Myosin contraction

Figure 10. Regulation of neutrophil migration by Rac/Cdc42 and Rho. Chemoattractant-induced G-protein activation stimulates Rac/Cdc42 and Rho. (1) Rac/Cdc42 activates actin polymerization in the lamellipodium by mechanisms proposed in Figure 8. (2) Rho activates myosin-based contraction in the uropod, while (3) Rac/Cdc42 activation of PAK inhibits myosin contraction by MLCK as described in Figure 9.

Figure 12. MAP kinase cascade in neutrophils. The MAP kinase cascade is a series of three protein kinases represented schematically in the left column. Active G-protein [G ] stimulates Ras and PI3K activity, which in turn mediate activation of three MAP kinase cascades, the JNK/SAPK pathway, the ERK1/2 pathway, and the p38 MAP kinase pathway. p38 MAP kinase phosphorylates MK2 and mediates activation of Akt/PKB and Hsp27 phosphorylation. Hsp27 phosphorylation may stimulate actin polymerization and/or myosin contraction.

Blaser, H., Reichman-Eried, M., Castanon, I., Dumstrei, K., Marlow, F.L., Kawakami, K., Solnica-Krezel, L., Heisenberg, C.P., and Raz, E. (2006) Migration of zebrafish primordial germ cells a role for myosin contraction and cytoplasmic flow. Developmental Cell, 11, 613-627. 

Payment i, Hoiden H M, Whittacker M, Yohn C B, Lorenz M, Hoimes K C and Miiiigan R A 1993 Structure of the actin-myosin compiex and its impiications for muscie contraction Science 261 58-65... 

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

Proteins can be broadly classified into fibrous and globular. Many fibrous proteins serve a stmctural role (11). CC-Keratin has been described. Fibroin, the primary protein in silk, has -sheets packed one on top of another. CoUagen, found in connective tissue, has a triple-hehcal stmcture. Other fibrous proteins have a motile function. Skeletal muscle fibers are made up of thick filaments consisting of the protein myosin, and thin filaments consisting of actin, troponin, and tropomyosin. Muscle contraction is achieved when these filaments sHde past each other. Microtubules and flagellin are proteins responsible for the motion of ciUa and bacterial dageUa. 

In the presence of calcium, the primary contractile protein, myosin, is phosphorylated by the myosin light-chain kinase initiating the subsequent actin-activation of the myosin adenosine triphosphate activity and resulting in muscle contraction. Removal of calcium inactivates the kinase and allows the myosin light chain to dephosphorylate myosin which results in muscle relaxation. Therefore the general biochemical mechanism for the muscle contractile process is dependent on the avaUabUity of a sufficient intraceUular calcium concentration. 

Muscle fibers contain myosin and actin which slide against each other during muscle contraction... 

Figure 14.11 The sliding filament model of muscle contraction. The actin (red) and myosin (green) filaments slide past each other without shortening.

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.

Rayment, 1., et al. Structure of the actin-myosin complex and its implications for muscle contraction. Science 261 58-65, 1996. 

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. ... 

In addition to the major proteins of striated muscle (myosin, actin, tropomyosin, and the troponins), numerous other proteins play important roles in the maintenance of muscle structure and the regulation of muscle contraction. Myosin and actin together account for 65% of the total muscle protein, and tropomyosin and the troponins each contribute an additional 5% (Table 17.1). The other regulatory and structural proteins thus comprise approximately 25% of the myofibrillar protein. The regulatory proteins can be classified as either myosin-associated proteins or actin-associated proteins. 

Contractile proteins Myosin 520 43 A band Contracts with actin... 

The molecular events of contraction are powered by the ATPase activity of myosin. Much of our present understanding of this reaction and its dependence on actin can be traced to several key discoveries by Albert Szent-Gyorgyi at the University of Szeged in Hungary in the early 1940s. Szent-Gyorgyi showed that solution viscosity is dramatically increased when solutions of myosin and actin are mixed. Increased viscosity is a manifestation of the formation of an actomyosin complex. 

However, release of ADP and P from myosin is much slower. Actin activates myosin ATPase activity by stimulating the release of P and then ADP. Product release is followed by the binding of a new ATP to the actomyosin complex, which causes actomyosin to dissociate into free actin and myosin. The cycle of ATP hydrolysis then repeats, as shown in Figure 17.23a. The crucial point of this model is that ATP hydrolysis and the association and dissociation of actin and myosin are coupled. It is this coupling that enables ATP hydrolysis to power muscle contraction. 

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.)... 

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