The contribution of kinesin-1 versus kinesin-3 to motility of the complex is influenced by the MT state. (A) Characterization of the MT populations in COS7 cells under various NZ treatments. COS7 cells were treated with NZ for the indicated times and then fixed and stained with antibodies to total tubulin (green) and K40-acetyled α-tubulin (red), which marks stable MTs. Bar, 10 µm. (B) Velocity distributions for kin1+kin3 complexes (two-color events) in untreated COS7 cells containing heterogeneous MT tracks (top; n = 106 events), NZ-treated cells containing stable MTs (middle; n = 84 events), and cells containing nascent MTs upon NZ washout (bottom; n = 90 events). The mean velocity ± SE of the kin1+kin3 complexes under each condition is indicated by yellow text in brackets at the top of each graph. The mean velocities ± SE of each peak in the distribution is indicated by black text above the peak. See Fig. S5 (C and D) for histograms of the single kin1 and kin3 motors under the different conditions.