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Weston TGR, Rees M, Gautel M, Fraternali F. Walking with giants: The challenges of variant impact assessment in the giant sarcomeric protein titin. WIREs Mech Dis 2024; 16:e1638. [PMID: 38155593 DOI: 10.1002/wsbm.1638] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2023] [Revised: 12/01/2023] [Accepted: 12/04/2023] [Indexed: 12/30/2023]
Abstract
Titin, the so-called "third filament" of the sarcomere, represents a difficult challenge for the determination of damaging genetic variants. A single titin molecule extends across half the length of a sarcomere in striated muscle, fulfilling a variety of vital structural and signaling roles, and has been linked to an equally varied range of myopathies, resulting in a significant burden on individuals and healthcare systems alike. While the consequences of truncating variants of titin are well-documented, the ramifications of the missense variants prevalent in the general population are less so. We here present a compendium of titin missense variants-those that result in a single amino-acid substitution in coding regions-reported to be pathogenic and discuss these in light of the nature of titin and the variant position within the sarcomere and their domain, the structural, pathological, and biophysical characteristics that define them, and the methods used for characterization. Finally, we discuss the current knowledge and integration of the multiple fields that have contributed to our understanding of titin-related pathology and offer suggestions as to how these concurrent methodologies may aid the further development in our understanding of titin and hopefully extend to other, less well-studied giant proteins. This article is categorized under: Cardiovascular Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Molecular and Cellular Physiology.
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Affiliation(s)
- Timir G R Weston
- Randall Centre for Cell & Molecular Biophysics, King's College London, London, UK
| | - Martin Rees
- Randall Centre for Cell & Molecular Biophysics, King's College London, London, UK
| | - Mathias Gautel
- Randall Centre for Cell & Molecular Biophysics, King's College London, London, UK
| | - Franca Fraternali
- Institute of Structural and Molecular Biology, University College London, London, UK
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2
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Herzog W, Schappacher-Tilp G. Molecular mechanisms of muscle contraction: A historical perspective. J Biomech 2023; 155:111659. [PMID: 37290181 DOI: 10.1016/j.jbiomech.2023.111659] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2023] [Accepted: 05/22/2023] [Indexed: 06/10/2023]
Abstract
Studies of muscle structure and function can be traced to at least 2,000 years ago. However, the modern era of muscle contraction mechanisms started in the 1950s with the classic works by AF Huxley and HE Huxley, both born in the United Kingdom, but not related and working independently. HE Huxley was the first to suggest that muscle contraction occurred through the sliding of two sets of filamentous structures (actin or thin filaments and myosin or thick filaments). AF Huxley then developed a biologically inspired mathematical model suggesting a possible molecular mechanism of how this sliding of actin and myosin might take place. This model then evolved from a two-state to a multi-state model of myosin-actin interactions, and from one that suggested a linear motor causing the sliding to a rotating motor. This model, the cross-bridge model of muscle contraction, is still widely used in biomechanics, and even the more sophisticated cross-bridge models of today still contain many of the features originally proposed by AF Huxley. In 2002, we discovered a hitherto unknown property of muscle contraction that suggested the involvement of passive structures in active force production, the so-called passive force enhancement. It was quickly revealed that this passive force enhancement was caused by the filamentous protein titin, and the three-filament (actin, myosin, and titin) sarcomere model of muscle contraction evolved. There are many suggestions of how these three proteins interact to cause contraction and produce active force, and one such suggestion is described here, but the molecular details of this proposed mechanism still need careful evaluation.
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Affiliation(s)
- Walter Herzog
- Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, Canada.
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3
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Abstract
Muscle has conventionally been viewed as a motor that converts chemical to kinetic energy in series with a passive spring, but new insights emerge when muscle is viewed as a composite material whose elastic elements are tuned by activation. New evidence demonstrates that calcium-dependent binding of N2A titin to actin increases titin stiffness in active skeletal muscles, which explains many long-standing enigmas of muscle physiology.
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Affiliation(s)
- Kiisa Nishikawa
- Department of Biological Sciences, Northern Arizona University, Flagstaff, Arizona
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4
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Williams CD, Holt NC. Spatial Scale and Structural Heterogeneity in Skeletal Muscle Performance. Integr Comp Biol 2018; 58:163-173. [DOI: 10.1093/icb/icy057] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Affiliation(s)
- C D Williams
- Allen Institute for Cell Science, 615 Westlake Ave N, Seattle, WA 98109, USA
| | - N C Holt
- Department of Biology, Northern Arizona University, S. San Francisco Street, Flagstaff, AZ 86011, USA
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5
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Lindstedt S, Nishikawa K. Huxleys’ Missing Filament: Form and Function of Titin in Vertebrate Striated Muscle. Annu Rev Physiol 2017; 79:145-166. [DOI: 10.1146/annurev-physiol-022516-034152] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Affiliation(s)
- Stan Lindstedt
- Center for Bioengineering Innovation, Northern Arizona University, Flagstaff, Arizona 86011-4185
| | - Kiisa Nishikawa
- Department of Biological Sciences, Northern Arizona University, Flagstaff, Arizona 86011-4185;
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6
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Schappacher-Tilp G, Leonard T, Desch G, Herzog W. Computing Average Passive Forces in Sarcomeres in Length-Ramp Simulations. PLoS Comput Biol 2016; 12:e1004904. [PMID: 27276390 PMCID: PMC4898704 DOI: 10.1371/journal.pcbi.1004904] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2015] [Accepted: 04/05/2016] [Indexed: 01/12/2023] Open
Abstract
Passive forces in sarcomeres are mainly related to the giant protein titin. Titin’s extensible region consists of spring-like elements acting in series. In skeletal muscles these elements are the PEVK segment, two distinct immunoglobulin (Ig) domain regions (proximal and distal), and a N2A portion. While distal Ig domains are thought to form inextensible end filaments in intact sarcomeres, proximal Ig domains unfold in a force- and time-dependent manner. In length-ramp experiments of single titin strands, sequential unfolding of Ig domains leads to a typical saw-tooth pattern in force-elongation curves which can be simulated by Monte Carlo simulations. In sarcomeres, where more than a thousand titin strands are arranged in parallel, numerous Monte Carlo simulations are required to estimate the resultant force of all titin filaments based on the non-uniform titin elongations. To simplify calculations, the stochastic model of passive forces is often replaced by linear or non-linear deterministic and phenomenological functions. However, new theories of muscle contraction are based on the hypothesized binding of titin to the actin filament upon activation, and thereby on a prominent role of the structural properties of titin. Therefore, these theories necessitate a detailed analysis of titin forces in length-ramp experiments. In our study we present a simple and efficient alternative to Monte Carlo simulations. Based on a structural titin model, we calculate the exact probability distributions of unfolded Ig domains under length-ramp conditions needed for rigorous analysis of expected forces, distribution of unfolding forces, etc. Due to the generality of our model, the approach is applicable to a wide range of stochastic protein unfolding problems. We provide a simple and stable algorithm to determine the exact solution of passive forces in a half sarcomere in length-ramp simulations. The approach is applicable to a wide range of stochastic models of protein unfolding.
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Affiliation(s)
- Gudrun Schappacher-Tilp
- Department for Mathematics and Computational Sciences, University of Graz, Graz, Austria
- * E-mail:
| | - Timothy Leonard
- Human Performance Laboratory, University of Calgary, Calgary, Canada
| | - Gertrud Desch
- Department for Mathematics and Computational Sciences, University of Graz, Graz, Austria
| | - Walter Herzog
- Human Performance Laboratory, University of Calgary, Calgary, Canada
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7
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Schappacher-Tilp G, Leonard T, Desch G, Herzog W. A novel three-filament model of force generation in eccentric contraction of skeletal muscles. PLoS One 2015; 10:e0117634. [PMID: 25816319 PMCID: PMC4376863 DOI: 10.1371/journal.pone.0117634] [Citation(s) in RCA: 70] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2014] [Accepted: 12/29/2014] [Indexed: 12/18/2022] Open
Abstract
We propose and examine a three filament model of skeletal muscle force generation, thereby extending classical cross-bridge models by involving titin-actin interaction upon active force production. In regions with optimal actin-myosin overlap, the model does not alter energy and force predictions of cross-bridge models for isometric contractions. However, in contrast to cross-bridge models, the three filament model accurately predicts history-dependent force generation in half sarcomeres for eccentric and concentric contractions, and predicts the activation-dependent forces for stretches beyond actin-myosin filament overlap.
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Affiliation(s)
| | - Timothy Leonard
- Human Performance Laboratory, University of Calgary, Calgary, Alberta, Canada
| | - Gertrud Desch
- Department of Mathematics and Scientific Computing, University of Graz, 8010 Graz, Austria
| | - Walter Herzog
- Human Performance Laboratory, University of Calgary, Calgary, Alberta, Canada
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8
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Nishikawa KC, Monroy JA, Uyeno TE, Yeo SH, Pai DK, Lindstedt SL. Is titin a 'winding filament'? A new twist on muscle contraction. Proc Biol Sci 2011; 279:981-90. [PMID: 21900329 DOI: 10.1098/rspb.2011.1304] [Citation(s) in RCA: 130] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Recent studies have demonstrated a role for the elastic protein titin in active muscle, but the mechanisms by which titin plays this role remain to be elucidated. In active muscle, Ca(2+)-binding has been shown to increase titin stiffness, but the observed increase is too small to explain the increased stiffness of parallel elastic elements upon muscle activation. We propose a 'winding filament' mechanism for titin's role in active muscle. First, we hypothesize that Ca(2+)-dependent binding of titin's N2A region to thin filaments increases titin stiffness by preventing low-force straightening of proximal immunoglobulin domains that occurs during passive stretch. This mechanism explains the difference in length dependence of force between skeletal myofibrils and cardiac myocytes. Second, we hypothesize that cross-bridges serve not only as motors that pull thin filaments towards the M-line, but also as rotors that wind titin on the thin filaments, storing elastic potential energy in PEVK during force development and active stretch. Energy stored during force development can be recovered during active shortening. The winding filament hypothesis accounts for force enhancement during stretch and force depression during shortening, and provides testable predictions that will encourage new directions for research on mechanisms of muscle contraction.
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Affiliation(s)
- Kiisa C Nishikawa
- Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011-5640, USA.
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9
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Roles of titin in the structure and elasticity of the sarcomere. J Biomed Biotechnol 2010; 2010:612482. [PMID: 20625501 PMCID: PMC2896707 DOI: 10.1155/2010/612482] [Citation(s) in RCA: 59] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2010] [Accepted: 05/13/2010] [Indexed: 11/18/2022] Open
Abstract
The giant protein titin is thought to play major roles in the assembly and function of muscle sarcomeres. Structural details, such as widths of Z- and M-lines and periodicities in the thick filaments, correlate with the substructure in the respective regions of the titin molecule. Sarcomere rest length, its operating range of lengths, and passive elastic properties are also directly controlled by the properties of titin. Here we review some recent titin data and discuss its implications for sarcomere architecture and elasticity.
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10
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Houmeida A, Baron A, Keen J, Khan GN, Knight PJ, Stafford WF, Thirumurugan K, Thompson B, Tskhovrebova L, Trinick J. Evidence for the oligomeric state of 'elastic' titin in muscle sarcomeres. J Mol Biol 2008; 384:299-312. [PMID: 18824175 DOI: 10.1016/j.jmb.2008.09.030] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2008] [Revised: 08/30/2008] [Accepted: 09/08/2008] [Indexed: 10/21/2022]
Abstract
The giant protein titin has important roles in muscle sarcomere integrity, elasticity and contractile activity. The key role in elasticity was highlighted in recent years by single-molecule mechanical studies, which showed a direct relationship between the non-uniform structure of titin and the hierarchical mechanism of its force-extension behavior. Further advances in understanding mechanisms controlling sarcomere structure and elasticity require detailed knowledge of titin arrangement and interactions in situ. Here we present data on the structure and self-interactive properties of an approximately 290 kDa ( approximately 100 nm long) tryptic fragment from the I-band part of titin that is extensible in situ. The fragment includes the conserved 'distal' tandem Ig segment of the molecule and forms side-by-side oligomers with distinctive 4 nm cross-striations. Comparisons between these oligomers and the end filaments seen at the tips of native thick filaments indicate identical structure. This shows that end-filaments are formed by the elastic parts of six titin molecules connecting each end of the thick filament to the Z-line. Self-association of elastic titin into stiff end-filaments adds a further hierarchical level in the mechanism of titin extensibility in muscle cells. Self-association of this part of titin may be required to prevent interference of the individual flexible molecules with myosin cross-bridges interacting with actin.
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Affiliation(s)
- Ahmed Houmeida
- Institute for Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT, UK
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11
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Tskhovrebova L, Houmeida A, Trinick J. Can the passive elasticity of muscle be explained directly from the mechanics of individual titin molecules? J Muscle Res Cell Motil 2007; 26:285-9. [PMID: 16465473 DOI: 10.1007/s10974-005-9034-5] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Recent progress in understanding the role of titin/connectin in muscle elasticity has been heavily based on results from single molecule mechanical experiments. The shape of force-extension curves from such data is similar to curves from muscle fibres and it has been tempting to assume that muscle elasticity can be extrapolated directly from the single molecule data. In this paper we discuss some of the factors that act on titin in the sarcomere that are likely to preclude such a direct extrapolation.
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Affiliation(s)
- Larissa Tskhovrebova
- Astbury Centre for Structural Molecular Biology and School of Biomedical Sciences, University of Leeds, LS2 9JT, Leeds, UKJ
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12
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Tskhovrebova L, Trinick J. Role of titin in vertebrate striated muscle. Philos Trans R Soc Lond B Biol Sci 2002; 357:199-206. [PMID: 11911777 PMCID: PMC1692937 DOI: 10.1098/rstb.2001.1028] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Titin is a giant muscle protein with a molecular weight in the megaDalton range and a contour length of more than 1 microm. Its size and location within the sarcomere structure determine its important role in the mechanism of muscle elasticity. According to the current consensus, elasticity stems directly from more than one type of spring-like behaviour of the I-band portion of the molecule. Starting from slack length, extension of the sarcomere first causes straightening of the molecule. Further extension then induces local unfolding of a unique sequence, the PEVK region, which is named due to the preponderance of these amino-acid residues. High speeds of extension and/or high forces are likely to lead to unfolding of the beta-sandwich domains from which the molecule is mainly constructed. A release of tension leads to refolding and recoiling of the polypeptide. Here, we review the literature and present new experimental material related to the role of titin in muscle elasticity. In particular, we analyse the possible influence of the arrangement and environment of titin within the sarcomere structure on its extensible behaviour. We suggest that, due to the limited conformational space, elongation and compression of the molecule within the sarcomere occur in a more ordered way or with higher viscosity and higher forces than are observed in solution studies of the isolated protein.
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Affiliation(s)
- L Tskhovrebova
- School of Biomedical Sciences, University of Leeds, Leeds LS2 9JT, UK
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13
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Cantino ME, Chew MWK, Luther PK, Morris E, Squire JM. Structure and nucleotide-dependent changes of thick filaments in relaxed and rigor plaice fin muscle. J Struct Biol 2002; 137:164-75. [PMID: 12064943 DOI: 10.1006/jsbi.2002.4474] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The myosin crossbridge array, positions of non-crossbridge densities on the backbone, and the A-band "end filaments" have been compared in chemically skinned, unfixed, uncryoprotected relaxed, and rigor plaice fin muscles using the freeze-fracture, deep-etch, rotary-shadowing technique. The images provide a direct demonstration of the helical packing of the myosin heads in situ in relaxed muscle and show rearrangements of the myosin heads, and possibly of other myosin filament proteins, when the heads lose ATP on going into rigor. In the H-zone these changes are consistent with crossbridge changes previously shown by others using freeze-substitution. In addition, new evidence is presented of protein rearrangements in the M-region (bare zone), associated with the transition from the relaxed to the rigor state, including a 27-nm increase in the apparent width of the M-region. This is interpreted as being mostly due to loss or rearrangement of a nonmyosin (M9) protein component at the M-region edge. The structure and titin periodicity of the end-filaments are described, as are suggestions of titin structure on the myosin filament backbone.
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Affiliation(s)
- M E Cantino
- Department of Physiology and Neurobiology, University of Connecticut, Storrs, Connecticut 06269-2242, USA.
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14
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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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15
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Liversage AD, Holmes D, Knight PJ, Tskhovrebova L, Trinick J. Titin and the sarcomere symmetry paradox. J Mol Biol 2001; 305:401-9. [PMID: 11152599 DOI: 10.1006/jmbi.2000.4279] [Citation(s) in RCA: 92] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Titin is thought to play a major role in myofibril assembly, elasticity and stability. A single molecule spans half the sarcomere and makes interactions with both a thick filament and the Z-line. In the unit cell structure of each half sarcomere there is one thick filament with 3-fold symmetry and two thin filaments with approximately 2-fold symmetry. The minimum number of titin molecules that could satisfy both these symmetries is 12. We determined the actual number of titin molecules in a unit cell from scanning transmission electron microscopy mass measurements of end-filaments. One of these emerges from each tip of the thick filament and is thought to be the in-register aggregate of the titin molecules associated with the filament. The mass per unit length of the end-filament (17.1 kDa/nm) is consistent with six titin molecules not 12. Thus the number of titin molecules present is insufficient to satisfy both symmetries. We suggest a novel solution to this paradox in which four of the six titin molecules interact with the two thin filaments in the unit cell, while the remaining two interact with the two thin filaments that enter the unit cell from the adjacent sarcomere. This arrangement would augment mechanical stability in the sarcomere.
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Affiliation(s)
- A D Liversage
- School of Biomedical Sciences, Leeds University, Leeds, LS2 9JT, UK
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16
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Gautel M, Mues A, Young P. Control of sarcomeric assembly: the flow of information on titin. Rev Physiol Biochem Pharmacol 1999; 138:97-137. [PMID: 10396139 DOI: 10.1007/bfb0119625] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/13/2023]
Affiliation(s)
- M Gautel
- European Molecular Biology Laboratory, Heidelberg, Germany
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17
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Gautel M, Mues A, Young P. Control of sarcomeric assembly: The flow of information on titin. Rev Physiol Biochem Pharmacol 1999. [DOI: 10.1007/bf02346661] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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18
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Rief M, Gautel M, Schemmel A, Gaub HE. The mechanical stability of immunoglobulin and fibronectin III domains in the muscle protein titin measured by atomic force microscopy. Biophys J 1998; 75:3008-14. [PMID: 9826620 PMCID: PMC1299971 DOI: 10.1016/s0006-3495(98)77741-0] [Citation(s) in RCA: 218] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
The domains of the giant muscle protein titin (connectin) provide interaction sites for other sarcomeric proteins and fulfill mechanical functions. In this paper we compare the unfolding forces of defined regions of different titin isoforms by single-molecule force spectroscopy. Constructs comprising six to eight immunoglobulin (Ig) domains located in the mechanically active I-band part of titin are compared to those containing fibronectin III (Fn3) and Ig domains from the A-band part. The high spatial resolution of the atomic force microscope allows us to detect differences in length as low as a few amino acids. Thus constructs of different lengths may be used as molecular rulers for structural comparisons with other modular proteins. The unfolding forces range between 150 and 300 pN and differ systematically between the constructs. Fn3 domains in titin exhibit 20% lower unfolding forces than Ig domains. Fn3 domains from tenascin, however, unfold at forces only half those of titin Fn3 domains. This indicates that the tightly folded titin domains are designed to maintain their structural integrity, even under the influence of stretching forces. Hence, at physiological forces, unfolding is unlikely unless the forces are applied for a long time (longer than minutes).
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Affiliation(s)
- M Rief
- Lehrstuhl für Angewandte Physik, Ludwig Maximilians Universität M unchen, 80799 München, Germany
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