The human chromokinesin Kid is a plus end‐directed microtubule‐based motor

Kid is a kinesin‐like DNA‐binding protein known to be involved in chromosome movement during mitosis, although its actual motor function has not been demonstrated. Here, we describe the initial characterization of Kid as a microtubule‐based motor using optical trapping microscopy. A bacterially expressed fusion protein consisting of a truncated Kid fragment (amino acids 1–388 or 1–439) is indeed an active microtubule motor with an average speed of ∼160 nm/s, and the polarity of movement is plus end directed. We could not detect processive movement of either monomeric Kid or dimerizing chimeric Kid; however, low levels of processivity (a few steps) cannot be detected with our method. These results are consistent with Kid having a role in chromosome congression in vivo , where it would be responsible for the polar ejection forces acting on the chromosome arms.