Subunit exchange between smooth muscle myosin filaments.

Area of Science:

• Biochemistry
• Cell Biology
• Muscle Physiology

Background:

• Phosphorylated smooth muscle myosin forms stable filaments, while dephosphorylated myosin disassembles in MgATP.
• The dynamic behavior and copolymer stability of smooth muscle myosin filaments were previously unknown.

Purpose of the Study:

• To investigate the stability of copolymers of phosphorylated and dephosphorylated smooth muscle myosin.
• To elucidate the exchange dynamics of myosin molecules within filaments.

Main Methods:

• Gel filtration, sedimentation velocity, and pelleting assays were employed to analyze filament composition.
• Electron microscopy with gold-labeled probes was used to visualize molecular exchange between filaments.

Main Results:

• MgATP dissociated dephosphorylated myosin from copolymers, regardless of other components.
• Myosin molecules were shown to rapidly and extensively exchange between filaments, indicating dynamic structures.
• Filament composition changes in response to solvent conditions, with MgATP promoting disassembly via folded monomers.

Conclusions:

• Smooth muscle myosin filaments are dynamic structures capable of altering composition.
• MgATP-induced disassembly occurs through the dissociation of dephosphorylated myosin, likely via a monomeric pool.