The Cell Surface Protein Ecm33 Is Involved in Negative Feedback Regulation of MAP Kinase Signalling and Development of the <i>In Vivo</i> Real-time Monitoring of MAP Kinase Signalling

Abstract

The mitogen-activated protein kinase (MAPK) pathways are signal transduction mechanisms that regulate many cellular processes in eukaryotic organisms, from yeasts to mammals. Multiple MAPKs regulate eukaryotic gene expression in response to various extracellular stimuli through phosphorylation of transcription factors. We have been studying the Pmk1 MAPK, a homologue of the mammalian ERK/MAPK in fission yeast. The Pmk1 MAPK regulates cell integrity and cell morphology. We have previously demonstrated that Atf1, a transcription factor downstream of the stress-activated MAPK pathway, serves also as a target of the Pmk1 MAPK signaling in fission yeast. Here, we identified ecm33⁺ gene, encoding a glycosyl-phosphatidylinositol (GPI)-anchored cell surface protein as a transcriptional target of Pmk1 and Atf1. The gene expression of ecm33⁺ is regulated by two transcription factors Atf1 and Mbx1. We also developed an in vivo real-time monitoring system of Atf1 or Mbx1 transcriptional activity, which enables to monitor the activation of the Pmk1 MAPK pathway by various stimuli. Finally, we demonstrated that Ecm33 is involved in the negative regulation of the Pmk1 MAPK signaling through the control of Ca²⁺ homeostasis. The ecm33 deleted cells displayed Ca²⁺ sensitivity and increased phosphorylation levels of Pmk1 MAPK. In addition, the Ecm33 overproducing cells displayed phenotypes closely similar to those of the pmk1 knockout cell. Collectively, Ecm33 plays a role in the negative feedback regulation of Pmk1 cell integrity signaling.