Origin of fast backward steps and detachment of kinesin motor under backward load

Kinesin-1 protein is a biological molecular motor that can step processively along microtubules toward the plus end (the forward direction) and can also step backward occasionally under small backward loads. Puzzlingly, recent high-temporal-resolution optical trapping data showed that under large backward loads, besides the conventional slow backward steps and detachment with dwell times (> 3 ms) dependent sensitively on the load and ATP concentration, the fast backward steps and detachment with dwell times (sub-milliseconds) independent on the load and ATP concentration are also present. While the slow backward steps and detachment can be understood easily, the origin of the fast backward steps and detachment is elusive. The explanation of the origin is critical to the mechanochemical coupling mechanism of the motor. Here, based on our proposed stepping pathway, we study numerically the dwell times and fractions of the forward steps, slow backward steps, slow detachment, fast backward steps and fast detachment under the backward loads in a wide range. The numerical results explain well the optical trapping data. The physical origin of the fast backward steps and detachment is explained.