Phosphorylation of G protein γ subunit Ste18 and the Ste5 scaffold form a braking system that governs pathway activation kinetics and switch‐like signaling 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, G protein signaling differentially activates two distinct 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), in combination with MAPK phosphorylation of Ste5, serves as a brake system that controls the rate and stability of scaffold association at the plasma membrane. Disabling the brake by precise point mutation of Ste18 phosphorylation sites or a MAPK docking site on Ste5 differentially governs the kinetics and amplitude of MAPK activation as well as the switch‐like morphological response to the stimulus. Complete elimination of the brake results in a response that is 5 times faster and ~4 times amplified over the response in wild type cells. Finally, we show that phosphorylation of Ste18 appeared late in evolutionary history ‐ at the same time as the MAPK docking site on Ste5, suggesting the possibility that both sides of the brake co‐evolved. Since phosphorylated intrinsically disordered tails are structurally conserved elements found in all Gγ subunits, including mammals, these results provide compelling evidence that such tails may function as intrinsic regulators of G protein signaling by altering the interactions of Gβγ subunits with their binding partners.Support or Funding InformationThis work was supported by funding from NIH grant GM117400 to M.T.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.