Ubiquitin-Dependent Lysosomal Membrane Protein Sorting and Degradation

Abstract

As an essential organelle in the cell, the lysosome is responsible for digestion and recycling of intracellular components, storage of nutrients, and pH homeostasis. The lysosome is enclosed by a special membrane to maintain its integrity, and nutrients are transported across the membrane by numerous transporters. Despite their importance in maintaining nutrient homeostasis and regulating signaling pathways, little is known about how lysosomal membrane protein lifetimes are regulated. We identified a yeast vacuolar amino acid transporter, Ypq1, that is selectively sorted and degraded in the vacuolar lumen following lysine withdrawal. This selective degradation process requires a vacuole anchored ubiquitin ligase (VAcUL-1) complex composed of Rsp5 and Ssh4. We propose that after ubiquitination, Ypq1 is selectively sorted into an intermediate compartment. The ESCRT machinery is then recruited to sort the ubiquitinated Ypq1 into intraluminal vesicles (ILVs). Finally, the compartment fuses with the vacuole and delivers ILVs into the lumen for degradation.

ESCRTs function directly on the lysosome membrane to downregulate ubiquitinated lysosomal membrane proteins

“…We co-expressed Ypq1-GFP and myc-Ub in doa4 Δ cells. Doa4 is a major deubiquitinase of the endomembrane system, and its deletion stabilizes ubiquitinated forms of Ypq1 (Amerik et al, 2000; Li et al, 2015b). We immunoprecipitated Ypq1-GFP before and after lysine withdrawal, and probed the precipitates for myc-Ub.…”

“…Rsp5 is recruited to the vacuole membrane by Ssh4, a PY-motif containing type-I transmembrane protein. After ubiquitination, Ypq1 is sorted into the vacuole lumen for degradation (Li et al, 2015b). We also reported that the zinc transporter Cot1 is ubiquitinated by Tul1, a RING-type E3 ligase, and is sorted into the vacuole lumen for degradation upon the depletion of zinc (Li et al, 2015a).…”

“…In the second model, Ypq1 is internalized directly into the vacuole lumen by the ESCRT machinery. Initially, we favored the recycling model (Li et al, 2015a, Li et al, 2015b) because some Ypq1-GFP puncta detached from vacuole membrane were detected after lysine withdrawal. The recycling model was also consistent with the previously characterized functional location of the ESCRT complexes at the late endosome (Babst et al, 2002, 2000; Cheever et al, 2001; Henne et al, 2012; Katzmann et al, 2003), and ESCRT function directly on the vacuole membrane has never been reported.…”

Rab GTPases and cell division

“…[2][3][4] Originally ESCRT complexes were identified to regulate multi-vesicular body formation and lysosomal sorting. 5 However several ESCRT proteins were demonstrated also to be required for cytokinesis. 6 Thus it was proposed that the recruitment of various ESCRT complex members, such as CHMP4, to the midbody, lead to a formation of filaments that mediate the final abscission step 7 ( Fig.…”

A mechanistic study on the biosynthetic regulation of bioactive metabolite Antroquinonol from edible and medicinal mushroom Antrodia camphorata

“…Second, the putative uncharacterised protein (gi|749901469), predicted protein (gi|170094730) and Hsp20/alpha crystalline family (gi|227085992) belong to heat shock proteins and function as molecular chaperone, which participate in the folding of nascent polypeptide chains and the transmembrane transport of proteins (Beckmann, Mizzen, & Welch, 1990;Fu, 2014). Predicted protein (gi|403418016) is the ubiquitin-dependent protein which is believed to have the protein catabolic function (Li, Rong, Chuang, Peng, & Emr, 2015). With the addition of coenzyme Q0, the expression level of the heat shock proteins were increased while that of ubiquitin-dependent protein was decreased.…”