes and the lethal larval trans-heterozygotes show abnormalities of muscle structure indicating that the Laker gene product is an important component of muscle.

Molecular genetics of actin function

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ADP-ribosylation of Drosophila indirect-flight-muscle actin and arthrin by Clostridium botulinum C2 toxin and Clostridium perfringens iota toxin

Purified Drosophila indirect-flight-muscle actin and arthrin, an actin-ubiquitin conjugate, were ADP-ribosylated by Clostridium botulinum C2 toxin and Clostridium perfringens iota toxin. Phalloidin treatment inhibited the ADP-ribosylation of Drosophila actin and arthrin. Like actin, the ADP-ribose-arthrin linkage was sensitive towards hydroxylamine treatment, indicating arginine as the amino acid acceptor. Actin translated in vitro from the indirect-flight-muscle-specific gene Act88F was ADP-ribosylated by C. botulinum C2 toxin and C. perfringens iota toxin. Actin from the R177Q mutant of Act88F translated in vivo was not ADP-ribosylated confirming Arg-177 as the ADP-ribose acceptor. Mutant L176M actin was modified by both toxins, indicating that amino acid 176 of actin does not define the substrate specificity of C. botulinum C2 toxin. Whereas the gene products of various C-terminal mutants of Act88F translated in vitro (E334K, V339I, E364K, G368E, R372H) were substrates for ADP-ribosylation by C. botulinum C2 toxin and by C. perfringens iota toxin, neither toxin modified the N-terminal O-12 deletion mutant.

The binding of mutant actins to profilin, ATP and DNase I

Twenty-five mutations were created in the Drosophila melanogaster Act88F actin gene by in vitro mutagenesis and the mutant actins expressed in vitro. The affinity of the mutant actins for ATP, profilin and DNase I was determined. They were also tested for conformational changes by non-denaturing gel electrophoresis. Mutations at positions 364 (highly conserved) and 366 (invariant) caused changes in conformation, reduced ATP binding and increased profilin binding. At position 362 (invariant) only the conservative change from tyrosine to phenylalanine had no effect; other changes at this position affected conformation, ATP and profilin binding. Although only glycine or serine occur naturally at position 368, changes to threonine or glutamine had no effect on the actin. The mutant in which Asp363 was replaced by His and that in which Glu364 was replaced by Lys decreased DNase I binding, yet neither amino acid occurs in the DNase I binding site. Likewise several mutations affect ATP and profilin binding but are distant from the binding sites. We conclude that, although actin has a highly conserved amino acid sequence, individual amino acids can have variable tolerance for substitutions. Also amino acid changes can exert significant effects on the binding of ligands to distant parts of the actin structure.

Polymorphism in a Drosophila indirect flight muscle-specific tropomyosin isozyme does not affect flight ability

We describe polymorphism in a Drosophila indirect flight muscle-specific tropomyosin isozyme, named TnH-34. Three variants of this protein differ in their mobilities as determined by 1-D and 2-D SDS-PAGE. Meiotic mapping places the polymorphism close to, if not within, the structural gene encoding this tropomyosin isozyme. The most likely site of the mutations is within a single C-terminal exon. Flight-testing of different genotypes reveals that this variation in TnH-34 does not affect flight ability. These results suggest that some sequence variation may be tolerated in this section of the protein and correlate with the variability of this protein in different insect species.

Post-translational processing of the amino terminus affects actin function

We have studied the importance of N-terminal processing for normal actin function using the Drosophila Act88F actin gene transcribed and translated in vitro. Despite having different charges as determined by two-dimensional (2D) gel electrophoresis, Act88F expressed in vivo and in vitro in rabbit reticulocyte lysate bind to DNase I with equal affinity and are able to copolymerise with bulk rabbit actin equally well. Using peptide mapping and thin-layer electrophoresis we have shown that bestatin [( 3-amino-2-hydroxy-4-phenyl-butanoyl]-L-leucine), an inhibitor of aminopeptidases, can inhibit actin N-terminal processing in rabbit reticulocyte lysate. Although processed and unprocessed actins translated in vitro are able to bind to DNase I equally well, unprocessed actins are less able to copolymerise with bulk actins. This effect is more pronounced when bulk rabbit actin is used but is still seen with bulk Lethocerus actin. Also, the unprocessed actins reduce the polymerisation of the processed actin translated in vitro with the bulk rabbit actin. This suggests that individual actins do interact, even in non-polymerising conditions. The reduced ability of unprocessed actin to polymerise shows that correct post-translational modification of the N terminus is required for normal actin function.

Characterisation of missense mutations in the Act88F gene of Drosophila melanogaster

We have created missense mutations in the indirect flight muscle (IFM)-specific Act88F actin gene of Drosophila melanogaster by random in vitro mutagenesis. Following P element-mediated transformation into wild-type flies and subsequent transfer of the inserts into Act88F null strains, the effects of the actin mutants on the structure and function of the IFMs were examined. All of the mutants were antimorphic for flight ability. E316K and G368E formed muscle with only relatively small defects in structure whilst the others produced IFMs with large amounts of disruption. E334K formed filaments but lacked Z discs. V339I formed no muscle structure in null flies and did not accumulate actin. E364K and G366D both had relatively stable actin but did not form myofibrils. Using an in vitro polymerisation assay we found no significant effects on the ability of the mutant actins to polymerise. E364K and G366D also caused a strong induction of heat shock protein (hsp) synthesis at normal temperatures and accumulated large amounts of hsp22 which, together with the mutant actin, was resistant to detergent extraction. Both E316K and E334K caused a weak induction of hsp synthesis. We discuss how the stability, structure and function of the different mutant actins affects myofibril assembly and function, and the induction of hsps.

Alteration in crossbridge kinetics caused by mutations in actin

The generation of force during muscle contraction results from the interaction of myosin and actin. The kinetics of this force generation vary between different muscle types and within the same muscle type in different species. Most attention has focused on the role of myosin isoforms in determining these differences. The role of actin isoforms has received little attention, largely because of the lack of a suitable cell type in which the myosin isoform remains constant yet the actin isoforms vary. An alternative approach would be to examine the effect of actin mutations, however, most of these cause such gross disruption of muscle structure that mechanical measurements are impossible. We have now identified two actin mutations which, despite involving conserved amino acids, can assemble into virtually normal myofibrils. These amino-acid changes in actin significantly affect the kinetics of force generation by muscle fibres. One of the mutations is not in the putative myosin-binding site, demonstrating the importance of long-range effects of amino acids on actin function.

Physiological properties of the dorsal longitudinal flight muscle and the tergal depressor of the trochanter muscle of Drosophila melanogaster

A prerequisite for using muscle mutants to study contraction in Drosophila melanogaster is a description of the mechanics of wild-type muscles. Here we describe the mechanics of two different wild-type muscles; the dorsal longitudinal flight muscle which is asynchronous (nerve impulses are not synchronised with each contraction), and a leg muscle, the tergal depressor of the trochanter, which is synchronous. We have compared their mechanics to those of the asynchronous flight and the synchronous leg muscle from the giant waterbug Lethocerus indicus. We found that the mechanics of the asynchronous flight muscles from the two species were similar. At rest length both muscles had a high relaxed stiffness, were partially activated by Ca2+ (low steady-state active tension) and, once activated, had a large delayed increase in tension, which was well maintained, in response to a rapid stretch. The rate constant for the delayed increase in tension was about 10 times greater for D. melanogaster than for L. indicus under the same conditions. The mechanics of the synchronous leg muscles from both species were different from those of the flight muscles and resembled those of other synchronous muscles such as vertebrate striated muscle. At rest length, both muscles had a lower relaxed stiffness than the flight muscles, were fully activated by Ca2+ (high steady-state active tension) and, once activated, had a small delayed increase in tension, which was less well maintained, in response to a rapid stretch. The rate constant for the delayed increase in tension was similar for the leg muscles of both species. The different mechanical properties of the flight and leg muscles must arise from differences in their contractile proteins. The demonstration that satisfactory mechanical responses can be obtained from the small (less than 1 mm long) muscles of D. melanogaster will enable future responses from mutant muscles to be tested.