A driving and coupling "Pac-Man" mechanism for chromosome poleward translocation in anaphase A.

During mitosis, chromatid harnesses its kinetochore translocation at the depolymerizing microtubule ends for its poleward movement in anaphase A. The force generation mechanism for such movement remains unknown. Analysis of the current experimental results shows that the bending energy release from the bound tubulin subunits alone cannot provide sufficient driving force. Additional contribution from effective electrostatic attractions between the kinetochore and the microtubule is needed for kinetochore translocation. Interestingly, as the kinetochore moves to inside the microtubule, the microtubule tip is free to bend outward so that the instantaneous distance between the kinetochore and the microtubule tip is much closer than the rest of the microtubule. This close contact yields much larger electrostatic attraction than that from the rest of the microtubule under physiological ionic conditions. As a result, the effective electrostatic interaction hinders the further kinetochore poleward translocation until the microtubule tip dissociates. Thus, the kinetochore translocation is strongly coupled at the depolymerizing microtubule end. This driving-coupling mechanism indicates that the kinetochore velocity is largely controlled by the microtubule dissociation rate, which explains the insensitivity of kinetochore velocity to its viscous drag and the large redundancy in its stalling force.