Myosin IIA drives membrane bleb retraction.

Membrane blebs form during various cellular processes, including division and migration. These structures arise when the plasma membrane detaches from the underlying actin cortex. Bleb formation involves three distinct phases: nucleation, growth, and retraction. During retraction, the actin cortex reassembles and pulls the bleb back into the cell. Nonmuscle myosin II (MII) is known to play a role in this process. Three MII paralogues—MIIA, MIIB, and MIIC—are present in mammalian cells, each with unique properties. Prior research has shown that MII contributes to cortical reassembly and bleb retraction. However, the specific role of each MII isoform in bleb dynamics remains unclear. This gap motivated investigations into which MII paralogue is responsible for bleb retraction. Understanding this distinction could clarify how different MII isoforms contribute to cell division and motility.

This study aimed to determine which MII isoform is specifically involved in bleb retraction during cytokinesis. The researchers focused on MIIA, MIIB, and MIIC, testing their roles in bleb dynamics. The goal was to identify the specific MII paralogue that drives bleb retraction. The team also sought to understand how MIIA functions in this process. They examined whether the motor domain or the nonhelical tailpiece of MIIA is necessary for bleb retraction. The study aimed to clarify the relationship between MIIA turnover and bleb retraction. This work addresses a key question in cytoskeletal mechanics: which MII isoform is essential for bleb retraction. The findings could help distinguish the functional roles of MII paralogues in cellular processes.

The researchers used live-cell imaging to observe bleb dynamics in mammalian cells. They employed CRISPR-based gene editing to selectively knock out MIIA, MIIB, and MIIC. Fluorescent tagging allowed tracking of MII isoform localization during bleb retraction. The team measured bleb retraction rates in cells lacking specific MII isoforms. They also tested the effects of disrupting the motor domain or the nonhelical tailpiece of MIIA. Cortical turnover of MIIA was quantified using time-lapse imaging. The experiments compared bleb retraction in control cells and cells with MII isoform knockouts. These methods enabled the researchers to identify MIIA as the driver of bleb retraction.

MIIA was found to be specifically responsible for bleb retraction during cytokinesis. Cells lacking MIIA showed significantly reduced bleb retraction. MIIB and MIIC did not compensate for the loss of MIIA. The motor domain of MIIA is necessary for bleb retraction. The nonhelical tailpiece of MIIA also contributes to its function. Faster turnover of MIIA at the cortex correlates with more efficient bleb retraction. Bleb retraction was not observed in MIIA-deficient cells. These findings indicate that MIIA has a unique role in this process.

The authors propose that MIIA is uniquely required for bleb retraction during cytokinesis. Their findings suggest that neither MIIB nor MIIC can substitute for MIIA in this process. The motor domain and nonhelical tailpiece of MIIA both contribute to its function. Faster MIIA turnover at the cortex is associated with more effective bleb retraction. These results clarify the distinct roles of MII isoforms in cellular mechanics. The study provides evidence that MIIA is specifically involved in bleb retraction. The authors suggest that MIIA’s unique properties make it well-suited for this role. These findings may help explain how different MII isoforms contribute to cell division.