Figure 3. GFP–TipAct allows linear arrays of actin bundles to stably and efficiently template microtubule organization.

Early (t=0) and late (t~40–50 min) time points of microtubule growth within linear arrays of actin bundles with (a), and without (b) GFP–TipAct. (c) Evolution of the distributions of microtubule orientation angle θ_MT, for the experiments in a and b, respectively. Colour gradients indicate the time evolution of the distribution. Each curve is shifted 0.17 a.u. relative to the previous one. Black curves show the time-averaged distribution of actin-bundle orientation angle θ_ACTIN. Evolution of microtubule alignment (d), and microtubule–actin overlap (e), as functions of 〈L_MT〉. Error bars are s.d. The gray-dashed line at 〈L_MT〉=19 μm in d indicates where the trends diverge. Normalized histograms of the difference between microtubule and mean F-actin orientation angle 〈θ_ACTIN〉, for short (f), and long (g), microtubules as defined in d. Data are the average of n=6/9 experiments, comprising 17/9 histograms for long microtubules, and 18/11 histograms for short microtubules, with/without GFP–TipAct, respectively. Error bars are s.d. Dark green and purple lines and insets show fits to equation (1) (see Methods). Data and fits without GFP–TipAct are shifted up 0.8 a.u. Composition of the experiments: 27 μM tubulin, 100 nM EB3 and 500 nM fascin; with (a) and without (b) 50 nM GFP–TipAct. Scale bars, 10 μm; time, h:min:s. MT, microtubule.