Troponin interaction with

Figure 8 Bending movements of a thin fiiament (a compiex of F-actin with tropomyosin and troponin) interacting with myosin fragments in the presence of ATP, where F-actin was iabeied with fiuorescent dye molecules to make it visibie under a fiuorescence microscope (a) inactive state in the absence of calcium ions (b) active state in the presence of caicium ions undergoing fast and large bending movements. The sequential micrographs were taken on 10 /u,m filaments at interval of 0.15 sec (a) and 0.10 sec (b1) and (b2). (From T. Yanagida, M. Nakase, N. Nishiyama, and F. Oosawa. Nature 307 58, 1984. With permission.)...

The Ca2+-binding subunit TN-C is homologous to calmodulin with four EF-hands. In contrast to calmodulin, which is ubiquitously expressed in multicellular eukaryotic organisms and interacts with many targets, troponin specifically regulates muscle contraction. There are some structural differences between Troponin C in skeletal and cardiac muscles reflecting their physiological differences. 

Relaxation occurs when sarcoplasmic Ca falls below 10 mol/L owing to its resequestration into the sarcoplasmic reticulum by Ca ATPase. TpC.dCa thus loses its Ca. Consequently, troponin, via interaction with tropomyosin, inhibits further myosin head and F-actin interaction, and in the presence of ATP the myosin head detaches from the F-actin. 

Troponin C Regulates muscle contraction complex formation triggers actin to realign and interact with myosin (389,390) — inhibited by relaxins, which are themselves Ca2+-binding proteins (v.s.)... 

Calcium effects. The biochemical effects of Ca "" in the cytoplasm are mediated by special Ca -binding proteins calcium sensors"). These include the annexins, calmodulin, and troponin C in muscle (see p. 334). Calmodulin is a relatively small protein (17 kDa) that occurs in all animal cells. Binding of four Ca "" ions (light blue) converts it into a regulatory element. Via a dramatic conformational change (cf 2a and 2b), Ca -calmodulin enters into interaction with other proteins and modulates their properties. Using this mechanism, Ca "" ions regulate the activity of enzymes, ion pumps, and components of the cytoskeleton. 

Sanders, C., and Smillie, L. B. (1984). Chicken gizzard tropomyosin Head-to-tail assembly and interaction with F-actin and troponin. Can. J. Biochem. Cell. Biol. 62, 443-448. 

Tanokura, M., Tawada, Y., Ono, A., and Ohtsuki, I. (1983). Chymotryptic subffagments of troponin T from rabbit skeletal muscle. Interaction with tropomyosin, troponin I and troponin C./. Biochem. (Tokyo) 93, 331-337. 

An increase in Ca2+ (e.g., from 10 8M to 10 5M) acts as the trigger. In striated muscle, Ca2+ is released from the endoplasmic reticulum into the cytoplasm following stimulation of the muscle cell via its attached motor nerve. The Ca2+ interacts with the troponin complex, causing a movement of tropomyosin to expose the myosin binding sites on the thin filaments (see Example 5.21). In smooth muscle, the released Ca2+ indirectly activates myosin light chain kinase which phosphorylates the light chains of myosin. Hence, the control is at the level of the thick filament. In some nonmuscle cells, the control by Ca2+ is at the level of the assembly of myosin into filaments. 

Both thin and thick filaments are present in the cytoplasm of the muscle cell. They are aligned with the long dimension of the muscle cell. In the resting state, troponin inhibits the interaction between actin and myosin filaments. When the concentration of Ca + rises, the conformation of troponin changes, permitting the filaments to interact with each other. The skeletal muscle relaxes when cytosolic Ca + levels return toward the basal level. This drop results in dissociation of the Ca-troponin complex. 

In the light of the available crystallographic studies, the amino acid sequences and NMR studies, certain general points have been noted for parvalbumin, Wasserman protein, calmodulin, troponin C and SIOO. (1) Each calcium site is formed from residues in a hand which includes a /3-strand. The sites contain backbone carbonyl and side-chain carboxylate. (2) Each binding site is linked to two helices. (3) Each -strand backs on to another strand to form a Ca-/3-sheet-Ca unit, which then involves four helices. (4) The helices interact with each other through largely hydrophobic surfaces. (5) The connections between remote ends of helices are relatively mobile strands, and differ from protein to protein. (6) The four-hand proteins, the calmodulins and troponins, are simileir to the sum of their two-handed fragments. 

Grand et al. (1982) reported the observation that Arg residues of the CN4 fragment were perturbed by actin, as shown by their proton magnetic resonance spectra (Fig. 3). They suggested that the C-terminal half of the CN4 region, which is rich in Arg residues, is mainly involved in the inhibitory action, whereas the N-terminal half is related to the interaction with troponin C. It is interesting to notice that both Lys-105 and Val-114, essential for the inhibitory action based on the synthetic peptide study, are not perturbed by actin. The side chains of these residues might interact with tropomyosin. 

There have been few reports on the interaction of troponin I with tropomyosin, in spite of the clear requirement of tropomyosin for the inhibitory action of troponin I. Ebashi et al. (1974) presented evidence from gel filtration showing that troponin I comigrates with tropomyosin. It was also reported that tropomyosin was retained in the column of troponin I-Sepharose 4B (Katayama, 1980). 

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