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

Abstract

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.