Kif18b₅₉₀ depolymerizes microtubules. (A) Example kymographs of GMP-CPP–stabilized microtubules in the absence and presence of 50 nM Kif18b. (B) Depolymerization rates for GMP-CPP–stabilized microtubules with no added motor (n = 97) and 50 nM Kif18b (n = 125; mean ± SEM). Unpaired t test significance: ****, P < 0.0001. (C) Example kymograph of Kif18b₅₉₀ in 0.1 mM ATP. (D) Histogram plot of Kif18b_1–590 velocities determined from kymograph in A; n = 229. (E) Run Length frequency (100-nm bins), single exponential fit, parameter 240 ± 10 nm; no run lengths were measurable <400 nm. n = 229. (F) Example kymographs of rhodamine-labeled dynamic microtubules in 7 µM tubulin with 0 and 50 nM dimeric Kif18b₅₉₀-GFP. (G) Measured growth speeds of dynamic extensions in the presence of 7 µM tubulin and 0, 12.5, and 50 nM dimeric Kif18b₅₉₀-GFP. Asterisks indicate ANOVA significance: ****, P < 0.0001. (H) Catastrophe frequencies of dynamic microtubule extensions in the presence of 7 µM tubulin and 0, 12.5, and 50 nM dimeric Kif18b₅₉₀-GFP. Each data point corresponds with the catastrophe frequency for an individual microtubule. Asterisks indicate Kolmogorov–Smirnov significance: ***, P < 0.001; ****, P < 0.0001. (I) Mean lengths of dynamic microtubule extensions in the presence of 7 µM tubulin and 0, 12.5, and 50 nM dimeric Kif18b₅₉₀-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 < 0.05. Error bars represent SEM.