An ER network is transported by myosin V during mitosis.

During mitosis, the movement of membrane-bound organelles within a cell is generally believed to slow down so that organelles can partition accurately into daughter cells. Using a new procedure for studying Xenopus oocyte extracts, Wöllert et al. now report on page 571 that the picture is more complicated.

Previous studies have focused primarily on microtubule-based organelle motility, whereas the new work examined F-actin–based movement. The authors used extracts from interphase or metaphase-arrested oocytes, and developed an in vitro procedure to produce stable three-dimensional microtubule-free F-actin networks from the extracts. Motion analysis shows that the movement of ER membranes and globular vesicles on F-actin is differentially regulated throughout the cell cycle. During mitosis, the ER speeds up while globular vesicles slow down. Myosin V is responsible for ER transport in this system, and the motor appears to be indirectly regulated by the kinase CaMKII.

Wöllert et al. propose that F-actin takes over the job of moving the ER during mitosis—a time when microtubules are diverted for mitotic spindle assembly and chromosome segregation. Unlike the Golgi apparatus, the ER does not fragment into small vesicles during mitosis, so its increase in motility and fusion may be a way of dividing the ER equally between daughter cells. ▪