FIG 7.

Model of the Sre1-mediated and Rim-mediated distinct responses to physiological pH. (A) The activating sensor for the sterol homeostasis pathway is unknown and could be linked to alkaline pH-induced reductions in ergosterol/membrane alterations or bioavailable iron. In response to alkaline pH, the Sre1 transcription factor is cleaved, activated, and localized to the nucleus to aid in the transcription of many genes involved in ergosterol biosynthesis and membrane homeostasis. This cleavage and activation are dependent on both the conserved transmembrane protein Scp1 and the basidiomycete-specific protease Stp1. (B) The Rim alkaline response pathway is signaled through the transmembrane pH sensor Rra1 and its interaction with the plasma membrane. At elevated pH levels, Rra1 is endocytosed, allowing it to interact with the downstream components of the pathway and propagate the signal to the endosomal membrane complex (ESCRT components, Rim23, and Rim20) and activate the Rim13 protease. This protease cleaves the Rim101 transcription factor, allowing it to translocate to the nucleus and induce the expression of genes involved in cell wall and surface remodeling.