| During mitosis, "polar ejection forces" (PEFs) are hypothesized to direct prometaphase chromosomes movements by pushing chromosome arms toward the spindle equator. PEFs are postulated to be caused by (a) plus-end directed microtubule (MT) based motor proteins on the chromosome arms, namely chromokinesins, and (b) the polymerization of spindle MTs into the chromosome. However, the exact role of PEFs is unclear, since little is known about the magnitude or form of PEFs. This study employs optical tweezers to recreate the lateral interaction between chromosome arms and MTs in vitro to obtain the first direct measurement of the speed and force of the PEFs developed on chromosome arms. The results include forces that frequently exceed 1 pN, maximum forces of 2--3 pN, and velocities of 83 +/- 56 nm/s; the movements exhibit a characteristic, non-continuous motion that includes displacements of >50 nm, stalls, and backwards slippage of the MT even under low loads. This activity is attributed to chromokinesin motors based on its ATP-dependence, antibody blocking experiments, and quantitative fluorescence. At first glance, this motor activity appears surprisingly weak and erratic, but it is ideally suited for producing PEFs that guide chromosome movements without severely deforming or damaging the local chromosome structure. |
