Microtubule depolymerization can drive poleward chromosome motion in fission yeast

Prometaphase kinetochores interact with spindle microtubules (MTs) to establish chromosome bi-orientation. Before becoming bi-oriented, chromosomes frequently exhibit poleward movements (P-movements), which are commonly attributed to minus end-directed, MT-dependent motors. In fission yeast there are three such motors: dynein and two kinesin-14s, Pkl1p and Klp2p. None of these enzymes is essential for viability, and even the triple deletion grows well. This might be due to the fact that yeasts kinetochores are normally juxtapolar at mitosis onset, removing the need for poleward chromosome movement during prometaphase. Anaphase P-movement might also be dispensable in a spindle that elongates significantly. To test this supposition, we have analyzed kinetochore dynamics in cells whose kinetochore-pole connections have been dispersed. In cells recovering from this condition, the maximum rate of poleward kinetochore movement was unaffected by the deletion of any or all of these motors, strongly suggesting that other factors, like MT depolymerization, can cause such movements in vivo. However, Klp2p, which localizes to kinetochores, contributed to the effectiveness of P-movement by promoting the shortening of kinetochore fibers.