What is the approximate step size of Myosin V during intracellular transport?
Myosin V stride size uniquely adapted to actin filament helical symmetry enabling straight long distance walks without spiraling around filament axis. The molecule possesses six IQ motifs per heavy chain creating extended lever about 24 nm much longer than myosin II lever. During processive movement heads alternate hand over hand trailing head detaches after ATP binding swings forward as lever rotates about 100 degrees. Actin filament repeats every 13 monomers corresponding to 36 nm axial distance where binding site orientation repeats facing same direction. Center of mass therefore advances 36 nm per ATP hydrolysis cycle matching repeat. Detailed single molecule tracking using optical trap and fluorescence showed individual head moves 72 nm per step while other remains attached ensuring dimer spans two helical repeats. This large step allows melanosomes vacuoles and mRNA to be carried efficiently through cortical actin and explains why artificial short lever mutants show reduced processivity and frequent detachment. Coordination via internal strain dependent gating prevents both heads detaching simultaneously maintaining high duty ratio.
Ref: Mehta et al., Nature 1999; Purcell et al., PNAS 2005 – Myosin V 36 nm center and 72 nm head step mechanism.