Actin filaments diffraction from

In thinking about X-ray diffraction from this assembly, a number of the sarcomere components contribute to the observed patterns in ways that have been the subject of detailed analysis. In the A-band, these include the myosin filament backbone, where the coiled-coil a-helical myosin rods pack together, the myosin head arrays in the bridge regions of the myosin filaments, the non-myosin A-band proteins titin and C-protein (MyBP-C), and the A-band parts of the actin filaments. Very little has been seen in X-ray patterns so far that appears to be related to the M-band, probably... 

Not all actin filaments have 13/6 helical symmetry. For example, in insect flight muscle, as exemplified by Lethocerus, the actin filaments form a 28/13 helix. This also occurs in the vertebrate striated muscle Z-band (Squire et al., this volume. Section III.E Luther and Squire, 2002). The differences between the diffraction patterns from helices with 13/6 and 28/13 symmetry are illustrated in Fig. 10. The diffraction patterns were generated by the program HELIX (Knupp and Squire, 2004), but the program MusLabel can also be used (Squire and Knupp, 2004). Another... 

The diffraction pattern from troponin is very different. Here, there is one troponin complex for each tropomyosin molecule, but the end-to-end repeat along the tropomyosin strands is about 385 A It is longer than the actin filament crossover repeat of just over 357 A in vertebrate muscles (Fig. 11A and B). Because much of the troponin complex is globular, unlike tropomyosin, it shows very marked discontinuous density every 385 A along each strand of the actin filament, with the troponins in opposite strands axially shifted by the actin monomer subunit translation h of... 

When negatively stained by uranyl acetate for electron microscopy, F-actin appears as twisted strings of beads whose diameter varies between 7 and 9 nm (Figure 19-3b). From the results of x-ray diffraction studies of actin filaments and the actin monomer structure shown in Figure... 

Figure Bl.17.12. Time-resolved visualization of the dissociation of myosin SI from filamentous actin (see also figure Bl.17.6). Shown are selected filament images before and after the release of a nucleotide analogue (AMPPNP) by photolysis (a) before flashing, (b) 20 ms, (c) 30 ms, (d) 80 ms and (e) 2 s after flashing. Note the change in obvious order (as shown by the diffraction insert in (a)) and the total dissociation of the complex in (e). The scale bar represents 35.4 mn. Picture with the courtesy of Academic Press.

Fig. 13. Illustrations of the possible arrangement of C-protein (MyBP-C) on the myosin filament backbone in projection down the axis (A) and in axial view (B). Of particular importance here is the possibility that the N-terminal half of C-protein extends out and binds to actin in relaxed muscle. (C) Simulation of the possible interactions of C-protein with binding sites on actin generated using the program MusLABEL (Squire and Knupp, 2004). (D) Left left half of the low-angle X-ray diffraction pattern from bony fish muscle (as in Fig. 11C), showing (right) the possible positions where the C-protein array in (D) might contribute. (From Squire elal, 2003d.)...

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