Effect of Syns on tubulin polymerization and depolymerization. (a–h) MTs undergoing polymerization with unlabeled tubulin in vitro were visualized using dark-field light microscopy. Tubulin polymerization was performed without Syns (a) and with αSyn (b), βSyn (c), γSyn (d), αSyn S129A (e), αSyn S129E (f), αSyn A30P (g), or αSyn E46K (h). MT length with or without Syns was measured and is shown beneath each image. The median values are indicated by red arrows. Scale bar: 30 μm. (i) Box-and-whisker plots of the lengths of MTs polymerized in vitro (N = 100 for each condition). (j–o) Effect of αSyns on MT stabilization. Unilaterally occurring spontaneous depolymerization was measured in vitro without αSyns (j) and with αSyn (k), αSyn S129A (l), αSyn S129E (m), αSyn A30P (n), or αSyn E46K (o). The distributions of the depolymerization velocities are shown beneath each image set. Spontaneous depolymerization proceeded unilaterally in each case. The pink arrowheads indicate the tips of depolymerizing MTs, and the dotted yellow lines indicate the original lengths of the MTs. The median values are indicated by red arrows. Scale bar: 5 μm. (p) Box-and-whisker plots of the spontaneous depolymerization velocities (N = 80 in each condition). P values in (i) and (p) were calculated with t-test of nonparametric test, mean ± SEM; ***p < 0.001, **p < 0.01, *p < 0.05, “NS” means not significant. See also Supplementary Videos 6 and 7.