Actin filaments myosin

Actin Filament Myosin Filament Actin Fll2uaent... 

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... 

A EXPERIMENTAL FIGURE 19-4 Decoration demonstrates the polarity of an actin filament. Myosin SI head domains bind to actin subunits in a particuiar orientation. When bound to aii the subunits in a fiiament, SI appears to spirai around the fiiament. This coating of myosin heads produces a series of arrowhead-iike decorations, most easiiy seen at the wide views of the fiiament. The poiarity in decoration defines a pointed (-) end and a barbed (-f) end the former corresponds to the top of the model in Figure 19-3c. [Courtesy of R. Craig.]... 

Analogues r of a /Z-membrane actin filament myosin filament ... 

Myosin heads form cross-bridges between the actin and myosin filaments... 

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... 

Figure 14.12 The swinging cross-bridge model of muscle contraction driven by ATP hydrolysis, (a) A myosin cross-bridge (green) binds tightly in a 45 conformation to actin (red), (b) The myosin cross-bridge is released from the actin and undergoes a conformational change to a 90 conformation (c), which then rebinds to actin (d). The myosin cross-bridge then reverts back to its 45° conformation (a), causing the actin and myosin filaments to slide past each other. This whole cycle is then repeated.

Figure 14.17 A sequence of events combining the swinging cross-bridge model of actin and myosin filament sliding with structural data of myosin with and without bound nucleotides.

Certain proteins endow cells with unique capabilities for movement. Cell division, muscle contraction, and cell motility represent some of the ways in which cells execute motion. The contractile and motile proteins underlying these motions share a common property they are filamentous or polymerize to form filaments. Examples include actin and myosin, the filamentous proteins forming the contractile systems of cells, and tubulin, the major component of microtubules (the filaments involved in the mitotic spindle of cell division as well as in flagella and cilia). Another class of proteins involved in movement includes dynein and kinesin, so-called motor proteins that drive the movement of vesicles, granules, and organelles along microtubules serving as established cytoskeletal tracks. ... 

FIGURE 17.23 The mechanism of skeletal muscle contraction. The free energy of ATP hydrolysis drives a conformational change in the myosin head, resulting in net movement of the myosin heads along the actin filament. Inset) A ribbon and space-filling representation of the actin—myosin interaction. (SI myosin image courtesy of Ivan Rayment and Hazel M. Holden, University of Wiseonsin, Madison.)... 

Movements of single myosin molecules along an actin filament can be measured by means of an optical trap consisting of laser beams focused on polystyrene beads attached to die ends of actin molecules. (Adapted from Finer et at., 1994. Nature 368 113- 119. See also Block, 1995. Nature 378 132 133.)... 

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. 

Studies on muscle contraction carried out between 1930 and 1960 heralded the modem era of research on cytoskeletal stmctures. Actin and myosin were identified as the major contractile proteins of muscle, and detailed electron microscopic studies on sarcomeres by H.E. Huxley and associates in the 1950s produced the concept of the sliding filament model, which remains the keystone to an understanding of the molecular mechanisms responsible for cytoskeletal motility. 

Fibronectin receptor is a two-chain glycoprotein of the integrin family that serves as a transmembrane linker by binding to talin on the cytoplasmic side and to fibronectin on the external side of the membrane. The pull exerted by stress fibers on attached structures may be produced by bipolar assemblies of nonmuscle myosin molecules producing a sliding of actin filaments of opposite polarity. 

Cytokinesis begins with astral relaxation of the cell cortex, perhaps triggered by the mitotic spindle, followed by the accumulation in a circumferential equatorial band of actin filaments and associated myosin molecules to form a contractile... 

Bundles of parallel actin filaments with uniform polarity. The microvilli of intestinal epithelial cells (enterocytes) are packed with actin filaments that are attached to the overlying plasma membrane through a complex composed of a 110-kD protein and calmodulin. The actin filaments are attached to each other through fimbrin (68 kD) and villin (95 kD). The actin bundles that emerge out of the roots of microvilli disperse horizontally to form a filamentous complex, the terminal web, in which several cytoskeletal proteins, spectrin (fodrin), myosin, actinin, and tropomyosin are present. Actin in the terminal web also forms a peripheral ring, which is associated with the plasma membrane on the lateral surfaces of the enterocyte (see Figure 5, p. 24). 

Cell migration and cytoplasmic movement involve predominantly actin filaments in the locomotion of neutrophilic granulocytes, both actin filaments and microtubules in the elongation of neuronal growth cones and migration of neurites, and both actin and myosin in cytokinesis and the contraction of skeletal and cardiac muscle. 

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