Nuclear envelope breakdown (NEBD) is an essential step during the G2/M transition in higher eukaryotic cells. Increasing evidence supports the notion that both microtubules and microtubuleassociated motor proteins are critical regulators of NEBD. Although it has been described that p150 Glued , the major component of the dynein/dynactin complex, localizes in the nuclear envelope during prophase, the exact role of p150 Glued and its regulation during NEBD are largely elusive. Polo-like kinase 1 (Plk1), the best characterized Ser/Thr kinase, is involved in mitotic entry in several systems; however, the targets of Plk1 during NEBD are unknown. Herein, we show that in mammalian cells both Plk1 and p150 Glued regulate NEBD and that Plk1 interacts with and phosphoryates p150 Glued during NEBD at prophase. Using various approaches, we showed that Plk1 phosphorylates p150 Glued at Ser-179 and that the pS179 epitope is generated at the nuclear envelope of prophase cells. Significantly, Plk1-mediated phosphorylation of p150 Glued at Ser-179 positively regulates its accumulation at the nuclear envelope during prophase. Finally, we found that cells expressing the Plk1-unphosphorylatable mutant (p150 Glued -S179A) arrest at G2, as indicated by reduced NEBD, increased levels of cyclin B and phospho-H3, but a decreased level of Cdc2 kinase activity. Taking these data together, we conclude that Plk1 phosphorylation of p150 Glued might be one major pathway of NEBD regulation. mitosis | microtubule dynamics | dynactin