Role of the DnaK and HscA homologs of Hsp70 chaperones in protein folding in E-coli

Folding of newly synthesized cytosolic proteins has been proposed to require assistance by Hsp70 chaperones, We investigated whether two Hsp70 homologs of Escherichia coli, DnaK and HscA, have this role in vivo. Double mutants lacking dnaK and hscA were viable and lacked defects in protein folding at intermediate temperature. After heat shock, a subpopulation of pre-existing proteins slowly aggregated in mutants lacking DnaK, but not HscA, whereas the bulk of newly synthesized proteins displayed wild-type solubility. For thermolabile firefly luciferase, DnaK was dispensable for de novo folding at 30 degrees C, but essential for aggregation prevention during heat shock and subsequent refolding. DnaK and HscA are thus not strictly essential for folding of newly synthesized proteins. DnaK instead has functions in refolding of misfolded proteins that are essential under stress.