Kinetochore kinesin CENP-E is a processive bi-directional tracker of dynamic microtubule tips

During vertebrate mitosis, the centromere-associated kinesin CENP-E transports misaligned chromosomes to the plus ends of spindle microtubules. Subsequently, the kinetochores that form at the centromeres establish stable associations with microtubule ends, which assemble and disassemble dynamically. Here we provide evidence that after chromosomes have congressed and bi-oriented, the CENP-E motor continues to play an active role at kinetochores, enhancing their links with dynamic microtubule ends. Using a combination of single molecule approaches and laser trapping in vitro we demonstrate that once reaching microtubule ends, CENP-E converts from a lateral transporter into a microtubule tip-tracker which maintains association with both assembling and disassembling microtubule tips. Computational modeling of this behavior supports our proposal that CENP-E tip-tracks bi-directionally via a “tethered motor” mechanism, which relies on both the motor and tail domains of CENP-E. Our results provide a molecular framework for CENP-E's contribution to the stability of attachments between kinetochores and dynamic microtubule ends.