Pom1 is not the size ruler

Cell size is under genetic control, and size at division may be mutationally changed so that cells divide at a larger or smaller size without any alteration in the generation time. 1 Larger and smaller cells thus double their size during the same time period, illustrating that larger cells grow faster than smaller cells. However, the increase of cellular growth rate with cell size in certain genetic backgrounds could amplify size heterogeneity of the population in the absence of additional regulation. Therefore, growing cells possess a homeo-static size control mechanism, which shortens the cycle time of larger cells, while it extends the cycle time for smaller ones (Fig. 1). This homeostasis works through the size control mechanisms that regulate the length of G 1 or G 2 phase, thereby controlling the timing of entry into DNA replication or mitosis, respectively (G 1 /s and G 2 /M size controls). The molecular mechanisms underlying these size controls are still obscure. A few years ago, 2 papers published in Nature demonstrated an intracellular concentration gradient of Pom1 kinase in rod-shaped fission yeast cells. The level of the mitotic inhibitory Pom1 kinase was found to be highest at cell tips and minimal around the nucleus in the middle of the cell. As cells grow, the Pom1 concentration at the cell center decreases, thus relieving its mitotic inhibitory effect. These observations lead to the proposal that the Pom1 gradient generated by a reaction–diffusion mechanism could serve as the basis of mitotic size control in fission yeast. A straightforward prediction of this hypothesis, that balanced growth and division should be compromised in Pom1-deleted cells, has been tested now by wood and Nurse 5 and published in the October 1, 2013 issue of Cell Cycle. By using live-cell imaging, the cycle time vs. birth size relationship (for example the green curve on Fig. 1) of Pom1-deleted cells was determined. This analysis showed no significant difference compared with wild-type cells, suggesting that Pom1 and its intracellular gradient cannot play a Figure 1. Mechanism of size homeostasis. The average mass doubling time (red curve) is independent of birth size, if larger cells can double their size during the same time as small ones. The size control mechanism modulates the time between 2 successive divisions (cycle time, green curve). Cells, born at the size where the 2 curves intersect (dashed line), double their size exactly until cell division. Cells …