(D) Ecm33 (P0.5R2.2) reporter expression is dependent on Pmk1-Atf1 signaling. RLM1-binding site in theecm33+promoter region and monitored the transcriptional activity of Atf1 or Mbx1 in living cells using a destabilized luciferase reporter gene fused to three tandem repeats of the CRE and six tandem repeats of the Rlm1-binding sequence, respectively. These reporter genes reflect the activation TP-10 of the Pmk1 pathway by various stimuli, thereby enabling the real-time monitoring of the Pmk1 cell integrity pathway. Notably, the ecm33cells displayed hyperactivation of the Pmk1 signaling together with hypersensitivity to Ca2+and an abnormal morphology, which were almost abolished by simultaneous deletion of the components of the Rho2/Pck2/Pmk1 pathway. Our results suggest that Ecm33 is usually involved in the negative feedback regulation of Pmk1 cell integrity signaling and is linked to cellular Ca2+signaling. == INTRODUCTION == The mitogen-activated protein kinase (MAPK) pathway is one of the most important intracellular signaling pathways that play a crucial role in cell proliferation, cell differentiation, and cell cycle regulation (Nishida and Gotoh, 1993;Marshall, 1994;Herskowitz, 1995;Levin and TP-10 Errede, 1995). MAPKs deliver extracellular signals from activated receptors to various cellular compartments, especially, the nucleus, where they regulate eukaryotic gene expression at the transcriptional and posttranscriptional levels (Pouyssegur, 2000;Sugiuraet al., 2003;Edmunds and Mahadevan, 2004;Satohet al., 2009). In the budding yeastSaccharomyces cerevisiae, the Slt2/Mpk1 MAPK pathway mediates cell TP-10 cycleregulated cell wall synthesis and responds to different signals, including cell cycle regulation, growth heat, changes in external osmolarity, and mating pheromones (Gustinet al., 1998). Signaling proteins involved in the pathway include TP-10 the GTP-binding protein Rho1, the protein kinase C homologue Pkc1, the MEKK Bck1p/Slk1p, the redundant pair of MAP/ERK kinases (MEKs) Mkk1 and Mkk2, the MAPK Slt2/Mpk1, and the transcription factor targets Rlm1 and SBF (Gustinet al., 1998). Moreover, signaling via Mpk1/Slt2-Rlm1 regulates the expression of at least 25 genes, most of which have been implicated in cell wall biogenesis (Jung and Levin, 1999;Junget al., 2002). We have been studying the Pmk1 MAPK signaling pathway in the fission yeastSchizosaccharomyces pombe. The Pmk1 MAPK, a homologue of the mammalian ERK/MAPK plays a central role in cell integrity in fission yeast (Todaet al., 1996;Zaitsevskaya-Carter and Cooper, 1997). The Pmk1 MAPK pathway is composed of MAPKKK Mkh1 (Sengaret al., 1997), MAPKK Pek1 (Sugiuraet al., 1999), and MAPK Pmk1/Spm1. The Pmk1 MAPK pathway also regulates ion homeostasis and morphogenesis (Satohet al., 2009) and is activated under multiple stresses, including heat shock, hyper- or hypotonic stresses, cell wall damage, or glucose deprivation (Todaet al., 1996;Sugiuraet al., 1999;Madridet al., 2006). We have previously exhibited that calcineurin and Pmk1 MAPK play antagonistic functions in Clhomeostasis (Sugiuraet al., 1998,2002) and genetic screening on the basis of the functional conversation between calcineurin and Pmk1 MAPK has resulted in the isolation of unfavorable regulators of the Pmk1 MAPK pathway, includingpmp1+, encoding a dual-specificity MAPK phosphatase (Sugiuraet al., 1998);pek1+, encoding a MAPK kinase (MAPKK;Sugiuraet al., 1999); andrnc1+, encoding a novel KH-type RNA-binding protein that stabilizes Pmp1 mRNA (Sugiuraet al., 2003,2004). Moreover, genetic screening forvic(viable in the presence of immunosuppressant and chloride ion) mutants revealed that thecpp1+gene, encoding a subunit of the protein farnesyltransferase, and its target Rho2 GTPase (Maet al., 2006) act as upstream regulators of the Pmk1-signaling pathway. Most recently, we have identified the Atf1 transcription factor as a downstream target of the Pmk1 MAPK pathway and exhibited that Atf1 is usually involved in cell integrity in addition to its well-established role in the stress responses mediated by the Sty1/Spc1 MAPK pathway in fission yeast (Takadaet al., 2007). Mbx2, an Rlm1 homologue in fission yeast, unlike in budding yeast, displayed only a modest sensitivity LIMD1 antibody to cell walldamaging brokers, suggesting that Mbx2 plays a minor role in this process (Takadaet al., 2007). Moreover, the intermediate phenotypes of the atf1cells in the cell integrity response suggest that other unidentified target(s) of Pmk1 must play a significant role in the cell integrity pathway in fission yeast. To identify novel genes involved in cell integrity.