New proline-rich proteins in isolated insect Z-discs

The function of elastic proteins in the oscillatory contraction of insect flight muscle

Oscillatory contraction of asynchronous insect flight muscle is activated by periodic stretches at constant low concentrations of Ca2+. The fibres must be relatively stiff to respond to small length changes occurring at high frequency. Several proteins in the flight muscle may determine the overall stiffness of the fibres. The Drosophila sallimus (sls) gene codes for multiple isoforms with a modular structure made up of immunoglobulin (Ig) and elastic PEVK domains, unique sequence, and a few fibronectin (Fn) domains at the end of the molecule. Kettin, derived from the sls gene, has Ig domains separated by linker sequences and is bound to actin near the Z-disc; the C-terminus is associated with the end of the A-band. Flight muscle also has longer isoforms of Sls, with extensible PEVK sequence, and C-terminal Fn domains; all extend from the Z-disc to the end of the A-band. Projectin, from a different gene, has repeating modules of Fn and Ig domains, and is associated with the end of thick filaments; tandem Ig and PEVK domains at the N-terminus are in the I-band. Projectin, kettin and other Sls isoforms form a mechanical link between thick and thin filaments; all are probably part of the connecting filaments, which branch from the thick filaments and are linked to actin near the Z-disc. The elasticity of fibres may depend on the relative amounts of those isoforms with extensible PEVK sequence. Flightin is bound on the outside of thick filaments and maintains the stiffness necessary for the transmission of stress along the filaments. Insect flight muscle has multiple elastic proteins to give the sarcomere the optimum compliance necessary for high frequency oscillatory contraction.

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.

Myofilin, a protein in the thick filaments of insect muscle

Thick filaments in striated muscle are myosin polymers with a length and diameter that depend on the fibre type. In invertebrates, the length of the thick filaments varies widely in different muscles and additional proteins control filament assembly. Thick filaments in asynchronous insect flight muscle have an extremely regular structure, which is likely to be essential for the oscillatory contraction of these muscles. The factors controlling the assembly of thick filaments in insect flight muscle are not known. We previously identified a thick filament core protein, zeelin 1, in Lethocerus flight and non-flight muscles. This has been sequenced, and the corresponding proteins in Drosophila and Anopheles have been identified. The protein has been re-named myofilin. Zeelin 2, which is on the outside of Lethocerus flight muscle thick filaments, has been sequenced and because of the similarity to Drosophila flightin, is re-named flightin. In Drosophila flight muscle, myofilin has a molecular weight of 20 kDa and is one of five isoforms produced from a single gene. In situ hybridisation of Drosophila embryos showed that myofilin RNA is first expressed late in embryogenesis at stage 15, a little later than myosin. Antibody to myofilin labelled the entire A-band, except for the H-zone, in cryosections of flight and non-flight muscle. The periodicity of myofilin in Drosophila flight muscle thick filaments was found to be 30 nm by measuring the spacing of gold particles in labelled cryosections; this is about twice the 14.5 nm spacing of myosin molecules. The molar ratio of myofilin to myosin in indirect flight muscle is 1:2, which is the same as that of flightin. We propose a model for the association of these proteins in thick filaments, which is consistent with the periodicity and stoichiometry. Myofilin is probably needed for filament assembly in all muscles, and flightin for stability of flight muscle thick filaments in adult flies.

Small, membrane-bound, alternatively spliced forms of ankyrin 1 associated with the sarcoplasmic reticulum of mammalian skeletal muscle

We have recently found that the erythroid ankyrin gene, Ank1, expresses isoforms in mouse skeletal muscle, several of which share COOH-terminal sequence with previously known Ank1 isoforms but have a novel, highly hydrophobic 72-amino acid segment at their NH2 termini. Here, through the use of domain-specific peptide antibodies, we report the presence of the small ankyrins in rat and rabbit skeletal muscle and demonstrate their selective association with the sarcoplasmic reticulum. In frozen sections of rat skeletal muscle, antibodies to the spectrin-binding domain (anti-p65) react only with a 210-kD Ank1 and label the sarcolemma and nuclei, while antibodies to the COOH terminus of the small ankyrin (anti-p6) react with peptides of 20 to 26 kD on immunoblots and decorate the myoplasm in a reticular pattern. Mice homozygous for the normoblastosis mutation (gene symbol nb) are deficient in the 210-kD ankyrin but contain normal levels of the small ankyrins in the myoplasm. In nb/nb skeletal muscle, anti-p65 label is absent from the sarcolemma, whereas anti-p6 label shows the same distribution as in control skeletal muscle. In normal skeletal muscle of the rat, anti-p6 decorates Z lines, as defined by antidesmin distribution, and is also present at M lines where it surrounds the thick myosin filaments. Immunoblots of the proteins isolated with rabbit sarcoplasmic reticulum indicate that the small ankyrins are highly enriched in this fraction. When expressed in transfected HEK 293 cells, the small ankyrins are distributed in a reticular pattern resembling the ER if the NH2-terminal hydrophobic domain is present, but they are uniformly distributed in the cytosol if this domain is absent. These results suggest that the small ankyrins are integral membrane proteins of the sarcoplasmic reticulum. We propose that, unlike the 210-kD form of Ank1, previously localized to the sarcolemma and believed to be a part of the supporting cytoskeleton, the small Ank1 isoforms may stabilize the sarcoplasmic reticulum by linking it to the contractile apparatus.

Cytoskeletal proteins of insect muscle: location of zeelins in Lethocerus flight and leg muscle

Asynchronous insect flight muscles produce oscillatory contractions and can contract at high frequency because they are activated by stretch as well as by Ca2+. Stretch activation depends on the high stiffness of the fibres and the regular structure of the filament lattice. Cytoskeletal proteins may be important in stabilising the lattice. Two proteins, zeelin 1 (35 kDa) and zeelin 2 (23 kDa), have been isolated from the cytoskeletal fraction of Lethocerus flight muscle. Both zeelins have multiple isoforms of the same molecular mass and different charge. Zeelin 1 forms micelles and zeelin 2 forms filaments when renatured in low ionic strength solutions. Filaments of zeelin 2 are ribbons 10 nm wide and 3 nm thick. The position of zeelins in fibres from Lethocerus flight and leg muscle was determined by immunofluorescence and immunoelectron microscopy. Zeelin 1 is found in flight and leg fibres and zeelin 2 only in flight fibres. In flight myofibrils, both zeelins are in discrete regions of the A-band in each half sarcomere. Zeelin 1 is across the whole A-band in leg myofibrils. Zeelins are not in the Z-disc, as was thought previously, but migrate to the Z-disc in glycerinated fibres. Zeelins are associated with thick filaments and analysis of oblique sections showed that zeelin 1 is closer to the filament shaft than zeelin 2. The antibody labelling pattern is consistent with zeelin molecules associated with myosin near the end of the rod region. Alternatively, the position of zeelins may be determined by other A-band proteins. There are about 2.0 to 2.5 moles of myosin per mole of each zeelin. The function of these cytoskeletal proteins may be to maintain the ordered structure of the thick filament.

Digestion of proteins associated with the Z-disc by calpain

The Z-disc of striated muscle is degraded by the Ca2(+)-activated proteinase, calpain, during autolysis of muscle fibres. The effect of calpain on proteins in preparations of Z-discs isolated from Lethocerus flight muscle has been studied. Calpain releases alpha-actinin from the Z-disc and digests two hydrophobic proteins associated with the Z-disc, zeelin 1 (35 kD) and zeelin 2 (23 kD). The Ca2+ sensitivity of zeelin digestion is shifted to lower Ca2+ concentrations (within the physiological range) in the presence of the phospholipids phosphatidyl inositol or phosphatidyl choline and diacylglycerol. The release of alpha-actinin is not affected by phospholipid. Preparations of isolated Z-discs have five times as much associated phospholipid (w/w) as myofibrils and the composition of the lipid differs from that of myofibrils. In muscle fibres the action of calpain on zeelins may be controlled by the composition of phospholipid in the fibres as well as by Ca2+.

Z-band proteins in the flight muscle and leg muscle of the honeybee

Monoclonal antibodies (mAb's) have been raised against proteins in preparations of Z-discs isolated from honeybee fibrillar flight muscle. These antibodies have identified four Z-disc antigens on immunoblots of honeybee fibrillar proteins. Antibody alpha binds to the 90-100 kD protein, alpha-actinin; mAb P interacts with the protein, projectin, an extremely large polypeptide (greater than 600kD) found in the connecting filaments which link thick filaments to the Z-band in insect asynchronous flight muscle. Two other mAb's recognize previously uncharacterized insect Z-band proteins. Monoclonal antibody Z(400) binds to a pair of proteins with molecular masses near 400 kD and 600 kD. Antibody Z(175) recognizes two components, 158 kD and 175 kD, that are not only immunologically similar but have nearly identical peptide maps. Indirect immunofluorescence microscopy studies show that the proteins recognized by mAb's alpha, Z(175) and Z(400) are located at the Z-band, while the mAb P antigen is found on either side of it. Three of the four antibodies we have obtained recognize leg muscle proteins. Monoclonal antibodies alpha and P comigrate on SDS gels with analogous components from flight muscle. Only the smaller of the two proteins identified in flight muscle by mAb Z(400) is found in leg muscle, however. Furthermore, no Z(175) antigens have been detected in the non-fibrillar tissue by either monoclonal or polyclonal antibodies. Immunofluorescence microscopy studies localize the alpha and Z(400) antigens at the Z-line in leg muscle fibrils. Surprisingly, however, mAb P binds within the A-bands of synchronous fibres, not between the A- and Z-bands as in asynchronous fibrillar muscle.

Characterization of components of Z-bands in the fibrillar flight muscle of Drosophila melanogaster

Twelve monoclonal antibodies have been raised against proteins in preparations of Z-disks isolated from Drosophila melanogaster flight muscle. The monoclonal antibodies that recognized Z-band components were identified by immunofluorescence microscopy of flight muscle myofibrils. These antibodies have identified three Z-disk antigens on immunoblots of myofibrillar proteins. Monoclonal antibodies alpha:1-4 recognize a 90-100-kD protein which we identify as alpha-actinin on the basis of cross-reactivity with antibodies raised against honeybee and vertebrate alpha-actinins. Monoclonal antibodies P:1-4 bind to the high molecular mass protein, projectin, a component of connecting filaments that link the ends of thick filaments to the Z-band in insect asynchronous flight muscles. The anti-projectin antibodies also stain synchronous muscle, but, surprisingly, the epitopes here are within the A-bands, not between the A- and Z-bands, as in flight muscle. Monoclonal antibodies Z(210):1-4 recognize a 210-kD protein that has not been previously shown to be a Z-band structural component. A fourth antigen, resolved as a doublet (approximately 400/600 kD) on immunoblots of Drosophila fibrillar proteins, is detected by a cross reacting antibody, Z(400):2, raised against a protein in isolated honeybee Z-disks. On Lowicryl sections of asynchronous flight muscle, indirect immunogold staining has localized alpha-actinin and the 210-kD protein throughout the matrix of the Z-band, projectin between the Z- and A-bands, and the 400/600-kD components at the I-band/Z-band junction. Drosophila alpha-actinin, projectin, and the 400/600-kD components share some antigenic determinants with corresponding honeybee proteins, but no honeybee protein interacts with any of the Z(210) antibodies.

Three-dimensional reconstruction of the Z disk of sectioned bee flight muscle

The three-dimensional structure of the central region of the Z disk of honeybee flight muscle has been determined to a resolution of 70 A by three-dimensional reconstruction from electron micrographs of tilted thin sections. The reconstructions show a complex assembly in which actin filaments terminate and are cross-linked together; a number of structural domains of this network are resolved in quantitative three-dimensional detail. The central region of the Z disk contains two sets of overlapping actin filaments of opposite polarity, which originate in the sarcomeres adjacent to the Z disk, and connections between these filaments. The filaments are deflected by the attachment of cross-links; spacing between filaments change by greater than 100 A during their passage through the Z disk. Each actin filament is linked by connecting structures to four filaments of opposite polarity and two filaments are of the same polarity. Four types of connecting density domain are observed in association with pairs of filaments of opposite polarity: C1, C2, C3, and C5. Two of these, C3 and C5, are associated with the ends of actin filaments. Another connection, C4, is associated with three filaments of the same polarity; C4 is threefold symmetric.

Characterization and fibre type distribution of a new myofibrillar protein of molecular weight 32 kDa

A new basic protein of molecular weight 32 kDa has been isolated and purified to homogeneity from skeletal muscles rich in type I fibres. By the use of a specific monoclonal antibody, the protein has been shown to be present in all type I fibres and some type II fibres, the number of which varies with the muscle and the region of the muscle sectioned. A protein of similar properties could not be isolated from rabbit muscles consisting predominantly of type II fibres. By fluorescence microscopy, the protein has been shown to be located in the Z-disc from which the presence of divalent cations, probably calcium, facilitates its extraction at low ionic strength. The protein is unusual in that its distribution does not correlate completely with the known muscle fibre types and in that as yet there is no evidence for the presence of an isoform in those cells that do not stain with the specific antibody for the 32 kDa protein isolated from slow muscles.

Arthrin: a new actin-like protein in insect flight muscle

There are one or more proteins of 50,000 to 60,000 Mr in the thin filaments of insect flight muscle. A protein of 55,000 Mr has been isolated from insect fibrillar flight muscle and called arthrin. Despite its higher molecular weight, arthrin is in many ways like actin. The amino acid composition of arthrin was similar to that of actin. There were similarities in the peptides produced by digesting the denatured proteins and mild digestion of polymerized proteins cleaved similar-sized fragments from arthrin and actin. Polymerized arthrin activated the Mg2+ ATPase of myosin to the same extent as actin and the ATPase was regulated by rabbit or Lethocerus troponin and tropomyosin. Arthrin did not itself act as troponin-T. Electron microscopy of negatively stained specimens showed that arthrin and actin filaments were similar in structure and that arthrin could be decorated by rabbit subfragment-1 to form normal-looking arrowheads. Arthrin formed paracrystals at an optimum concentration of MgCl2 (25 mM) that was somewhat lower than the optimum for actin paracrystals. Optical diffraction showed that the structure of the paracrystals was similar to those formed from actin. The mass of arthrin and actin filaments relative to phage fd was measured by scanning transmission electron microscopy; the relative mass of arthrin and actin was 1.33, in agreement with molecular weight estimations. Therefore arthrin has the properties of a heavy form of actin. The proportion of actin, arthrin and troponin-T in Lethocerus myofibrils was six moles of actin to one mole of arthrin and one mole of troponin-T. The function of arthrin is not known.

Functionality of muscle proteins in gelation mechanisms of structured meat products

Recent advances in muscle biology concerning the discoveries of a large variety of proteins have been described in this review. The existence of polymorphism in several muscle proteins is now well established. Various isoforms of myosin not only account for the difference in physiological functions and biochemical activity of different fiber types or muscles, but also seem to differ in functional properties in food systems. The functionality of various muscle proteins, especially myosin and actin in the gelation process in modal systems which simulate structured meat products, is discussed at length. Besides, the role of different subunits and subfragments of myosin molecule in the gelation mechanism, and the various factors affecting heat-induced gelation of actomyosin in modal systems are also highlighted. Finally, the areas which need further investigation in this discipline have been suggested.

The elasticity of relaxed insect fibrillar flight muscle

The mechanical properties of glycerol-extracted fibres from the dorsal longitudinal muscle of Lethocerus have been determined by sinusoidal and transient analysis in the time range 1 ms-1000 s, and from rest length to 10% strain for fibres in relaxing and rigor solutions. The fibres behave reversibly up to strains of about 5%, but reach an elastic limit in the range 5-9% strain, depending upon the rate of strain. Electron micrographs of fibres at different degrees of stretch, and after partial extraction of the contractile proteins, suggest that a connexion between the end of the A filament and the Z line, named a C filament, is responsible for the high stiffness of the relaxed muscle. Estimates are made of the compliance of the A, I and C filaments. The mechanical response of the relaxed muscle, over the entire frequency range studied, is assignable to the C filaments. An analysis of the stiffness of the fibres at different tensions in activating and relaxing solutions, and in fibres relaxed by orthovanadate, shows that the C filaments still exert their mechanical effect in the active muscle. That is, the response of the active muscle consists of the contribution from the cross-bridges plus that of the C filaments, acting mechanically in parallel. This situation is incompatible with earlier explanations of the fully activated mechanical dynamics of fibrillar muscle. Alternative explanations at the cross-bridge level are described in the paper that follows this one.

Salivary proline-rich proteins

Proline-rich proteins are major components of parotid and submandibular saliva in humans as well as other animals. They can be divided into acidic, basic and glycosylated proteins. The primary structure of the acidic proline-rich proteins is unique and shows that the proteins do not belong to any known family of proteins. The proline-rich proteins are apparently synthesized the acinar cells of the salivary glands and their phenotypic expression is under complex genetic control. The acidic proline-rich proteins will bind calcium with a strength which indicates that they may be important in maintaining the concentration of ionic calcium in saliva. Moreover they can inhibit formation of hydroxyapatite, whereby growth of hydroxyapatite crystals on the tooth surface in vivo may be avoided. Both of these activities as well as the binding site for hydroxyapatite are located in the N-terminal proline-poor part of the protein. Little is known about the functions of the glycosylated and basic proline-rich proteins.