Titin/ˈtaɪtɪn/, also known as connectin, is a protein that, in humans, is encoded by the TTNgene. Titin is a giant protein, greater than 1 µm in length, that functions as a molecular spring which is responsible for the passive elasticity of muscle. It is composed of 244 individually folded protein domains connected by unstructured peptide sequences. These domains unfold when the protein is stretched and refold when the tension is removed.
Titin is important in the contraction of striated muscle tissues. It connects the Z line to the M line in the sarcomere. The protein contributes to force transmission at the Z line and resting tension in the I band region. It limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle. Variations in the sequence of titin between different types of muscle (e.g., cardiac or skeletal) have been correlated with differences in the mechanical properties of these muscles.
After myosin and actin, titin is the third most abundant protein in muscle and an adult human contains approximately 0.5 kg of titin. With its length of ~27,000 to ~33,000 amino acids (depending on the splice isoform), titin is the largest known protein. Furthermore, the gene for titin contains the largest number of exons (363) discovered in any single gene, as well as the longest single exon (17,106 bp).
Reiji Natori in 1954 was the first to propose an elastic structure in muscle fiber to account for the return to the resting state when muscles are stretched and then released. In 1977, Koscak Maruyama and coworkers isolated an elastic protein from muscle fiber which they called connectin. Two years later, Kuan Wang and coworkers identified a doublet band on electrophoresis gel corresponding to a high molecular weight elastic protein which they named titin.
Labeit in 1990 isolated a partial cDNA clone of titin. In 1995, Labeit and Kolmerer determined the cDNA sequence of human cardiac titin. Bang and coworkers in 2001 determined the complete sequence of the human titin gene.
A number of titin isoforms are produced in different striated muscle tissues as a result of alternative splicing. All but one of these isoforms are in the range of ~27,000 to ~36,000 amino acid residues in length. The exception is the small cardiac novex-3 isoform which is only 5,604 amino acid residues in length. The following table lists the known titin isoforms:
Titin is the largest known protein; its human variant consists of 34,350 amino acids, with the molecular weight of the mature "canonical" isoform of the protein being approximately 3,816,188.13 Da. Its mouse homologue is even larger, comprising 35,213 amino acids with a MW of 3,906,487.6 Da. It has a theoretical isoelectric point of 6.01. The protein's empirical chemical formula is C169 723H270 464N45 688O52 243S912. It has a theoretical instability index (II) of 42.41, classifying the protein as unstable. The protein's in vivohalf-life, the time it takes for half of the amount of protein in a cell to break down after its synthesis in the cell, is predicted to be approximately 30 hours (in mammalianreticulocytes).
N-terminal I-band: acts as the elastic part of the molecule and is composed mainly of type II modules. More specifically the I-band contains two regions of tandem type II immunoglobulin domains on either side of a PEVK region that is rich in proline, glutamate, valine and lysine.
C-terminal A-band: is thought to act as a protein-ruler and possesses kinase activity. The A-band is composed of alternating type I and II modules with super-repeat segments. These have been shown to align to the 43 nm axial repeats of myosin thick filaments with immunoglobulin domains correlating to myosin crowns.
Sliding filament model of muscle contraction. (Titin labeled at upper right.)
Titin is a large abundant protein of striated muscle. An N-terminal Z-disc region and a C-terminal M-line region bind to the Z-line and M-line of the sarcomere, respectively, so that a single titin molecule spans half the length of a sarcomere. Titin also contains binding sites for muscle-associated proteins so it serves as an adhesion template for the assembly of contractile machinery in muscle cells. It has also been identified as a structural protein for chromosomes. Considerable variability exists in the I-band, the M-line and the Z-disc regions of titin. Variability in the I-band region contributes to the differences in elasticity of different titin isoforms and, therefore, to the differences in elasticity of different muscle types. Of the many titin variants identified, five are described with complete transcript information available.
Mutations anywhere within the unusually long sequence of this gene can cause premature stop codons or other defects. Titin mutations are associated with hereditary myopathy with early respiratory failure, early-onset myopathy with fatal cardiomyopathy, core myopathy with heart disease, centronuclear myopathy, Limb-girdle muscular dystrophy type 2J, familialdilated cardiomyopathy 9, hypertrophic cardiomyopathy and tibial muscular dystrophy. Further research also suggests that no genetically linked form of any dystrophy or myopathy can be safely excluded from being caused by a mutation on the TTN gene. Truncating mutations in dilated cardiomyopathy patients are most commonly found in the A region; although truncations in the upstream I region might be expected to prevent translation of the A region entirely, alternative splicing creates some transcripts that do not encounter the premature stop codon, ameliorating its effect.
Autoantibodies to titin are produced in patients with the autoimmune disease scleroderma.
^ abcLabeit S, Barlow DP, Gautel M, Gibson T, Holt J, Hsieh CL, Francke U, Leonard K, Wardale J, Whiting A (1990). "A regular pattern of two types of 100-residue motif in the sequence of titin". Nature345 (6272): 273–6. doi:10.1038/345273a0. PMID2129545.
^ abItoh-Satoh M, Hayashi T, Nishi H, Koga Y, Arimura T, Koyanagi T, Takahashi M, Hohda S, Ueda K, Nouchi T, Hiroe M, Marumo F, Imaizumi T, Yasunami M, Kimura A (2002). "Titin mutations as the molecular basis for dilated cardiomyopathy". Biochem. Biophys. Res. Commun.291 (2): 385–93. doi:10.1006/bbrc.2002.6448. PMID11846417.
^ abcBang ML, Centner T, Fornoff F, Geach AJ, Gotthardt M, McNabb M, Witt CC, Labeit D, Gregorio CC, Granzier H, Labeit S (2001). "The complete gene sequence of titin, expression of an unusual approximately 700-kDa titin isoform, and its interaction with obscurin identify a novel Z-line to I-band linking system". Circ. Res.89 (11): 1065–72. doi:10.1161/hh2301.100981. PMID11717165.
^Bennett PM, Gautel M (1996). "Titin domain patterns correlate with the axial disposition of myosin at the end of the thick filament". J. Mol. Biol.259 (5): 896–903. doi:10.1006/jmbi.1996.0367. PMID8683592.
^Hinson JT, Chopra A, Nafissi N, Polacheck WJ, Benson CC, Swist S, et al. (2015). "Titin mutations in iPS cells define sarcomere insufficiency as a cause of dilated cardiomyopathy". Science349 (6251): 982–6. doi:10.1126/science.aaa5458. PMID26315439.
^Kontrogianni-Konstantopoulos A, Bloch RJ (2003). "The hydrophilic domain of small ankyrin-1 interacts with the two N-terminal immunoglobulin domains of titin". J. Biol. Chem.278 (6): 3985–91. doi:10.1074/jbc.M209012200. PMID12444090.
^ abMiller MK, Bang ML, Witt CC, Labeit D, Trombitas C, Watanabe K, Granzier H, McElhinny AS, Gregorio CC, Labeit S (2003). "The muscle ankyrin repeat proteins: CARP, ankrd2/Arpp and DARP as a family of titin filament-based stress response molecules". J. Mol. Biol.333 (5): 951–64. doi:10.1016/j.jmb.2003.09.012. PMID14583192.
^Ono Y, Shimada H, Sorimachi H, Richard I, Saido TC, Beckmann JS, Ishiura S, Suzuki K (1998). "Functional defects of a muscle-specific calpain, p94, caused by mutations associated with limb-girdle muscular dystrophy type 2A". J. Biol. Chem.273 (27): 17073–8. doi:10.1074/jbc.273.27.17073. PMID9642272.
^Sorimachi H, Kinbara K, Kimura S, Takahashi M, Ishiura S, Sasagawa N, Sorimachi N, Shimada H, Tagawa K, Maruyama K (1995). "Muscle-specific calpain, p94, responsible for limb girdle muscular dystrophy type 2A, associates with connectin through IS2, a p94-specific sequence". J. Biol. Chem.270 (52): 31158–62. doi:10.1074/jbc.270.52.31158. PMID8537379.
^Lange S, Auerbach D, McLoughlin P, Perriard E, Schäfer BW, Perriard JC, Ehler E (2002). "Subcellular targeting of metabolic enzymes to titin in heart muscle may be mediated by DRAL/FHL-2". J. Cell. Sci.115 (Pt 24): 4925–36. doi:10.1242/jcs.00181. PMID12432079.
^Mayans O, van der Ven PF, Wilm M, Mues A, Young P, Fürst DO, Wilmanns M, Gautel M (1998). "Structural basis for activation of the titin kinase domain during myofibrillogenesis". Nature395 (6705): 863–9. doi:10.1038/27603. PMID9804419.
^Zou P, Gautel M, Geerlof A, Wilmanns M, Koch MH, Svergun DI (2003). "Solution scattering suggests cross-linking function of telethonin in the complex with titin". J. Biol. Chem.278 (4): 2636–44. doi:10.1074/jbc.M210217200. PMID12446666.
^Mues A, van der Ven PF, Young P, Fürst DO, Gautel M (1998). "Two immunoglobulin-like domains of the Z-disc portion of titin interact in a conformation-dependent way with telethonin". FEBS Lett.428 (1-2): 111–4. doi:10.1016/S0014-5793(98)00501-8. PMID9645487.
^Centner T, Yano J, Kimura E, McElhinny AS, Pelin K, Witt CC, Bang ML, Trombitas K, Granzier H, Gregorio CC, Sorimachi H, Labeit S (2001). "Identification of muscle specific ring finger proteins as potential regulators of the titin kinase domain". J. Mol. Biol.306 (4): 717–26. doi:10.1006/jmbi.2001.4448. PMID11243782.
Sorimachi H, Ono Y, Suzuki K (2000). "Skeletal muscle-specific calpain, p94, and connectin/titin: their physiological functions and relationship to limb-girdle muscular dystrophy type 2A". Adv. Exp. Med. Biol.481: 383–95; discussion 395–7. doi:10.1007/978-1-4615-4267-4_23. PMID10987085.
Wu Y, Labeit S, Lewinter MM, Granzier H (2002). "Titin: an endosarcomeric protein that modulates myocardial stiffness in DCM". J. Card. Fail.8 (6 Suppl): S276–86. doi:10.1054/jcaf.2002.129278. PMID12555133.