Movie S10.

TubRC does not catalyze the addition of TubZ subunits to filaments. Filament seeds (magenta) emanate from (green, minus end) TubRC, whereas TubZ filament extensions grow using differently labeled TubZ (cyan). At equimolar ratios of TubZ to TubRC, cyan extensions are only observed at ends opposite of TubRC; they are hence only extended at the plus end. The motion of the filaments is due to the flow applied to add differently labeled TubZ (cyan). Note that the bottom filament breaks during the flush, forming an antiparallel arrangement (two cyan extensions) in which TuRC molecules move toward each other. The movie plays at 20-fold real time.