Formation of Non-Nucleoplasmic Proteasome Foci during the Late Stage of Hyperosmotic Stress.

Jeeyoung Lee,Eunkyoung Kim,Ly Thi Huong Luu Le,Min Jae Lee

doi:10.3390/cells10092493

Abstract

Cellular stress induces the formation of membraneless protein condensates in both the nucleus and cytoplasm. The nucleocytoplasmic transport of proteins mainly occurs through nuclear pore complexes (NPCs), whose efficiency is affected by various stress conditions. Here, we report that hyperosmotic stress compartmentalizes nuclear 26S proteasomes into dense nuclear foci, independent of signaling cascades. Most of the proteasome foci were detected between the condensed chromatin mass and inner nuclear membrane. The proteasome-positive puncta were not colocalized with other types of nuclear bodies and were reversibly dispersed when cells were returned to the isotonic medium. The structural integrity of 26S proteasomes in the nucleus was slightly affected under the hyperosmotic condition. We also found that these insulator-body-like proteasome foci were possibly formed through disrupted nucleus-to-cytosol transport, which was mediated by the sequestration of NPC components into osmostress-responding stress granules. These data suggest that phase separation in both the nucleus and cytosol may be a major cell survival mechanism during hyperosmotic stress conditions.

Highlights

Cells have multiple systems to maintain proteome integrity in response to various types of environmental stress
While investigating the dynamics of nuclear proteasomes, we observed from multiple cell lines that hyperosmotic stress (200 mM NaCl for ~6 h) resulted in reduced levels of proteasome subunits in the nuclear fractions, and spatial changes of nuclear proteasome-positive signals that originally dispersed in the nucleoplasm to become condensed into multiple foci near the anterior nuclear periphery (Figure S1)
Cells treated with 100 mM NaCl had only a limited number of nuclear speckles, but completely lacked the perinuclear foci (Figure S2B). These results indicate that nuclear proteasome foci formation upon hyperosmotic stress is mainly correlated with the severity of hyperosmotic stress and does not differentially respond to diverse osmolytes

Summary

Introduction

Cells have multiple systems to maintain proteome integrity in response to various types of environmental stress. Misfolded proteins arising from stress-induced denaturation, mutation, and posttranslational modifications are cleared preferentially by the ubiquitinproteasome system [1,2,3]. Reversible compartmentalization of potentially toxic proteins into membraneless organelles such as nuclear stress bodies and cytoplasmic stress granules is one of the pro-survival mechanisms of cells under substantial cellular stress [4]. Many of these adaptation strategies seem to have been evolutionarily conserved from yeast to mammals. Liquid-liquid phase separation (LLPS) and promiscuous interactions among intrinsically disordered proteins are considered to be the main driving force behind the formation of stress-induced membraneless organelles including stress granules [7,8]

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