Maintaining a neutral range disperses myosin molecules under salt-free conditions

Skeletal muscle myosin is generally considered insoluble under physiological, low ionic strength, or salt-free conditions due to its tendency to self-assemble into filamentous polymers in vitro. Our previous study showed that myosin can be solubilized in low ionic strength solutions containing l-histidine. However, another report suggested that 1-methylhistidine could not solubilize myosin, and the factors essential for myosin solubilization remain unclear. To elucidate the role of l-histidine in the water solubilization of myosin, we examined myosin solubility and the molecular properties of its rod domain, l-meromyosin, using structurally related buffer compounds. Under salt-free conditions, solubility depended heavily on the acid dissociation constant of buffer, indicating that maintaining a neutral pH is critical. The rod domain showed molecular elongation regardless of the buffer type, yet surface charge and hydrophobicity remained comparable to conditions with high ionic strength. These results suggest that myosin is inherently soluble and maintains its structural integrity under neutral, salt-free conditions. The apparent insolubility under such conditions is likely to result from hydrochloric acid used for pH adjustment. Since l-histidine and imidazole achieve neutrality without acid addition, they are ideal buffers for myosin solubilization.