Step-size is determined by neck length in myosin V

Area of Science:

• Molecular Biology
• Biophysics
• Cellular Motor Proteins

Background:

• Myosin V is a highly processive motor protein essential for intracellular transport.
• Its long neck region, featuring six calmodulin-binding IQ motifs, enables multiple steps per cycle.
• Understanding myosin V's processivity mechanism is crucial for deciphering its role in cellular functions.

Purpose of the Study:

• To investigate the relationship between myosin V neck length and its step-size during movement on actin filaments.
• To elucidate the contribution of IQ motifs to myosin V's processive motility.
• To test the swinging lever-arm and hand-over-hand models of myosin V movement.

Main Methods:

• Engineered myosin V constructs with varying numbers of IQ motifs (2, 4, 6, 8) and lacking the globular tail domain.
• Fluorescently labeled myosin V mutants using single Cy3-labeled calmodulin.
• Measured step-size and kinetics of individual myosin V mutants using Fluorescence Imaging with One Nanometer Accuracy (FIONA).

Main Results:

• Step-size was directly proportional to the neck length (number of IQ motifs) in myosin V constructs.
• Kinetics data supported the hand-over-hand model of myosin V's two-headed action.
• The 8IQ mutant exhibited a broad distribution of step-sizes, indicating multiple actin binding site choices.

Conclusions:

• Myosin V's step-size is significantly influenced by its neck region's length, not solely by actin's structure.
• The study provides strong evidence for the swinging lever-arm model and supports the hand-over-hand mechanism in myosin V motility.
• Neck length is a key determinant of myosin V's step size and adaptability on actin filaments.