(157bc) A Rab Escort Protein in Yeast Regulates the MAPK Pathway That Controls Filamentous Growth

Signal transduction pathways control the response to stress and cell differentiation into specialized cell types. In yeast and other fungal species, signaling pathways control filamentous growth, which in pathogens is critical for host–cell attachment, invasion into tissues, and virulence. As commonly seen during cell differentiation, multiple pathways operate in an integrated network to regulate filamentous growth. A MAPK pathway, called the fMAPK pathway, is among the pathways that regulate filamentous growth. fMAPK pathway shares a subset of common components with two other MAPK pathways in yeast (mating and HOG pathways), including the Rho-type GTPase Cdc42, PAK Ste20, and MAPKKK Ste11. A key gap in understanding fMAPK pathway specification is connecting upstream components to downstream pathway-specific kinases, like Ste7 and Kss1. To address this gap, a genome-wide screen (of 4416 yeast genes) was performed. The screen led to identifying genes that, when overexpressed, induce a growth reporter (FUS1-HIS3) that responds to ERK-type MAPK pathways (Mating/fMAPK) but not p38-type MAPK pathways (HOG) in yeast. The screen analysis and target validation identified new regulators of ERK-type pathways that were not previously implicated in MAPK signaling, including MRS6, a Rab escort protein. Mrs6 plays an established role in protein trafficking and signaling, but has not been already connected to MAPK pathway regulation. We found out that Mrs6 overexpression stimulated phosphorylation of the MAP kinase that regulates fMAPK pathway, Kss1, but not the phosphorylation of the MAP kinases that regulate the mating or HOG pathways, indicating that it may be a specific regulator of the fMAPK pathway. Mrs6 interacted with the PAK kinase Ste20 and MAPKK Ste7 by two-hybrid analysis. The interaction between Mrs6 and Ste7 may contribute to ERK-type (STE20->STE11->STE7) signaling but not p38-type (STE20->STE11->PBS2) signaling. In mammals, loss of the MRS6 homolog REP1/CHM leads to choroideremia, which causes progressive vision loss. Intriguingly, REP1/CHM regulates EGFR, which is a major regulator of the Grb-SOS-RAS-MEK-ERK pathway. Given that EGFR also signals through RAS-MEK-ERK, our screen may have identified a new regulator of ERK-type MAPK pathways.