Analysis of novel hyperosmotic shock response suggests 'beads in liquid' cytosol structure.

A I Alexandrov,R N Chuprov-Netochin,S V Leonov,Tyurin-Kuzmin A Pyotr,M D Ter-Avanesyan,A A Dergalev,E V Grosfeld,M O Agaphonov,I I Kireev,V V Kushnirov

doi:10.1242/bio.044529

Abstract

ABSTRACTProteins can aggregate in response to stresses, including hyperosmotic shock. Formation and disassembly of aggregates is a relatively slow process. We describe a novel instant response of the cell to hyperosmosis, during which chaperones and other proteins form numerous foci with properties uncharacteristic of classical aggregates. These foci appeared/disappeared seconds after shock onset/removal, in close correlation with cell volume changes. Genome-wide and targeted testing revealed chaperones, metabolic enzymes, P-body components and amyloidogenic proteins in the foci. Most of these proteins can form large assemblies and for some, the assembled state was pre-requisite for participation in foci. A genome-wide screen failed to identify genes whose absence prevented foci participation by Hsp70. Shapes of and interconnections between foci, revealed by super-resolution microscopy, indicated that the foci were compressed between other entities. Based on our findings, we suggest a new model of cytosol architecture as a collection of numerous gel-like regions suspended in a liquid network. This network is reduced in volume in response to hyperosmosis and forms small pockets between the gel-like regions.

Highlights

Systems biology aspires to understand life in quantitative detail, eventually allowing complete modeling of living cells in silico
Earlier it was observed that hyperosmotic shock causes aggregation of cellular proteins and model amyloidogenic proteins in C. elegans (Burkewitz et al, 2011), as well as influences disappearance and appearance of prion amyloids in yeast (Newnam et al, 2011; Tyedmers et al, 2008)
Since some chaperone proteins were shown to bind to aggregates of misfolded proteins, one could expect that green fluorescent protein (GFP) fusions of such chaperones would decorate aggregates formed in response to hyperosmotic shock, allowing monitoring of aggregate formation in vivo

Summary

Introduction

Systems biology aspires to understand life in quantitative detail, eventually allowing complete modeling of living cells in silico. Even for simple cytosolic processes, this has been difficult to achieve due to various aspects of cytoplasmic heterogeneity (for a review, see (Luby-Phelps, 1999; Luby-Phelps, 2013) and differing views on the nature of the cytosol, i.e. is it a simple solution, crowded liquid, or a hydrogel (Grygorczyk et al, 2015). While all of these models describe some properties of the cytosol, they are difficult to unite in a single framework and there is a lack of comprehensible mechanistic models. Another difficulty that adds to the complexity of cytosolic structure is the ability of the cytosol to change its viscosity in a dramatic manner, such as during changes of pH (Munder et al, 2016; Parry et al, 2014) or osmotic pressure (Miermont et al, 2013)

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