Diffraction pattern from myosin head array

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... 

Myosin filament structure has been described by Squire et al. (2005). In vertebrate striated muscles the myosin filaments can be described approximately as three-stranded 9/1 helices. The helix pitch is 1287 A, but, because there are three strands and nine subunits in each strand, the structure repeats after C = 1287/3 = 429 A. Figure 12 shows the expected form of the low-angle diffraction pattern from such filaments. The modeling of this structure by X-ray diffraction was described by Squire et al. in terms of the three crowns of heads within each 429 A repeat. The crown repeat of 143 A gives rise to an m = +1 meridional reflection, which has been labeled as the M3 reflection in many muscle studies (as in Fig. 12). The myosin head array also gives rise to layer lines at orders of the repeat of 429 A. The first myosin layer line (ML1) is at 1/429 A-1, the second (ML2) at 2/429 = 1/ 214.5 A-1, and so on. The M3 reflection occurs on the third layer line at 3/429 = 1/143 A-1. 

It was shown in Fig. 27 of Squire et al. (this volume) that the myosin filaments in different muscle types, particularly in invertebrate muscles, have their heads arranged on different surface lattices. There can be different numbers of helical strands and also different axial repeats. However, in all of these other cases the head arrays appear to be perfectly helical. The vertebrate striated muscle myosin filaments are different in that their heads do not lie on perfect helical tracks there is a perturbation (described in Squire et al., this volume, see Fig. 20) that makes the three crowns within a 429 A repeat nonequivalent. This shows up very obviously in the X-ray diffraction patterns from vertebrate striated muscles. If the structure was helical, meridional reflections would be expected to occur only at positions where m = 0, 1, 2, etc., that is, at multiples of 3/429 A-1. In fact, meridional reflections are observed on all of the first few orders of the 429 A repeat, in particular with a strong peak on the second myosin layer line at 214.5 A (M2), and with others at M4, M5, M7, and so on. These are all indications that the helix is not quite perfect. 

The origin of these closely spaced peaks was very quickly shown to be the interference effects observed within the sarcomere because, in the case of C-protein, the diffraction patterns from the two C-zones in a single A-band would interfere, and, in the case of troponin, the diffraction patterns from the two troponin arrays across the Z-band would interfere. Also, in the case of the M3 multiple, the diffraction from the myosin heads in the two bridge regions of a single A-band would interfere (see summary in Squire, 1981 pages 576-582). In the case of the C-zone interference, illustrated in Fig. 21A—C), the diffraction intensity profile from a single C-zone (A) would have a prominent peak at 430 A, but the two C-zones in one A-band would be centered a distance L apart (Fig. 21C). The two C-zones could then be considered as... 

---