A mitotic cell reshapes itself twice. Roubinet et al. identify two molecular networks that help govern these transformations.
Early in mitosis, the cortex tightens, causing the cell to round up. During anaphase, the poles relax, and the cell equator contracts, allowing the cell to stretch and eventually split. Moesin, which belongs to a family of proteins that link the actin cytoskeleton to the plasma membrane, helps control the cell's shape. Roubinet et al. investigated what regulates the activity and location of moesin.
Moesin's position reflects the cell's form, the researchers found. Until early anaphase, the protein is spread around the cortex, helping to keep the cell spherical. But then moesin begins to disappear from the poles and build up at the equator. Previous work showed that the enzyme Slik switches moesin on early in mitosis. Roubinet et al. discovered that the phosphatase Pp1-87B turns moesin off. Pp1-87B settles on segregating chromsomes during anaphase, which puts it in position to shut down moesin at the cell poles.
Two other enzymes, Pten and Skittles, boost the amounts of the phospholipid PI(4,5)P2 at the cell's midsection, attracting moesin to that part of the cell. The researchers think that the two networks help ensure that moesin's activity surges all around the cortex before concentrating at the equator, allowing the cell to start out spherical and then to elongate and divide.