Actin interaction with myosin

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. 

The diffraction data were also used to guide the selection of the best preserved e.m. images of decorated actin which were then used for a 3-D reconstruction (Amos et al., 1982). In this work, it was suggested that a myosin head interacts with two actin monomers (while still retaining a 1 1 stoichiometry), but this point has not been proved definitively. 

Other major differences between kinesins and myosin II heads involve kinetics180 181 and processivity.173 Dimeric kinesin is a processive molecule. It moves rapidly along microtubules in 8-nm steps but remains attached.182 1823 Myosins V and VI are also proces-sive1 83, a3e but myosin II is not. It binds, pulls on actin, and then releases it. The many myosin heads interacting with each actin filament accomplish muscle contraction with a high velocity in spite of the short time of attachment. Ned and Kar3 are also nonprocessive and slower than the plus end-oriented kinesins.184... 

Various nonmuscle forms of myosin also interact with actin without formation of the myofibrils of muscle.299 In most higher organisms nonmuscle myosins often consist of two 200-kDa subunits plus two pairs of light chains of 17 and 24 kDa each. These may form bipolar aggregates, which may bind to pairs of actin filaments to cause relative movement of two parts of a cell.303 Movement depending upon the cytoskeleton is complicated by the presence of a bewildering array of actin-binding proteins, some of which are listed in Table 19-1. 

Fig. 7. General structures of a number of different motors (A) myosin II interacting with actin, (B) kinesin carrying a cargo and interacting with a microtubule, and (C) cytoplasmic dynein, with its associated cargo-laden dynactin complex, interacting with a microtubule.

An enhancement of ATPase action comes through the phosphorylation of myosin light chains (MW 18,000). The phosphorylation is achieved because the high cellular [Ca2+] activates myosin kinase, an enzyme that contains calmodulin, a Ca2+-binding subunit. Phosphorylation of myosin is absolutely required for smooth muscle contraction, though not for the contraction of skeletal or cardiac muscle, because smooth muscle has no troponin. Thus, whereas contraction and relaxation in skeletal and cardiac muscle are achieved principally via the action of Ca2+ on troponin, in smooth muscle they must depend solely on the Ca2+-dependent phosphorylation of myosin. In skeletal and cardiac muscle, once the stimulus to the sarcolemma is removed, [Ca2+] in sarcoplasm drops rapidly back to 10 7 or 10 8 M via various Ca2+ pump mechanisms present in the sarcoplasmic reticulum, and tropomyosin can once again interfere with the myosin-actin interaction. 

When muscle is relaxed, troponin does not bind Ca because sarcoplasmic [Ca2+] is very low. This permits the tropomyosin to return to the relaxed state in the groove of the thin filaments and interfere with myosin-actin interaction. Since the thin filaments slide across (over) the thick filaments during contraction, the I zone, which represents the thin filaments, will decrease greatly in size. 

All except tropomyosin form larger molecules. Tubulin, under the influence of GTP, aggregates into microtubules G-actin polymerizes to F-actin myosin forms thick filaments by tail-to-tail interaction and /3-actin polymerizes to microfilaments. /3-Actin is similar but not identical to G-actin. Tropomyosin is a fibrous protein associated with the F-actin polymer. It controls myosin-actin interaction under the influence of troponin. 

An alternative mechanism by which cAMP may act to inhibit platelet function was proposed by Hathaway et al. (81). In this model (Figure 12), it is suggested that an increase in cAMP results in inhibition of myosin phosphorylation and consequent inhibition of platelet contractile activity (since unphos-phorylated myosin cannot interact with actin). Thus, it was proposed that cAMP causes the activation of a protein kinase which in turn phosphorylates myosin kinase. In the phosphory-lated form, myosin kinase is less capable of binding calmodulin and therefore is not as effective in phosphorylating myosin. 

Still, there is some controversy as to whether unphosphorylated myosin can interact with actin. Most studies suggest that myosin has to be phosphorylated for interaction with actin (Sobieszek and Small 1977, Sherry et al. 1978, Sellers et al. 1982, Chacko and Rosenfeld 1982, Sellers et al. 1985). However, under conditions where the unphosphorylated myosin is filamentous, Vmax of the MgATPase activity is about one-half of that of phosphorylated myosin as already mentioned (Wagner and Vu 1986,1987). This puzzle may be resolved on the basis of a recent study in which the minimal molecular requirement for myosin to be in the off-state was determined (Trybus et al. 1997). Mutants of myosin with different lengths of the rod showed that a length approximately equal to the myosin head was necessary to achieve a completely off-state . It was concluded that the myosin rod mediates specific interactions with the head that are required to obtain a completely inactive state of vertebrate smooth myosins. If this interaction could be prevented, e.g. by constraints imposed by the native thick filament structure or accessory proteins, then partial activation of the actomyosin ATPase and slowly cycling of unphosphorylated cross-bridges could occur. 

The rapid decrease in the level of ATP following death has two consequences. First, the cytosolic level of calcium rises rapidly because the Ca i-ATPase pumps in the plasma membrane and sarcoplasmic reticulum membrane no longer operate. High Ca, through troponin and tropomyosin, enables myosin to interact with actin. Second, a large proportion of SI heads -will be associated with actin. Recall that ATP is required to dissociate the actomyosin complex. In the absence of ATP, skeletal muscle is locked in the contracted (rigor) state. 

Since single F-actin filaments were made directly visible under an optical microscope, various experiments were undertaken to observe dynamic behaviors of F-actin interacting with myosin and other actin-binding proteins. 

Ishijima A, Kojima H, Funatsu T, Tokunaga M, Higuchi H, Tanaka H and Yanagida T 1998 Simultaneous observation of individual ATPase and mechanical events by a single myosin molcule during interaction with actin Ce//92 161-71... 

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. 

The interaction with myosin motors enables F-actin to transport molecules as well as to change or maintain the shape of the cell by exerting tension. Thus, myosin-I motors move to the barbed end and can transport cargoes such as vesicles. When immobilized at the cargo site... 

Myosin Subftagment-I Interacts With Two G-Actin Molecules Oligomers of G-Actin and S] Are the Second Intennediates in F-Actin-Si Assembly Conclusion... 

Myosin Subfragment-1 Interacts With Two G-Actin Molecules... 

Valentin-Ranc, C., Combeau, C., Carlier, M.-F., Pantaloni, D. (1991). Myosin subftagment-1 interacts with two G-actin molecules in the absence of ATP. J. Biol. Chem. 266,17872—17879. 

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.

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