A Rab Escort Protein Regulates the MAPK Pathway That Controls Filamentous Growth in Yeast

MAPK pathways regulate different responses yet can share a subset of common components. In this study, a genome-wide screen was performed to identify genes that, when overexpressed, induce a growth reporter (FUS1-HIS3) that responds to ERK-type MAPK pathways (Mating/filamentous growth or fMAPK) but not p38-type MAPK pathways (HOG) in yeast. Approximately 4,500 plasmids overexpressing individual yeast genes were introduced into strains containing the FUS1-HIS3 reporter by high-throughput transformation. Candidate genes were identified by measuring the degree of growth, which was a reflection of reporter activity. Of fourteen genes identified and validated by re-testing, two were metabolic controls (HIS3 and ATR1), five had established roles in regulating ERK-type pathways (STE4, STE7, BMH1, BMH2, MIG2) and seven represent potentially new regulators of MAPK signaling (RRN6, CIN5, MRS6, KAR2, TFA1, RSC3, RGT2). MRS6 encodes a Rab escort protein and effector of the TOR pathway that plays an established role in nutrient signaling. MRS6 overexpression stimulated filamentous/invasive growth and phosphorylation of the ERK-type fMAPK, Kss1. Overexpression of MRS6 reduced the osmotolerance of cells and phosphorylation of the p38/HOG pathway MAPK, Hog1. Mrs6 interacted with the PAK kinase Ste20 and MAPKK Ste7 by two-hybrid analysis. Collectively, the data indicate that Mrs6 may function to selectively propagate an ERK-dependent signal. Generally speaking, the identification of new MAPK pathway regulators by genetic screening in yeast may be a useful resource for understanding signaling pathway regulation.