A motor-domain construct of KIF1A, a single-headed kinesin superfamily protein, was demonstrated to take more than 600 steps before detaching from a microtubule. However, its molecular mechanism remained unclear. Here we demonstrate the nucleotide-dependent binding between the lysine-rich, highly positively charged loop 12 of the KIF1A motor domain (K-loop) and the glutamate-rich, highly negatively charged C-terminal region of tubulin (E-hook). This binding did not contribute in the strong binding state but only in the weak binding state. This binding was demonstrated to be essential for the single-headed processivity by functioning as the anchor for the one-dimensional simple Brownian movement in the weak binding state. This Brownian movement will allow the small KIF1A motor domain to span the distance between the binding sites on microtubule and also will give the diffusive nature to the movement of single KIF1A molecules. These observations quantitatively fitted well to the predictions made from our Brownian motor model on the mechanism of the single-headed processive movement.
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