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J. Biol. Chem., Vol. 257, Issue 23, 13880-13883, Dec, 1982
JR Sellers, E Eisenberg and RS Adelstein
Phosphorylation of the 20,000-dalton light chains of smooth muscle heavy
meromyosin (HMM) from turkey gizzards results in a large increase in the
actin-activated MgATPase activity over that observed with unphosphorylated
HMM. In an attempt to define which step in the kinetic cycle is affected by
phosphorylation, we have measured the binding of both unphosphorylated and
phosphorylated HMM to actin in the presence of ATP using sedimentation.
There was only a 4-fold difference in the actin binding constants of
unphosphorylated HMM (5.35 x 10(3) M-1) and fully phosphorylated HMM (2.35
x 10(4) M-1). In contrast, the maximum rate of the actin-activated MgATPase
activity (Vmax) of phosphorylated HMM was 25 times greater than that for
unphosphorylated HMM. These data rule out a mechanism whereby the
unphosphorylated light chain of myosin regulates actin-myosin interaction
by directly or indirectly blocking the binding of HMM to actin. This
implies that some step in the kinetic cycle other than the binding of HMM
to actin must be regulated. We have also measured the rate constant for ATP
hydrolysis (the initial phosphate burst) under the same conditions and
found that this step was very fast compared to the steady state ATPase rate
and was unaffected by phosphorylation. This suggests that the step which is
regulated by phosphorylation is either phosphate release or a step
preceding phosphate release but following ATP hydrolysis.
The binding of smooth muscle heavy meromyosin to actin in the presence of ATP. Effect of phosphorylation
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