Regulation of MAPK Function by Direct Interaction with the Mating-Specific Gα in Yeast

Abstract

The mating response of the budding yeast Saccharomyces cerevisiae is mediated by a prototypical heterotrimeric GTP-binding protein (G protein) and mitogen-activated protein kinase (MAPK) cascade. Although signal transmission by such pathways has been modeled in detail, postreceptor down-regulation is less well understood. The pheromone-responsive G protein alpha subunit (Galpha) of yeast down-regulates the mating signal, but its targets are unknown. We have found that Galpha binds directly to the mating-specific MAPK in yeast cells responding to pheromone. This interaction contributes both to modulation of the mating signal and to the chemotropic response, and it demonstrates direct communication between the top and bottom of a Galpha-MAPK pathway.

A walk-through of the yeast mating pheromone response pathway

“…In the yeast mating pathway, putative D-sites are also been found in Gpa1 Gα [102], the Ptp3 phosphatase [154], and the Dig1 and Dig2 transcriptional regulators [79]. Hence, D-sites appear to be portable, modular motifs that mediate the interaction of MAPKs with multiple binding partners, contributing to both signal transmission and specificity.…”

“…As is true for virtually all other GPCR/G-protein modules in eukaryotes, receptor occupancy stimulates the Gα subunit of the G protein to exchange GDP for GTP; GTP-bound Gα then releases the Gβγ heterodimer (see [32] for a recent review of G-protein level events). Gα may also have additional roles in mating besides just regulatingGβγ release [55,102]. Furthermore, Gα may not truly release Gβγ [78]; instead, Gα may remain loosely bound to (and in regulatory communication with) Gβγ and perhaps the receptor as well.…”

Modelling the dynamics of the yeast pheromone pathway

“…Fus3PP phosphorylates and regulates various nuclear and cytoplasmatic proteins (Dohlman, 2002): Sst2 (necessary for G protein inactivation), Ste5 (the scaffold protein of the MAPK cascade; Kranz et al, 1994), Ste11 and Ste7 , Far1 (required for morphological changes and for cell cycle arrest; Peter et al, 1993), Ste12 (involved in the transcriptional activation; Metodiev et al, 2002), Dig1/Rst1 and Dig2/Rst2 (necessary for transcriptional inhibition; Tedford et al, 1997) and others.…”

Gβ phosphorylation is critical for efficient chemotropism in yeast

“…More recently, we discovered that the activated forms of the mating-specific Ga protein and the Fus3 MAPK interact directly. A mutant form of Ga that is severely defective in binding Fus3, Ga DSD (Ga-Docking Site Disrupted; previously Ga K21E R22E ), confers a defect in partner discrimination, indicating a problem in directional sensing and/or directed growth (Metodiev et al, 2002;Strickfaden and Pryciak, 2008;Yu et al, 2008). Ga DSD also results in hypophosphorylation and reduced levels of Gb, as does fus3D.…”