Heterologous Expression of Thermolabile Proteins Enhances Thermotolerance in <i>Escherichia coli</i>

Abstract

Heat shock proteins (HSPs) play important roles in the mechanism of cellular protection against various environmental stresses. It is well known that accumulation of misfolded proteins in a cell triggers the HSPs expression in prokaryotes as well as eukaryotes. In this study, we heterologously expressed two proteins in E. coli , namely, citrate synthase ( Cp CSY) and malate dehydrogenase ( Cp MDH) from a psychrophilic bacterium Colwellia psychrerythraea 34H (optimal growth temperature 8°C). Our analyses using circular dichromism along with temperature-dependant enzyme activities measured in purified or direct cell extracts confirmed that the Cp CSY and Cp MDH are thermolabile and present in misfolded form even at physiological growth temperature. We observed that the cellular levels of HSPs, both GroEL and DnaK cheperonins were increased. Similarly, higher levels were observed for sigma factor s 32 which is specific to heat-shock protein expression. These results suggest that the misfolded-thermolabile proteins expressed in E. coli induced the heat shock response. Furthermore, heat treatment (53°C) to wild type E. coli noticeably delayed their growth recovery but cells expressing Cp CSY and Cp MDH recovered their growth much faster than that of wild type E. coli . This reveals that the HSPs expressed in response to misfolded-thermolabile proteins protected E. coli against heat-induced damage. This novel approach may be a useful tool for investigating stress-tolerance mechanisms of E. coli .