Titin

Up to 400 myosin molecules are arranged in the thick hlaments (/ 1500 nm, d 12 nm). By bringing the tails together, a major cord is formed and on its surface the heads are spirally located. The distance between two adjacent heads on such a spiral is 14.3 nm, and that between the two repeating heads in the same row or line is 42.9 nm. Their association is reversible under certain conditions. Myosin is stabihzed by titin during muscle contraction (cf. Fig. 12.9b). 

Apart from actin and myosin, titin is the third hl-ament in the sarcomere (Fig. 12.9b). It connects the myosin hlaments with the Z hne and forms an elastic region with actin. Therefore, titin is the backbone of the sarcomere. As a result of its size (Mr = 3 x 10 ), it moves very slowly 

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Rief M, Gautel M, Oesterhelt F, Fernandez J M and Gaub FI E 1997 Reversible unfolding of individual titin immunoglobulin domains by AFM Science 276 1109... 

The SMD simulations were based on an NMR structure of the Ig domain 127 of the cardiac titin I-band (Improta et ah, 1996). The Ig domains consist of two /9-sheets packed against each other, with each sheet containing four strands, as shown in Fig. 8b. After 127 was solvated and equilibrated, SMD simulations were carried out by fixing one terminus of the domain and applying a force to the other in the direction from the fixed terminus to the other terminus. Simulations were performed as described by Eq. (1) with V = 0.5 A/ps and if = 10 ksT/A 414 pN/A. The force-extension profile from the SMD trajectory showed a single force peak as presented in Fig. 8a. This feature agrees well with the sawtooth-shaped force profile exhibited in AFM experiments. 

Improta et al., 1996] Improta, S., Politou, A., and Pasture, A. Immunoglobulinlike modules from titin I-band extensible components of muscle elasticity. Structure. 4 (1996) 323-337... 

Kellermayer et aJ., 1997] Kellermayer, M., Smith, S., Granzier, H., and Bustamante, C. Folding-unfolding transition in single titin modules characterized with laser tweezers. Science. 276 (1997) 1112-1116... 

Labeit et al., 1997] Labeit, S., Kolmerer, B., and Linke, W. The giant protein titin emerging roles in physiology and pathophysiology. Circulation Research. 80 (1997) 290-294... 

Maruyama, 1997] Maruyama, K. Connectin/titin, a giant elastic protein of muscle. FASEB J. 11 (1997) 341-345... 

Tskhovrebova et al., 1997] Tskhovrebova, L., Trinick, J., Sleep, J., and Simmons, R. Elasticity and unfolding of single molecules of the giant protein titin. Nature. 387 (1997) 308-312... 

Special applications, such as in high-magnetic fields, require special thermometers. The carbon-glass and strontium-titinate resistance thermometers have the least magnetoresistance effects. 

Two cytoskeletal proteins, tltln (also known as connectm) and nebulm, account for 15% of the total protein in the myofibril. Together these proteins form a flexible filamentous network that surrounds the myofibrils. Titin is an elastic protein and can stretch under tension. Its discovery and characteriza-... 

Cytoskeletal proteins Titin 1 2800 10 A-I interaction Links myosin filament... 

FIGURE 17.21 A drawing of the arrangement of the elastic protein titin in the skeletal mnscle sarcomere. Titin filaments originate at the periphery of the M band and extend along the myosin filaments to the Z lines. These titin filaments produce the passive tension existing in myofibrils that have been stretched so that the thick and thin filaments no longer overlap and cannot interact. (Adapted from Ohtsuki, ., Maruyama, K, and Ebashi,. S ., 1986. Advances ia Protein Chemisti y 38 1—67.)... 

Muscle contraction is initiated by a signal from a motor nerve. This triggers an action potential, which is propagated along the muscle plasma membrane to the T-tubule system and the sarcotubular reticulum, where a sudden large electrically excited release of Ca " into the cytosol occurs. Accessory proteins closely associated with actin (troponins T, I, and C) together with tropomyosin mediate the Ca -dependent motor command within the sarcomere. Other accessory proteins (titin, nebulin, myomesin, etc.) serve to provide the myofibril with both stability... 

Both the thick and thin filaments contain other proteins. For example, the thick filament contains titin (molecular weight about 3,000,000) and the thin filament contains nebulin (although not in cardiac muscle), and the regulatory proteins troponin (molecular weight about 33,000) and tropomyosin (molecular weight about 70,000). Nebulin and titin are thought to be ruler proteins, that is, they determine the overall length of the thin and the thick filament, respectively. The... 

The copolymerization parameters for copolymerization of dilactide and e-caprolactone catalyzed by stannous octoate, stannous chloride, and tetrabutyl titinate have been determined (5). 

Fig. 22 Photo-crosslinked titin-mimetic hydrogel, with the circles indicating the folded immuno-globulin-like domains and the wavy lines indicating the resilin-like domains. Reproduced from [235] with permission from Nature Publishing Group, copyright 2010...

A number of additional proteins play various roles in the structure and function of muscle. They include titin (the largest protein known), nebufin, a-actinin, desmin, dystrophin, and calcineurin. Some properties of these proteins are summarized in Table 49-2. 

Titin Reaches from the Z line to the M line Largest protein in body. Role in relaxation of muscle. 

Myosinbinding protein C Arranged transversely in sarcomere A-bands Binds myosin and titin. Plays a role in maintaining the structural integrity of the sarcomere. 

Nave, R., Weber, K., and Potschka, M., Universal calibration of size-exclusion chromatography for proteins in guanidinium hydrochloride including the high-molecular-mass proteins titin and nebulin,. Chromatogr. A, 654, 229, 1993. 

Some proteins may be very large, indeed. Titin, for example, a muscle protein and the largest known protein, includes in its molecule over 26,000 combined amino acid units. 

Recently, aqueous solution conformational studies of three short peptides from 9 to 12 residues corresponding to sequences present in titin... 

Lu, H. Isralewitz, B. Krammer, A. Vogel, V. Schulten, K., Unfolding of titin immunoglobulin domains by steered molecular dynamics simulation, Biophys. J. 1998, 75, 662-671... 

Protein domains are the common currency of protein structure and function. Protein domains are discrete structural units that fold up to form a compact globular shape. Experiments on protein structure and function have been greatly aided by consideration of the modular nature of proteins. This has allowed very large proteins to be studied. The expression of individual domains has allowed the intractable giant muscle protein titin to be structurally studied (Pfuhl and Pastore, 1995). Protein domains can be found in a variety of contexts, (Fig. 1), in association with a range of unrelated domains and in a variety of orders. Ultimately protein domains are defined at the level of three-dimensional structure however, many protein domains have been described at the level of sequence. The success of sequence-based methods has been demonstrated by numerous confirmations, by elucidation of the three-dimensional structure of the domain. 

Fig. 8. (a) Schematic of the AFM pulling experiments and expected unraveling of an individual nucleosome as a result of pulling on the DNA. (b) Example force-extension curves on isolated chicken erythrocyte chromatin fibers redrawn from Ref [69]. (c) Idealized schematic of a typical force-extension curve obtained on pulling single titin moleeules, as in the experiments of Rief et al. [71]. (d) Explanation of the titin force curve by successive unfolding of individual protein domains (see text). 

The experimental approach used to mechanically stretch a chromatin fiber with the AFM is depicted schematically in Fig. 8a, and some example curves obtained with native chicken erythrocyte chromatin fibers are presented in Fig. 8b. These curves exhibited a saw-tooth pattern, similar to the patterns obtained upon stretching of multi-domain proteins like titin [71] or tenascin [72] (Fig. 8c). Each of... 

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