Vacuolar hydrolysis and efflux: current knowledge and unanswered questions

Abstract

Hydrolysis within the vacuole in yeast and the lysosome in mammals is required for the degradation and recycling of a multitude of substrates, many of which are delivered to the vacuole/lysosome by autophagy. In humans, defects in lysosomal hydrolysis and efflux can have devastating consequences, and contribute to a class of diseases referred to as lysosomal storage disorders. Despite the importance of these processes, many of the proteins and regulatory mechanisms involved in hydrolysis and efflux are poorly understood. In this review, we describe our current knowledge of the vacuolar/lysosomal degradation and efflux of a vast array of substrates, focusing primarily on what is known in the yeast Saccharomyces cerevisiae. We also highlight many unanswered questions, the answers to which may lead to new advances in the treatment of lysosomal storage disorders. Abbreviations: Ams1: α-mannosidase; Ape1: aminopeptidase I; Ape3: aminopeptidase Y; Ape4: aspartyl aminopeptidase; Atg: autophagy related; Cps1: carboxypeptidase S; CTNS: cystinosin, lysosomal cystine transporter; CTSA: cathepsin A; CTSD: cathepsin D; Cvt: cytoplasm-to-vacuole targeting; Dap2: dipeptidyl aminopeptidase B; GS-bimane: glutathione-S-bimane; GSH: glutathione; LDs: lipid droplets; MVB: multivesicular body; PAS: phagophore assembly site; Pep4: proteinase A; PolyP: polyphosphate; Prb1: proteinase B; Prc1: carboxypeptidase Y; V-ATPase: vacuolar-type proton-translocating ATPase; VTC: vacuolar transporter chaperone.

Keywords: Autophagy; efflux; glutathione; hydrolase; lysosome; polyphosphate; proteolysis; vacuole.

Figure 1.

Macro- and microautophagy in S. cerevisiae. Macroautophagy consists of induction and nucleation at the phagophore assembly site (PAS) followed by elongation and closure of the phagophore membrane around cellular cargo to form a completed autophagosome. The outer membrane of the autophagosome fuses with the vacuole membrane, releasing a single-membrane autophagic body into the vacuole lumen. Microautophagy occurs via direct invagination of the vacuole membrane and scission to release a vesicle into the vacuole lumen. In both types of autophagy, resident vacuolar hydrolases degrade the autophagic body and its contents, which are then exported back into the cytoplasm for reuse by the cell. See text for more detail.

Figure 2.

Numerous vacuolar hydrolases undergo proteolytic processing. Arrowheads indicate cleavage sites (the exact site of cleavage within Ape4 is unknown), with proteases involved in cleavage listed above. Gray boxes indicate signal sequences, black boxes indicate propeptides, maize boxes indicate cytoplasmic domains, blue boxes indicate transmembrane domains, and white boxes indicate mature vacuolar domains. aa, amino acid. See text for details.