During cell division, adherent animal cells round up to create the precise spatial geometry required for accurate chromosome segregation, a process that depends on the coordinated remodeling of the cytoskeleton and plasma membrane. 

It is generally assumed that the plasma membrane passively follows cytoskeletal reorganization during this process. However, whether the membrane is subject to active regulation, and the precise mechanism underlying the coordination between the two components, remain unclear.

In a study published in Journal of Cell Biology, a research team led by Prof. LI Hongchang from the Shenzhen Institute of Advanced Technology of the Chinese Academy of Sciences uncovered a fundamental plasma membrane remodeling mechanism that governs the mitotic cell rounding.

Numb is a protein known as a fate determinant in asymmetric cell division. Researchers found that it underwent significant phosphorylation at Ser7, Ser265, and Ser284 during symmetric mitosis. Numb phosphorylation was crucial not only for its proper localization at the plasma membrane but also for efficient membrane retraction during mitotic cell rounding. 

Overexpression of a non-phosphorylatable Numb mutant which remained stably localized at the membrane significantly disrupted membrane retraction in mitotic cell. And impaired Numb phosphorylation disrupted mitotic cell rounding and led to defects in spindle orientation and chromosome segregation.

Through mass spectrometry, co-immunoprecipitation, and actin cortex binding assays, researchers elucidated the molecular mechanism linking Numb phosphorylation to membrane remodeling. They revealed that Numb phosphorylation was not mediated by previously reported kinases aPKC or Plk1, but by Aurora A, a central regulator of mitosis progression.

Numb directly bound to myosin I, a key family of motor proteins that link the plasma membrane to the actin cortex, preventing myosin I from binding to F-actin, thereby weakening the membrane-cytoskeleton linkage. At the onset of mitosis, Aurora A precisely reversed this state by phosphorylating Numb, triggering its dissociation from the plasma membrane. Therefore, myosin I was released and free to rebind the actin cortex. 

This coordinated molecular switch enabled the synchronized retraction of the plasma membrane and remodeling of the cytoskeleton, facilitating efficient mitotic cell rounding. "Our work offers insights into the mechanisms of membrane remodeling in cell division, and may provide potential strategies for mitosis-targeted cancer therapies," said Prof. LI.

Moreover, researchers demonstrated that Numb phosphorylation regulated cell shape during protease-induced rounding, underscoring the relevance of this mechanism across diverse cellular processes involving morphological remodeling.