FH1FH2mDia2 stabilizes MTs by decreasing rates of MT disassembly. (A–D) Disassembly assays of MTs elongating from axonemal fragments and induced to disassemble by dilution in the presence or absence of the indicated concentrations of His-FH1FH2mDia2 or buffer alone. The rate of MT shortening was monitored by real-time video DIC microscopy. (A) A series of micrographs taken from a real-time video recording of MTs undergoing shortening. Arrowheads point to MT ends shortening in the presence of 2 μM His-FH1FH2mDia2 (mDia2) or buffer alone. Time is shown in seconds. (B) Histogram of the frequency of final shortening velocities of MT plus and minus ends after dilution into 2 μM His-FH1FH2Dia2 or buffer alone. Arrows indicate means for each condition. Data are normalized percentages of total MTs (to account for differences in sample size; n ≥ 53 ends for each condition). Velocities >90 μm/min are pooled in the last bin. (C) Graphs showing the dependence of shortening velocity of MT plus and minus ends on concentration of His-FH1FH2mDia2 (mDia2). Data are mean ± SE of shortening velocities (n ranges from 3 to 27 for each data point). (D) Plots of life histories of representative MT plus ends induced to disassemble by dilution into buffer alone or into buffer with 2 μM His-FH1FH2mDia2. Bar, 5 μm.