ain . In fission yeast, full activation and proper localization of For3 
is achieved by binding of Cdc42 to its N-terminus and of Bud6 to the C-terminal DAD domain, relieving the autoinhibitory conformation. Cdc42 binding protein Pob1 is also required for For3 localization to the tips and facilitates Cdc42-mediated activation of For3. Actin cables nucleated by formins act as linear tracks along which exocytic vesicles can be transported by myosin-V towards the sites of actin nucleation. Vesicles are then tethered by the exocyst complex to the cell membrane. Proper localization of the exocyst to the growth sites depends on both actin cables and Cdc42. Therefore, Cdc42 is required for actin cable nucleation by For3 formin and also for the recruitment of the exocyst components to the growth zones. The amount of active GTP-bound Cdc42 is regulated by guanine nucleotide exchange factors and GTPase activating proteins . Whereas the two known GEFs of Cdc42 in S. pombe, Scd1 and Gef1, localize to cell tips, the inhibitory GAP Rga4 localizes to the cell sides. Rga4 is excluded from the tips by a mechanism that requires the Pom1 kinase and prevents ectopic growth away from the cell ends. The NDR kinase Orb6 spatially restricts growth to cell tips by preventing Gef1 localization to the cell sides, thus providing PP2A Role in S. pombe Morphogenesis further spatial control of cell growth. Rga4 is also required for the normal localization of For3 and for the normal organization of the actin cytoskeleton. In addition to the spatial control of Cdc42 activity, temporal regulation of Cdc42 GAPs and GEFs activity by Cdk1 might provide the cells with a mechanism that coordinates pattern of cell growth with cell cycle progression. Once cell polarity has been established, feedback mechanisms involving Cdc42 regulation maintain this polarized state. In this study, we investigate the role of protein phosphatase type 2A in the regulation of cell polarity and cell size in S. pombe. PP2A is one of the major serine/threonine phosphatases in eukaryotic cells. It is involved in the regulation of a variety of cellular processes including cell cycle progression, cytokinesis, stress response and morphogenesis. PP2A is a holoenzyme formed by a catalytic subunit, a structural subunit and a regulatory subunit, which confers substrate specificity and regulates the subcellular localization of the PP2A complex. Different B and C subunits provide the cell with a set of distinct PP2A complexes with different substrate specificity. Proper assembly and activation of the PP2A holoenzyme also requires posttranslational modification of the catalytic subunit by a Pomalidomide site methyltransferase that catalyzes a reversible methylation of the C subunit at carboxyterminal leucine. In addition, binding of a phosphatase activator to both catalytic and scaffolding subunit is further required for the proper assembly and activity of PP2A complex. The regulatory function of PTPAs is conserved through evolution from yeast to humans. In this work, we characterize in S. pombe the function of a previously undescribed PP2A regulatory subunit PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22189254 of PTPA type that we named Pta2. Based on the in vivo Pta2 association with the PP2A complex and on the sequence analysis, we propose that pta2 is the homologue of S. cerevisiae PTPA, RRD2/YPA2. pta2D cells are cold-sensitive and have a number of morphogenetic phenotypes, including changes in cell shape and altered pattern of growth, as well as a difference in size at divis
