Abstract

Mitotic chromosome alignment is essential for the robust separation of genetic material into daughter cells. In mammalian cells, this process requires the function of Kif18A, a kinesin-8 motor protein. Kif18A confines chromosome movement to the mitotic spindle equator by accumulating at the plus-ends of kinetochore microtubule bundles (K-fibers), where it functions to suppress K-fiber dynamics. It is not understood how the motor accumulates at K-fiber plus-ends, a difficult feat requiring the motor to navigate protein dense microtubule tracks. Our data indicate that Kif18As relatively long (17 amino acid) neck linker is required for the motors accumulation at K-fiber plus-ends. Shorter neck linker (sNL) variants of Kif18A display a deficiency in K-fiber accumulation, especially on K-fibers near the center of the spindle. This pattern correlates with the more uniform concentration of the microtubule bundling protein HURP on central K-fibers compared to peripheral K-fibers. Depletion of HURP permits Kif18A sNL to accumulate on central K-fibers, while HURP overexpression reduces wild-type Kif18As ability to accumulate on this same K-fiber subset. Furthermore, single molecule assays indicate that Kif18A sNL motors are less proficient at navigating microtubules coated with the microtubule associated protein tau. Taken together, these results support a model in which Kif18As neck linker length permits efficient navigation of obstacles such as HURP to reach K-fiber ends during mitosis.\n\nSignficiance StatementKinesin motor proteins play key roles in controlling chromosome alignment and segregation during cell division. The kinesin Kif18A confines chromosomes to the middle of the spindle by accumulating at the ends of microtubules attached to chromosomes. We show here that Kif18As ability to accumulate at the end of these microtubules requires navigation of microtubule-associated protein obstacles, and that this activity is imparted by a relatively long neck linker region. These findings demonstrate a molecular mechanism for navigation of densely populated microtubules inside a cell.