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Growth‐driven displacement of protein aggregates along the cell length ensures partitioning to both daughter cells in Caulobacter crescentus
Stockholm University, Faculty of Science, Department of Molecular Biosciences, The Wenner-Gren Institute. Stockholm University, Science for Life Laboratory (SciLifeLab).ORCID iD: 0000-0003-1858-7770
Stockholm University, Faculty of Science, Department of Molecular Biosciences, The Wenner-Gren Institute. Stockholm University, Science for Life Laboratory (SciLifeLab).
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2019 (English)In: Molecular Microbiology, ISSN 0950-382X, E-ISSN 1365-2958, Vol. 111, no 6, p. 1430-1448Article in journal (Refereed) Published
Abstract [en]

All living cells must cope with protein aggregation, which occurs as a result of experiencing stress. In previously studied bacteria, aggregated protein collects at the cell poles and is retained throughout consecutive cell divisions only in old pole‐inheriting daughter cells, resulting in aggregation‐free progeny within a few generations. In this study we describe the in vivo kinetics of aggregate formation and elimination following heat and antibiotic stress in the asymmetrically dividing bacterium Caulobacter crescentus. Unexpectedly, in this bacterium protein aggregates form as multiple distributed foci located throughout the cell volume. Time‐lapse microscopy revealed that under moderate stress, the majority of these protein aggregates are short‐lived and rapidly dissolved by the major chaperone DnaK and the disaggregase ClpB. Severe stress or genetic perturbation of the protein quality control machinery induces the formation of long‐lived aggregates. Importantly, the majority of persistent aggregates neither collect at the cell poles nor are they partitioned to only one daughter cell type. Instead, we show that aggregates are distributed to both daughter cells in the same ratio at each division, which is driven by the continuous elongation of the growing mother cell. Therefore, our study has revealed a new pattern of protein aggregate inheritance in bacteria.

Place, publisher, year, edition, pages
2019. Vol. 111, no 6, p. 1430-1448
Keywords [en]
protein aggregation, molecular chaperones, Caulobacter crescentus, aggregate inheritance, cellular aging
National Category
Biological Sciences
Research subject
Molecular Bioscience
Identifiers
URN: urn:nbn:se:su:diva-166029DOI: 10.1111/mmi.14228ISI: 000471131800004OAI: oai:DiVA.org:su-166029DiVA, id: diva2:1287732
Available from: 2019-02-11 Created: 2019-02-11 Last updated: 2019-07-08Bibliographically approved
In thesis
1. Stress response regulation and protein aggregate inheritance in Caulobacter crescentus
Open this publication in new window or tab >>Stress response regulation and protein aggregate inheritance in Caulobacter crescentus
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Many stress conditions a cell encounters threaten the continuation of basic biological processes ultimately endangering its survival. Heat shock and antibiotic exposure can lead to a sudden surge of protein un- and misfolding, while nutrient starvation directly causes a lack of energy and molecular building blocks. Our understanding of how cells integrate environmental stress signals, execute protective functions and handle persistent damage is still far from comprehensive. In this thesis the model bacterium Caulobacter crescentus was used to answer basic questions about the regulation and execution of bacterial stress responses and damage clearance.

Persistent larger protein aggregates can be maintained as remnants of a past stress exposure and in all of the few bacteria studied to date these particles collect at the poles. In the symmetrically dividing bacterium E. coli this aggregate localization pattern was shown to lead to an old pole lineage-specific retention. In paper I, we studied aggregate formation and inheritance in an asymmetrically dividing bacterium. While aggregates are dissolved by molecular chaperones following moderate heat stress, intense stress induces the emergence of long-lived aggregates. Surprisingly, we find that the majority of persistent aggregates do not collect at the old poles but instead describe a mechanism by which they are constantly displaced towards the new pole. This causes inheritance of aggregates by old and new pole cells at a stable rate without lineage-specific retention, a previously unknown pattern of aggregate inheritance in bacteria.

While we found that deletion of most chaperones in C. crescentus does not affect viability in the absence of stress, the mechanistic basis for why DnaK, like in other bacteria, is also required in the absence of stress remains unclear. In paper II, we show that DnaK's function as a negative regulator of the heat shock sigma factor σ32 is essential for viability at physiological temperatures and uncover potential new layers of σ32 regulation. We find that the σ32-dependent response comprises a reallocation of resources from proliferative to maintenance functions and in addition to its known function in blocking DNA replication also affects other processes like protein translation, a process vulnerable to proteotoxic stress. Prolonged unrestricted activity of this stress response induced by the absence of DnaK is lethal. We conclude that while DnaK is essential for protein folding at elevated temperatures, its evolutionarily newer function in balancing the cell's proliferative and maintenance programs is a requirement for survival.

Growth and cell cycle progression is also regulated in response to nutrient limitation. Like under heat shock conditions, we show in paper III that carbon starvation during entry into stationary phase leads to a block of DNA replication for which, in contrast to heat stress, the molecular basis was not yet understood. We find that downregulation of DnaA levels is achieved by an as yet unknown nutrient availability sensing process involving the 5' untranslated region, inhibiting translation of the dnaA mRNA, which combined with constant degradation of DnaA by the protease Lon results in its elimination. This study provided new mechanistic insight into nutrient-dependent control of DNA replication and shows that the same regulatory outcomes can be achieved through different means depending on the stress response.

In conclusion this thesis describes the discovery of an unanticipated alternative way of protein aggregate inheritance with implications for our view on damage segregation in bacterial populations as well as new mechanistic insight into how cells balance proliferative with protective functions in response to heat shock and nutrient limitation.

Place, publisher, year, edition, pages
Stockholm: Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, 2019. p. 81
Keywords
stress, protein aggregation, cellular aging, molecular chaperones, heat shock response, DnaK, sigma factor, suppressor genes, DNA replication, starvation, Caulobacter crescentus
National Category
Biological Sciences
Research subject
Molecular Bioscience
Identifiers
urn:nbn:se:su:diva-166030 (URN)978-91-7797-586-1 (ISBN)978-91-7797-587-8 (ISBN)
Public defence
2019-03-29, Vivi Täckholmsalen (Q-salen) NPQ-huset, Svante Arrhenius väg 20, Stockholm, 10:00 (English)
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Available from: 2019-03-06 Created: 2019-02-12 Last updated: 2019-04-12Bibliographically approved

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Schramm, Frederic D.Schroeder, KristenJonas, Kristina
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