To step or not to step? How biochemistry and mechanics influence processivity in Kinesin and Eg5

Abstract

Conventional kinesin and Eg5 are essential nanoscale motor proteins. Single-molecule and presteady-state kinetic experiments indicate that both motors use similar strategies to generate movement along microtubules, despite having distinctly different in vivo functions. Single molecules of kinesin, a long-distance cargo transporter, are highly processive, binding the microtubule and taking 100 or more sequential steps at velocities of up to 700 nm/s before dissociating, whereas Eg5, a motor active in mitotic spindle assembly, is also processive, but takes fewer steps at a slower rate. By dissecting the structural, biochemical and mechanical features of these proteins, we hope to learn how kinesin and Eg5 are optimized for their specific biological tasks, while gaining insight into how biochemical energy is converted into mechanical work.

Figure 1

Schematic of Kinesin-1 towing its cargo along a microtubule (left) while Eg5 crosslinks two anti-parallel microtubules (right). By walking to the plus-ends of each microtubule, Eg5 can slide the two microtubules apart.

Figure 2

Stepping models for Kinesin and Eg5. For kinesin, a processive run begins when a motor head collides with the microtubule and releases its ADP. ATP binding at the forward head leads to neck linker docking and plus-end directed movement of the rearward head that promotes binding at the next site on the microtubule. Release of ADP at step K4 results in a state with both heads bound tightly to the microtubule and the development of intermolecular strain between the two motor domains. ATP hydrolysis occurs on the rearward head followed by phosphate release and rearward head detachment, which returns the motor to K1, having taken a single step. For kinesin, phosphate release is rate-limiting. In contrast to kinesin, Eg5 collides with the microtubule but cannot bind ATP until a conformational change occurs, which is believed to involve movement of the neck linker. This isomerization establishes a processive run (step E2), allowing ATP binding on the forward head. Successive steps in the cycle are similar to those for kinesin but with rate-limiting ATP hydrolysis.