No 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.

Summary Proteasomes are complex molecular machines that consist of 66 subunits. The assembly of these complexes is highly coordinated in a process that requires at least ten proteasome-specific molecular chaperones. One of the challenges in studying assembly intermediates is their relatively low abundance as compared to the proteasome holoenzyme. Therefore, superior separating techniques are crucial for analyses of proteasomal complexes in general and studies in the assembly in particular. For this reason, native gel analyses have been abundantly used in studying proteasomes, as they provide a high resolution. Native gels are very versatile and can be used in various combinatorial approaches. In this chapter, we outline two approaches to prepare samples for native gels. The first is a yeast cryo-grinding method and the second a core particle (CP) base reconstitution approach. We describe the native gel electrophoresis, as well as various downstream analyses, including 2D native-SDS-PAGE. These techniques and approaches that can all be used, often in parallel, to gain a variety of information about activity and composition of the complexes separated by native gel. The potential combined approaches are highlighted in the summary flow chart in figure 1.

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The proteasome selectively degrades proteins. It consists of a core particle (CP), which contains proteolytic active sites that can associate with different regulators to form various complexes. How these different complexes are regulated and affected by changing physiological conditions, however, remains poorly understood. In this study, we focused on the activator Blm10 and the regulatory particle (RP). In yeast, increased expression of Blm10 outcompeted RP for CP binding, which suggests that controlling the cellular levels of Blm10 can affect the relative amounts of RP-bound CP. While strong overexpression of BLM10 almost eliminated the presence of RP-CP complexes, the phenotypes this should induce were not observed. Our results show this was due to the induction of Blm10-CP autophagy under prolonged growth in YPD. Similarly, under conditions of endogenous BLM10 expression, Blm10 was degraded through autophagy as well. This suggests that reducing the levels of Blm10 allows for more CP-binding surfaces and the formation of RP-CP complexes under nutrient stress. This work provides important insights into maintaining the proteasome landscape and how protein expression levels affect proteasome function.