CDK1-mediated phosphorylation of EPLIN is crucial for mitotic spindle positioning

Correct anchoring of the mitotic spindle to the cell cortex is essential for proper cell division. However, how the architecture of the cell cortex is coordinately remodeled during mitotic progression remains poorly understood. Here, we show that depletion of EPLIN (an actin-bundling protein) causes spindle deviation and an arrest in metaphase, accompanied with abnormal cortical actin accumulation and blebbing. We identify the Ser203 of EPLIN as the major phosphorylation site for CDK1 in mitosis and demonstrate that this phosphorylation event is required for EPLIN to maintain proper actin cortex and correct spindle positioning. Moreover, EPLIN phosphorylation at Ser203 enhances its capability to reorganize branched actin filaments into astral structures in vitro, with which EPLIN may sequester actin bundles and the Arp2/3 complex, and thereby prevent excessive de novo actin polymerization. Thus, this study unveils an important role for EPLIN in spindle positioning via its function to maintain proper architecture of the actin cortex during mitosis.