Lateral and longitudinal compaction of PRC1 overlap zones drives stabilization of interzonal microtubules.

During anaphase, antiparallel-overlapping midzone microtubules elongate and form bundles, contributing to chromosome segregation and the location of contractile ring formation. Midzone microtubules are dynamic in early but not late anaphase; however, the kinetics and mechanisms of stabilization are incompletely understood. Using photoactivation of cells expressing PA-EGFP-α-tubulin we find that immediately after anaphase onset, a single highly dynamic population of midzone microtubules is present; as anaphase progresses, both dynamic and stable populations of midzone microtubules coexist. By mid-cytokinesis, only static, non-dynamic microtubules are detected. The velocity of microtubule sliding also decreases as anaphase progresses, becoming undetectable by late anaphase. Following depletion of PRC1, midzone microtubules remain highly dynamic in anaphase and fail to form static arrays in telophase despite furrowing. Cells depleted of Kif4a contain elongated PRC1 overlap zones and fail to form static arrays in telophase. Cells blocked in cytokinesis form short PRC1 overlap zones that do not coalesce laterally; these cells also fail to form static arrays in telophase. Together, our results demonstrate that dynamic turnover and sliding of midzone microtubules is gradually reduced during anaphase and that the final transition to a static array in telophase requires both lateral and longitudinal compaction of PRC1 containing overlap zones.