A novel study led by Kristina Jonas (Stockholm University/SciLifeLab Fellow) shows that the molecular chaperone DnaK (Hsp70) is required for growth in the fresh water bacterium Caulobacter crescentus, but that the specific function of DnaK differs depending on the environmental conditions of the cells.

DnaK belongs to a group of proteins called heat shock protein, which have counterparts also in humans. Heat shock proteins are known to play a key role in thermal stress adaption by assisting the folding and refolding of other proteins. In this way, they help to prevent the accumulation of damaged proteins whose folding is disrupted by the increased temperature and that potentially become toxic. 

In various bacteria, including Caulobacter, DnaK is required for cell growth and proliferation at all temperatures. In the current study the authors found that DnaK’s function as a folding catalyst is only required for growth under stress conditions, but not under favorable conditions. Instead, it is DnaK’s function as a regulator of gene expression that determines cell growth under such conditions. It is well-established that in addition to its folding activity DnaK is a negative regulator of the bacterial heat shock sigma factor σ³². The new study demonstrates that this regulatory function of DnaK is a growth requirement and that in the absence of DnaK unleashed σ³² activity inhibits growth and proliferation.

The study shows how Caulobacter and likely many other bacteria appoint the heat-stress responsive protein DnaK to gene regulatory functions to boost cell growth under normal temperature conditions when its role in protein folding and stability is not necessary.

Read the full article in PLOS Genetics