Kif18b accumulates at the tips of growing microtubules and acts as a catastrophe factor to reduce microtubule length. (A) Kif18b tracks the tips of growing microtubules. Example kymographs of rhodamine-labeled dynamic microtubules in 7 µM tubulin with 0 and 100 nM dimeric Kif18b-GFP. (B) Measured growth speeds of dynamic extensions in the presence of 7 µM tubulin and 0, 50, and 100 nM dimeric Kif18b-GFP. Asterisks indicate ANOVA significance: ****, P < 0.0001. (C) Catastrophe frequencies of dynamic microtubule extensions in the presence of 7 µM tubulin and 0, 50, and 100 nM dimeric Kif18b-GFP. Each data point corresponds with the catastrophe frequency for an individual microtubule. Asterisks indicate Kolmogorov–Smirnov significance: ***, P < 0.001. (D) Mean lengths of dynamic microtubule extensions in the presence of 7 µM tubulin and 0, 50, and 100 nM dimeric Kif18b-GFP. Each data point corresponds with the mean length of the rhodamine-labeled extension from a Hilyte₆₄₇-labeled seed over the course of a kymograph. Asterisks indicate Kolmogorov–Smirnov significance: **, P < 0.002. Error bars represent SEM. (E) Table containing the microtubule dynamic parameters (mean + SEM) in the absence and presence of 50 and 100 nM Kif18b-GFP.