Arrestin-related proteins mediate pH signaling in fungi

Metazoan arrestins bind to seven-transmembrane (7TM) receptors to regulate function. Aspergillus nidulans PaIF, a protein involved in the fungal ambient pH signaling pathway, contains arrestin N-terminal and C-terminal domains and binds strongly to two different regions within the C-terminal cytoplasmic tail of the 7TM, putative pH sensor PaIH. Upon exposure to alkaline ambient pH, PaIF is phosphorylated and, like mammalian beta-arrestins, ubiquitinated in a signal-dependent and 7TM protein-dependent manner. Substitution in PaIF of a highly conserved arrestin N-terminal domain Ser residue prevents PaIF-PaIH interaction and pH signaling in vivo. Thus, PalF is the first experimentally documented fungal arrestin-related protein, dispelling the notion that arrestins are restricted to animal proteomes. Epistasis analyses demonstrate that PaIF posttranslational modification is partially dependent on the 4TM protein Pall but independent of the remaining pH signal transduction pathway proteins PaA, PaIB, and PaIC, yielding experimental evidence bearing on the order of participation of the six components of the pH signal transduction pathway. Our data strongly implicate PaIH as an ambient pH sensor, possibly with the cooperation of PaIl.