Myosin aggregation

In muscle cells, molecules of myosin aggregate to form structures called thick filaments (Fig. 5-30a). These rodlike structures serve as the core of the con-... 

Actin is present in all eukaryotic cells where it has structural and mobility functions. Most movement associated with microfilaments requires myosin. The myosin-to-actin ratio is much lower in nonmuscle cells, and myosin bundles are much smaller (10-20 molecules rather than about 500), but the interaction between myosin and actin in nonmuscle cells is generally similar to that in muscle. As in smooth muscle, myosin aggregation and activation of the actin-myosin interaction are regulated primarily by light chain phosphorylation. Myosins involved in transporting organelles along actin filaments are often activated by Ca-CaM. 

Ramhez, J.A., Martin-Polo, M.O., and Bandman, E. 2000. Fish myosin aggregation as affected by freezing and initial physical state. Journal of Food Science 65 556-560. 

Similar, the effect of electric field on the cultures of animal cells, deposed on the same type of electrode, was investigated. It was found that the cells proliferation and the morphologic changes are affected by applying small differences of potential between electrodes. The phenomenon was ascribed to the structural changes of cellular cytoskeleton, in last instance, to the modification actin-myosin aggregates. 

Huxley suggested that crossbridges can move out in this way and bind to actin because S-2 of HMM acted as a flexible link between LMM in the thick filament backbone and S-1. This was based on the observation that heavy meromyosin could be digested by chymotrypsin into two further subffagments (Lowey et al., 1966), S-1 and S-2, as described above, and that S-1 contained the ATPase and actin binding sites, whereas S-2 did not moreover, S-2 did not self-aggregate, as did the rod or LMM portion of myosin. 

When myosin is digested with trypsin, two myosin fragments (meromyosins) are generated. L ht mero-myosin (LMM) consists of aggregated, insoluble a-he-hcal fibers from the tail of myosin (Figure 49 ). LMM... 

Figure 13.6 Myosin and actin molecules and myosin crossbridges. Each kind of filament is composed of a different protein myosin in the thick filaments and actin in the thin filaments. In the case of actin, the individual F-actins are more or less spherical but a large number of these combine to produce a long chain, two of which wind around each other, rather like a rope, to produce the thin filament. The myosin molecule is more complex and shaped somewhat like a golf club. To form the thick filament, the shafts aggregate to leave the heads protruding on all sides. These heads form the cross-bridges and are responsible for pulling the thin filaments into the spaces between the thick filaments (see Figure 13.5).

In the keratins, large parts of the peptide chain show right-handed a-helical coiling. Two chains each form a left-handed superhelix, as is also seen in myosin (see p. 65). The superhelical keratin dimers join to form tetramers, and these aggregate further to form protofilaments, with a diameter of 3 nm. Finally, eight protofilaments then form an intermediate filament, with a diameter of 10 nm (see p.204). 

The thick filaments. Dissociated myosin molecules can be induced to aggregate into rods similar to the thick filaments of muscle.129 Since the filaments... 

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. 

Myosin. The main protein of the thick filaments in a muscle myofibril. It is composed of two coiled subunits (Mt. about 220,000) that can aggregate to form a thick filament that is globular at each end. 

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. 

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