Isolation of an extragenic suppressor of the rna1-1 mutation in Saccharomyces cerevisiae

Constitutive activation of the pH-responsive Rim101 pathway in yeast mutants defective in late steps of the MVB/ESCRT pathway

In many fungi, transcriptional responses to alkaline pH are mediated by conserved signal transduction machinery. In the homologous system in Saccharomyces cerevisiae, the zinc-finger transcription factor Rim101 is activated under alkaline conditions to regulate transcription of target genes. The activation of Rim101 is exerted through proteolytic processing of its C-terminal inhibitory domain. Regulated processing of Rim101 requires several proteins, including the calpain-like protease Rim13/Cpl1, a putative protease scaffold Rim20, putative transmembrane proteins Rim9, and Rim21/Pal2, and Rim8/Pal3 of unknown biochemical function. To identify new regulatory components and thereby determine the order of action among the components in the pathway, we screened for suppressors of rim9Delta and rim21Delta mutations. Three identified suppressors-did4/vps2, vps24, and vps4-all belonged to "class E" vps mutants, which are commonly defective in multivesicular body sorting. These mutations suppress rim8, rim9, and rim21 but not rim13 or rim20, indicating that Rim8, Rim9, and Rim21 act upstream of Rim13 and Rim20 in the pathway. Disruption of DID4, VPS24, or VPS4, by itself, uncouples pH sensing from Rim101 processing, leading to constitutive Rim101 activation. Based on extensive epistasis analysis between pathway-activating and -inactivating mutations, a model for architecture and regulation of the Rim101 pathway is proposed.

Protein transport from the late Golgi to the vacuole in the yeast Saccharomyces cerevisiae

The late Golgi compartment is a major protein sorting station in the cell. Secreted proteins, cell surface proteins, and proteins destined for endosomes or lysosomes must be sorted from one another at this compartment and targeted to their correct destinations. The molecular details of protein trafficking pathways from the late Golgi to the endosomal system are becoming increasingly well understood due in part to information obtained by genetic analysis of yeast. It is now clear that proteins identified in yeast have functional homologues (orthologues) in higher organisms. We will review the molecular mechanisms of protein targeting from the late Golgi to endosomes and to the vacuole (the equivalent of the mammalian lysosome) of the budding yeast Saccharomyces cerevisiae.

Ubiquitin-dependent sorting into the multivesicular body pathway requires the function of a conserved endosomal protein sorting complex, ESCRT-I

The multivesicular body (MVB) pathway is responsible for both the biosynthetic delivery of lysosomal hydrolases and the downregulation of numerous activated cell surface receptors which are degraded in the lysosome. We demonstrate that ubiquitination serves as a signal for sorting into the MVB pathway. In addition, we characterize a 350 kDa complex, ESCRT-I (composed of Vps23, Vps28, and Vps37), that recognizes ubiquitinated MVB cargo and whose function is required for sorting into MVB vesicles. This recognition event depends on a conserved UBC-like domain in Vps23. We propose that ESCRT-I represents a conserved component of the endosomal sorting machinery that functions in both yeast and mammalian cells to couple ubiquitin modification to protein sorting and receptor downregulation in the MVB pathway.