Invertebrate connectin spans as much as 3.5 microm in the giant sarcomeres of crayfish claw muscle

A nanobody toolbox to investigate localisation and dynamics of Drosophila titins and other key sarcomeric proteins

Measuring the positions and dynamics of proteins in intact tissues or whole animals is key to understanding protein function. However, to date, this is challenging, as the accessibility of large antibodies to dense tissues is often limited, and fluorescent proteins inserted close to a domain of interest may affect protein function. These complications apply in particular to muscle sarcomeres, arguably one of the most protein-dense assemblies in nature, which complicates studying sarcomere morphogenesis at molecular resolution. Here, we introduce a toolbox of nanobodies recognising various domains of the two Drosophila titin homologs, Sallimus and Projectin, as well as the key sarcomeric proteins Obscurin, α-Actinin, and Zasp52. We verified the superior labelling qualities of our nanobodies in muscle tissue as compared to antibodies. By applying our toolbox to larval muscles, we found a gigantic Sallimus isoform stretching more than 2 µm to bridge the sarcomeric I-band, while Projectin covers almost the entire myosin filaments in a polar orientation. Transgenic expression of tagged nanobodies confirmed their high affinity-binding without affecting target protein function. Finally, adding a degradation signal to anti-Sallimus nanobodies suggested that it is difficult to fully degrade Sallimus in mature sarcomeres; however, expression of these nanobodies caused developmental lethality. These results may inspire the generation of similar toolboxes for other large protein complexes in Drosophila or mammals.

Kettin, the large actin-binding protein with multiple immunoglobulin domains, is essential for sarcomeric actin assembly and larval development in Caenorhabditis elegans

Among many essential genes in the nematode Caenorhabditis elegans, let-330 is located on the left arm of chromosome V and was identified as the largest target of a mutagen in this region. However, let-330 gene has not been characterized at the molecular level. Here, we report that two sequenced let-330 alleles are nonsense mutations of ketn-1, a previously characterized gene encoding kettin. Kettin is a large actin-binding protein of 472 kDa with 31 immunoglobulin domains and is expressed in muscle cells in C. elegans. let-330/ketn-1 mutants are homozygous lethal at the first larval stage with mild defects in body elongation. These mutants have severe defects in sarcomeric actin and myosin assembly in striated muscle. However, α-actinin and vinculin, which are components of the dense bodies anchoring actin to the membranes, were not significantly disorganized by let-330/ketn-1 mutation. Kettin localizes to embryonic myofibrils before α-actinin is expressed, and α-actinin deficiency does not affect kettin localization in larval muscle. Depletion of vinculin minimally affects kettin localization but significantly reduces colocalization of actin with kettin in embryonic muscle cells. These results indicate that kettin is an essential protein for sarcomeric assembly of actin filaments in muscle cells.

A great legacy of muscle research and education from Dr. Sumiko Kimura

Dr. Sumiko Kimura, a former professor of biology at Chiba University, was known as a distinguished biochemist who contributed considerably to our knowledge about the cytoskeleton of muscle cells, especially through her work on connectin (also called titin) and actin regulatory proteins. Sadly, she suddenly passed away in Tokyo on November 1, 2018 at the age of 71. She succumbed to multiple organ failure caused by a bacterial infection following a third operation on her heart. Dr. Kimura had been continuing her research into connectin right up until several months before her decease.

Nuclear connectin novex-3 promotes proliferation of hypoxic foetal cardiomyocytes

Loss of cardiomyocyte proliferative capacity after birth is a major obstacle for therapeutic heart regeneration in adult mammals. We and others have recently shown the importance of hypoxic in utero environments for active foetal cardiomyocyte proliferation. Here, we report the unexpected expression of novex-3, the short splice variant of the giant sarcomeric protein connectin (titin), in the cardiomyocyte nucleus specifically during the hypoxic foetal stage in mice. This nuclear localisation appeared to be regulated by the N-terminal region of novex-3, which contains the nuclear localisation signal. Importantly, the nuclear expression of novex-3 in hypoxic foetal cardiomyocytes was repressed at the postnatal stage following the onset of breathing and the resulting elevation of oxygen tension, whereas the sarcomeric expression remained unchanged. Novex-3 knockdown in foetal cardiomyocytes repressed cell cycle-promoting genes and proliferation, whereas novex-3 overexpression enhanced proliferation. Mechanical analysis by atomic force microscopy and microneedle-based tensile tests demonstrated that novex-3 expression in hypoxic foetal cardiomyocytes contributes to the elasticity/compliance of the nucleus at interphase and facilitates proliferation, by promoting phosphorylation-induced disassembly of multimer structures of nuclear lamins. We propose that novex-3 has a previously unrecognised role in promoting cardiomyocyte proliferation specifically at the hypoxic foetal stage.

An Adaptable Spectrin/Ankyrin-Based Mechanism for Long-Range Organization of Plasma Membranes in Vertebrate Tissues

Ankyrins are membrane-associated proteins that together with their spectrin partners are responsible for micron-scale organization of vertebrate plasma membranes, including those of erythrocytes, excitable membranes of neurons and heart, lateral membrane domains of columnar epithelial cells, and striated muscle. Ankyrins coordinate functionally related membrane transporters and cell adhesion proteins (15 protein families identified so far) within plasma membrane compartments through independently evolved interactions of intrinsically disordered sequences with a highly conserved peptide-binding groove formed by the ANK repeat solenoid. Ankyrins are coupled to spectrins, which are elongated organelle-sized proteins that form mechanically resilient arrays through cross-linking by specialized actin filaments. In addition to protein interactions, cellular targeting and assembly of spectrin/ankyrin domains also critically depend on palmitoylation of ankyrin-G by aspartate-histidine-histidine-cysteine 5/8 palmitoyltransferases, as well as interaction of beta-2 spectrin with phosphoinositide lipids. These lipid-dependent spectrin/ankyrin domains are not static but are locally dynamic and determine membrane identity through opposing endocytosis of bulk lipids as well as specific proteins. A partnership between spectrin, ankyrin, and cell adhesion molecules first emerged in bilaterians over 500 million years ago. Ankyrin and spectrin may have been recruited to plasma membranes from more ancient roles in organelle transport. The basic bilaterian spectrin-ankyrin toolkit markedly expanded in vertebrates through gene duplications combined with variation in unstructured intramolecular regulatory sequences as well as independent evolution of ankyrin-binding activity by ion transporters involved in action potentials and calcium homeostasis. In addition, giant vertebrate ankyrins with specialized roles in axons acquired new coding sequences by exon shuffling. We speculate that early axon initial segments and epithelial lateral membranes initially were based on spectrin-ankyrin-cell adhesion molecule assemblies and subsequently served as "incubators," where ion transporters independently acquired ankyrin-binding activity through positive selection.

Regulation of structure and function of sarcomeric actin filaments in striated muscle of the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has been used as a valuable system to study structure and function of striated muscle. The body wall muscle of C. elegans is obliquely striated muscle with highly organized sarcomeric assembly of actin, myosin, and other accessory proteins. Genetic and molecular biological studies in C. elegans have identified a number of genes encoding structural and regulatory components for the muscle contractile apparatuses, and many of them have counterparts in mammalian cardiac and skeletal muscles or striated muscles in other invertebrates. Applicability of genetics, cell biology, and biochemistry has made C. elegans an excellent system to study mechanisms of muscle contractility and assembly and maintenance of myofibrils. This review focuses on the regulatory mechanisms of structure and function of actin filaments in the C. elegans body wall muscle. Sarcomeric actin filaments in C. elegans muscle are associated with the troponin-tropomyosin system that regulates the actin-myosin interaction. Proteins that bind to the side and ends of actin filaments support ordered assembly of thin filaments. Furthermore, regulators of actin dynamics play important roles in initial assembly, growth, and maintenance of sarcomeres. The knowledge acquired in C. elegans can serve as bases to understand the basic mechanisms of muscle structure and function.

Genomic- and protein-based approaches for connectin (titin) identification in the ascidian Ciona intestinalis

Determining the complete primary structure of large proteins is difficult because of the large sequence size and low sequence homology among animals, as is the case with connectin (titin)-like proteins in invertebrate muscles. Conventionally, large proteins have been investigated using immuno-screenings and plaque hybridization screenings that require significant time and labor. Recently, however, the genomic sequences of various invertebrates have been determined, leading to changes in the strategies used to elucidate the complete primary structures of large proteins. In this paper, we describe our methods for determining the sequences of large proteins by elucidating the primary structure of connectin from the ascidian Ciona intestinalis as an example. We searched for genes that encode connectin-like proteins in the C. intestinalis genome using the BLAST search program. Subsequently, we identified some domains present in connectin and connectin-like proteins, such as immunoglobulin (Ig), fibronectin type 3 (Fn) and kinase domains in C. intestinalis using the SMART program and manual estimation. The existence of these domains and the unique sequences between each domain were confirmed using RT-PCR. We also examined the localization of mRNA using whole-mount in situ hybridization (WISH) and protein expression using SDS-PAGE. These analyses indicate that the domain structure and molecular weight of ascidian connectin are similar to those of vertebrate connectin and that ascidian connectin is also expressed in heart muscle, similarly to vertebrate connectin. The methods described in this study can be used to determine the primary structures of large proteins, such as novel connectin-like proteins in invertebrates.

Amphioxus connectin exhibits merged structure as invertebrate connectin in I-band region and vertebrate connectin in A-band region

Connectin is an elastic protein found in vertebrate striated muscle and in some invertebrates as connectin-like proteins. In this study, we determined the structure of the amphioxus connectin gene and analyzed its sequence based on its genomic information. Amphioxus is not a vertebrate but, phylogenetically, the lowest chordate. Analysis of gene structure revealed that the amphioxus gene is approximately 430 kb in length and consists of regions with exons of repeatedly aligned immunoglobulin (Ig) domains and regions with exons of fibronectin type 3 and Ig domain repeats. With regard to this sequence, although the region corresponding to the I-band is homologous to that of invertebrate connectin-like proteins and has an Ig-PEVK region similar to that of the Neanthes sp. 4000K protein, the region corresponding to the A-band has a super-repeat structure of Ig and fibronectin type 3 domains and a kinase domain near the C-terminus, which is similar to the structure of vertebrate connectin. These findings revealed that amphioxus connectin has the domain structure of invertebrate connectin-like proteins at its N-terminus and that of vertebrate connectin at its C-terminus. Thus, amphioxus connectin has a novel structure among known connectin-like proteins. This finding suggests that the formation and maintenance of the sarcomeric structure of amphioxus striated muscle are similar to those of vertebrates; however, its elasticity is different from that of vertebrates, being more similar to that of invertebrates.

Extensive and modular intrinsically disordered segments in C. elegans TTN-1 and implications in filament binding, elasticity and oblique striation

TTN-1, a titin like protein in Caenorhabditis elegans, is encoded by a single gene and consists of multiple Ig and fibronectin 3 domains, a protein kinase domain and several regions containing tandem short repeat sequences. We have characterized TTN-1's sarcomere distribution, protein interaction with key myofibrillar proteins as well as the conformation malleability of representative motifs of five classes of short repeats. We report that two antibodies developed to portions of TTN-1 detect an approximately 2-MDa polypeptide on Western blots. In addition, by immunofluorescence staining, both of these antibodies localize to the I-band and may extend into the outer edge of the A-band in the obliquely striated muscle of the nematode. Six different 300-residue segments of TTN-1 were shown to variously interact with actin and/or myosin in vitro. Conformations of synthetic peptides of representative copies of each of the five classes of repeats--39-mer PEVT, 51-mer CEEEI, 42-mer AAPLE, 32-mer BLUE and 30-mer DispRep--were investigated by circular dichroism at different temperatures, ionic strengths and solvent polarities. The PEVT, CEEEI, DispRep and AAPLE peptides display a combination of a polyproline II helix and an unordered structure in aqueous solution and convert in trifluoroethanol to alpha-helix (PEVT, CEEEI, DispRep) and beta-turn (AAPLE) structures, respectively. The octads in BLUE motifs form unstable alpha-helix-like structures coils in aqueous solution and negligible heptad-based, alpha-helical coiled-coils. The alpha-helical structure, as modeled by threading and molecular dynamics simulations, tends to form helical bundles and crosses based on its 8-4-2-2 hydrophobic helical patterns and charge arrays on its surface. Our finding indicates that APPLE, PEVT, CEEEI and DispRep regions are all intrinsically disordered and highly reminiscent of the conformational malleability and elasticity of vertebrate titin PEVK segments. The proposed presence of long, modular and unstable alpha-helical oligomerization domains in the BLUE region of TTN-1 could bundle TTN-1 and stabilize oblique striation of the sarcomere.

A constitutive model for muscle properties in a soft-bodied arthropod

In this paper, we examine the mechanical properties of muscles in a soft-bodied arthropod under both passive and stimulated conditions. In particular, we examine the ventral interior lateral muscle of the tobacco hornworm caterpillar, Manduca sexta, and show that its response is qualitatively similar to the behaviour of particle-reinforced rubber. Both materials are capable of large nonlinear elastic deformations, show a hysteretic behaviour and display stress softening during the first few cycles of repeated loading. The Manduca muscle can therefore be considered as different elastic materials during loading and unloading and is best described using the theory of pseudo-elasticity. We summarize the basic equations for transversely isotropic pseudo-elastic materials, first for general deformations and then for the appropriate uniaxial specialization. The constitutive relation proposed is in good agreement with the experimental data for both the passive and the stimulated conditions.

Chicken Breast Muscle Connectin: Passive Tension and I-Band Region Primary Structure

We performed cDNA cloning of chicken breast muscle connectin. Together with previous results, our analysis elucidated a 24.2 kb sequence encoding the amino terminus of the protein. This corresponded to the I-band region of the skeletal muscle sarcomere, which is involved in extension and contraction between the Z-line and the A-I junction. There were fewer middle immunoglobulin domains and amino acid residues in the PEVK segment of chicken breast muscle connectin than in human skeletal muscle connectin, but more than in human cardiac muscle connectin. We measured passive tension generation by stretching mechanically skinned myofibril bundles. This revealed that appreciable tension development in chicken breast muscle began at longer sarcomere spacings than in rabbit cardiac muscle, but at shorter spacings than in rabbit psoas and soleus muscles. We suggest that the chicken breast muscle sarcomere remains in a relatively extended state even in unstrained sarcomeres. This would explain why chicken breast muscle does not extend under force to the same degree as rabbit psoas and soleus muscles.

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.

Molecular and biochemical characterization of kettin in Caenorhabditis elegans

Kettin is a unique member of the connectin/titin family of muscle elastic proteins, which has repetitive immunoglobulin-like domains that are separated by weakly conserved linker sequences. In striated muscles of insects and crayfish, kettin binds to actin filaments and localizes to the Z-disc and its adjacent region in the I-band. Recent sequence analysis of invertebrate connectin/titin (also known as SLS proteins) has revealed that kettin is a splice variant of connectin/titin. In contrast, in the nematode Caenorhabditis elegans, the kettin gene is independent of the genes for other connectin/titin-related proteins. Immunofluorescent localization of kettin shows that it localizes to the I-bands in the obliquely striated body wall muscle. Therefore, C. elegans is an attractive model system to study specific functions of kettin in muscle cells.

Caenorhabditis elegans kettin, a large immunoglobulin-like repeat protein, binds to filamentous actin and provides mechanical stability to the contractile apparatuses in body wall muscle

Kettin is a large actin-binding protein with immunoglobulin-like (Ig) repeats, which is associated with the thin filaments in arthropod muscles. Here, we report identification and functional characterization of kettin in the nematode Caenorhabditis elegans. We found that one of the monoclonal antibodies that were raised against C. elegans muscle proteins specifically reacts with kettin (Ce-kettin). We determined the entire cDNA sequence of Ce-kettin that encodes a protein of 472 kDa with 31 Ig repeats. Arthropod kettins are splice variants of much larger connectin/titin-related proteins. However, the gene for Ce-kettin is independent of other connectin/titin-related genes. Ce-kettin localizes to the thin filaments near the dense bodies in both striated and nonstriated muscles. The C-terminal four Ig repeats and the adjacent non-Ig region synergistically bind to actin filaments in vitro. RNA interference of Ce-kettin caused weak disorganization of the actin filaments in body wall muscle. This phenotype was suppressed by inhibiting muscle contraction by a myosin mutation, but it was enhanced by tetramisole-induced hypercontraction. Furthermore, Ce-kettin was involved in organizing the cytoplasmic portion of the dense bodies in cooperation with alpha-actinin. These results suggest that kettin is an important regulator of myofibrillar organization and provides mechanical stability to the myofibrils during contraction.

Partial sequence of connectin-like 1200K-protein in obliquely striated muscle of a polychaete (Annelida): evidence for structural diversity from vertebrate and invertebrate connectins

Vertebrate striated muscle contains the giant elastic protein connectin that maintains the position of the A-band at the center of the sarcomere during repeated muscular contraction and relaxation. Connectin-like molecules may perform conserved functions in vertebrate and invertebrate striated and oblique muscles, although less is known about the structure of invertebrate connectins at present. The protein that maintains such a structure is present not only in vertebrate striated muscle, but also in invertebrate striated and oblique muscle. In the present study, we analyzed the partial primary structure of a 1200K-protein, which is a connectin-like protein that is expressed in Neanthes sp. body wall muscle that is in turn composed of oblique muscle. Antibody screening of a cDNA library of Neanthes sp. body wall muscle identified two different clones. Both clones coded for a sequence predominantly comprised of the four amino acids proline (P), glutamate (E), valine (V) and lysine (K). One clone included a PEVK-like repeat sequence flanked by an Ig domain, while the other clone comprised a distinct 14 amino acid repeat rich in PEVK residues, flanked by a non-repetitive unique sequence. The PEVK region is found in vertebrate connectin and is thought to generate elasticity and be responsible for passive tension of the muscle. The antibodies produced against a portion of each clone both reacted with bands corresponding to 1200 kDa present in Neanthes sp. body wall muscle. Therefore, our results demonstrate that this 1200K-protein is a connectin-like elastic protein and includes specific PEVK-like fragment. We suggest that this 1200K-protein plays a major role in maintaining the structure of oblique muscle in invertebrates.

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.

The entire cDNA sequences of projectin isoforms of crayfish claw closer and flexor muscles and their localization

Projectin is a giant protein related to twitchin and titin/connectin, that is found in arthropod striated muscle. The complete sequence of a 1 MDa projectin from Drosophila muscle was recently deduced from a thorough analysis of the genomic DNA (Southgate and Ayme-Southgate, 2001). Here we report the complete sequence for projectin from crayfish claw closer muscle (8625 residues; 962,634 Da). The N-terminal sequence contains 12 unique 19-residue repeats rich in glutamic acid (E) and lysine (K). This region, termed the EK region, is clearly distinguishable from the PEVK-like domain of Drosophila projectin. The sequence of crayfish flexor projectin differs from that of closer muscle projectin in that there is a 114-residue deletion and a 35-residue insertion in the N-terminal region. Immunofluorescence microscopy demonstrated that projectin is mainly localized within the sarcomeric A band in both closer and flexor muscles, although the N-terminal region was shown to extrude into the I band region. In the closer muscles, invertebrate connectin (D-titin) connects the Z line to the edge of the A band (Fukuzawa et al., 2001). We have shown that invertebrate connectin is also present in flexor muscle sarcomeres, although in very low abundance.

Calcium-dependent molecular spring elements in the giant protein titin

Titin (also known as connectin) is a giant protein with a wide range of cellular functions, including providing muscle cells with elasticity. Its physiological extension is largely derived from the PEVK segment, rich in proline (P), glutamate (E), valine (V), and lysine (K) residues. We studied recombinant PEVK molecules containing the two conserved elements: approximately 28-residue PEVK repeats and E-rich motifs. Single molecule experiments revealed that calcium-induced conformational changes reduce the bending rigidity of the PEVK fragments, and site-directed mutagenesis identified four glutamate residues in the E-rich motif that was studied (exon 129), as critical for this process. Experiments with muscle fibers showed that titin-based tension is calcium responsive. We propose that the PEVK segment contains E-rich motifs that render titin a calcium-dependent molecular spring that adapts to the physiological state of the cell.

Localization of projectin in locust flight muscle

Projectin is a giant filamentous protein of arthropod striated muscle. By using immunofluorescence microscopy, projectin was shown to span between the I band and the A band in locust (Locusta migratoria) flight muscle sarcomeres. The N- and C-terminal regions of projectin molecules were localized in the I band and A band, respectively. This observation explains the controversial reports of previous studies that projectin is localized either in the I band or in the A band of locust flight muscle sarcomeres. It is also observed that the N-terminal region of projectin is located in the I band of locust leg muscle sarcomeres.

Malleable conformation of the elastic PEVK segment of titin: non-co-operative interconversion of polyproline II helix, beta-turn and unordered structures

To understand the structural basis of molecular elasticity and protein interaction of the elastic PEVK (Pro-Glu-Val-Lys) segment of the giant muscle protein titin, we carried out a detailed analysis of a representative PEVK module and a 16-module PEVK protein under various environmental conditions. Three conformational states, polyproline II (PPII) helix, beta-turn and unordered coil were identified by CD and NMR. These motifs interconvert without long-range co-operativity. As a general trend, the relative content of PPII increases with lower temperature and higher polarity, beta-turn increases with lower temperature and lower polarity, and unordered coil increases with higher temperature and higher polarity. NMR studies demonstrate that trans -proline residues are the predominant form at room temperature (22 degrees C), with little trans -to- cis isomerization below 35 degrees C. Ionic strength affects salt bridges between charged side chains, but not the backbone conformation. We conclude that titin PEVK conformation is malleable and responds to subtle environmental changes without co-operativity. This gradual conformational transition may represent a regulatory mechanism for fine-tuning protein interactions and elasticity.

Titin: properties and family relationships

In striated muscles, the rapid production of macroscopic levels of force and displacement stems directly from highly ordered and hierarchical protein organization, with the sarcomere as the elemental contractile unit. There is now a wealth of evidence indicating that the giant elastic protein titin has important roles in controlling the structure and extensibility of vertebrate muscle sarcomeres.

Varieties of elastic protein in invertebrate muscles

Elastic proteins in the muscles of a nematode (Caenorhabditis elegans), three insects (Drosophila melanogaster, Anopheles gambiae, Bombyx mori) and a crustacean (Procambus clarkii) were compared. The sequences of thick filament proteins, twitchin in the worm and projectin in the insects, have repeating modules with fibronectin-like (Fn) and immunoglobulin-like (Ig) domains conserved between species. Projectin has additional tandem Igs and an elastic PEVK domain near the N-terminus. All the species have a second elastic protein we have called SLS protein after the Drosophila gene, sallimus. SLS protein is in the I-band. The N-terminal region has the sequence of kettin which is a spliced product of the gene composed of Ig-linker modules binding to actin. Downstream of kettin, SLS protein has two PEVK domains, unique sequence, tandem Igs, and Fn domains at the end. PEVK domains have repeating sequences: some are long and highly conserved and would have varying elasticity appropriate to different muscles. Insect indirect flight muscle (IFM) has short I-bands and electron micrographs of Lethocerus IFM show fine filaments branching from the end of thick filaments to join thin filaments before they enter the Z-disc. Projectin and kettin are in this region and the contribution of these to the high passive stiffness of Drosophila IFM myofibrils was measured from the force response to length oscillations. Kettin is attached both to actin near the Z-disc and to the end of thick filaments, and extraction of actin or digestion of kettin leads to rapid decrease in stiffness; residual tension is attributable to projectin. The wormlike chain model for polymer elasticity fitted the force-extension curve of IFM myofibrils and the number of predicted Igs in the chain is consistent with the tandem Igs in Drosophila SLS protein. We conclude that passive tension is due to kettin and projectin, either separate or linked in series.

Single-molecule measurement of elasticity of serine-, glutamate- and lysine-rich repeats of invertebrate connectin reveals that its elasticity is caused entropically by random coil structure

Invertebrate connectin (I-connectin) is a 1960 kDa elastic protein linking the Z line to the tip of the myosin filament in the giant sarcomere of crayfish claw closer muscle (Fukuzawa et al., 2001 EMBO J 20: 4826-4835). I-Connectin can be extended up to 3.5 microns upon stretch of giant sarcomeres. There are several extensible regions in I-connectin: two long PEVK regions, one unique sequence region and Ser-, Glu- and Lys-rich 68 residue-repeats called SEK repeats. In the present study, the force measurement of the single recombinant SEK polypeptide containing biotinylated BDTC and GST tags at the N and C termini, respectively, were performed by intermolecular force microscopy (IFM), a refined AFM system. The force vs. extension curves were well fit to the wormlike chain (WLC) model and the obtained persistence length of 0.37 +/- 0.01 nm (n = 11) indicates that the SEK region is a random coil along its full length. This is the first observation of an entropic elasticity of a fully random coil region that contributes to the physiological function of I-connectin.

Titins in C.elegans with unusual features: coiled-coil domains, novel regulation of kinase activity and two new possible elastic regions

We report that there are previously unrecognized proteins in Caenorhabditis elegans that are similar to the giant muscle proteins called titins, and these are encoded by a single approximately 90kb gene. The gene structure was predicted by GeneMark.hmm and then experimentally verified. The Ce titin gene encodes polypeptides of 2.2MDa, 1.2MDa and 301kDa. The 2.2MDa isoform resembles twitchin and UNC-89 in that it contains multiple Ig (56) and FnIII (11) domains, and a single protein kinase domain. In addition, however, the 2.2MDa isoform contains four classes of short, 14-51 residue, repeat motifs arranged mostly in many tandem copies. One of these tandem repeat regions is similar to the PEVK regions of vertebrate and fly titins. As the PEVK region is one of the main elastic elements of the titins and is also composed of short tandem repeats, this suggests that the repeat motifs in the Ce titins may have a similar elastic function. An interesting aspect of the two largest Ce titin isoforms, is that in contrast to other members of the twitchin/titin family, there are multiple regions which are likely to form coiled-coil structure. In transgenic animals, the first approximately 100 residues of the largest isoforms targets to dense bodies, the worm analogs of Z-discs. Anti-Ce titin antibodies show localization to muscle I-bands beginning at the L2-L3 larval stages and this pattern continues into adult muscle. Ce titins may not have a role in early myofibril assembly: (1) Ce titins are too short to span half a sarcomere, and the onset of their expression is well after the initial assembly of thick filaments. (2) Ce titins are not localized to I-bands in embryonic or L1 larval muscle. The Ce titin protein kinase domain is most similar to the kinase domains of the twitchins and projectin. The Ce titin kinase has protein kinase activity in vitro, and this activity is regulated by a novel mechanism.