Drosophilaprojectin: relatedness to titin and twitchin and correlation withlethal (4) 102 CDaandbent-Dominantmutants

Genomic organization of the autonomous regulatory domain of eyeless locus in Drosophila melanogaster

In Drosophila, expression of eyeless (ey) gene is restricted to the developing eyes and central nervous system. However, the flanking genes, myoglianin (myo), and bent (bt) have different temporal and spatial expression patterns as compared to the ey. How distinct regulation of ey is maintained is mostly unknown. Earlier, we have identified a boundary element intervening myo and ey genes (ME boundary) that prevents the crosstalk between the cis-regulatory elements of myo and ey genes. In the present study, we further searched for the cis-elements that define the domain of ey and maintain its expression pattern. We identify another boundary element between ey and bt, the EB boundary. The EB boundary separates the regulatory landscapes of ey and bt genes. The two boundaries, ME and EB, show a long-range interaction as well as interact with the nuclear architecture. This suggests functional autonomy of the ey locus and its insulation from differentially regulated flanking regions. We also identify a new Polycomb Response Element, the ey-PRE, within the ey domain. The expression state of the ey gene, once established during early development is likely to be maintained with the help of ey-PRE. Our study proposes a general regulatory mechanism by which a gene can be maintained in a functionally independent chromatin domain in gene-rich euchromatin.

Localization of the Elastic Proteins in the Flight Muscle of Manduca sexta

The flight muscle of Manduca sexta (DLM₁) is an emerging model system for biophysical studies of muscle contraction. Unlike the well-studied indirect flight muscle of Lethocerus and Drosophila, the DLM₁ of Manduca is a synchronous muscle, as are the vertebrate cardiac and skeletal muscles. Very little has been published regarding the ultrastructure and protein composition of this muscle. Previous studies have demonstrated that DLM₁ express two projectin isoform, two kettin isoforms, and two large Salimus (Sls) isoforms. Such large Sls isoforms have not been observed in the asynchronous flight muscles of Lethocerus and Drosophila. The spatial localization of these proteins was unknown. Here, immuno-localization was used to show that the N-termini of projectin and Salimus are inserted into the Z-band. Projectin spans across the I-band, and the C-terminus is attached to the thick filament in the A-band. The C-terminus of Sls was also located in the A-band. Using confocal microscopy and experimental force-length curves, thin filament lengths were estimated as ~1.5 µm and thick filament lengths were measured as ~2.5 µm. This structural information may help provide an interpretive framework for future studies using this muscle system.

A versatile two-step CRISPR- and RMCE-based strategy for efficient genome engineering in Drosophila

The development of clustered, regularly interspaced, short palindromic repeats (CRISPR)/CRISPR-associated (Cas) technologies promises a quantum leap in genome engineering of model organisms. However, CRISPR-mediated gene targeting reports in Drosophila melanogaster are still restricted to a few genes, use variable experimental conditions, and vary in efficiency, questioning the universal applicability of the method. Here, we developed an efficient two-step strategy to flexibly engineer the fly genome by combining CRISPR with recombinase-mediated cassette exchange (RMCE). In the first step, two sgRNAs, whose activity had been tested in cell culture, were co-injected together with a donor plasmid into transgenic Act5C-Cas9, Ligase4 mutant embryos and the homologous integration events were identified by eye fluorescence. In the second step, the eye marker was replaced with DNA sequences of choice using RMCE enabling flexible gene modification. We applied this strategy to engineer four different locations in the genome, including a gene on the fourth chromosome, at comparably high efficiencies. Our data suggest that any fly laboratory can engineer their favorite gene for a broad range of applications within approximately 3 months.

An ongoing role for structural sarcomeric components in maintaining Drosophila melanogaster muscle function and structure

Animal muscles must maintain their function while bearing substantial mechanical loads. How muscles withstand persistent mechanical strain is presently not well understood. The basic unit of muscle is the sarcomere, which is primarily composed of cytoskeletal proteins. We hypothesized that cytoskeletal protein turnover is required to maintain muscle function. Using the flight muscles of Drosophila melanogaster, we confirmed that the sarcomeric cytoskeleton undergoes turnover throughout adult life. To uncover which cytoskeletal components are required to maintain adult muscle function, we performed an RNAi-mediated knockdown screen targeting the entire fly cytoskeleton and associated proteins. Gene knockdown was restricted to adult flies and muscle function was analyzed with behavioural assays. Here we analyze the results of that screen and characterize the specific muscle maintenance role for several hits. The screen identified 46 genes required for muscle maintenance: 40 of which had no previously known role in this process. Bioinformatic analysis highlighted the structural sarcomeric proteins as a candidate group for further analysis. Detailed confocal and electron microscopic analysis showed that while muscle architecture was maintained after candidate gene knockdown, sarcomere length was disrupted. Specifically, we found that ongoing synthesis and turnover of the key sarcomere structural components Projectin, Myosin and Actin are required to maintain correct sarcomere length and thin filament length. Our results provide in vivo evidence of adult muscle protein turnover and uncover specific functional defects associated with reduced expression of a subset of cytoskeletal proteins in the adult animal.

Drosophila melanogaster muscle LIM protein and alpha-actinin function together to stabilize muscle cytoarchitecture: a potential role for Mlp84B in actin-crosslinking

Stabilization of tissue architecture during development and growth is essential to maintain structural integrity. Because of its contractile nature, muscle is especially susceptible to physiological stresses, and has multiple mechanisms to maintain structural integrity. The Drosophila melanogaster Muscle LIM Protein (MLP), Mlp84B, participates in muscle maintenance, yet its precise mechanism of action is still controversial. Through a candidate approach, we identified α-actinin as a protein that functions with Mlp84B to ensure muscle integrity. α-actinin RNAi animals die primarily as pupae, and Mlp84B RNAi animals are adult viable. RNAi knockdown of Mlp84B and α-actinin together produces synergistic early larval lethality and destabilization of Z-line structures. We recapitulated these phenotypes using combinations of traditional loss-of-function alleles and single-gene RNAi. We observe that Mlp84B induces the formation of actin loops in muscle cell nuclei in the absence of nuclear α-actinin, suggesting Mlp84B has intrinsic actin cross-linking activity, which may complement α-actinin cross-linking activity at sites of actin filament anchorage. These results reveal a molecular mechanism for MLP stabilization of muscle and implicate reduced actin crosslinking as the primary destabilizing defect in MLP-associated cardiomyopathies. Our data support a model in which α-actinin and Mlp84B have important and overlapping functions at sites of actin filament anchorage to preserve muscle structure and function.

The myofibrillar protein, projectin, is highly conserved across insect evolution except for its PEVK domain

All striated muscles respond to stretch by a delayed increase in tension. This physiological response, known as stretch activation, is, however, predominantly found in vertebrate cardiac muscle and insect asynchronous flight muscles. Stretch activation relies on an elastic third filament system composed of giant proteins known as titin in vertebrates or kettin and projectin in insects. The projectin insect protein functions jointly as a "scaffold and ruler" system during myofibril assembly and as an elastic protein during stretch activation. An evolutionary analysis of the projectin molecule could potentially provide insight into how distinct protein regions may have evolved in response to different evolutionary constraints. We mined candidate genes in representative insect species from Hemiptera to Diptera, from published and novel genome sequence data, and carried out a detailed molecular and phylogenetic analysis. The general domain organization of projectin is highly conserved, as are the protein sequences of its two repeated regions-the immunoglobulin type C and fibronectin type III domains. The conservation in structure and sequence is consistent with the proposed function of projectin as a scaffold and ruler. In contrast, the amino acid sequences of the elastic PEVK domains are noticeably divergent, although their length and overall unusual amino acid makeup are conserved. These patterns suggest that the PEVK region working as an unstructured domain can still maintain its dynamic, and even its three-dimensional, properties, without the need for strict amino acid conservation. Phylogenetic analysis of the projectin proteins also supports a reclassification of the Hymenoptera in relation to Diptera and Coleoptera.

Structure-function relations of the giant elastic protein titin in striated and smooth muscle cells

The striated muscle sarcomere contains, in addition to thin and thick filaments, a third myofilament comprised of titin. The extensible region of titin spans the I-band region of the sarcomere and develops passive force in stretched sarcomeres. This force positions the A-bands in the middle of the sarcomere, maintains sarcomere length homogeneity and, importantly, is responsible for myocardial passive tension that determines diastolic filling. Recent work suggests that smooth muscle expresses a truncated titin isoform with a short extensible region that is predicted to develop high passive force levels. Several mechanisms for tuning the titin-based passive tension have been discovered that involve alternative splicing as well as posttranslational modification, mechanisms that are at play both during normal muscle function as well as during disease.

In indirect flight muscles Drosophila projectin has a short PEVK domain, and its NH2-terminus is embedded at the Z-band

Insect indirect flight muscles (IFM) contain a third filament system made up of elastic connecting or C-filaments. The giant protein projectin is the main, if not the only, component of these structures. In this study we found that projectin is oriented within the IFM sarcomere with its NH2-terminus embedded in the Z-bands. We demonstrate that this protein has an elastic region that can be detected by the movement of specific epitopes following stretch. One possible elastic region is the PEVK-like domain located close to the NH2-terminus. The amino acid length of this region is short, and 52% of its residues are P, E, V or K. We propose a model in which projectin extends from the Z-band to the lateral borders of the A-band. The PEVK-like domain and a series of Ig domains spanning the intervening I-band may provide the elastic properties of projectin.

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.

Assembly of the giant protein projectin during myofibrillogenesis in Drosophila indirect flight muscles

BACKGROUND

Projectin is a giant modular protein of Drosophila muscles and a key component of the elastic connecting filaments (C-filaments), which are involved in stretch activation in insect Indirect Flight Muscles. It is comparable in its structure to titin, which has been implicated as a scaffold during vertebrate myofibrillogenesis.

METHODS

We performed immunofluorescence studies on Drosophila pupal tissue squashes and isolated myofibrils to identify the pattern of appearance and assembly for projectin and several other myofibrillar proteins, using both wild type and mutant fly stocks.

RESULTS AND CONCLUSIONS

In the first step of assembly, projectin immunolocalization appears as random aggregates colocalizing with alpha-actinin, kettin and Z(210), as well as, F-actin. In the second step of assembly, all these proteins become localized within discrete bands, leading ultimately to the regularly spaced I-Z-I regions of myofibrils. This assembly process is not affected in myosin heavy chain mutants, indicating that the anchoring of projectin to the thick filament is not essential for the assembly of projectin into the developing myofibrils. In the actin null mutation, KM88, the early step involving the formation of the aggregates takes place despite the absence of the thin filaments. All tested Z-band proteins including projectin are present and are colocalized over the aggregates. This supports the idea that interactions of projectin with other Z-band associated proteins are sufficient for its initial assembly into the forming myofibrils. In KM88, though, mature Z-bands never form and projectin I-Z-I localization is lost at a later stage during pupal development. In contrast, treatment of adult myofibrils with calpain, which removes the Z-bands, does not lead to the release of projectin. This suggests that after the initial assembly with the Z-bands, projectin also establishes additional anchoring points along the thick and/or thin filaments. In conclusion, during pupation the initial assembly of projectin into the developing myofibril relies on early association with Z-band proteins, but in the mature myofibrils, projectin is also held in position by interactions with the thick and/or the thin filaments.

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.

Alternative splicing of an amino-terminal PEVK-like region generates multiple isoforms of Drosophila projectin

Drosophila projectin is an extremely large protein found within the muscle sarcomeric unit, parallel with the actin and myosin filaments. Projectin has been suggested as the elastic component of C-filaments in insect indirect flight muscles, which is consistent with its localization from the Z band to the tip of the A band in these muscles. Here, we describe the completion of the projectin sequence analysis, which defines projectin as a 1 MDa protein, composed of 39 immunoglobulin and 39 fibronectin III domains. This analysis led also to the identification of a domain rich in the amino acids P, E, V and K within the NH(2) terminus of projectin. The length of the projectin PEVK-like region varies from 100 to 624 amino acid residues, following a complex pattern of alternative splicing events. PEVK domains were first identified in vertebrate titin and they have been associated with the elasticity of the protein. The PEVK-like domain of the projectin isoforms in indirect flight muscles may contribute to the elastic function of the C-filaments. The synchronous projectin isoforms contain a PEVK-like region, and the possible non-elastic function(s) of this domain in synchronous muscles are discussed.

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.

Drosophila projectin: a look at protein structure and sarcomeric assembly

The large projectin protein is found in all Drosophila muscles; however, it shows a dual sarcomeric localization depending on the muscle type. In larval and adult synchronous muscles, projectin is found localized over the A-band. Initial in vitro binding assays indicate interactions of several projectin regions with themselves and myosin heavy chain. These interactions might be critical for the assembly of projectin over the myosin filament during embryonic myofibrillogenesis and larval growth. On the other hand, projectin localizes over the I-Z-I region in indirect flight muscles. Correspondingly, projectin is found in association with forming Z-bands during pupation and colocalizes with alpha-actinin and kettin.

Links in the chain: the contribution of kettin to the elasticity of insect muscles

Asynchronous flight muscle fibers are activated by periodic stretches and need to be stiff for strain to be transmitted to the contractile system. Kettin associated with thin filaments and projectin with thick filaments contribute to fiber stiffness. Kettin extends along thin filaments with the N-terminus in the Z-disc and the C-terminus outside. C filaments connecting thick filaments to the Z-disc contain projectin but not kettin. Insect flight myofibrils have a titin PEVK epitope which is only exposed on stretch, suggesting it is short and inaccessible. It is concluded that kettin stiffens thin filaments near the Z-disc and projectin and titin provide elasticity to C filaments.

D-Titin: a giant protein with dual roles in chromosomes and muscles

Previously, we reported that chromosomes contain a giant filamentous protein, which we identified as titin, a component of muscle sarcomeres. Here, we report the sequence of the entire titin gene in Drosophila melanogaster, D-Titin, and show that it encodes a two-megadalton protein with significant colinear homology to the NH(2)-terminal half of vertebrate titin. Mutations in D-Titin cause chromosome undercondensation, chromosome breakage, loss of diploidy, and premature sister chromatid separation. Additionally, D-Titin mutants have defects in myoblast fusion and muscle organization. The phenotypes of the D-Titin mutants suggest parallel roles for titin in both muscle and chromosome structure and elasticity, and provide new insight into chromosome structure.

Drosophila stretchin-MLCK is a novel member of the Titin/Myosin light chain kinase family

Members of the titin/myosin light chain kinase family play an essential role in the organization of the actin/myosin cytoskeleton, especially in sarcomere assembly and function. In Drosophila melanogaster, projectin is so far the only member of this family for which a transcription unit has been characterized. The locus of another member of this family, a protein related to Myosin light chain kinase, was also identified. The cDNA and genomic sequences published explain only the shorter transcripts expressed by this locus. Here, we report the complete molecular characterization of this transcription unit, which spans 38 kb, includes 33 exons and accounts for transcripts up to 25 kb in length. This transcription unit contains both the largest exon (12,005 nt) and the largest coding region (25,213 nt) reported so far for Drosophila. This transcription unit features both internal promoters and internal polyadenylation signals, which enable it to express seven different transcripts, ranging from 3.3 to 25 kb in size. The latter encodes a huge, titin-like, 926 kDa kinase that features two large PEVK-rich repeats, 32 immunoglobulin and two fibronectin type-III domains, which we designate stretchin-MLCK. In addition, the 3' end of the stretchin-MLCK transcription unit expresses shorter transcripts that encode 86 to 165 kDa isoforms of stretchin-MLCK that are analogous to vertebrate Myosin light chain kinases. Similarly, the 5' end of the Stretchin-Mlck transcription unit can also express transcripts encoding kettin and Unc-89-like isoforms, which share no sequences with the MLCK-like transcripts. Thus, this locus can be viewed as a single transcription unit, Stretchin-Mlck (genetic abbreviation Strn-Mlck), that expresses large, composite transcripts and protein isoforms (sequences available at http://www.academicpress.com/jmb), as well as a complex of two independent transcription units, the Stretchin and Mlck transcription units (Strn and Mlck, respectively) the result of a "gene fission" event, that encode independent transcripts and proteins with distinct structural and enzymatic functions.

Structural analysis of the titin gene in hypertrophic cardiomyopathy: identification of a novel disease gene

Hypertrophic cardiomyopathy (HCM) is characterized by ventricular hypertrophy accompanied by myofibrillar disarrays. Molecular genetic analyses have revealed that mutations in 8 different genes cause HCM. Mutations in these disease genes, however, could be found in about half of HCM patients, suggesting that there are other unknown disease gene(s). Because the known disease genes encode sarcomeric proteins expressed in the cardiac muscle, we searched for a disease-associated mutation in the titin gene in 82 HCM patients who had no mutation in the known disease genes. A G to T transversion in codon 740, from CGC to CTC, replacing Arginine with Leucine was found in a patient. This mutation was not found in more than 500 normal chromosomes and increased the binding affinity of titin to alpha-actitin in the yeast two-hybrid assay. These observations suggest that the titin mutation may cause HCM in this patient via altered affinity to alpha-actinin.

Assembly of thick filaments and myofibrils occurs in the absence of the myosin head

We investigated the importance of the myosin head in thick filament formation and myofibrillogenesis by generating transgenic Drosophila lines expressing either an embryonic or an adult isoform of the myosin rod in their indirect flight muscles. The headless myosin molecules retain the regulatory light-chain binding site, the alpha-helical rod and the C-terminal tailpiece. Both isoforms of headless myosin co-assemble with endogenous full-length myosin in wild-type muscle cells. However, rod polypeptides interfere with muscle function and cause a flightless phenotype. Electron microscopy demonstrates that this results from an antimorphic effect upon myofibril assembly. Thick filaments assemble when the myosin rod is expressed in mutant indirect flight muscles where no full-length myosin heavy chain is produced. These filaments show the characteristic hollow cross-section observed in wild type. The headless thick filaments can assemble with thin filaments into hexagonally packed arrays resembling normal myofibrils. However, thick filament length as well as sarcomere length and myofibril shape are abnormal. Therefore, thick filament assembly and many aspects of myofibrillogenesis are independent of the myosin head and these processes are regulated by the myosin rod and tailpiece. However, interaction of the myosin head with other myofibrillar components is necessary for defining filament length and myofibril dimensions.

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 Drosophila projectin protein: isoforms and implication for projectin filament assembly

The protein composition of the various muscle types in Drosophila melanogaster has been studied quite thoroughly and the analysis has revealed many differences involving the usage of muscle specific isoforms of a given protein, as well as the presence of proteins restricted to one muscle type. Drosophila projectin, the giant protein component of the third filament is quite unusual as it not only shows specific isoforms in various muscle types, but these isoforms are located at different sarcomeric locations, I band in the IFM and A band in synchronous muscles. This may suggest distinct functions for the projectin protein in various muscles, as well as a different set of protein interactions for each projectin isoform. Projectin is encoded by a single gene and the isoforms were proposed to be the result of alternative splicing of a primary transcript. Here, we report the nearly complete sequence of Drosophila projectin, as well as the possible splicing patterns used to generate different isoforms. The overall domain organization in projectin is composed of repeated motifs I and II in a few specific patterns, similar to its Caenorhabditis homolog, twitchin. Sequence similarity between twitchin and projectin further suggests how some domains may possibly be important for protein interactions and/or functions. Alternative splicing operates at the COOH terminus, leading to a shorter projectin protein lacking some of the terminal motifs II and unique sequence. These isoforms are discussed in view of projectin differential size and localization.

Human autoantibodies reveal titin as a chromosomal protein

Assembly of the higher-order structure of mitotic chromosomes is a prerequisite for proper chromosome condensation, segregation and integrity. Understanding the details of this process has been limited because very few proteins involved in the assembly of chromosome structure have been discovered. Using a human autoimmune scleroderma serum that identifies a chromosomal protein in human cells and Drosophila embryos, we cloned the corresponding Drosophila gene that encodes the homologue of vertebrate titin based on protein size, sequence similarity, developmental expression and subcellular localization. Titin is a giant sarcomeric protein responsible for the elasticity of striated muscle that may also function as a molecular scaffold for myofibrillar assembly. Molecular analysis and immunostaining with antibodies to multiple titin epitopes indicates that the chromosomal and muscle forms of titin may vary in their NH2 termini. The identification of titin as a chromosomal component provides a molecular basis for chromosome structure and elasticity.

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

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

Immunohistochemical study and western blotting analysis of titin-like proteins in the striated muscle of Drosophila melanogaster and in the striated and smooth muscle of the oligochaete Eisenia foetida

The presence and distribution of titin-like proteins have been examined in transversely striated muscle of Drosophila melanogaster, in obliquely striated muscles (body wall and inner muscular layer of the pseudoheart) and smooth muscle (outer muscular layer of the pseudoheart) from the earthworm Eisenia foetida by means of Western blotting analysis, light microscopy immunohistochemistry, and electron microscopy immunogold labeling, using antibodies anti vertebrate (chicken) titin (3,000 kDa) and arthropod (D. melanogaster) mini-titin (twitchin or projectin) (700 kDa). To determine whether these antibodies immunoreact non-specifically against vertebrate titin, mouse skeletal muscle was also studied. As negative control, mouse smooth muscle was used. Immunoreaction to mini-titin was found in all the invertebrate muscles studied. For each of these muscles, Western blotting analysis of mini-titin showed a single band, at approximately 700 kDa. Electron microscopy immunolabeling to this protein was observed along the whole sarcomere length (A bands and I bands) in both transversely striated muscles of the insect and obliquely striated muscles of the earthworm, although the number of immunogold particles was more abundant in the insect muscles. Mini-titin immunolabeling was also observed in the smooth muscle cells that formed the outer layer of the earthworm pseudoheart although in lower amounts than in the obliquely striated muscle. The absence of true sarcomeres in the smooth muscle cells did not permit to determine the extension of mini-titin immunolabeling. No immunoreaction to this protein was found in the striated and smooth muscles of the mouse. Immunoreaction to titin was only observed in the mouse skeletal muscle, in which both A bands and I bands appeared immunolabeled. Present results show that mini-titin in the invertebrate muscles studied differs immunohistochemically from vertebrate titin and, in contrast with titin, mini-titin is also present in invertebrate smooth muscles.

The Caenorhabditis elegans gene unc-89, required fpr muscle M-line assembly, encodes a giant modular protein composed of Ig and signal transduction domains

Mutations in the Caenorhabditis elegans gene unc-89 result in nematodes having disorganized muscle structure in which thick filaments are not organized into A-bands, and there are no M-lines. Beginning with a partial cDNA from the C. elegans sequencing project, we have cloned and sequenced the unc-89 gene. An unc-89 allele, st515, was found to contain an 84-bp deletion and a 10-bp duplication, resulting in an in-frame stop codon within predicted unc-89 coding sequence. Analysis of the complete coding sequence for unc-89 predicts a novel 6,632 amino acid polypeptide consisting of sequence motifs which have been implicated in protein-protein interactions. UNC-89 begins with 67 residues of unique sequences, SH3, dbl/CDC24, and PH domains, 7 immunoglobulins (Ig) domains, a putative KSP-containing multiphosphorylation domain, and ends with 46 Ig domains. A polyclonal antiserum raised to a portion of unc-89 encoded sequence reacts to a twitchin-sized polypeptide from wild type, but truncated polypeptides from st515 and from the amber allele e2338. By immunofluorescent microscopy, this antiserum localizes to the middle of A-bands, consistent with UNC-89 being a structural component of the M-line. Previous studies indicate that myofilament lattice assembly begins with positional cues laid down in the basement membrane and muscle cell membrane. We propose that the intracellular protein UNC-89 responds to these signals, localizes, and then participates in assembling an M-line.

Transdifferentiation of chicken embryonic cells into muscle cells by the 3' untranslated region of muscle tropomyosin

Transfection with a plasmid encoding the 3' untranslated region (3' UTR) of skeletal muscle tropomyosin induces chicken embryonic fibroblasts to express skeletal tropomyosin. Such cells become spindle shaped, fuse, and express titin, a marker of striated muscle differentiation. Skeletal muscle tropomyosin and titin organize in sarcomeric arrays. When the tropomyosin 3' UTR is expressed in osteoblasts, less skeletal muscle tropomyosin is expressed, and titin expression is delayed. Some transfected osteoblasts become spindle shaped but do not fuse nor organize these proteins into sarcomeres. Transfected cells expressing muscle tropomyosin organize muscle and nonmuscle isoforms into the same structures. Thus, the skeletal muscle tropomyosin 3' UTR induces transdifferentiation into a striated muscle phenotype in a cell-type-specific context.

Both synchronous and asynchronous muscle isoforms of projectin (the Drosophila bent locus product) contain functional kinase domains

In Drosophila, the large muscle protein, projectin, has very different localizations in synchronous and asynchronous muscles, suggesting that projectin has different functions in different muscle types. The multiple projectin isoforms are encoded by a single gene; however they differ significantly in size (as detected by gel mobility) and show differences in some peptide fragments, presumably indicating alternative splicing or termination. We now report additional sequence of the projectin gene, showing a kinase domain and flanking regions highly similar to equivalent regions of twitchin, including a possible autoinhibitory region. In spite of apparent differences in function, all isoforms of projectin have the kinase domain and all are capable of autophosphorylation in vitro. The projectin gene is in polytene region 102C/D where the bentD phenotype maps. The recessive lethality of bentD is associated with a breakpoint that removes sequence of the projectin kinase domain. We find that different alleles of the highly mutable recessive lethal complementation group, l(4)2, also have defects in different parts of the projectin sequence, both NH2-terminal and COOH-terminal to the bentD breakpoint. These alleles are therefore renamed as alleles of the bent locus. Adults heterozygous for projectin mutations show little, if any, effect of one defective gene copy, but homozygosity for any of the defects is lethal. The times of death can vary with allele. Some alleles kill the embryos, others are larval lethal. These molecular studies begin to explain why genetic studies suggested that l(4)2 was a complex (or pseudoallelic) locus.

Connectin, an elastic protein of striated muscle

Connectin, also called titin, a giant elastic protein of striated muscle (approximately 3000 kDa) mainly consists of fibronectin type III and immunoglobulin C2 globular domains, the beta-sheets of which are parallel to the main axis of the molecule. One connectin molecule runs through the I band and binds onto the myosin filament up to the M line starting from the Z line. It positions the myosin filament at the center of a sarcomere. Connectin is also responsible for resting tension generation. Biodiversity of the connectin family exists in invertebrate muscle.

The evolution of titin and related giant muscle proteins

Titin and twitchin are giant proteins expressed in muscle. They are mainly composed of domains belonging to the fibronectin class III and immunoglobulin c2 families, repeated many times. In addition, both proteins have a protein kinase domain near the C-terminus. This paper explores the evolution of these and related muscle proteins in an attempt to determine the order of events that gave rise to the different repeat patterns and the order of appearance of the proteins. Despite their great similarity at the level of sequence organization, titin and twitchin diverged from each other at least as early as the divergence between vertebrates and nematodes. Most of the repeating units in titin and twitchin were estimated to derive from three original domains. Chicken smooth-muscle myosin light-chain kinase (smMLCK) also has a kinase domain, several immunoglobulin domains, and a fibronectin domain. From a comparison of the kinase domains, titin is predicted to have appeared first during the evolution of the family, followed by twitchin and with the vertebrate MLCKs last to appear. The so-called C-protein from chicken is also a member of this family but has no kinase domain. Its origin remains unclear but it most probably pre-dates the titin/twitchin duplication.

Mini-titins in striated and smooth molluscan muscles: structure, location and immunological crossreactivity

Invertebrate mini-titins are members of a class of myosin-binding proteins belonging to the immunoglobulin superfamily that may have structural and/or regulatory properties. We have isolated mini-titins from three molluscan sources: the striated and smooth adductor muscles of the scallop, and the smooth catch muscles of the mussel. Electron microscopy reveals flexible rod-like molecules about 0.2 micron long and 30 A wide with a distinctive polarity. Antibodies to scallop mini-titin label the A-band and especially the A/I junction of scallop striated muscle myofibrils by indirect immunofluorescence and immuno-electron microscopy. This antibody crossreacts with mini-titins in scallop smooth and Mytilus catch muscles, as well as with proteins in striated muscles from Limulus, Lethocerus (asynchronous flight muscle), and crayfish. It labels the A/I junction (I-region in Lethocerus) in these striated muscles as well as in chicken skeletal muscle. Antibodies to the repetitive immunoglobulin-like regions and also to the kinase domain of nematode twitchin crossreact with scallop mini-titin and label the A-band of scallop myofibrils. Electron microscopy of single molecules shows that antibodies to twitchin kinase bind to scallop mini-titin near one end of the molecule, suggesting how the scallop structure might be aligned with the sequence of nematode twitchin.

Connectin, giant elastic protein, in giant sarcomeres of crayfish claw muscle

In the giant sarcomeres (sarcomere length, 10 microns at rest) of crayfish claw muscle, 3000 kDa connectin-like protein but not projectin (mini-titin) appears to be responsible for passive tension generation. Proteolysis of crayfish connectin in skinned fibres was parallel with disappearance of resting tension. Immunofluorescence observations using the antiserum to crayfish connectin showed that crayfish connectin linked the A band to the Z line in a giant sarcomere. It appears that crayfish connectin exerts a centering force on the A band in a sarcomere. Very thin filaments in the I band were visualized after the actin filaments had been removed by the treatment with plasma gelsolin. Crayfish connectin was partially purified and its rotary shadowed image was a very long filament. Projectin was localized on the A band of crayfish giant sarcomeres and remained unmoved during stretch or contraction. However, on dissolution of myosin filaments, projectin moved to the Z line together with crayfish connectin. It seems that projectin binds to connectin on the myosin filament. In regular size of sarcomeres (sarcomere lengths, 3-4 microns at rest) of crayfish stretcher muscle, projectin linked the A band to the Z line, as in insect flight muscle.

The major myosin-binding domain of skeletal muscle MyBP-C (C protein) resides in the COOH-terminal, immunoglobulin C2 motif

A common feature shared by myosin-binding proteins from a wide variety of species is the presence of a variable number of related internal motifs homologous to either the Ig C2 or the fibronectin (Fn) type III repeats. Despite interest in the potential function of these motifs, no group has clearly demonstrated a function for these sequences in muscle, either intra- or extracellularly. We have completed the nucleotide sequence of the fast type isoform of MyBP-C (C protein) from chicken skeletal muscle. The deduced amino acid sequence reveals seven Ig C2 sets and three Fn type III motifs in MyBP-C. alpha-chymotryptic digestion of purified MyBP-C gives rise to four peptides. NH2-terminal sequencing of these peptides allowed us to map the position of each along the primary structure of the protein. The 28-kD peptide contains the NH2-terminal sequence of MyBP-C, including the first C2 repeat. It is followed by two internal peptides, one of 5 kD containing exclusively spacer sequences between the first and second C2 motifs, and a 95-kD fragment containing five C2 domains and three fibronectin type III motifs. The C-terminal sequence of MyBP-C is present in a 14-kD peptide which contains only the last C2 repeat. We examined the binding properties of these fragments to reconstituted (synthetic) myosin filaments. Only the COOH-terminal 14-kD peptide is capable of binding myosin with high affinity. The NH2-terminal 28-kD fragment has no myosin-binding, while the long internal 100-kD peptide shows very weak binding to myosin. We have expressed and purified the 14-kD peptide in Escherichia coli. The recombinant protein exhibits saturable binding to myosin with an affinity comparable to that of the 14-kD fragment obtained by proteolytic digestion (1/2 max binding at approximately 0.5 microM). These results indicate that the binding to myosin filaments is mainly restricted to the last 102 amino acids of MyBP-C. The remainder of the molecule (1,032 amino acids) could interact with titin, MyBP-H (H protein) or thin filament components. A comparison of the highly conserved Ig C2 domains present at the COOH-terminus of five MyBPs thus far sequenced (human slow and fast MyBP-C, human and chicken MyBP-H, and chicken MyBP-C) was used to identify residues unique to these myosin-binding Ig C2 repeats.