The amount of actin portion dissociated from myosin B [prepared from carp and white croaker] in the presence of NaCl

When myosin B was kept at 2-30°C in a medium containing 40mM Tris-maleate (pH7.0) and 0.3-2.0M NaCl, soluble proteins in a medium of low ionic strength (50mM KCl, pH7.0) increased. SDS-polyacrylamide gel electrophoretograms exhibited that there was a large amount of actin in this soluble fraction. The relative amount (%) of soluble actin on the basis of actin content in native myosin B was then estimated by a staining density on the gel rod. It was found that the amount of actin dissociated from myosin B increased as the treatment with NaCl proceeded. Thus, the maximum liberation of actin amounted to approximately 60% within 1-2h upon addition of 2M NaCl (for carp) or 1M NaCl (for white croaker). It was also found that the actin component in such a soluble form does not show activating effect on myosin Mg-ATPase nor inhibiting effect on myosin EDTA-ATPase. Based on the above findings, we confirmed that actin portion of white croaker myosin B is more sensitive to NaCl compared to that of carp myosin B.