ESCRT-I components of the endocytic machinery are required for Rim101-dependent ambient pH regulation in the yeast Yarrowia lipolytica

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Abstract
Ambient pH signalling involves a cascade of conserved Rim or Pal products in ascomycetous yeasts or filamentous fungi, respectively. Insertional mutagenesis in the yeastYarrowia lipolyticaidentified two components of the endosome-associated ESCRT-I complex involved in multivesicular body (MVB) vesicle formation, YlVps28p and YlVps23p. They were shown to be required at alkaline pH, like Rim factors, for transcriptional activation of alkaline-induced genes and repression of acid-induced genes. The constitutively activeYlRIM101-1119allele, which suppresses the pH-signalling defects ofYlrimmutations, also suppressesYlvpsdefects in pH response, but not in endocytosis. The contribution of the ESCRT-III component Snf7p could not be assessed due to the essential nature of this component inY. lipolytica. Unlike Rim factors, YlVps4p, a component of the MVB pathway acting downstream from ESCRT complexes, seems not to be required for the alkaline response. InY. lipolytica, allvpsmutations including those affectingYlVPS4, affected growth at acidic pH, a feature not exhibited byYlrimmutations. These results suggest that Rim and Vps pathways cooperate in ambient pH signalling and that this relation is conserved across the full range of hemiascomycetous yeasts.