Processivity of the single-headed kinesin KIF1A through biased binding to tubulin

Conventional isoforms of the motor protein kinesin behave functionally not as ‘single molecules’ but as ‘two molecules’ paired. This dimeric structure poses a barrier to solving its mechanism1,2,3,4. To overcome this problem, we used an unconventional kinesin KIF1A (refs 5, 6) as a model molecule. KIF1A moves processively as an independent monomer7,8, and can also work synergistically as a functional dimer9. Here we show, by measuring its movement with an optical trapping system10, that a single ATP hydrolysis triggers a single stepping movement of a single KIF1A monomer. The step size is distributed stochastically around multiples of 8 nm with a gaussian-like envelope and a standard deviation of 15 nm. On average, the step is directional to the microtubule's plus-end against a load force of up to 0.15 pN. As the source for this directional movement, we show that KIF1A moves to the microtubule's plus-end by ∼3 nm on average on binding to the microtubule, presumably by preferential binding to tubulin on the plus-end side. We propose a simple physical formulation to explain the movement of KIF1A.