Figure 7. Phosphorylated Ste18^Nt and Ste5^FBD Constitute a Dynamic Phosphoregulatory System for Pheromone Signaling.

(A) In response to pheromone, Ste18 is rapidly phosphorylated at its N-terminal tail (P-lollipop). Ste5 is simultaneously phosphorylated via negative feedback controlled by Fus3/Ste5^FBD docking (P-lollipops). Together, this constitutes a phospho-inhibitory system that prevents otherwise rapid Ste5/PM association. While not shown outright here, previous work implicates pheromone-stimulated expression of Ptc1 phosphatase and removal of inhibitory phosphorylation on Ste5 as the inhibition release (Malleshaiah et al., 2010) (dashed orange box). Consequently, the mating pathway is activated with a kinetic delay, as evident by the slower rate of Ste5 association at the membrane and delayed peak activation of Fus3.

(B) Cells engineered to prevent activation of the Ste18/Ste5 system (Ste18^3A/Ste5^ND) respond ~6 times faster with ~4 times greater intensity than observed in wild-type cells—a response that demonstrates synergy between the two phospho-regulatory elements (phosphorylated Ste18 and Ste5) in the system.