To comprehensively address these questions we have used quantitative proteomics. Our results demonstrate that within the first 3 hr of amino acid starvation many integral plasma membrane proteins, including high-affinity amino acid permeases, glucose transporters and G-protein coupled receptors, were selectively removed from the cell surface by endocytosis and subsequently targeted into vacuoles via the ESCRT-dependent MVB pathway and degraded, while others remained stable or were up-regulated (e.g., the general amino acid permease, Gap1). This comprehensive and selective remodeling of the plasma membrane appeared to be completed within 3–4 hr of starvation. Autophagy was also immediately activated upon starvation and remained active throughout starvation. Surprisingly, early during starvation the selective degradation of membrane proteins via the MVB pathway was mainly responsible to maintain critical levels of free intracellular amino acids that were sufficient to uphold protein synthesis and promote the corresponding adaptation of the proteome. Most notably this included the de novo synthesis of vacuolar hydrolases, which boosted the proteolytic activity of vacuoles to support the efficient degradation of autophagic cargo. The continuous delivery and degradation of autophagic cargo further enhanced intracellular amino acid recycling and was ultimately essential to restore intracellular amino acid pools of cells during extended starvation. These findings reveal an unexpected role for the MVB pathway in maintaining intracellular amino acid homeostasis and thereby promoting the up-regulation of vacuolar hydrolases early during starvation, which is tightly coordinated with autophagy. This catabolic cascade is ultimately required to allow starving cells to complete their cell division cycle and enter a quiescent state for survival. - See more at: http://elifesciences.org/content/4/e07736.full#sthash.o8PsAY1C.dpuf