Abstract
The proteasome is responsible for the degradation of many cellular proteins. If and how this abundant and normally stable complex is degraded by cells is largely unknown. Here we show that in yeast, upon nitrogen starvation, proteasomes are targeted for vacuolar degradation through autophagy. Using GFP-tagged proteasome subunits, we observed that autophagy of a core particle (CP) subunit depends on the deubiquitinating enzyme Ubp3, although a regulatory particle (RP) subunit does not. Furthermore, upon blocking of autophagy, RP remained largely nuclear, although CP largely localized to the cytosol as well as granular structures within the cytosol. In all, our data reveal a regulated process for the removal of proteasomes upon nitrogen starvation. This process involves CP and RP dissociation, nuclear export, and independent vacuolar targeting of CP and RP. Thus, in addition to the well characterized transcriptional up-regulation of genes encoding proteasome subunits, cells are also capable of down-regulating cellular levels of proteasomes through proteaphagy.
Highlights
Using GFP-tagged proteasome subunits, we observed that autophagy of a core particle (CP) subunit depends on the deubiquitinating enzyme Ubp3, a regulatory particle (RP) subunit does not
We observed the appearance of free GFP for Rpn11-GFP, indicating these results reflect a common property of proteasomes and not a unique feature of specific proteasome subunits (Fig. 1, A and C)
Proteasomes relocalize upon starvation; nitrogen starvation leads to vacuolar localization and glucose starvation to proteasome storage granules (PSGs)
Summary
All strains have the DF5 background genotype (lys801 leu 112 ura his3-⌬200 trp). All strains used are from this work. SJR786 sJR858 sJR861 sJR869 sJR875 sJR876 sJR877 sJR878 sJR879 sJR880 sJR881 sJR882 sJR883 sJR889 sJR890 sJR896 sJR897 sJR900 sJR904 sRJ925. We characterized how nitrogen starvation, but not glucose starvation, induces autophagy of proteasomes. Our data indicate that the vacuolar targeting of proteasomes requires nuclear export of proteasomes that are targeted for autophagy. CP and RP are targeted to the vacuole through different mechanisms, as only CP autophagy depends on the deubiquitinating enzyme Ubp. Our data show the autophagic removal of proteasomes from cells upon nitrogen starvation is not simple non-selective bulk autophagy but is a highly regulated process
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