Chimeras of Kinesin-6 and Kinesin-14 reveal head-neck-tail domain functions and dysfunctions that lead to aneuploidy in fission yeast 

Priyanka Sasmal

Kinesin motors play diverse roles in cells, including spindle assembly and chromosome segregation. Each kinesin has three general domains – the motor head, neck, and tail. As microtubule (MT) motors, kinesins have directionality, walking toward the plus- or minus-end of a MT. Plus-end kinesins have their motor head at the N-terminus, while minus-end kinesins have their motor head at the C-terminus. Interestingly, in vitro data indicate that the motor head does not dictate directionality. Here, we seek to understand the cellular function of each kinesin domain. We systematically created chimeras of fission yeast kinesin-6 Klp9 (a plus-end kinesin localized at the spindle midzone to slide the MTs and elongate the spindle) and kinesin-14 Pkl1 (a minus-end kinesin localized at the spindle poles to focus MTs). Our in vivo data reveal that the tail dictates cellular localization, and in some cases directionality of the motor head; the motor head produces binding and sliding forces affecting spindle function; and the neck modulates the forces of the motor head. Specifically, Pkl1-head, when put on Klp9-neck-tail, walks toward the spindle midzone and slides MTs faster than the wild-type Klp9. This results in spindle breakage and aneuploidy. In contrast, Klp9-head, when put on Pkl1-neck-tail, localizes to the spindle poles, but failed to properly focus MTs, leading to abnormal MT protrusions. This results in asymmetric displacement of the chromosomes and aneuploidy. In vitro reconstitution of purified motors confirms these domain-dependent effects on directionality, speed, and microtubule-binding affinity. Together, these findings reveal distinct mechanistic pathways by which kinesin domain architecture governs motor function and how their dysfunction drives mitotic errors and aneuploidy.