Filament Structure and the M-Band

One of the tasks of structural biologists studying muscle contraction is to determine the organization and shapes of the myosin head in muscle under different physiological conditions. The technique of low-angle X-ray diffraction has unique advantages in this process, particularly since it can be applied to living muscle, which can be stimulated to produce active force or can be studied under a variety of different steady-state conditions. The main problem with X-ray fiber diffraction, as detailed in Squire and 

Knupp (2005 this volume), is one of interpretation. Unlike X-ray crystallography, low-angle fiber diffraction does not lead directly to images of the diffracting object or to the generation of electron density maps. Setting up models with whatever prior knowledge is available is required, and then the unknowns must be set up as parameters and searched for the best values of these parameters to provide a model that satisfactorily accounts for the observed diffraction patterns. 

A typical low-angle diffraction pattern from relaxed bony fish muscle is shown in Fig. 4B. Much of the intensity that is seen comes from the organization of the myosin heads on the myosin filaments in the resting state (probably mainly MADP.Pi). We know that the myosin heads lie approximately on three co-axial helices of subunit translation 143 A and repeat 429 A. This is most easily represented by the radial net shown in Fig. 16B-D. The radial net in D is like an opened-out surface view of the filament in B. Here the helical tracks become straight lines, and the black blobs represent the origins on the myosin filament surface of the pairs of myosin heads in each myosin molecule. From early studies it is known that the three crowns within the 429 A repeat are not exactly the same and that there is a perturbation. 

One possible way to conduct such a search is to systematically change every single parameter in appropriately small steps and to calculate the diffraction pattern in each case. However, with as many as 20 or so 

Myosin Head Organization in Relaxed Vertebrate Myosin Filaments 

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