Kinetic asymmetry drives kinesin-1's unidirectional and processive movement

Kinesin-1 is a dimeric motor protein that uses ATP hydrolysis energy to move along microtubules in a hand-over-hand manner1. The unidirectional movement of kinesin-1 has traditionally been explained by an ATP-dependent power stroke action of the neck linker2-4, connecting its two catalytic domains (heads), that biases the diffusional motion forward (biased-diffusion). However, recent studies on synthetic molecular motors have supported a Brownian ratchet mechanism based on kinetic asymmetry between two locations (biased- binding)5-9, and which of these mechanisms applies to biological motors remains debated10-12. Here, we engineered a two-headed kinesin that alternately uses these mechanisms to step forward, allowing us to investigate how they contribute to unidirectional movement. The tethered head that uses biased-diffusion frequently rebound to the rear-binding site but eventually stepped forward, as the front head remained securely bound to the microtubule. The biased-binding mechanism proved more efficient by preventing rebinding of the detached head and was independent of ATP binding. Instead, ATP hydrolysis energy is primarily consumed to ensure preferential detachment of the rear head. These findings demonstrate that kinesin-1 functions as an information ratchet based on kinetic asymmetry in microtubule-binding and detachment of the heads, while power strokes serve to enhance movements under load.