Developmental Biology - Protein Regulation|
How Cells Keep Growing Even When Under Attack
New findings on how stress turns energy into growth...
In a surprise observation, biochemists at the University of Massachusetts at Amherst, observed when stressed bacteria develop an overrun and blocked damage-containment system their cells turn on different cellular pathways to insure normal growth continues.
Recent experiments verify how bacteria switch gears in response to different stresses in order to maintain normal cell functions such as replicating DNA. The research appears in the journal Cell Molecular Cell.
Professor Peter Chien, Director of the Models to Medicine Center at Amherst, explains that all cells must maintain normal growth under stress. All cells also contain clean-up proteases to degrade used proteins and other cell waste. So, similar protective backup systems may regulate other biological responses.
"Cancer cells also are constantly growing under protein stress conditions. Understanding how cells take advantage of protease competition to respond to stress leads to tempting speculation that we may be able to inhibit similar pathways and block uncontrolled growth."
Peter Chien PhD, Director, Models to Medicine Center, Institute of Applied Life Sciences; Department of Biochemistry and Molecular Biology Program, University of Massachusetts Amherst, Massachusetts, USA.
In bacteria, a protease known as Lon destroys damaged proteins to protect the cell from potential toxic consequences. It degrades some normal signaling proteins as well.
After studying the Lon protease and pathways it uses during cell stress, i.e. antibiotic attacks or extreme heat, the authors show that when bacteria are stressed, the increase in damaged proteins ends up temporarily swamping Lon protease. This results in stabilized signaling proteins normally degraded by Lon setting off a cascade of responses.
"Misfolded proteins are canaries in the coal mines. When they build up so high that Lon proteases become blocked, cells turn on pathways needed to ensure growth. In particular, cells increase the amount of deoxynucleotides - the 'DN' of DNA - building blocks of DNA replication."
Peter Chien PhD
Quite by surprise, the researchers discovered this new pathway exploring the essential character of different genes dependent on Lon protease.
"Rilee used a new approach that looks at the fitness cost of each gene in different mutant backgrounds. Surprisingly, he found loss of a normally essential deoxynucleotide synthesis gene, is tolerated in cells missing Lon protease."
This suggests decreasing Lon activity, cells compensate by making more deoxynucleotides, a result researchers confirmed using metabolomics a procedure measuring hundreds of chemicals in a cell all at once.
Explains Chien: "Metabolomics tells us there is a substantial shift in all building blocks for DNA synthesis when Lon activity is compromised. At the same time, we see when cells are stressed they make more of these molecules."
That connection led researchers to determine damaged proteins arise when stress blocks Lon.
Loss of Lon increases dNTP pools to permit loss of normally essential dCTP deaminase
Stabilization of CcrM increases RNR expression and protects against hydroxyurea
Misfolded proteins can competitively inhibit CcrM degradation by the Lon protease
Stabilizing transcription factors by titration of Lon promotes stress responses
During proteotoxic stress, bacteria maintain critical processes like DNA replication while removing misfolded proteins, which are degraded by the Lon protease. Here, we show that in Caulobacter crescentus Lon controls deoxyribonucleoside triphosphate (dNTP) pools during stress through degradation of the transcription factor CcrM. Elevated dNTP/nucleotide triphosphate (NTP) ratios in Delta Lon cells protects them from deletion of otherwise essential deoxythymidine triphosphate (dTTP)-producing pathways and shields them from hydroxyurea-induced loss of dNTPs. Increased dNTP production in Delta Lon results from higher expression of ribonucleotide reductase driven by increased CcrM. We show that misfolded proteins can stabilize CcrM by competing for limited protease and that Lon-dependent control of dNTPs improves fitness during protein misfolding conditions. We propose that linking dNTP production with availability of Lon allows Caulobacter to maintain replication capacity when misfolded protein burden increases, such as during rapid growth. Because Lon recognizes misfolded proteins regardless of the stress, this mechanism allows for response to a variety of unanticipated conditions.
Rilee D. Zeinert, Hamid Baniasadi, Benjamin P. Tu and PeterChien.
This work was funded by the National Institute of General Medical Sciences in the form of a MIRA grant to Chien and the Chemistry-Biology Training Program, which also supported Zeinert. The MIRA program does not fund individual projects, but broad programs of basic discovery research, to encourage researchers to propose more long-term, innovative, creative projects and to worry less about short-term goals and results.
D.M.G. and M.Z.-G. are grateful to Caltech for start-up funding. Work in the M.Z.-G. laboratory is also funded by grants from the Wellcome Trust (207415/Z/17/Z); Open Philanthropy; the Weston Havens Foundation; and the Curci Foundation. B.A.T.W. is supported by the Gates Cambridge Trust.
The authors declare they have no competing interests.
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