Tropomyosin binding sites

Tropomyosin is thought to lie in the groove formed between the associated actin strands. The sites at which the myosin crossbridges attach are affected by the relationship between tropomyosin and the actin strands. The role of tropomyosin in smooth muscle is completely undefined while in striated muscle it is clearly involved in the activation of contraction. The difference is made clear by the absence from smooth muscle of the protein, troponin, which in striated muscle provides the binding site for the triggering calcium. 

When the sarcolemma is excited by a nerve impulse, the signal is transmitted into the T tubule system and a release channel in the nearby sarcoplasmic reticulum opens, releasing Ca + from the sarcoplasmic reticulum into the sarcoplasm. The concentration of Ca in the sarcoplasm rises rapidly to 10 mol/L. The Ca -binding sites on TpC in the thin filament are quickly occupied by Ca +. The TpC-4Ca + interacts with Tpl and TpT to alter their interaction with tropomyosin. Accordingly, tropomyosin moves out of the way or alters the conformation of F-actin so that the myosin head-ADP-P (Figure 49-6) can interact with F-actin to start the contraction cycle. 

When the action potentials in the alpha motor neuron cease, stimulation of muscle fiber is ended. Ca++ ions are pumped back into the sarcoplasmic reticulum and troponin and tropomyosin return to their original positions. As a result, the myosin-binding sites on the actin are covered once again. The thin filaments return passively to their original positions, resulting in muscle relaxation. 

Function of calcium Reposition troponin/tropomyosin to uncover myosin binding sites on actin Phosphorylate and activate myosin to bind with actin Phosphorylate and activate myosin to bind with actin... 

In skeletal muscle, calcium binds to troponin and causes the repositioning of tropomyosin. As a result, the myosin-binding sites on the actin become uncovered and crossbridge cycling takes place. Although an increase in cytosolic calcium is also needed in smooth muscle, its role in the mechanism of contraction is very different. Three major steps are involved in smooth muscle contraction ... 

In smooth muscle, caldesmon plays an analogous role to that of troponin in striated muscle in that it blocks the myosin binding sites. The CaCM complex removes caldesmon from its binding on the thin actin filaments allowing tropomyosin to reposition in the helical grooves of F-actin leading to myosin ATP ase activation. 

Calcium binds to troponin C on tropomyosin, causing a conformational change that exposes myosin-binding sites on actin. 

The decreased [Ca2+] causes tropomyosin to resume its previous configuration, blocking the myosin-binding site. 

Figure 13.7 A diagram of the actin helix showing position of the tropomyosin. Both actin chains are flanked by tropomyosin molecules, which are long string-like molecules that span seven actin monomers. The troponin complex is attached to the tropomyosin but is not shown. From this diagram, it should be clear how the tropomyosin molecule can conceal the actin-binding sites for the myosin cross-bridges in the relaxed condition. A small conformational change in tropomyosin exposes the sites for attachment of the cross-bridges.

Figure 13.16 A summary of the control of muscle contraction by the motor neurone. When an electrical impulse arrives at the junction between a nerve axon and a muscle fibre, a small amount of acetylcholine is released. This initiates an action potential which is transmitted throughout the fibre via the T-tubules. This causes the sarcoplasmic reticulum to release Ca ions which initiate contraction of the myofibrils via changes in troponin and tropomyosin. Thus sites on the actin for binding of the myosin cross-bridges are exposed.

The interaction between actin and myosin must be regulated so that contraction occurs only in response to appropriate signals from the nervous system. The regulation is mediated by a complex of two proteins, tropomyosin and troponin. Tropomyosin binds to the thin filament, blocking the attachment sites for the myosin head groups. Troponin is a Ca2+-binding protein. 

A nerve impulse causes release of Ca2+ from the sarcoplasmic reticulum. The released Ca2+ binds to troponin (another protein-ligand interaction) and causes a conformational change in the tropomyosin-troponin complexes, exposing the myosin-binding sites on the thin filaments. Contraction follows. 

Stewart, M., and McLachlan, A. D. (1975). Fourteen actin-binding sites on tropomyosin Nature 257, 331—333. 

Smillie, L.B., Pato, M.D., Pearlstone, J.R., Mak, A.S. 1980. Periodicity of alpha-helical potential in tropomyosin sequence correlates with alternating actin binding-sites. Journal of Molecular Biology 136(2) 199-202. 

In skeletal muscle in the relaxed state, the sarcoplasm has a high Mg ATP2 -concent rat ion, but the concentration of calcium is below the threshold required for initiation of contraction. The myosin head, under resting conditions is unable to react with actin of the thin filaments because in the absence of calcium the tropomyosin molecule masks the myosin binding site on G-actin monomer or holds it in a conformation that is unreactive, through the action of TN-1 subunit of troponin. One tropomyosin molecule inhibits the myosin binding activity of seven G-actin monomers. 

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. 

Figure 6.3. Stractuie and function of myofilaments, a Arrangement of proteins within the filaments, b, c Mechanism of myofilament motion. Calcium binds to troponin, which in tnm causes tropomyosin to move and expose the myosin binding site on actin. Binding to actin canses the myosin heads to kink, which translates into a sliding motion, c The kinked conformation of myosin cleaves ATP. In the process, myosin releases itself from actin and letnms to the extended conformation it then binds to another actin monomer, and the cycle is repeated.

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