Relationship of DFG16 to the Rim101p pH response pathway in Saccharomyces cerevisiae and Candida albicans

Many fungal pH responses depend upon conserved Rim101p/PacC transcription factors, which are activated by C-terminal proteolytic processing. The means by which environmental pH is sensed by this pathway are not known. Here, we report a screen of the Saccharomyces cerevisiae viable deletion mutant library that has yielded a new gene required for processed Rim101p accumulation, DFG16. An S. cerevisiae dfg16 Delta mutant expresses Rim101p-repressed genes at elevated levels. In addition, Candida albicans dfg16 Delta/dfg16 Delta mutants are defective in alkaline pH-induced filamentation, and their defect is suppressed by expression of truncated Rim101-405p. Thus, Dfg16p is a functionally conserved Rim101p pathway member. Many proteins required for processed Rim101p accumulation are members of the ESCRT complex, which functions in the formation of multivesicular bodies (MVBs). Staining with the dye FM4-64 indicates that the S. cerevisiae dfg16 Delta mutant does not have an MVB defect. We find that two transcripts, PRY1 and ASN1, respond to mutations that affect both the Rim101p and MVB pathways but not to mutations that affect only one pathway. The S. cerevisiae dfg16 Delta mutation does not affect PRY1 and ASN1 expression, thus confirming that Dfg16p function is restricted to the Rim101p pathway. Dfg16p is homologous to Aspergillus nidulans PalH, a component of the well-characterized PacC processing pathway. We verify that the previously recognized PalH homolog, Rim21p, also functions in the S. cerevisiae Rim101p pathway. Dfg16p is predicted to have seven membrane-spanning segments and a long hydrophilic C-terminal region, as expected if Dfg16p were a G-protein-coupled receptor.