Single-headed scallop myosin and regulation

Area of Science:

• Biochemistry
• Molecular Biology
• Muscle Physiology

Background:

• Scallop myosin is a motor protein crucial for muscle contraction.
• Calcium ions (Ca2+) play a regulatory role in scallop muscle function.
• Understanding myosin structure-function relationships is key to elucidating muscle mechanics.

Purpose of the Study:

• To characterize the biochemical and biophysical properties of single-headed scallop myosin (shM).
• To investigate the role of myosin head number and rod attachment in Ca2+ regulation.
• To compare the Ca2+ sensitivity and ATPase kinetics of shM with intact scallop myosin.

Main Methods:

• Purification of single-headed scallop myosin (shM) using papain digestion and hydrophobic interaction chromatography.
• Electron microscopy to determine the structure of shM.
• Calcium-binding assays to measure association constants.
• Enzymatic assays to determine ATPase activity and kinetics under varying Ca2+ concentrations.
• Single-turnover experiments using radiolabeled ATP.

Main Results:

• shM preparation contained equimolar essential and regulatory light chains with an intact heavy chain and rod.
• shM displayed a higher Ca2+ binding affinity (11 x 10^6 M-1) compared to intact myosin (5 x 10^6 M-1).
• Ca2+-activated ATPase activity of shM showed a 3-fold increase and closely paralleled Ca2+ binding, unlike the cooperative activation of intact myosin.
• Single-turnover kinetics revealed a significantly faster ATPase cycle for shM (<30 s) compared to intact myosin (~3 min).

Conclusions:

• The regulatory mechanism of scallop myosin by Ca2+ is influenced by the presence of two heads.
• The attachment of the myosin head to the coiled-coil rod is critical for the cooperative Ca2+ regulation observed in intact scallop myosin.
• shM provides a valuable tool for dissecting the contributions of individual myosin heads to enzymatic activity and regulation.