Myosin molecule

Funatsu T, Flarada Y, Tokunaga M, Saito K and Yanagida T 1995 Imaging of single fluorescent molecules and individual ATP turnovers by single myosin molecules in aqueous solution Nature 374 555-9... 

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 14.14 Sci ematic diagram of the myosin molecule, comprising two heavy chains (green) that form a coiled-coil tail with two globular heads and four light chains (gray) of two slightly differing sizes, each one bound to each heavy-chain globular head.

FIGURE 17.18 The packing of myosin molecules in a thick filament. Adjoining molecules are offset by approximately 14 nm, a distance corresponding to 98 residues of the coiled coil. 

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.)... 

Finer, J. T., Simmons, R. M., and Spndich, J. A., 1994. Single myosin molecule mechanics Piconewton forces and nanometer steps. Nature 368 113-119. 

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... 

Organization into macromolecular structures. There are no apparent templates necessary for the assembly of muscle filaments. The association of the component proteins in vitro is spontaneous, stable, and relatively quick. Filaments will form in vitro from the myosins or actins from all three kinds of muscle. Yet in vitro smooth muscle myosin filaments are found to be stable only in solutions somewhat different from in vivo conditions. The organizing principles which govern the assembly of myosin filaments in smooth muscle are not well understood. It is clear, however, a filament is a sturdy structure and that individual myosin molecules go in and out of filaments whose structure remains in a functional steady-state. As described above, the crossbridges sticking out of one side of a smooth muscle myosin filament are all oriented and presumably all pull on the actin filament in one direction along the filament axis, while on the other side the crossbridges all point and pull in the opposite direction. The complement of minor proteins involved in the structure of the smooth muscle myosin filament is unknown, albeit not the same as that of skeletal muscle since C-protein and M-protein are absent. 

Phosphorylation-dephosphorylation. The site on myosin which is phosphory-lated is not the same as the site by which it attaches to actin. Therefore, there are two geometrically separate reactions in regulation and from the Law of Reversibility there must be at least some myosin molecules in at least four different states ... 

Figure 8. (Continued). As described above, the packing of myosin molecules into the thick filament is such that a layer of heads is seen every 14.3 nm, and this reflection is thought to derive from this packing. Off the meridian the 42.9 nm myosin based layer line is shown. This arises from the helical pitch of the thick filament, due to the way in which the myosin molecules pack into the filament. The helical pitch is 42.9 nm. c) Meridional reflections from actin. Actin based layer lines can be seen at 35.5 nm, 5.9 nm and 5.1 nm (1st, 6th, and 7th layer lines)and they all arise from the various helical repeats along the thin filament. Only the 35.5 nm layer line is shown here.The 5.9 nm and 5.1 nm layer lines arise from the monomeric repeat. The 35.5 nm layer line arises from the long pitch helical repeat and is roughly equivalent to seven actin monomers. A meridional spot at 2.8 nm can also be seen, d) The equatorial reflections, 1,0 and 1,1 which arise from the spacings between crystal planes seen in cross section of muscle.

Figure 13, A schematic diagram of the motility assay. Myosin molecules (HMM or S-1 are also used) stick to glass coverslips coated with nitrocellulose. Actin, in solution, is then added to the glass coverslip and it binds to the myosin molecules. When ATP is added, actin can move over the surface, propelled by the myosin molecules.

Smooth muscles have molecular structures similar to those in striated muscle, but the sarcomeres are not aligned so as to generate the striated appearance. Smooth muscles contain a-actinin and tropomyosin molecules, as do skeletal muscles. They do not have the troponin system, and the fight chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Regulation of smooth muscle contraction is myosin-based, unlike striated muscle, which is actin-based. However, like striated muscle, smooth muscle contraction is regulated by Ca. ... 

Thick filaments. Each thick filament contains 200 to 300 myosin molecules. Each myosin molecule is made up of two identical subunits shaped like golf clubs two long shafts wound together with a myosin head, or crossbridge, on the end of each. These molecules are arranged so that the shafts are bundled together and oriented toward the center of the thick filament. The myosin heads project outward from either end of the thick filament (see Figure 11.1, panel a). 

Actin and myosin molecules, and thrombosthenin, are contractile proteins that enable platelets to contract. 

Some of the molecules responsible for hair-cell transduction have been identified. A few key molecules, some already described, have been identified as part of the transduction complex (Fig. 51-6). Myosin molecules clearly play essential roles, and hair cells express a variety of myosin isoforms. Because it is located at the tips of the stereocilia, near the tip-link anchors, myosin-1 c is the best candidate for the adaptation motor. Selective inhibition of a sensitized myosin- lc mutant with an ADP analog proved that this myosin participates in adaptation [14], although contribution by other myosins has yet to be ruled out. For example, mice with near-null mutations in myosin-7a have defects in auditory transduction that are consistent with alterations in the adaptation machinery, suggesting a central role for this myosin too [15]. 

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