Figure 1. Working Model of the Mating Pathway of Saccharomyces cerevisae.

To facilitate clarity, a contemporary model of the pheromone pathway is shown here, which is inferred from prior studies described in the text. The mating pathway of Saccharomyces cerevisae is triggered in response to pheromone-dependent activation of the G-protein-coupled receptor, Ste2, and subsequent recruitment of Ste5 to the plasma membrane (PM), mediated by Ste5^PM and Ste5^PH domains as well as the Ste5^RING domain that binds directly to free Gβ/Ste4. Under resting conditions, Ste5/PM association and MAPK activation is disfavored due to hyper-phosphorylation at positions surrounding Ste5^PM (sites marked as sticks), auto-inhibition of the Ste5^PH domain through a competitive interaction with Ste5^VWA, and the lack of free Gβ/Ste4 that is sequestered by GDP-bound Gα/Gpa1. Pathway activation is also inhibited by the Ste5^FBD, which allosterically activates Fus3 to promote phosphorylation of Ste5^T287 and three other phosphosites proximal to Ste5^FBD and Ste5^RING (P-lollipops). Upon pheromone stimulation, cascade activation of MAP kinases relies on PM recruitment and stable association of Ste5 that is concomitant with several state changes, including (but not limited to): (1) upregulated expression of the phosphatase Ptc1, which competes with Fus3 for binding to Ste5^FBD and promotes Ste5 de-phosphorylation; (2) a large conformational change in Ste5 accompanied by de-inhibition of Ste5^VWA; and (3) MAPK cascade activation of Fus3 and Kss1. For simplicity, interactions involving proteins such as Ste20, Far1, Cdc42, Cdc24, and many more involved in shmoo formation are not shown. Central components of the model discussed extensively here as a framework for this work include the Ste5^FBD and phosphoregulatory control by Fus3 and Ptc1, captured in part by Bhattacharyya et al. (2006), Coyle et al. (2013), and Malleshaiah et al. (2010); conformational dynamics and PM association of Ste5, captured in part by Takahashi and Pryciak (2008) and Zalatan et al. (2012); and the role of Gβγ/Ste5^RING as an essential feature for Ste5/PM association, captured in part by Winters et al. (2005). Additional, but not all, contributions have been cited in the Introduction, Discussion, and Supplemental Information.