Self-assembly of a fluxing bipolar MT array with dynamic MTs and kinesin-5. (A) Simulations were initiated with MTs in an antiparallel array centered in the zone of high RanGTP concentration (yellow dashed line). MT color indicates orientation (MTs with plus-ends pointed to the right in red, to the left in green) and region (minus-end region in white). (B) MTs experience repulsion at close range (0–50 nm) and attraction between 50 and 90 nm. The shaded area represents the interaction zone. (C) Homotetrameric kinesin-5 (orange) are simple plus end–directed motors, and slide apart antiparallel MTs. (D) Kymograph of selected MT minus-ends (green) and kinesin-5 motors (orange) shows that kinesin-5 remains stationary while bound MTs slide poleward. (E) Bipolar MT structure formed by MTs, cross-linking force, and kinesin-5. Single MTs extend far from the midzone, and poles are not defined.