Combinatorial Phosphorylation-Dependent Synergy Between a G Protein Subunit and the Ste5 Scaffold Regulates Differential MAP-Kinase Activation in Yeast

Abstract

Heterotrimeric G proteins (Gαβγ) play a critical role as transducers in G protein signaling systems responsible for activating a wide range of physiological processes including neurotransmitter and hormone signaling. In yeast, heterotrimeric G protein signaling results in the distinctly different activation of two mitogen-activated protein kinases (MAPKs) - Fus3 and Kss1 - a phenomenon that requires conformational switching of the scaffold protein Ste5. Here we show that rapid receptor-activated phosphorylation of the intrinsically disordered N-terminal tail of the yeast Gγ subunit (Ste18-Nt) has co-evolved to synergize with MAPK binding on Ste5 to control the kinetics and amplitude of Fus3 and Kss1 signaling. By modulating this synergistic component genetically, we show that the activation kinetics and amplitude of either kinase can be controlled - leading to preferential hyperactivation of Fus3 or Kss1. These results further resolve the mechanism by which scaffolds control pathway activation and provide compelling evidence that the intrinsically disordered N-terminal tails of Gγ subunits can serve as regulators of G protein signaling.