Systematic analysis of ESCRT mutants reveals an essential function for the MVB pathway in amino acid recycling.

The ESCRT machinery drives the multivesicular body (MVB) pathway in eukaryotic cells and thus is required for the degradation of ubiquitinated membrane proteins in lysosomes. To systematically characterize how cells respond to loss of ESCRT function, we used quantitative proteomics and gene expression profiling in yeast. We find that ESCRT mutants display severe defects in amino acid homeostasis, resulting in lower levels of free intracellular amino acids. This deficit renders the growth of ESCRT mutants highly sensitive to nutrient limitation. Further proteomic analysis revealed that the MVB pathway essentially contributes to proteome remodeling under nutrient limitation. The rapid decline of protein synthesis upon starvation no longer enables ESCRT mutants to complete their cell cycle and properly enter a quiescent state, which strongly affects their long-term survival. Thus, the MVB pathway functions to selectively down-regulate integral membrane proteins and, together with autophagy and proteasomal degradation, considerably contributes to amino acid recycling.

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