Ordering of the myofilament lattice in muscle fibers

The effect of pH on the muscle filament lattice in skinned rabbit psoas fibers was studied by X-ray diffraction. In relaxed fibers, the intensity of the 11 equatorial reflection, I 11, remained constant between pH 7.0 and pH 6.0 and fell markedly when the pH was decreased to 5.5. The intensity of the 10 reflection was almost constant over this pH range. These results indicate that the thick-filament lattice is more stable than that of the thin filaments, and that the thin filaments are positioned within the thick-filament lattice by a charge-dependent force. In rigor fibers, the decrease in I 11 over this pH range was much smaller, which shows that the thin filament lattice can also be stabilized by the presence of actomyosin crossbridges. These conclusions were confirmed by electron microscopy. Thus, the thin filaments can be positioned in the trigonal positions of the thick-filament lattice by two different mechanisms, one electrostatic and the other steric.