The myosin cardiac loop participates functionally in the actomyosin interaction.

Area of Science:

• Biochemistry
• Molecular Biology
• Cellular Biophysics

Background:

• Myosin, a motor protein, converts chemical energy from ATP into mechanical work via actin interaction.
• The actomyosin interaction is mediated by myosin's actin-binding site, including surface loops that influence affinity and ATPase activity.
• The cardiac or C-loop, a structured surface loop on beta-cardiac myosin, is implicated in myosin ATPase regulation and actomyosin interaction.

Purpose of the Study:

• To investigate the functional role of the C-loop in beta-cardiac myosin and smooth muscle myosin.
• To determine the C-loop's contribution to actomyosin affinity and the regulation of myosin ATPase velocity.

Main Methods:

• Proteolytic digestion of the C-loop in beta-cardiac myosin.
• Site-directed mutagenesis of a charged residue within the C-loop of smooth muscle myosin.
• Assays for actin-activated myosin ATPase and actomyosin affinity in rigor.

Main Results:

• Proteolytic digestion of the C-loop abolished actin-activated myosin ATPase and reduced actomyosin affinity over 100-fold.
• Mutagenesis of a charged C-loop residue in smooth muscle myosin altered actomyosin affinity.
• Mutagenesis studies showed a doubling of in vitro motility and actin-activated ATPase velocities.

Conclusions:

• The C-loop is essential for actomyosin interaction, significantly impacting affinity and ATPase regulation.
• A charged region within the C-loop appears to be involved in electrostatic binding to actin.
• The C-loop is likely a crucial component of the energy transduction machinery in myosins.