fered from each other in their growth patterns after separation. After most divisions, one daughter grew in a monopolar 193022-04-7 manner using its old tip, and the other grew in a bipolar manner from 
the beginning of the cell cycle. In the remaining 24% of divisions, cells grew either at both old ends or at one new and one old end. Lineage analysis of pta2D cells revealed that monopolar and bipolar cells were both able to generate one monopolar and one bipolar daughter after division. All bipolar daughter cells contained the scar from the previous division, similar to what has been reported for for3D mutant, though how the scar contributes to cell polarity is not clear. 6 PP2A Role in S. pombe Morphogenesis Surprisingly, monopolar and bipolar daughters enter mitosis at different sizes. Bipolar cells always divided at volumes around 15% larger than their monopolar sisters. This is the first example, to our knowledge, where in the fission yeast two daughter cells enter mitosis at different sizes after a symmetric division in a previous mitosis. Analysis of growth rates and cell division timing in the two daughters has revealed that growth rates were very different in monopolar and bipolar cells , whereas the timing of mitotic entry was similar in the two daughter cells . We interpret these observations as evidence that G2/M cell size control checkpoint that coordinates growth and division in S. pombe is unable to detect and/or compensate for the difference in growth rates between the two daughters in pta2D background. Thus, lack of PP2A function in S. pombe, in agreement with previously reported results, is negatively regulating cell size at division. Some of this regulation may be due to the ability of PP2A to regulate growth pattern and growth rates. The premature mitotic entry in pta2D cells resembles the reported phenotypes of both the catalytic and regulatory B-type subunits of PP2A phosphatase, consistent with synthetic lethality that we observe in pta2Dwee1-50 cells and with a reported function of PP2A in the regulation of phosphorylation state and activity of Cdc25 that has been described in S. cerevisiae and X. laevis. It is possible that dependence of division size on the growth pattern could be a consequence of advanced mitosis in the pta2 mutant. Deregulation of the normal mitotic control could potentially sensitize cell size checkpoint in pta2D to the asymmetry between the daughters. Thus, PP2A-Pta2 regulates activation of growth at the new ends formed by cell division. It seems to promote identical growth pattern in the wild type daughters by preventing premature new end growth in one daughter cell. Since cells lacking for3 formin display similar asymmetric growth patterns in the two daughters, it is possible that changes in Cdc42 dependent actin cable nucleation by formin are responsible for the asymmetric growth of daughters in pta2D. Consistent with this hypothesis, deletion of cdc42 activator gef1 in pta2D cells significantly decreased the number of divisions producing asymmetric bipolar/monopolar growth of daughter cells, from 76% to 33%. A recent report suggesting similar changes in growth pattern upon ectopic activation of Cdc42 caused by rga4 PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22189367 deletion is providing additional support to our model. In contrast, mutants in polarity factors that also alter growth patterns but do not interfere with Cdc42 regulation only generate monopolar cells. growth. This additive phenotype is similar to what is seen when actin cables are compro
