Interplay of actin, ADP and Mg interactions with striated muscle myosin: Implications of their roles in ATPase.

The effects of Mg on the interaction between ADP, a product of the ATPase reaction, and striated muscle myosin-subfragment 1 (S1) were investigated with both functional and spectroscopic methods. Mg inhibited striated muscle myosin ATPase in the presence of F-actin. Significant effects of Mg were observed in both rate constants of NOE build-up and maximal intensities in WaterLOGSY NMR experiments as F-actin concentration increased. In the absence of F-actin, myosin S1 with Mg bound to a fluorescent ADP analog about five-times tighter than without Mg. In the presence of F-actin, the affinity of myosin S1 toward the ADP analog significantly decreased both with and without Mg. The equilibrium titration of myosin-S1 into F-actin revealed that in the presence of ADP the apparent dissociation constant (K) without Mg was more than five-fold smaller than with Mg. Further, we examined effects of F-actin, ADP and Mg binding to myosin on the tertiary structure of myosin-S1 using near UV circular dichroism (CD) spectroscopy. Both in the presence and absence of ADP, there was a Mg-dependent difference in the near UV CD spectra of actomyosin. Our results show that Mg affects myosin-ADP and actin-myosin interactions which may be reflected in myosin ATPase activity.