Abstract
Kinesin motors and their associated microtubule tracks are essential for long-distance transport of cellular cargos 1. Intracellular activity and proper recruitment of kinesins is regulated by biochemical signaling, cargo adaptors and microtubule associated proteins2–4. Here we report on a novel regulatory mechanism of kinesin’s load-bearing capacity by forces across the microtubule track. Using optical tweezers and the three-bead assay5,6 to specifically apply forces parallel to the long-axis of the microtubule, we found that the median attachment duration between kinesin and microtubules under opposing forces is up to 10-fold longer than observed previously using the more conventional single-bead assay, which is likely due to vertical forces imposed by the single bead 7. Using the three-bead assay, we also found that not all the protofilaments are equivalent interacting substrates for kinesin and that the median attachment duration of kinesin varies by more than 10-fold, depending on the relative angular position of the forces along the circumference of the microtubule. Thus, depending on the geometry of forces across the microtubule, kinesin can switch from a fast detaching motor (< 0.2 s) to a persistent motor that sustains attachment (> 3 s) at high forces (5 pN). Our data show that the load-bearing capacity of the kinesin motor is highly variable and can be dramatically affected by off-axis forces and forces across the microtubule lattice which has implications for a range of cellular activites including cell division and organelle transport.