Myosin drawing

FIGURE 17.21 A drawing of the arrangement of the elastic protein titin in the skeletal mnscle sarcomere. Titin filaments originate at the periphery of the M band and extend along the myosin filaments to the Z lines. These titin filaments produce the passive tension existing in myofibrils that have been stretched so that the thick and thin filaments no longer overlap and cannot interact. (Adapted from Ohtsuki, ., Maruyama, K, and Ebashi,. S ., 1986. Advances ia Protein Chemisti y 38 1—67.)... 

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 muscle is a highly organized tissue, built up of individual cells known as fibres, which are held together by connective tissue. Each muscle fibre consists of a high number of single strands of myofibrils. The myofibrils are again comprised of myofilaments. The myofilaments are divided into thin and thick filaments, which mainly contain two filamentary proteins, actin and myosin, respectively. The myofibrils occupy approximately 80% of the muscle cell volume, and the majority of the water, which makes up about 75% of the muscle, is located in the spaces between thin and thick filaments. A schematic drawing of muscle structure is shown in Fig. 1. 

You may be asked to draw a diagram of the sarcomere. It is made up of actin and myosin filaments, as shown below. The thick myosin filaments contain many crossbridges, which, when activated, bind to the thin actin filaments. Tropomyosin molecules (containing troponin) run alongside the actin filaments and play an important role in excitation-contraction coupling. 

FIGURE 5-33 Molecular mechanism of muscle contraction. Conformational changes in the myosin head that are coupled to stages in the ATP hydrolytic cycle cause myosin to successively dissociate from one actin subunit, then associate with another farther along the actin filament. In this way the myosin heads slide along the thin filaments, drawing the thick filament array into the thin filament array (see Fig. 5-32). 

Figure 19-10 (A) An approximate scale drawing of the myosin molecule. The "hinge" is a region that is rapidly attacked by trypsin to yield the light and heavy meromyosins (LMM and HMM). Total length -160 nm, molecular mass, 470 kDa two -200-kDa heavy chains two pairs of 16- to 21-kDa light chains heads -15 x 4 x 3 nm. (B) Electron micrograph of rabbit myosin monomers that became dissociated from thick filaments in the presence of ATP, fixed and shadowed with platinum.127 Courtesy of Tsuyoshi Katoh.

The drawing is not to scale any given cell would have many more myosin molecules on its surface. 

Figure 8.10 Schematic drawing of the myosin molecule. Note that trypsin produces two fragments, light and heavy meromyosins.

Comparison of the amino acid sequences of myosins, kinesins, and dyneins did not reveal significant relationships between these protein families but, after their three-dimensional structures were determined, members of the myosin and kinesin families were found to have remarkable similarities. In particular, both myosin and kinesin contain P-loop NTPase cores homologous to those found in G proteins. Sequence analysis of the dynein heavy chain reveals it to be a member of the AAA subfamily of P-loop NTPases that we encountered previously in the context of the 19S proteasome (Section 23.2.21. Dynein has six sequences encoding such P-loop NTPase domains arrayed along its length. Thus, we can draw on our knowledge of G proteins and other P-loop NTPases as we analyze the mechanisms of action of these motor proteins. 

To understand how a muscle contracts, consider the Interactions between one myosin head (among the hundreds In a thick filament) and a thin (actin) filament as diagrammed In Figure 3-25. During these cyclical Interactions, also called the cross-bridge cycle, the hydrolysis of ATP Is coupled to the movement of a myosin head toward the Z disk, which corresponds to the (+) end of the thin filament. Because the thick filament Is bipolar, the action of the myosin heads at opposite ends of the thick filament draws the thin filaments toward the center of the thick filament and therefore toward the center of the sarcomere (Figure 19-23). This movement shortens the sarcomere until the ends of the thick filaments abut the Z disk or the (—) ends of the thin filaments overlap at the center of the A band. Contraction of an Intact muscle results from the activity of hundreds of myosin heads on a single thick filament, amplified by the hundreds of thick and thin filaments In a sarcomere and thousands of sarcomeres In a muscle fiber. 

Fig. 4. Muscle proteins. Positions of myosin heads (S-1), actin monomers (A) and troponin (circies) in active and relaxed states shown in relationship to a cross section of decorated actin. S-1 a, S-1 b and S-1 c indicate the domains of the HMM. Aa and Ab are the domains of the actin monomers. The circles labeled a" show troponin fibrils in the active position b" and "c are possible positions in the relaxed state. This drawing is not a helical projection but a schematic view of two actin monomers (the one on top is 27 A above the other one and rotated by about 167° around the filament axis) and their associated S1s. The SI on the left is therefore 27 A above the SI on the right hand side of the drawing. In three dimensions, additional S1s would be in contact with both of the actin subunits shown. Therefore it only appears in this two-dimensional drawing that the tropomyosin and SI do not make equivalent contacts with the two ac-tins. [Used with permission from E.H.Egelman J. Muse. Res. Cell. Moti7. 6(1985) 129-151]...

Fig. 9A, B A Optically-sectioned SHG image series showing the alternation of signal intensity as the angle between the myosin/actin filaments and fundamental polarization (shown in arrow) varies. B Schematic drawing showing the relative directions of the assigned parameter. Scale bar. 20 pm...

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