Actin filaments myosin interactions with

This ATPase activity [EC 3.6.1.32] is directly responsible for muscle contraction. In the absence of actin filaments, myosin is a feeble ATPase with a kcat of only 0.05 s because product release is much slower than the rapid release of a proton in the P—O—P bond-cleavage step forming ADP and Pi from bound ATP. Interaction with... 

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 absence of troponin I inhibits the thin filament from interacting with myosin. Troponin T is also involved in this inhibition. Troponin T, on one hand, binds to tropomyosin and, on the other hand, binds to troponin I. The action of troponin T is mosdy to adjust the relation of troponin I to tropomyosin—actin for the inhibitory state in the absence of Ca. ... 

Actin occurs in multiple isoforms within individual smooth muscle cells. All of these isoforms are capable of forming filamentous actin that can interact with myosin to generate force. Although the functional importance of the different actin isoforms is presently unclear, there is evidence to suggest that they may serve to "customize actin filaments to serve different functional roles within the cell by determining its interactions with different binding proteins. The first part of this section will review the molecular structure of the thin filament. The structure of actin and the relationship to the other protein constituents of the thin filament to actin... 

Within each sarcomere the relative sliding of thick and thin filaments is brought about by "cross-bridges," parts of the myosin molecules that stick out from the myosin filaments and interact cyclically with the thin actin filaments, transporting them hy a kind of rowing action. During this process, the hydrolysis of ATP to ADP and phosphate couples the conformational... 

The structure and arrangement of the actin and myosin filaments in muscle. During muscle contraction the cyclic interaction of myosin crossbridges with actin filaments draws the actin filaments across the myosin filaments. 

The crossbridge cycle in muscle. Myosin crossbridges interact cyclically with binding sites on actin filaments. Note that the energy release step—when ATP is broken down to ADP—recocks the crossbridge head. 

Figure 7. Length tension relationship. A schematic diagram showing how force varies with sarcomere length, and how this is explained by the relative amount of overlap between the thick and the thin filaments, and hence the numbers of myosin crossbridges in the thick filaments that can interact with actin in the thin filaments.

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. 

Although not organized as in muscle, actin filaments in nonmuscle cells interact with myosin to cause cellular movements. 

Because there are no sarcomeres in smooth muscle, there are no Z lines. Instead, the actin filaments are attached to dense bodies. These structures, which contain the same protein as Z lines, are positioned throughout the cytoplasm of the smooth muscle cell as well as attached to the internal surface of the plasma membrane. Myosin filaments are associated with the actin filaments, forming contractile bundles oriented in a diagonal manner. This arrangement forms a diamond-shaped lattice of contractile elements throughout the cytoplasm. Consequently, the interaction of actin and myosin during contraction causes the cell to become shorter and wider. 

Tropomyosin and troponin are proteins located in the thin filaments, and together with Ca2+, they regulate the interaction of actin and myosin (Fig. 43-3) [5]. Tropomyosin is an a-helical protein consisting of two polypeptide chains its structure is similar to that of the rod portion of myosin. Troponin is a complex of three proteins. If the tropomyosin-troponin complex is present, actin cannot stimulate the ATPase activity of myosin unless the concentration of free Ca2+ increases substantially, while a system consisting solely of purified actin and myosin does not exhibit any Ca2+ dependence. Thus, the actin-myosin interaction is controlled by Ca2+ in the presence of the regulatory troponin-tropomyosin complex [6]. 

Actin filaments are the thinnest of the cytoskeletal filaments, and therefore also called microfilaments. Polymerized actin monomers form long, thin fibers of about 8 nm in diameter. Along with the above-mentioned function of the cytoskeleton, actin interacts with myosin ( thick ) filaments in skeletal muscle fibers to provide the force of muscular contraction. Actin/Myosin interactions also help produce cytoplasmic streaming in most cells. 

Assembly of the actin network merely by interaction with these binding proteins can itself account for pseudopodia formation and propulsive movement. However, there is some evidence to suggest that F-actin-myosin interactions are required for vectorial movement hence it has been demonstrated that pseudopodia contain filament networks comprising actin and myosin. Myosin plays a role in the contractile movement of neutrophils in a... 

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