Editing Cell growth (section)

==== Cell polarity factors ====
Cell polarity factors positioned at the cell tips provide spatial cues to limit Cdr2 distribution to the cell middle. In fission yeast ''[[Schizosaccharomyces pombe]]'' (''S. Pombe''), cells divide at a defined, reproducible size during mitosis because of the regulated activity of Cdk1.<ref>{{cite journal |doi=10.1016/S0168-9525(02)02745-2 |author=Rupes I |title=Checking cell size in yeast |journal=Trends Genet. |volume=18 |issue=9 |pages=479–85 |date=September 2002 |pmid=12175809 }}</ref> The cell polarity protein kinase [[Pom1]], a member of the dual-specificity tyrosine-phosphorylation regulated kinase (DYRK) family of kinases, localizes to cell ends. In Pom1 knockout cells, Cdr2 was no longer restricted to the cell middle, but was seen diffusely through half of the cell. From this data it becomes apparent that Pom1 provides inhibitory signals that confine Cdr2 to the middle of the cell. It has been further shown that Pom1-dependent signals lead to the phosphorylation of Cdr2. Pom1 knockout cells were also shown to divide at a smaller size than wild-type, which indicates a premature entry into mitosis.<ref name=Moseley09/>

Pom1 forms polar gradients that peak at cell ends, which shows a direct link between size control factors and a specific physical location in the cell.<ref>{{cite journal |vauthors=Padte NN, Martin SG, Howard M, Chang F |title=The cell-end factor pom1p inhibits mid1p in specification of the cell division plane in fission yeast |journal=Curr. Biol. |volume=16 |issue=24 |pages=2480–7 |date=December 2006 |pmid=17140794 |doi=10.1016/j.cub.2006.11.024 |doi-access=free |bibcode=2006CBio...16.2480P }}</ref> As a cell grows in size, a gradient in Pom1 grows. When cells are small, Pom1 is spread diffusely throughout the cell body. As the cell increases in size, Pom1 concentration decreases in the middle and becomes concentrated at cell ends. Small cells in early G2 which contain sufficient levels of Pom1 in the entirety of the cell have inactive Cdr2 and cannot enter mitosis. It is not until the cells grow into late G2, when Pom1 is confined to the cell ends that Cdr2 in the medial cortical nodes is activated and able to start the inhibition of Wee1. This finding shows how cell size plays a direct role in regulating the start of mitosis. In this model, Pom1 acts as a molecular link between cell growth and mitotic entry through a Cdr2-Cdr1-Wee1-Cdk1 pathway.<ref name=Moseley09/> The Pom1 polar gradient successfully relays information about cell size and geometry to the Cdk1 regulatory system. Through this gradient, the cell ensures it has reached a defined, sufficient size to enter mitosis.