Delineating the role of the k-loop in the KIF1A chemomechanical cycle

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The kinesin-3 KIF1A has three prominent biophysical characteristics: fast velocity, superprocessivity, and low resistance to load. Using a combination of single molecule motility assay and stopped-flow spectroscopy, we measured the discrete steps of the chemomechanical cycle. The fast rear-head detachment suggests that the fast velocity originates from a reduced waiting time in the 2-head-bound state, resulting in KIF1A predominantly residing in the vulnerable 1-head-bound state during its stepping cycle and therefore being susceptible to load; while the superprocessivity arises from the relatively high microtubule affinity in the post-hydrolysis state.