Biology subjects

Waite, K. A.

Publications and source records attributed to Waite, K. A..

3 recordsLinked to original sources

bioRxiv2023

Proteasome condensate formation is driven by multivalent interactions with shuttle factors and K48-linked ubiquitin chains

Stress conditions can cause the relocalization of proteasomes to condensates in yeast and mammalian cells. The interactions that facilitate the formation of proteasome condensates, however, are unclear. Here, we show that the formation of proteasome condensates in yeast depends on long K48-linked ubiquitin chains together with the proteasome shuttle factors Rad23 and Dsk2. These shuttle factors colocalize to these condensates. Strains deleted for the third shuttle factor gene, DDI1, show proteasome condensates in the absence of cellular stress, consistent with the accumulation of substrates with long K48-linked ubiquitin chains that accumulate in this mutant. We propose a model where the long K48-linked ubiquitin chains function as a scaffold for the ubiquitin binding domains of the shuttle factors and the proteasome, allowing for the multivalent interactions that further drive condensate formation. Indeed, we determined different intrinsic ubiquitin receptors of the proteasome (Rpn1, Rpn10, and Rpn13) are critical under different condensate inducing conditions. In all, our data support a model where the cellular accumulation of substrates with long ubiquitin chains, potentially due to reduced cellular energy, allows for proteasome condensate formation. This suggests that proteasome condensates are not simply for proteasome storage, but function to sequester soluble ubiquitinated substrates together with inactive proteasomes. SignificanceStress conditions can cause the relocalization of proteasomes to condensates in yeast as well as mammalian cells. Our work shows that the formation of proteasome condensates in yeast depends on long K48-linked ubiquitin chains, the proteasome binding shuttle factors Rad23 and Dsk2 and proteasome intrinsic ubiquitin receptors. Here, different receptors are critical for different condensate inducers. These results indicate distinct condensates can form with specific functionality. Our identification of key factors involved in the process is crucial for understanding the function of proteasome relocalization to condensates. We propose that cellular accumulation of substrates with long ubiquitin chains results in the formation of condensates comprising those ubiquitinated substrates, proteasomes, and proteasome shuttle factors, where the ubiquitin chains serve as the scaffold for condensate formation.

cell biology↗

bioRxiv2022

Proteasome granular localization is regulated through mitochondrial respiration and kinase signaling

In yeast, proteasomes are enriched in cell nuclei where they execute important cellular functions. Nutrient-stress can change this localization indicating proteasomes respond to the cells metabolic state. However, the signals that connect these processes remain poorly understood. Carbon starvation triggers a reversible translocation of proteasomes to cytosolic condensates known as proteasome storage granules (PSGs). Surprisingly, we observed strongly reduced PSG levels when cells had active cellular respiration prior to starvation. This suggests the mitochondrial activity of cells is a determining factor in the response of proteasomes to carbon starvation. Consistent with this, upon inhibition of mitochondrial function we observed proteasomes relocalize to granules. These links between proteasomes and metabolism involve specific signaling pathways, as we identified a MAP kinase cascade that is critical to the formation of proteasome granules after respiratory growth but not following glycolytic growth. Furthermore, the yeast homolog of AMP kinase, Snf1, is important for proteasome granule formation induced by mitochondrial inhibitors, while dispensable for granule formation following carbon starvation. We propose a model where mitochondrial activity promotes proteasome nuclear localization. SummaryWe determined a role for mitochondrial respiration in regulating proteasome granule formation and identified the cell integrity MAP kinase pathway and Snf1 kinase as regulatory factors.

cell biology↗

bioRxiv2021

Proteasome autophagy is specifically regulated and requires factors dispensible for general autophagy

Changing physiological conditions can increase the need for protein degradative capacity in eukaryotic cells. Both the ubiquitin-proteasome system and autophagy contribute to protein degradation. However, proteasomes are also an autophagy substrate. Thus, these processes must be differentially regulated depending on the physiological conditions presented. The signals and molecular mechanisms that govern proteasome autophagy are only partly elucidated. Our data indicate that chemical inhibition of TORC1 with rapamycin induces a bi-phasic response where proteasome levels are upregulated followed by an autophagy-dependent reduction. Surprisingly, several conditions that result in inhibited TORC1 exclusively induce proteasome autophagy (i.e. without any proteasome upregulation), suggesting a convergence of signals upstream of proteasome autophagy under different physiological conditions. Indeed, several conditions that activate general autophagy did not induce proteasome autophagy further distinguishing between proteasome autophagy and general autophagy. Consistent with this, we found that Atg11, the receptor for selective autophagy, and the map kinases Mpk1, Mkk1, and Mkk2, all play a role in autophagy of proteasomes, while they are dispensible for general autophagy. In all, our data provide new insights into the molecular regulation of proteasome autophagy by demonstrating that these complexes are specifically regulated under different autophagy inducing conditions.

biochemistry↗