The Role of Cdc42 Regulation in Cell Growth Pattern and Timing

In fission yeast, polarized cell growth stops during division and resumes after cell separation at the end that existed in the previous generation. It is unclear how growth occurs specifically at this end after cell division. Via a pedigree analysis of mutant cells which display a unique polarity phenotype, we show the existence of a memory of growth. This memory of growth exists at the pre-existing cell end, which grew in the previous generation, and facilitates growth at that end in the next generation. This memory of growth gives the pre-existing end some advantage in a competition for active Cdc42, which is a major regulator of polarized growth. In a mutant that has more available active Cdc42, we observe precociously bipolar growth, indicating that, while the memory of growth gives the pre-existing cell end an advantage, this advantage is immaterial when there is enough Cdc42 activity for both cell ends.

The memory of growth promotes growth, but growth does not occur during cell division. Once cell division is complete, the memory of growth is able to promote growth. How is growth activated after cell division? To investigate this, we uncoupled cytokinesis and growth, which are normally sequential events, by delaying cytokinesis with a temporary Latrunculin A treatment. Mitotic cells recovering from treatment initiate end growth without cell separation, displaying a polar elongation sans separation (PrESS) phenotype. PrESS cell ends reactivate Cdc42 before cell separation, but at a fixed time after anaphase B. A candidate screen implicates Rga4, a negative regulator of Cdc42, in this process. We show that Rga4 appears punctate at the cell sides during G2, but is diffuse during mitosis, extending to the ends. While the Morphogenesis Orb6 (MOR) pathway is known to promote cell separation and growth by activating protein synthesis, we find that for polarized growth, removal of Rga4 from the ends is also necessary. Therefore, we propose that growth resumes after division once the MOR pathway is activated and the ends lose Rga4 in a cell-cycle-dependent manner.