mised in the Tea1 deletion. To check whether pta2D and for3D were epistatic for their cell morphology phenotypes, we produced double mutant pta2Dfor3D. Spores of pta2Dfor3D germinated and slowly proliferated, forming microcolonies. Microscopic observation of these cells showed that most of them eventually became unpolarized. The strong negative interaction between pta2 and for3 genes is reminiscent of complete loss of cell polarity in double mutants 
of for3 with exocyst components, raising the possibility that PP2A might be regulating exocyst function. Thus, cell polarization defects of pta2D cells are additive to defects in microtubule and actin-dependent mechanisms of cell polarization. PP2A-Pta2 regulates levels and activity of Cdc42 Cdc42 is a key regulator of polarity and cdc42 mutants in fission yeast exhibit abnormal morphology. At the molecular level, Cdc42 is required for actin cable formation and also for tethering exocytic GSK461364 vesicles to the sites of growth. In addition to the described genetic interactions of pta2D with both MT- and actindependent mechanisms of cell polarization, we also found a negative genetic interaction between pta2D and the deletion of the exocyst component Exo70. Thus, we tested whether changes in cell morphology and growth site selection seen in pta2D cells can involve changes in the Cdc42 activity. To this end, we analyzed genetic interactions between pta2D and two regulators of the Cdc42 pathway, gef1D and rga4D . Apart from partial suppression of daughter cell asymmetry, no significant changes were seen in gef1Dpta2D compared to pta2D cells. In contrast, morphological defects of pta2D cells were exacerbated in pta2Drga4D double mutant, resulting in a complete loss of polarity. Neither gef1D nor rga4D suppressed cold-sensitivity of pta2D. Since Gef1 is an activator of Cdc42 and Rga4 is an inhibitor, the phenotypes of double mutants are consistent with higher Cdc42 activity in the pta2D. To directly measure the amounts of Cdc42 in pta2D cells we assayed total levels of Cdc42 by western blot using HA-Cdc42 and levels of active Cdc42 by a pull down assay using bacterially expressed PBD-domain fused to GST, which binds GTP-bound Cdc42, as PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22189214 a bait. The levels of active Cdc42 were increased in pta2-deleted strain, consistent with the above genetic interaction data. Surprisingly, the total amounts of Cdc42 were also elevated. To check whether the changes in total Cdc42 protein levels observed in pta2D cells were due to transcriptional upregulation or to changes in protein synthesis or degradation, we analyzed by northern blot Cdc42 mRNA levels in the pta2 deletion. We found no changes in cdc42 mRNA levels in the pta2D background relative to those in wild type, suggesting that PP2A might be directly or indirectly regulating Cdc42 translation or turnover. The two distinct morphogenetic phenotypes seen in pta2D cells, altered growth pattern and loss of cell polarity are both consistent with the role of Pta2 in downregulating Cdc42. In summary, pta2D cells have several phenotypes highlighting the involvement of PP2A in the regulation of morphogenesis and the cell cycle. Alterations in cell polarity and cell shape in pta2D cells as well as the altered growth pattern suggest a role for PP2A in re-organizing actin machinery after division and in maintaining polarized growth zones. Morphological defects of pta2D are additive to defects in MT-dependent mechanism of cell polarization and also to the absence of actin cab