The modulatory effect of MgATP on heterotrimeric smooth muscle myosin phosphatase activity.

Regulation of the enzymatic activity of heterotrimeric smooth muscle myosin phosphatase (SMMP) by MgATP was examined using phosphorylated myosin (P-myosin), heavy meromyosin (P-HMM), subfragment-1 (P-S1), and 20 kDa myosin light chain (P-MLC(20)) as substrates. The activity toward P-myosin and P-HMM was dose-dependently reduced by MgATP, whereas that toward P-S1 or P-MLC(20) was unchanged. The reduction was mainly due to a decrease in the affinity of SMMP for the substrate with the unchanged maximum activity. This regulation is entirely new in the respect that the responsible molecule is the substrate, not SMMP. Because P-myosin derived from myosin stored in 50% glycerol at -20 degrees C was insensitive to MgATP, the proper integrity of P-myosin is required. Coexisting myosin did not affect this regulation, but it inhibited the SMMP activity in the absence of MgATP. With P-myosin, the enzyme activity was biphasically steeply dependent on the ionic strength. This requires that determinations are conducted with a fixed ionic strength. The Q(10) value was about 2, which was quite similar to that for myosin light chain kinase. These results suggest that the rate of dephosphorylation of P-myosin is lowered at rest, but that it may reach a value comparable to the rate of phosphorylation of myosin in the sarcoplasm with the increased level of P-myosin during muscle activation. This regulation by MgATP may underlie the "latch mechanism" in some respects.