201
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Grama L, Somogyi B, Kellermayer MSZ. Direct Visualization of Surface-Adsorbed Single Fluorescently Labeled Titin Molecules. ACTA ACUST UNITED AC 2001. [DOI: 10.1002/1438-5171(200107)2:2<79::aid-simo79>3.0.co;2-l] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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202
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Abstract
Repeating motifs of 26-28 amino acids have been identified in the PEVK region of the giant elastic protein titin. These motifs, termed PPAK for the four amino acids that often constitute the beginning of the motif, occur 60 times in human soleus titin. PPAK motifs occur in groups of 2-12 that are separated by regions rich in glutamic acid (approximately 45%) and termed polyE segments. The fluctuation of the net charge between the PPAK and polyE regions suggests ionic interactions between these segments and their involvement in the elastic function of titin. Proteins 2001;43:145-149.
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Affiliation(s)
- M Greaser
- University of Wisconsin, Muscle Biology Laboratory, Madison, Wisconsin.
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203
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Joseph C, Stier G, O'Brien R, Politou AS, Atkinson RA, Bianco A, Ladbury JE, Martin SR, Pastore A. A structural characterization of the interactions between titin Z-repeats and the alpha-actinin C-terminal domain. Biochemistry 2001; 40:4957-65. [PMID: 11305911 DOI: 10.1021/bi002739r] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Titin and alpha-actinin, two modular muscle proteins, are with actin the major components of the Z-band in vertebrate striated muscles where they serve to organize the antiparallel actin filament arrays in adjacent sarcomeres and to transmit tension between sarcomeres during activation. Interactions between titin and alpha-actinin have been mainly localized in a 45-amino acid multiple motif (Z-repeat) in the N-terminal region of titin and the C-terminal region of alpha-actinin. In this study, we provide the first quantitative characterization of alpha-actinin-Z-repeat recognition and dissect the interaction to its minimal units. Different complementary techniques, such as circular dichroism, calorimetry, and nuclear magnetic spectroscopy, were used. Two overlapping alpha-actinin constructs (Act-EF34 and Act-EF1234) containing two and four EF-hand motifs, respectively, were produced, and their folding properties were examined. Complex formation of Act-EF34 and Act-EF1234 with single- and double-Z-repeat constructs was studied. Act-EF34 was shown quantitatively to be necessary and sufficient for binding to Z-repeats, excluding the presence of additional high-affinity binding sites in the remaining part of the domain. The binding affinities of the different Z-repeats for Act-EF34 range from micromolar to millimolar values. The strongest of these interactions are comparable to those observed in troponin C-troponin I complexes. The binding affinities for Act-EF34 are maximal for Zr1 and Zr7, the two highly homologous sequences present in all muscle isoforms. No cooperative or additional contributions to the interaction were observed for Z-repeat double constructs. These findings have direct relevance for evaluating current models of Z-disk assembly.
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Affiliation(s)
- C Joseph
- NIMR, The Ridgeway, London NW7 1AA, UK
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204
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Haravuori H, Vihola A, Straub V, Auranen M, Richard I, Marchand S, Voit T, Labeit S, Somer H, Peltonen L, Beckmann JS, Udd B. Secondary calpain3 deficiency in 2q-linked muscular dystrophy: titin is the candidate gene. Neurology 2001; 56:869-77. [PMID: 11294923 DOI: 10.1212/wnl.56.7.869] [Citation(s) in RCA: 136] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
BACKGROUND Tibial muscular dystrophy (TMD), a late-onset dominant distal myopathy, is caused by yet unknown mutations on chromosome 2q, whereas MD with myositis (MDM) is a muscular dystrophy of the mouse, also progressing with age and linked to mouse chromosome 2. For both disorders, linkage studies have implicated titin as a potential candidate gene. METHODS The authors analyzed major candidate regions in the titin gene by sequencing and Southern blot hybridization, and performed titin immunohistochemistry on TMD patient material to identify the underlying mutation. Western blot studies were performed on the known titin ligands in muscle samples of both disorders and controls, and analysis of apoptosis was also performed. RESULTS The authors identified almost complete loss of calpain3, a ligand of titin, in the patient with limb-girdle MD (LGMD) with a homozygous state of TMD haplotype when primary calpain3 gene defect was excluded. Apoptotic myonuclei with altered distribution of transcription factor NF-kB and its inhibitor IkBalpha were encountered in muscle samples of patients with either heterozygous or homozygous TMD haplotype. Similar findings were confirmed in the MDM mouse. CONCLUSIONS These results imply that titin mutations may be responsible for TMD, and that the pathophysiologic pathway following calpain3 deficiency may overlap with LGMD2A. The loss of calpain3 could be a downstream effect of the deficient TMD gene product. The significance of the secondary calpain3 defect for the pathogenesis of TMD was emphasized by similar calpain3 deficiency in the MDM mouse, which is suggested to be a mouse model for TMD. Homozygous mutation at the 2q locus may thus be capable of producing yet another LGMD.
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Affiliation(s)
- H Haravuori
- Department of Human Molecular Genetics, National Public Health Institute, University of Helsinki, Finland
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205
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Mayans O, Wuerges J, Canela S, Gautel M, Wilmanns M. Structural evidence for a possible role of reversible disulphide bridge formation in the elasticity of the muscle protein titin. Structure 2001; 9:331-40. [PMID: 11525170 DOI: 10.1016/s0969-2126(01)00591-3] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
BACKGROUND The giant muscle protein titin contributes to the filament system in skeletal and cardiac muscle cells by connecting the Z disk and the central M line of the sarcomere. One of the physiological functions of titin is to act as a passive spring in the sarcomere, which is achieved by the elastic properties of its central I band region. Titin contains about 300 domains of which more than half are folded as immunoglobulin-like (Ig) domains. Ig domain segments of the I band of titin have been extensively used as templates to investigate the molecular basis of protein elasticity. RESULTS The structure of the Ig domain I1 from the I band of titin has been determined to 2.1 A resolution. It reveals a novel, reversible disulphide bridge, which is neither required for correct folding nor changes the chemical stability of I1, but it is predicted to contribute mechanically to the elastic properties of titin in active sarcomeres. From the 92 Ig domains in the longest isoform of titin, at least 40 domains have a potential for disulphide bridge formation. CONCLUSIONS We propose a model where the formation of disulphide bridges under oxidative stress conditions could regulate the elasticity of the I band in titin by increasing sarcomeric resistance. In this model, the formation of the disulphide bridge could refrain a possible directed motion of the two beta sheets or other mechanically stable entities of the I1 Ig domain with respect to each other when exposed to mechanical forces.
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Affiliation(s)
- O Mayans
- EMBL Hamburg Outstation, Germany
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206
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Centner T, Yano J, Kimura E, McElhinny AS, Pelin K, Witt CC, Bang ML, Trombitas K, Granzier H, Gregorio CC, Sorimachi H, Labeit S. Identification of muscle specific ring finger proteins as potential regulators of the titin kinase domain. J Mol Biol 2001; 306:717-26. [PMID: 11243782 DOI: 10.1006/jmbi.2001.4448] [Citation(s) in RCA: 301] [Impact Index Per Article: 13.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
The giant myofibrillar protein titin contains within its C-terminal region a serine-threonine kinase of unknown function. We have identified a novel muscle specific RING finger protein, referred to as MURF-1, that binds in vitro to the titin repeats A168/A169 adjacent to the titin kinase domain. In myofibrils, MURF-1 is present within the periphery of the M-line lattice in close proximity to titin's catalytic kinase domain, within the Z-line lattice, and also in soluble form within the cytoplasm. Yeast two-hybrid screens with MURF-1 as a bait identified two other highly homologous MURF proteins, MURF-2 and MURF-3. MURF-1,2,3 proteins are encoded by distinct genes, share highly conserved N-terminal RING domains and in vitro form dimers/heterodimers by shared coiled-coil motifs. Of the MURF family, only MURF-1 interacts with titin repeats A168/A169, whereas MURF-3 has been reported to affect microtubule stability. Association of MURF-1 with M-line titin may potentially modulate titin's kinase activity similar to other known kinase-associated proteins, whereas differential expression and heterodimerization of MURF1, 2 and 3 may link together titin kinase and microtubule-dependent signal pathways in striated muscles.
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Affiliation(s)
- T Centner
- European Molecular Biology Laboratory, Heidelberg, D-69117, Germany
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207
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Minajeva A, Kulke M, Fernandez JM, Linke WA. Unfolding of titin domains explains the viscoelastic behavior of skeletal myofibrils. Biophys J 2001; 80:1442-51. [PMID: 11222304 PMCID: PMC1301335 DOI: 10.1016/s0006-3495(01)76116-4] [Citation(s) in RCA: 140] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
The elastic section of the giant muscle protein titin contains many immunoglobulin-like domains, which have been shown by single-molecule mechanical studies to unfold and refold upon stretch-release. Here we asked whether the mechanical properties of Ig domains and/or other titin regions could be responsible for the viscoelasticity of nonactivated skeletal-muscle sarcomeres, particularly for stress relaxation and force hysteresis. We show that isolated psoas myofibrils respond to a stretch-hold protocol with a characteristic force decay that becomes more pronounced following stretch to above 2.6-microm sarcomere length. The force decay was readily reproducible by a Monte Carlo simulation taking into account both the kinetics of Ig-domain unfolding and the worm-like-chain model of entropic elasticity used to describe titin's elastic behavior. The modeling indicated that the force decay is explainable by the unfolding of only a very small number of Ig domains per titin molecule. The simulation also predicted that a unique sequence in titin, the PEVK domain, may undergo minor structural changes during sarcomere extension. Myofibrils subjected to 1-Hz cycles of stretch-release exhibited distinct hysteresis that persisted during repetitive measurements. Quick stretch-release protocols, in which variable pauses were introduced after the release, revealed a two-exponential time course of hysteresis recovery. The rate constants of recovery compared well with the refolding rates of Ig-like or fibronectin-like domains measured by single-protein mechanical analysis. These findings suggest that in the sarcomere, titin's Ig-domain regions may act as entropic springs capable of adjusting their contour length in response to a stretch.
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Affiliation(s)
- A Minajeva
- Institute of Physiology and Pathophysiology, University of Heidelberg, D-69120 Heidelberg, Germany
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208
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Keller TC, Eilertsen K, Higginbotham M, Kazmierski S, Kim KT, Velichkova M. Role of titin in nonmuscle and smooth muscle cells. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2001; 481:265-77; discussion 278-81. [PMID: 10987078 DOI: 10.1007/978-1-4615-4267-4_16] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/17/2023]
Abstract
Extensive investigation of vertebrate striated muscle titin has yielded significant insight into its structure and function in striated muscle. We have begun to investigate other members of the titin protein family found in vertebrate smooth muscle and nonmuscle cells. Smooth and nonmuscle titins resemble striated muscle titin in molecular size and morphology but differ in their interactions with myosin II filaments and in the structural contexts in which they exist in vivo. Divergence of these titins from the muscle titin paradigm demonstrates the versatility of this remarkable family of giant proteins.
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Affiliation(s)
- T C Keller
- Department of Biological Science, Florida State University, Tallahassee, USA
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209
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McElhinny AS, Labeit S, Gregorio CC. Probing the functional roles of titin ligands in cardiac myofibril assembly and maintenance. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2001; 481:67-86; discussion 86-8. [PMID: 10987067 DOI: 10.1007/978-1-4615-4267-4_5] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Sarcomeres of cardiac muscle are comprised of numerous proteins organized in an elegantly precise order. The exact mechanism of how these proteins are assembled into myofibrils during heart development is not yet understood, although existing in vitro and in vivo model systems have provided great insight into this complex process. It has been proposed by several groups that the giant elastic protein titin acts as a "molecular template" to orchestrate sarcomeric organization during myofibrillogenesis. Titin's highly modular structure, composed of both repeating and unique domains that interact with a wide spectrum of contractile and regulatory ligands, supports this hypothesis. Recent functional studies have provided clues to the physiological significance of the interaction of titin with several titin-binding proteins in the context of live cardiac cells. Improved models of cardiac myofibril assembly, along with the application of powerful functional studies in live cells, as well as the characterization of additional titin ligands, is likely to reveal surprising new functions for the titin third filament system.
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Affiliation(s)
- A S McElhinny
- Department of Cell Biology and Anatomy, University of Arizona, Tucson, USA
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210
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Kellermayer MS, Smith S, Bustamante C, Granzier HL. Mechanical manipulation of single titin molecules with laser tweezers. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2001; 481:111-26; discussion 127-8. [PMID: 10987069 DOI: 10.1007/978-1-4615-4267-4_7] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/17/2023]
Abstract
Titin (also known as connectin) is a giant filamentous polypeptide of multi-domain construction spanning between the Z- and M-lines of the vertebrate muscle sarcomere. The molecule is significant in maintaining sarcomeric structural integrity and generating passive muscle force via its elastic properties. Here we summarize our efforts to characterize titin's elastic properties by manipulating single molecules with force-measuring laser tweezers. The titin molecules can be described as an entropic spring in which domain unfolding occurs at high forces during stretch and refolding at low forces during release. Statistical analysis of a large number (> 500) of stretch-release experiments and comparison of experimental data with the predictions of the wormlike chain theory permit the estimation of unfolded titin's mean persistence length as 16.86 A (+/- 0.11 SD). The slow rates of unfolding and refolding compared with the rates of stretch and release, respectively, result in a state of non-equilibrium and the display of force hysteresis. Folding kinetics as the source of non-equilibrium is directly demonstrated here by the abolishment of force hysteresis in the presence of chemical denaturant. Experimental observations were well simulated by superimposing a simple domain folding kinetics model on the wormlike chain behavior of titin and considering the characteristics of the compliant laser trap. The original video presentation of this paper may be viewed on the web at http:¿www.pote.hu/mm/prezentacio/mkpres/++ +mkpres.htm.
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211
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Machado C, Andrew DJ. Titin as a chromosomal protein. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2001; 481:221-32; discussion 232-6. [PMID: 10987075 DOI: 10.1007/978-1-4615-4267-4_13] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
We identified titin as a chromosomal protein using a human autoimmune scleroderma serum. We cloned the corresponding gene in the fruitfly, Drosophila melanogaster. We have demonstrated that titin is not only expressed and localized in striated muscle but is also distributed uniformly on condensed mitotic chromosomes using multiple antibodies directed against different domains of both Drosophila and vertebrate titin. Titin is a giant sarcomeric protein responsible for the elasticity of striated muscle. Titin may also function as a molecular scaffold during myofibril assembly. We hypothesize that titin is a component of chromosomes that may function to determine chromosome structure and provide elasticity, playing a role similar to that proposed for titin in muscle. We have identified mutations in Drosophila Titin (D-Titin) and are characterizing phenotypes in muscle and chromosomes.
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Affiliation(s)
- C Machado
- Department of Cell Biology and Anatomy, Johns Hopkins University School of Medicine, Baltimore, MD, USA
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212
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Trombitás K. Connecting filaments: a historical prospective. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2001; 481:1-23. [PMID: 10987063 DOI: 10.1007/978-1-4615-4267-4_1] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/12/2023]
Abstract
This short review covers the development of the extensible filament research from the very beginning until the most recent results. This work emphasizes the milestones of discovery, which led us from initial observations that were solely ultrastructural to the molecular understanding of the extensible process of these filaments.
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Affiliation(s)
- K Trombitás
- Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Washington State University, Pullman, USA
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213
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Granzier H, Helmes M, Cazorla O, McNabb M, Labeit D, Wu Y, Yamasaki R, Redkar A, Kellermayer M, Labeit S, Trombitás K. Mechanical properties of titin isoforms. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2001; 481:283-300; discussion 300-4. [PMID: 10987079 DOI: 10.1007/978-1-4615-4267-4_17] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/12/2023]
Abstract
Titin is a giant filamentous polypeptide of multi-domain construction spanning between the Z- and M-lines of the sarcomere. As a result of differential splicing, length variants of titin are expressed in different skeletal and cardiac muscles. Here we first briefly review some of our previous work that has revealed that titin develops force in sarcomeres either stretched beyond their slack length (passive force) or shortened to below the slack length (restoring force) and that titin's force underlies a large fraction of the diastolic force of cardiac muscle. Next we present our mechanical and immunoelectron microscopical (IEM) studies of skeletal and cardiac muscles that express titin isoforms. The previously deduced molecular properties of titin were used to model titin's extensible region in the sarcomere as serially linked WLCs: rigid segments (containing folded Ig/Fn domains) and more flexible segments (PEVK segment). The model was tested on skeletal muscle fibers that express titin isoforms with tandem Ig and PEVK length variants. The model adequately predicts titin's behavior along a wide sarcomere length range in skeletal muscle, but at long sarcome lengths (SLs), predicted forces are much higher than those determined experimentally. IEM reveals that this may result from Ig domain unfolding. Experiments were also performed on cardiac myocytes from mouse and cow that express predominantly a small cardiac titin isoform (N2B titin) or a large isoform (N2BA titin), respectively. The passive tension-SL relation of myocytes was found to increase more steeply with SL in mouse than in cow. IEM revealed an additional source of extensibility within both of these cardiac titins: the unique N2B sequence (absent in skeletal muscle). Furthermore, the PEVK segment of the N2BA isoform extended to a maximal length of approximately 200 nm, as opposed to approximately 60 nm for the N2B isoform. We propose that, along the physiological SL range, the long PEVK segment found in N2BA titins results in a low PEVK fractional extension and that this underlies the lower passive tensions of N2BA-expressing cow myocytes.
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Affiliation(s)
- H Granzier
- Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Washington State University, Pullman, USA
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214
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Centner T, Fougerousse F, Freiburg A, Witt C, Beckmann JS, Granzier H, Trombitás K, Gregorio CC, Labeit S. Molecular tools for the study of titin's differential expression. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2001; 481:35-49; discussion 50-2. [PMID: 10987065 DOI: 10.1007/978-1-4615-4267-4_3] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/17/2023]
Abstract
Although vertebrate genomes appear to contain only one titin gene, a large variety of quite distinct titin isoforms are expressed in striated muscle tissues. The isoforms appear to be generated by a series of complex, not yet fully characterized differential splicing mechanisms. Here, we provide an overview of the titin-specific antibodies that have been raised by our laboratory to study individual differentially expressed isoforms of titin. The staining patterns obtained in different tissues will contribute to the identification of both the particular titin isoforms that are expressed in the different tissues, as well as their intracellular distributions. In addition, antibodies to titin that are available are rapidly allowing for the refinement of our knowledge of titin's elastic spring properties. Knowledge of the nature and structure of vertebrate titins that may also be expressed in nonmuscle tissues may be broadened using these antibodies.
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Affiliation(s)
- T Centner
- European Molecular Biology Laboratory, Heidelberg, Germany
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215
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Linke WA. Titin elasticity in the context of the sarcomere: force and extensibility measurements on single myofibrils. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2001; 481:179-202; discussion 203-6. [PMID: 10987073 DOI: 10.1007/978-1-4615-4267-4_11] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/17/2023]
Abstract
Skeletal-muscle titin contains in its I-band section two main elastic elements, stretches of Ig-like domains and the PEVK segment. Both elements contribute to the extensibility and passive force development of relaxed skeletal muscle fibers during stretch. To explore the nature of elasticity of the segments, their force-extension relation was determined with immunofluorescence and immunoelectron microscopy, combined with isolated myofibril mechanics. The results were then fitted with recent models of biopolymer elasticity. Whereas an entropic-spring mechanism may account for the elasticity of the Ig-domain segments, PEVK-titin elasticity appears to have both entropic and enthalpic origins. The modeling explains why the two elements extend sequentially upon stretch: elongation of the Ig-domain regions (with folded modules) is followed by unraveling of the PEVK domain. I-band titin in cardiac muscle is expressed in two main isoforms, N2-A and N2-B. The N2-A isoform is similar to that found in skeletal muscle, whereas the N2-B titin is distinguished by cardiac-specific Ig-motifs and nonmodular sequences within the central I-band section. By examining the extensibility of N2-B titin, it was found that this isoform extends by recruiting three distinct elastic elements: poly-Ig regions and the PEVK domain at low to modest stretch, and in addition, a unique 572-residue sequence insertion at higher physiological stretch. Extension of all three elements allows cardiac titin to stretch fully reversibly at physiological sarcomere lengths, without the need to unfold individual Ig domains.
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Affiliation(s)
- W A Linke
- Institute of Physiology II, University of Heidelberg, Germany
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216
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Bullard B, Goulding D, Ferguson C, Leonard K. Links in the chain: the contribution of kettin to the elasticity of insect muscles. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2001; 481:207-18; discussion 219-20. [PMID: 10987074 DOI: 10.1007/978-1-4615-4267-4_12] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
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.
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Affiliation(s)
- B Bullard
- European Molecular Biology Laboratory, Heidelberg, Germany
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217
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Kellermayer MS, Smith SB, Bustamante C, Granzier HL. Mechanical fatigue in repetitively stretched single molecules of titin. Biophys J 2001; 80:852-63. [PMID: 11159452 PMCID: PMC1301283 DOI: 10.1016/s0006-3495(01)76064-x] [Citation(s) in RCA: 77] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
Abstract
Relaxed striated muscle cells exhibit mechanical fatigue when exposed to repeated stretch and release cycles. To understand the molecular basis of such mechanical fatigue, single molecules of the giant filamentous protein titin, which is the main determinant of sarcomeric elasticity, were repetitively stretched and released while their force response was characterized with optical tweezers. During repeated stretch-release cycles titin becomes mechanically worn out in a process we call molecular fatigue. The process is characterized by a progressive shift of the stretch-force curve toward increasing end-to-end lengths, indicating that repeated mechanical cycles increase titin's effective contour length. Molecular fatigue occurs only in a restricted force range (0-25 pN) during the initial part of the stretch half-cycle, whereas the rest of the force response is repeated from one mechanical cycle to the other. Protein-folding models fail to explain molecular fatigue on the basis of an incomplete refolding of titin's globular domains. Rather, the process apparently derives from the formation of labile nonspecific bonds cross-linking various sites along a pre-unfolded titin segment. Because titin's molecular fatigue occurs in a physiologically relevant force range, the process may play an important role in dynamically adjusting muscle's response to the recent history of mechanical perturbations.
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Affiliation(s)
- M S Kellermayer
- Department of Biophysics, Pécs University Medical School, Pécs, H-7624 Hungary
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218
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Rudy DE, Yatskievych TA, Antin PB, Gregorio CC. Assembly of thick, thin, and titin filaments in chick precardiac explants. Dev Dyn 2001; 221:61-71. [PMID: 11357194 DOI: 10.1002/dvdy.1125] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
De novo cardiac myofibril assembly has been difficult to study due to the lack of available cell culture models that clearly and accurately reflect heart muscle development in vivo. However, within precardiac chick embryo explants, premyocardial cells differentiate and commence beating in a temporal pattern that corresponds closely with myocyte differentiation in the embryo. Immunofluorescence staining of explants followed by confocal microscopy revealed that distinct stages of cardiac myofibril assembly, ranging from the earliest detection of sarcomeric proteins to the late appearance of mature myofibrils, were consistently recognized in precardiac cultures. Assembly events involved in the early formation of sarcomeres were clearly visualized and accurately reflected observations described by others during chick heart muscle development. Specifically, the early colocalization of alpha-actinin and titin dots was observed near the cell periphery representing I-Z-I-like complex formation. Myosin-containing thick filaments assembled independently of actin-containing thin filaments and appeared centered within sarcomeres when titin was also linearly aligned at or near cell borders. An N-terminal epitope of titin was detected earlier than a C-terminal epitope; however, both epitopes were observed to alternate near the cell periphery concomitant with the earliest formation of myofibrils. Although vascular actin was detected within cells during early assembly stages, cardiac actin predominated as the major actin isoform in mature thin filaments. Well-aligned thin filaments were also observed in the absence of organized staining for tropomodulin at thin filament pointed ends, suggesting that tropomodulin is not required to define thin filament lengths. Based on these findings, we conclude that the use of the avian precardiac explant system accurately allows for direct investigation of the mechanisms regulating de novo cardiac myofibrillogenesis.
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Affiliation(s)
- D E Rudy
- Department of Cell Biology and Anatomy, University of Arizona, Tucson, Arizona, USA
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219
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Reedy MC, Bullard B, Vigoreaux JO. Flightin is essential for thick filament assembly and sarcomere stability in Drosophila flight muscles. J Cell Biol 2000; 151:1483-500. [PMID: 11134077 PMCID: PMC2150682 DOI: 10.1083/jcb.151.7.1483] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Flightin is a multiply phosphorylated, 20-kD myofibrillar protein found in Drosophila indirect flight muscles (IFM). Previous work suggests that flightin plays an essential, as yet undefined, role in normal sarcomere structure and contractile activity. Here we show that flightin is associated with thick filaments where it is likely to interact with the myosin rod. We have created a null mutation for flightin, fln(0), that results in loss of flight ability but has no effect on fecundity or viability. Electron microscopy comparing pupa and adult fln(0) IFM shows that sarcomeres, and thick and thin filaments in pupal IFM, are 25-30% longer than in wild type. fln(0) fibers are abnormally wavy, but sarcomere and myotendon structure in pupa are otherwise normal. Within the first 5 h of adult life and beginning of contractile activity, IFM fibers become disrupted as thick filaments and sarcomeres are variably shortened, and myofibrils are ruptured at the myotendon junction. Unusual empty pockets and granular material interrupt the filament lattice of adult fln(0) sarcomeres. Site-specific cleavage of myosin heavy chain occurs during this period. That myosin is cleaved in the absence of flightin is consistent with the immunolocalization of flightin on the thick filament and biochemical and genetic evidence suggesting it is associated with the myosin rod. Our results indicate that flightin is required for the establishment of normal thick filament length during late pupal development and thick filament stability in adult after initiation of contractile activity.
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Affiliation(s)
- Mary C. Reedy
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710
| | - Belinda Bullard
- European Molecular Biology Laboratory, Heidelberg 69012, Germany
| | - Jim O. Vigoreaux
- Department of Biology, University of Vermont, Burlington, Vermont 05405
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220
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Wu Y, Cazorla O, Labeit D, Labeit S, Granzier H. Changes in titin and collagen underlie diastolic stiffness diversity of cardiac muscle. J Mol Cell Cardiol 2000; 32:2151-62. [PMID: 11112991 DOI: 10.1006/jmcc.2000.1281] [Citation(s) in RCA: 169] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Small (N2B) and large (N2BA) cardiac titin isoforms are differentially expressed in a species-specific and heart location-specific manner. To understand how differential expression of titin isoforms may influence passive stiffness of cardiac muscle we investigated the mechanical properties of mouse left ventricular (MLV) wall muscle (expressing predominantly the small titin isoform), bovine left atrial (BLA) wall muscle (predominantly the large isoform), and bovine left ventricular (BLV) wall muscle (expressing small and large isoforms at similar levels). Results indicate that the overall passive muscle stiffness of the muscle types varies nearly ten-fold, with stiffness increasing in the following order: BLA, BLV and MLV. To investigate the basis of the variation in the overall muscle stiffness, the contributions of titin and collagen to muscle stiffness were determined. Results showed that increased muscle stiffness results from increases in both titin- and collagen-based passive stiffness, indicating that titin and collagen change in a co-ordinated fashion. The expression level of the small titin isoform correlates with titin's contribution to overall muscle stiffness, suggesting that differential expression of titin isoforms is an effective means to modulate the filling behavior of the heart.
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Affiliation(s)
- Y Wu
- Department of VCAPP, Washington State University, Pullman, WA 99164-6520, USA
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221
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Trombitás K, Redkar A, Centner T, Wu Y, Labeit S, Granzier H. Extensibility of isoforms of cardiac titin: variation in contour length of molecular subsegments provides a basis for cellular passive stiffness diversity. Biophys J 2000; 79:3226-34. [PMID: 11106626 PMCID: PMC1301197 DOI: 10.1016/s0006-3495(00)76555-6] [Citation(s) in RCA: 75] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
Abstract
Titin is a giant polypeptide that spans between the Z- and M-lines of the cardiac muscle sarcomere and that develops force when extended. This force arises from titin's extensible I-band region, which consists mainly of three segment types: serially linked immunoglobulin-like domains (Ig segments), interrupted by the PEVK segment, and the N2B unique sequence. Recently it was reported that the myocardium of large mammals co-expresses small (N2B) and large (N2BA) cardiac isoforms and that the passive stiffness of cardiac myocytes varies with the isoform expression ratio. To understand the molecular basis of the differences in passive stiffness we investigated titin's extensibility in bovine atrium, which expresses predominantly N2BA titin, and compared it to that of rat, which expresses predominantly N2B titin. Immunoelectron microscopy was used with antibodies that flank the Ig segments, the PEVK segment, and the unique sequence of the N2B element. The extension of the various segments was then determined as a function of sarcomere length (SL). When slack sarcomeres of bovine atrium were stretched, the PEVK segment extended much more steeply and the unique N2B sequence less steeply than in rat, while the Ig segments behaved similarly in both species. However, the extensions normalized with the segment's contour length (i.e., the fractional extensions) of Ig, PEVK, and unique sequence segments all increase less steeply with SL in cow than in rat. Considering that fractional extension determines the level of entropic force, these differences in fractional extension are expected to result in shallow and steep passive force-SL curves in myocytes that express high levels of N2BA and N2B titin, respectively. Thus, the findings provide a molecular basis for passive stiffness diversity.
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Affiliation(s)
- K Trombitás
- Department of Veterinary and Comparative Anatomy, Pharmacology, and Physiology, Washington State University, Pullman, Washington 99164-6520, USA
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222
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Person V, Kostin S, Suzuki K, Labeit S, Schaper J. Antisense oligonucleotide experiments elucidate the essential role of titin in sarcomerogenesis in adult rat cardiomyocytes in long-term culture. J Cell Sci 2000; 113 Pt 21:3851-9. [PMID: 11034912 DOI: 10.1242/jcs.113.21.3851] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
An essential role of titin as a molecular ruler in sarcomerogenesis has been frequently discussed. In this study, we tested the hypothesis that the expression of titin is a prerequisite for thick filament incorporation into sarcomeres by using an antisense oligonucleotide approach to interfere with titin translation in the de-/redifferentiation model of adult rat cardiomyocytes (ARC) in long-term culture. As a first step, the growth pattern ranging from rod shape to round and later to spreading cells and the cell surface area of ARC were quantitatively evaluated and standardized. This represents the basis for experiments interfering with sarcomere formation using three different antisense phosphorothioate oligonucleotides (S-ODN) at a dosage of 10 microM specific for titin mRNA. Presence of fluorescein labeled S-ODN in ARC indicated cellular uptake and both, antisense and random S-ODN, induced a significant increase in cell size as compared with control untreated ARC. At days 12 and 16 in culture, antisense S-ODN treatment resulted in reduced expression of titin and disturbance of myosin incorporation into sarcomeres, evident by diffuse myosin labeling and a significantly decreased area of regular myosin cross-striation (control 75%, day 12 S-ODN 20%, day 16 14%) shown by laser scanning confocal microscopy. Cellular integrity indicated by presence of alpha-actinin was not disturbed. These findings provide evidence for the role of titin as a template for myosin incorporation and therefore as a prerequisite for sarcomerogenesis.
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Affiliation(s)
- V Person
- Max-Planck-Institut für Physiologische und Klinische Forschung, Abteilung für Experimentelle Kardiologie, Bad Nauheim, Germany
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223
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Machado C, Andrew DJ. D-Titin: a giant protein with dual roles in chromosomes and muscles. J Cell Biol 2000; 151:639-52. [PMID: 11062264 PMCID: PMC2185597 DOI: 10.1083/jcb.151.3.639] [Citation(s) in RCA: 115] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2000] [Accepted: 09/20/2000] [Indexed: 11/28/2022] Open
Abstract
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.
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Affiliation(s)
- C Machado
- Department of Cell Biology and Anatomy, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205-2196, USA
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224
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Zatz M, Vainzof M, Passos-Bueno MR. Limb-girdle muscular dystrophy: one gene with different phenotypes, one phenotype with different genes. Curr Opin Neurol 2000; 13:511-7. [PMID: 11073356 DOI: 10.1097/00019052-200010000-00002] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Among 14 limb-girdle muscular dystrophy genes that have been mapped, 10 (three autosomal dominant and seven autosomal recessive) have so far had their product identified. This review will focus on the most recent data in the field and on our own experience of more than 200 patients studied with autosomal recessive-limb-girdle muscular dystrophy, classified from calpainopathy to telethoninopathy. Genotype: phenotype correlations in this highly heterogeneous group show a similar clinical course among patients with different forms, whereas a discordant phenotype may be seen in unrelated patients or in affected sibs carrying the same mutation. Understanding such similarities or differences remains a major challenge. It will depend on future knowledge of gene-protein functions, on protein interactions and on identifying modifying genes and other factors underlying clinical variability.
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Affiliation(s)
- M Zatz
- Departamento de Biologia, Instituto de Biociências, Universidade de São Paulo, Brazil.
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225
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Abstract
Muscular dystrophies represent a heterogeneous group of disorders, which have been largely classified by clinical phenotype. In the last 10 years, identification of novel skeletal muscle genes including extracellular matrix, sarcolemmal, cytoskeletal, cytosolic, and nuclear membrane proteins has changed the phenotype-based classification and shed new light on the molecular pathogenesis of these disorders. A large number of genes involved in muscular dystrophy encode components of the dystrophin-glycoprotein complex (DGC) which normally links the intracellular cytoskeleton to the extracellular matrix. Mutations in components of this complex are thought to lead to loss of sarcolemmal integrity and render muscle fibers more susceptible to damage. Recent evidence suggests the involvement of vascular smooth muscle DGC in skeletal and cardiac muscle pathology in some forms of sarcoglycan-deficient limb-girdle muscular dystrophy. Intriguingly, two other forms of limb-girdle muscular dystrophy are possibly caused by perturbation of sarcolemma repair mechanisms. The complete clarification of these various pathways will lead to further insights into the pathogenesis of this heterogeneous group of muscle disorders.
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Affiliation(s)
- R D Cohn
- Howard Hughes Medical Institute, Department of Physiology and Biophysics and of Neurology, University of Iowa College of Medicine, 400 EMRB, Iowa City, Iowa 52242, USA
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226
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Zhang Y, Featherstone D, Davis W, Rushton E, Broadie K. Drosophila D-titin is required for myoblast fusion and skeletal muscle striation. J Cell Sci 2000; 113 ( Pt 17):3103-15. [PMID: 10934048 DOI: 10.1242/jcs.113.17.3103] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
An ethylmethane sulfonate (EMS) mutagenesis of Drosophila melanogaster aimed at discovering novel genes essential for neuromuscular development identified six embryonic lethal alleles of one genetic locus on the third chromosome at 62C. Two additional lethal P element insertion lines, l(3)S02001 and l(3)j1D7, failed to complement each other and each of the six EMS alleles. Analysis of genomic sequence bracketing the two insertion sites predicted a protein of 16,215 amino acid residues, encoded by a 70 kb genomic region. This sequence includes the recently characterized kettin, and includes all known partial D-Titin sequences. We call the genetic locus, which encodes both D-Titin and kettin, D-Titin. D-Titin has 53 repeats of the immunoglobulin C2 domain, 6 repeats of the fibronectin type III domain and two large PEVK domains. Kettin appears to be the NH2-terminal one third of D-Titin, presumably expressed via alternative splicing. Phenotype assays on the allelic series of D-Titin mutants demonstrated that D-Titin plays an essential role in muscle development. First, D-Titin has an unsuspected function in myoblast fusion during myogenesis and, second, D-Titin later serves to organize myofilaments into the highly ordered arrays underlying skeletal muscle striation. We propose that D-Titin is instrumental in the development of the two defining features of striated muscle: the formation of multi-nucleate syncitia and the organization of actin-myosin filaments into striated arrays.
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Affiliation(s)
- Y Zhang
- Department of Biology, University of Utah, Salt Lake City, Utah 84112-0840, USA
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227
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Abstract
One of the most fascinating examples of cytoskeletal assembly is the myofibril, the contractile structure of striated (i.e. skeletal and cardiac) muscle. Myofibrils are composed of repeating contractile units known as sarcomeres, perhaps the most highly ordered macromolecular structures in eukaryotic cells. When skeletal and cardiac muscle cells differentiate, thousands of structural and regulatory molecules assemble into the semicrystalline sarcomeric contractile units. As a consequence of this precise assembly, many different classes of proteins function together to convert the molecular interactions of actin and myosin efficiently into the macroscopic movements of contractile activity.
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Affiliation(s)
- C C Gregorio
- Dept of Cell Biology and Anatomy, The University of Arizona, Tucson, AZ, USA.
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228
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Ojima K, Lin Z, Bang ML, Holtzer S, Matsuda R, Labeit S, Sweeney H, Holtzer H. Distinct families of Z-line targeting modules in the COOH-terminal region of nebulin. J Cell Biol 2000; 150:553-66. [PMID: 10931867 PMCID: PMC2175182 DOI: 10.1083/jcb.150.3.553] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
Abstract
To learn how nebulin functions in the assembly and maintenance of I-Z-I bands, MYC- and GFP- tagged nebulin fragments were expressed in primary cultured skeletal myotubes. Their sites of incorporation were visualized by double staining with anti-MYC, antibodies to myofibrillar proteins, and FITC- or rhodamine phalloidin. Contrary to expectations based on in vitro binding studies, none of the nebulin fragments expressed in maturing myotubes were incorporated selectively into I-band approximately 1.0-micrometer F-alpha-actin-containing thin filaments. Four of the MYC/COOH-terminal nebulin fragments were incorporated exclusively into periodic approximately 0.1-micrometer Z-bands. Whereas both anti-MYC and Rho-phalloidin stained intra-Z-band F-alpha-actin oligomers, only the latter stained the pointed ends of the polarized approximately 1.0-micrometer thin filaments. Z-band incorporation was independent of the nebulin COOH-terminal Ser or SH3 domains. In vitro cosedimentation studies also demonstrated that nebulin SH3 fragments did not bind to F-alpha-actin or alpha-actinin. The remaining six fragments were not incorporated into Z-bands, but were incorporated (a) diffusely throughout the sarcoplasm and into (b) fibrils/patches of varying lengths and widths nested among normal striated myofibrils. Over time, presumably in response to the mediation of muscle-specific homeostatic controls, many of the ectopic MYC-positive structures were resorbed. None of the tagged nebulin fragments behaved as dominant negatives; they neither blocked the assembly nor induced the disassembly of mature striated myofibrils. Moreover, they were not cytotoxic in myotubes, as they were in the fibroblasts and presumptive myoblasts in the same cultures.
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Affiliation(s)
- K. Ojima
- Department of Cell and Developmental Biology, The School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104
| | - Z.X. Lin
- Department of Cell Biology, Beijing Institute for Cancer Research, Beijing Medical University, Beijing 100034, China
| | | | - S. Holtzer
- Department of Cell and Developmental Biology, The School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104
| | - R. Matsuda
- Department of Life Science, University of Tokyo, Tokyo, Japan 153-8092
| | - S. Labeit
- Department of Anesthesiology and Intensive Operative Care, Klinikum, Mannheim, Germany
| | - H.L. Sweeney
- Department of Physiology, The School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104
| | - H. Holtzer
- Department of Cell and Developmental Biology, The School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104
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229
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Champagne MB, Edwards KA, Erickson HP, Kiehart DP. Drosophila stretchin-MLCK is a novel member of the Titin/Myosin light chain kinase family. J Mol Biol 2000; 300:759-77. [PMID: 10891286 DOI: 10.1006/jmbi.2000.3802] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
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.
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Affiliation(s)
- M B Champagne
- Department of Cell Biology, Duke University Medical Center, Durham, NC, 27710-0001, USA
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230
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Johnson RC, Penzes P, Eipper BA, Mains RE. Isoforms of kalirin, a neuronal Dbl family member, generated through use of different 5'- and 3'-ends along with an internal translational initiation site. J Biol Chem 2000; 275:19324-33. [PMID: 10777487 DOI: 10.1074/jbc.m000676200] [Citation(s) in RCA: 83] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Kalirin is a neuron-specific GDP/GTP exchange factor for Rho subfamily GTP-binding proteins. The major Kalirin transcripts in adult rat brain were identified. Most include a Sec14p-like putative lipid-binding motif followed by nine spectrin-like repeats and a Dbl homology/pleckstrin homology (DH-PH) domain. Kalirin proteins with four different NH(2) termini are generated through the use of five different 5'-ends; three of the proteins differ only at the extreme NH(2) terminus, and one is truncated because translation is initiated at a methionine in the 5th spectrin repeat. Four different 3'-ends yield Kalirin proteins with additional functional domains. Kalirin-7 (7-kilobase pair mRNA) terminates with a PDZ-binding motif, which in Kalirin-8 is replaced by an SH3 domain. Kalirin-9 contains another pair of DH-PH and SH3 domains. Kalirin-12 additionally encodes a putative Ser/Thr protein kinase. Antisera specific for different COOH termini established Kalirin-7 as the most abundant in cortex, with significant amounts of Kalirin-9 and Kalirin-12; Kalirin-7 was less prevalent in cerebellum and olfactory bulb. Kalirin proteins lacking the Sec14p-like domain and first four spectrin-like repeats were much less prevalent. Form-specific antisera demonstrated that different forms of Kalirin were localized to distinct subcellular regions of cultured neurons. Members of the family of Kalirin proteins may subserve different functions at these different locations.
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Affiliation(s)
- R C Johnson
- Department of Neuroscience, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
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231
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Weiwad WK, Linke WA, Wussling MH. Sarcomere length-tension relationship of rat cardiac myocytes at lengths greater than optimum. J Mol Cell Cardiol 2000; 32:247-59. [PMID: 10722801 DOI: 10.1006/jmcc.1999.1069] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The study was aimed at determining both passive and Ca(2+)-activated forces of single skinned rat cardiac cells. Particular attention was paid to the descending limb of the active length-tension curve while the sarcomeric order of stretched cells was investigated before and during contraction. To analyse sarcomere length and sarcomere-length inhomogeneity, a fast Fourier transform (FFT) was employed. The fundamental frequency in the FFT spectrum is a measure of sarcomere length. The full-width-half-maximum of the first-order line is a measure of sarcomere-length inhomogeneity. In relaxing buffer, the sarcomere-length inhomogeneity of skinned cells increased linearly with mean sarcomere length. Upon Ca(2+)-dependent activation of skinned cells contracting isometrically, mean sarcomere length decreased slightly and inhomogeneity increased; both effects were greater at higher Ca(2+)concentrations. Maximum activation was reached at sarcomere lengths between 2.2 and 2.4 microm, whereas the descending limb of the active length-tension curve approached zero force already at approximately 2.8 microm. This steep force decline could not be explained by overly inhomogeneous sarcomere lengths in very long, contracting cells. Rather, the results of mechanical measurements on single cardiac myofibrils implied that high stretching is accompanied by irreversible structural alterations within cardiac sarcomeres, most likely thick-filament disarray and disruption of binding sites between myosin and titin due to changes in titin's tertiary structure. Loss of a regular thick-filament organization may then impair active force generation. We conclude that the descending limb of the cardiac length-tension curve is determined both by the degree of actin-myosin overlap and by the intrinsic properties of titin filaments.
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Affiliation(s)
- W K Weiwad
- Julius Bernstein Institute of Physiology, Martin Luther University Halle-Wittenberg, Halle, D-06097, Germany
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232
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Moreira ES, Wiltshire TJ, Faulkner G, Nilforoushan A, Vainzof M, Suzuki OT, Valle G, Reeves R, Zatz M, Passos-Bueno MR, Jenne DE. Limb-girdle muscular dystrophy type 2G is caused by mutations in the gene encoding the sarcomeric protein telethonin. Nat Genet 2000; 24:163-6. [PMID: 10655062 DOI: 10.1038/72822] [Citation(s) in RCA: 246] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Autosomal recessive limb-girdle muscular dystrophies (AR LGMDs) are a genetically heterogeneous group of disorders that affect mainly the proximal musculature. There are eight genetically distinct forms of AR LGMD, LGMD 2A-H (refs 2-10), and the genetic lesions underlying these forms, except for LGMD 2G and 2H, have been identified. LGMD 2A and LGMD 2B are caused by mutations in the genes encoding calpain 3 (ref. 11) and dysferlin, respectively, and are usually associated with a mild phenotype. Mutations in the genes encoding gamma-(ref. 14), alpha-(ref. 5), beta-(refs 6,7) and delta (ref. 15)-sarcoglycans are responsible for LGMD 2C to 2F, respectively. Sarcoglycans, together with sarcospan, dystroglycans, syntrophins and dystrobrevin, constitute the dystrophin-glycoprotein complex (DGC). Patients with LGMD 2C-F predominantly have a severe clinical course. The LGMD 2G locus maps to a 3-cM interval in 17q11-12 in two Brazilian families with a relatively mild form of AR LGMD (ref. 9). To positionally clone the LGMD 2G gene, we constructed a physical map of the 17q11-12 region and refined its localization to an interval of 1.2 Mb. The gene encoding telethonin, a sarcomeric protein, lies within this candidate region. We have found that mutations in the telethonin gene cause LGMD 2G, identifying a new molecular mechanism for AR LGMD.
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Affiliation(s)
- E S Moreira
- [1] Centro de Estudos do Genoma Humano, Universidade de São Paulo, São Paulo, Brazil
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233
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Luther PK. Three-dimensional structure of a vertebrate muscle Z-band: implications for titin and alpha-actinin binding. J Struct Biol 2000; 129:1-16. [PMID: 10675292 DOI: 10.1006/jsbi.1999.4207] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The Z-band in vertebrate striated muscles, mainly comprising actin filaments, alpha-actinin, and titin, serves to organise the antiparallel actin filament arrays in adjacent sarcomeres and to transmit tension between sarcomeres during activation. Different Z-band thicknesses, formed from different numbers of zigzag crosslinking layers and found in different fibre types, are thought to be associated with the number of repetitive N-terminal sequence domains of titin. In order to understand myofibril formation it is necessary to correlate the ultrastructures and sequences of the actin filaments, titin, and alpha-actinin in characteristic Z-bands. Here electron micrographs of the intermediate width, basketweave Z-band of plaice fin muscle have been subject to a novel 3D reconstruction process. The reconstruction shows that antiparallel actin filaments overlap in the Z-band by about 22-25 nm. There are three levels of Z-links (probably alpha-actinin) in which at each level two nearly diametrically opposed links join an actin filament to two of its antiparallel neighbours. One set of links is centrally located in the Z-band and there are flanking levels orthogonal to this. A 3D model of the observed structure shows how Z-bands of different widths may be formed and it provides insights into the structural arrangements of titin and alpha-actinin in the Z-band. The model shows that the two observed symmetries in different Z-bands, c2 and p12(1), may be attributed respectively to whether the number of Z-link levels is odd or even.
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Affiliation(s)
- P K Luther
- Biophysics Group, Blackett Laboratory, Imperial College, Exhibition Road, London, SW7 2AZ, United Kingdom.
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234
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Cazorla O, Freiburg A, Helmes M, Centner T, McNabb M, Wu Y, Trombitás K, Labeit S, Granzier H. Differential expression of cardiac titin isoforms and modulation of cellular stiffness. Circ Res 2000; 86:59-67. [PMID: 10625306 DOI: 10.1161/01.res.86.1.59] [Citation(s) in RCA: 291] [Impact Index Per Article: 12.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Extension of the I-band segment of titin gives rise to part of the diastolic force of cardiac muscle. Previous studies of human cardiac titin transcripts suggested a series of differential splicing events in the I-band segment of titin leading to the so-called N2A and N2B isoform transcripts. Here we investigated titin expression at the protein level in a wide range of mammalian species. Results indicate that the myocardium coexpresses 2 distinct titin isoforms: a smaller isoform containing the N2B element only (N2B titin) and a larger isoform with both the N2B and N2A elements (N2BA titin). The expression ratio of large N2BA to small N2B titin isoforms was found to vary greatly in different species; eg, in the left ventricle the ratio is approximately 0.05 in mouse and approximately 1.5 in pig. Differences in the expression ratio were also found between atria and ventricles and between different layers of the ventricular wall. Immunofluorescence experiments with isoform-specific antibodies suggest that coexpression of these isoforms takes place at the single-myocyte level. The diastolic properties of single cardiac myocytes isolated from various species expressing high levels of the small (rat and mouse) or large (pig) titin isoform were studied. On average, pig myocytes are significantly less stiff than mouse and rat myocytes. Gel analysis indicates that this result cannot be explained by varying amounts of titin in mouse and pig myocardium. Rather, low stiffness of pig myocytes can be explained by its high expression level of the large isoform: the longer extensible region of this isoform results in a lower fractional extension for a given sarcomere length and hence a lower force. Implications of our findings to cardiac function are discussed.
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Affiliation(s)
- O Cazorla
- Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Washington State University, Pullman, Wash., USA
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235
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Trombitás K, Freiburg A, Centner T, Labeit S, Granzier H. Molecular dissection of N2B cardiac titin's extensibility. Biophys J 1999; 77:3189-96. [PMID: 10585940 PMCID: PMC1300589 DOI: 10.1016/s0006-3495(99)77149-3] [Citation(s) in RCA: 81] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
Titin is a giant filamentous polypeptide of multidomain construction spanning between the Z- and M-lines of the cardiac muscle sarcomere. Extension of the I-band segment of titin gives rise to a force that underlies part of the diastolic force of cardiac muscle. Titin's force arises from its extensible I-band region, which consists of two main segment types: serially linked immunoglobulin-like domains (tandem Ig segments) interrupted with a proline (P)-, glutamate (E)-, valine (V)-, and lysine (K)-rich segment called PEVK segment. In addition to these segments, the extensible region of cardiac titin also contains a unique 572-residue sequence that is part of the cardiac-specific N2B element. In this work, immunoelectron microscopy was used to study the molecular origin of the in vivo extensibility of the I-band region of cardiac titin. The extensibility of the tandem Ig segments, the PEVK segment, and that of the unique N2B sequence were studied, using novel antibodies against Ig domains that flank these segments. Results show that only the tandem Igs extend at sarcomere lengths (SLs) below approximately 2.0 microm, and that, at longer SLs, the PEVK and the unique sequence extend as well. At the longest SLs that may be reached under physiological conditions ( approximately 2.3 microm), the PEVK segment length is approximately 50 nm whereas the unique N2B sequence is approximately 80 nm long. Thus, the unique sequence provides additional extensibility to cardiac titins and this may eliminate the necessity for unfolding of Ig domains under physiological conditions. In summary, this work provides direct evidence that the three main molecular subdomains of N2B titin are all extensible and that their contribution to extensibility decreases in the order of tandem Igs, unique N2B sequence, and PEVK segment.
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Affiliation(s)
- K Trombitás
- Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Washington State University, Pullman, Washington 99164-6520, USA
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236
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Abstract
Recent studies of the giant protein titin have shed light on its roles in muscle assembly and elasticity and include the surprising findings described here. We now know that the titin kinase domain, which has long been a puzzle, has a novel regulation mechanism. A substrate, telethonin, has been identified that is located over one micron away from the kinase domain in mature muscle. Single-molecule studies have demonstrated the fascinating process of reversible mechanical unfolding of titin. Lastly, and most surprisingly, it has been claimed that titin controls assembly and elasticity in chromosomes.
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Affiliation(s)
- J Trinick
- School of Biomedical Sciences, Leeds University, Leeds, UK LS2 9JT.
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237
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Djinović-Carugo K, Young P, Gautel M, Saraste M. Structure of the alpha-actinin rod: molecular basis for cross-linking of actin filaments. Cell 1999; 98:537-46. [PMID: 10481917 DOI: 10.1016/s0092-8674(00)81981-9] [Citation(s) in RCA: 206] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
We have determined the crystal structure of the two central repeats in the alpha-actinin rod at 2.5 A resolution. The repeats are connected by a helical linker and form a symmetric, antiparallel dimer in which the repeats are aligned rather than staggered. Using this structure, which reveals the structural principle that governs the architecture of alpha-actinin, we have devised a plausible model of the entire alpha-actinin rod. The electrostatic properties explain how the two alpha-actinin subunits assemble in an antiparallel fashion, placing the actin-binding sites at both ends of the rod. This molecular architecture results in a protein that is able to form cross-links between actin filaments.
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Affiliation(s)
- K Djinović-Carugo
- Structural Biology Programme, European Molecular Biology Laboratory, Heidelberg, Germany
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238
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Linke WA, Rudy DE, Centner T, Gautel M, Witt C, Labeit S, Gregorio CC. I-band titin in cardiac muscle is a three-element molecular spring and is critical for maintaining thin filament structure. J Cell Biol 1999; 146:631-44. [PMID: 10444071 PMCID: PMC2150553 DOI: 10.1083/jcb.146.3.631] [Citation(s) in RCA: 184] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/1999] [Accepted: 07/08/1999] [Indexed: 11/22/2022] Open
Abstract
In cardiac muscle, the giant protein titin exists in different length isoforms expressed in the molecule's I-band region. Both isoforms, termed N2-A and N2-B, comprise stretches of Ig-like modules separated by the PEVK domain. Central I-band titin also contains isoform-specific Ig-motifs and nonmodular sequences, notably a longer insertion in N2-B. We investigated the elastic behavior of the I-band isoforms by using single-myofibril mechanics, immunofluorescence microscopy, and immunoelectron microscopy of rabbit cardiac sarcomeres stained with sequence-assigned antibodies. Moreover, we overexpressed constructs from the N2-B region in chick cardiac cells to search for possible structural properties of this cardiac-specific segment. We found that cardiac titin contains three distinct elastic elements: poly-Ig regions, the PEVK domain, and the N2-B sequence insertion, which extends approximately 60 nm at high physiological stretch. Recruitment of all three elements allows cardiac titin to extend fully reversibly at physiological sarcomere lengths, without the need to unfold Ig domains. Overexpressing the entire N2-B region or its NH(2) terminus in cardiac myocytes greatly disrupted thin filament, but not thick filament structure. Our results strongly suggest that the NH(2)-terminal N2-B domains are necessary to stabilize thin filament integrity. N2-B-titin emerges as a unique region critical for both reversible extensibility and structural maintenance of cardiac myofibrils.
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Affiliation(s)
- W A Linke
- Physiologisches Institut II, Universität Heidelberg, D-69120 Heidelberg, Germany.
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239
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Fraternali F, Pastore A. Modularity and homology: modelling of the type II module family from titin. J Mol Biol 1999; 290:581-93. [PMID: 10390355 DOI: 10.1006/jmbi.1999.2876] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We report the homology modelling of the structures of the 162 type II modules from the giant multi-domain protein titin (also known as connectin). The package MODELLER was used and implemented in an automated fashion using four experimentally determined structures as templates. Validation of the models was assessed in terms of divergence from the templates and consensus of the alignments. The homology within the whole family of type II modules as well as with the templates is relatively high (20-35% identity and ca 50% similarity). Comparison between the models of domains for which an NMR structure has been solved and the experimental solution gives an estimate of the quality of the modelling. Our results allow us to distinguish between a set of structurally relevant residues, which are conserved throughout the whole family and buried in the hydrophobic core, from the residues that are conserved and exposed. These latter residues are potentially functionally important. Comparison of exposed conserved patches for modules in different regions of the titin molecule suggests potential interaction surfaces. Our results may be tested directly for those modules whose binding partner is known.
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240
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Helmes M, Trombitás K, Centner T, Kellermayer M, Labeit S, Linke WA, Granzier H. Mechanically driven contour-length adjustment in rat cardiac titin's unique N2B sequence: titin is an adjustable spring. Circ Res 1999; 84:1339-52. [PMID: 10364572 DOI: 10.1161/01.res.84.11.1339] [Citation(s) in RCA: 134] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The giant elastic protein titin is largely responsible for passive forces in cardiac myocytes. A number of different titin isoforms with distinctly different structural elements within their central I-band region are expressed in human myocardium. Their coexpression has so far prevented an understanding of the respective contributions of the isoforms to myocardial elasticity. Using isoform-specific antibodies, we find in the present study that rat myocardium expresses predominantly the small N2B titin isoform, which allows us to characterize the elastic behavior of this isoform. The extensibility and force response of N2B titin were studied by using immunoelectron microscopy and by measuring the passive force-sarcomere length (SL) relation of single rat cardiac myocytes under a variety of mechanical conditions. Experimental results were compared with the predictions of a mechanical model in which the elastic titin segment behaves as two wormlike chains, the tandem immunoglobulin (Ig) segments and the PEVK segment (rich in proline [P], glutamate [E], valine [V], and lysine [K] residues), connected in series. The overall contour length was predicted from the sequence of N2B cardiac titin. According to mechanical measurements, above approximately 2.2 microm SL titin's elastic segment extends beyond its predicted contour length. Immunoelectron microscopy indicates that a prominent source of this contour-length gain is the extension of the unique N2B sequence (located between proximal tandem Ig segment and PEVK), and that Ig domain unfolding is negligible. Thus, the elastic region of N2B cardiac titin consists of three mechanically distinct extensible segments connected in series: the tandem Ig segment, the PEVK segment, and the unique N2B sequence. Rate-dependent and repetitive stretch-release experiments indicate that both the contour-length gain and the recovery from it involve kinetic processes, probably unfolding and refolding within the N2B segment. As a result, the contour length of titin's extensible segment depends on the rate and magnitude of the preceding mechanical perturbations. The rate of recovery from the length gain is slow, ensuring that the adjusted length is maintained through consecutive cardiac cycles and that hysteresis is minimal. Thus, as a result of the extensible properties of the unique N2B sequence, the I-band region of the N2B cardiac titin isoform functions as a molecular spring that is adjustable.
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Affiliation(s)
- M Helmes
- Department of Veterinary and Comparative Anatomy, Pharmacology, and Physiology, Washington State University, Pullman, WA, USA
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241
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Almenar-Queralt A, Gregorio CC, Fowler VM. Tropomodulin assembles early in myofibrillogenesis in chick skeletal muscle: evidence that thin filaments rearrange to form striated myofibrils. J Cell Sci 1999; 112 ( Pt 8):1111-23. [PMID: 10085247 DOI: 10.1242/jcs.112.8.1111] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Actin filament lengths in muscle and nonmuscle cells are believed to depend on the regulated activity of capping proteins at both the fast growing (barbed) and slow growing (pointed) filament ends. In striated muscle, the pointed end capping protein, tropomodulin, has been shown to maintain the lengths of thin filaments in mature myofibrils. To determine whether tropomodulin might also be involved in thin filament assembly, we investigated the assembly of tropomodulin into myofibrils during differentiation of primary cultures of chick skeletal muscle cells. Our results show that tropomodulin is expressed early in differentiation and is associated with the earliest premyofibrils which contain overlapping and misaligned actin filaments. In addition, tropomodulin can be found in actin filament bundles at the distal tips of growing myotubes, where sarcomeric alpha-actinin is not always detected, suggesting that tropomodulin caps actin filament pointed ends even before the filaments are cross-linked into Z bodies by alpha-actinin. Tropomodulin staining exhibits an irregular punctate pattern along the length of premyofibrils that demonstrate a smooth phalloidin staining pattern for F-actin. Strikingly, the tropomodulin dots often appear to be located between the closely spaced, dot-like Z bodies that are stained for (α)-actinin. Thus, in the earliest premyofibrils, the pointed ends of the thin filaments are clustered and partially aligned with respect to the Z bodies (the location of the barbed filament ends). At later stages of differentiation, the tropomodulin dots become aligned into regular periodic striations concurrently with the appearance of striated phalloidin staining for F-actin and alignment of Z bodies into Z lines. Tropomodulin, together with the barbed end capping protein, CapZ, may function from the earliest stages of myofibrillogenesis to restrict the lengths of newly assembled thin filaments by capping their ends; thus, transitions from nonstriated to striated myofibrils in skeletal muscle are likely due principally to filament rearrangements rather than to filament polymerization or depolymerization. Rearrangements of actin filaments capped at their pointed and barbed ends may be a general mechanism by which cells restructure their actin cytoskeletal networks during cell growth and differentiation.
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Affiliation(s)
- A Almenar-Queralt
- Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037, USA
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