Key Points
- To investigate whether LC20 light chain phosphorylation is required to activate the actin-dependent ATPase activity of filamentous calf aorta smooth muscle myosin.
- Measured actin-activated MgATPase activity across filamentous calf aorta myosin in unphosphorylated (5% LC20 phosphorylated) and phosphorylated states.
- Determined maximum reaction velocity (Vmax) and the actin concentration required for half-maximal velocity (Kapp) in the presence and absence of tropomyosin.
- Unphosphorylated filamentous aorta myosin demonstrated substantial actin-activated ATPase activity, reaching a Vmax that was one-half (50%) of that observed for phosphorylated myosin.
- Phosphorylation of LC20 produced up to a 10-fold decrease in Kapp in the presence of tropomyosin, substantially increasing the myosin's apparent affinity for actin.
- At low concentrations of tropomyosin and actin, stimulation of actin-activated ATPase activity increased linearly with the fraction of phosphorylated LC20.
Structured PICO
PPopulationCalf aorta myosins (unphosphorylated and phosphorylated)
IInterventionActin and tropomyosin
CComparatorUnphosphorylated vs phosphorylated myosin
OOutcomeActin-activated ATPase activity (Vmax and Kapp)surrogate
The relatively high actin-activated ATPase activity of unphosphorylated aorta myosin suggests that proteins other than LC20 phosphorylation may be involved in regulating smooth muscle contraction.