Heat-Induced Aggregation and Interaction with Membranes of Cytoplasmic Proteins and the Suppressive Effect of Heat Shock Proteins in Escherichia coli Cells.

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We investigated the change in the location of intracellular proteins in Escherichia coli cells after heat treatment at 55°C in a buffer. After heat treatment, the amount of soluble proteins in cell extracts decreased and those of urea-soluble proteins, which were sedimented by highspeed centrifugation and then solubilized with 6 M urea solution, increased correspondingly. Based on the sucrose density gradient ultracentrifugation analysis, two separate peak fractions of the sedimentary intracellular proteins appeared. One was sedimented at the bottom and the other was cosedimented with the membranes. The sedimentary intracellular proteins in a rpoH mutant of E. coli after heat shock to 45°C in a growth medium were also separated in two peak fractions, as in its parent strain heated at 55°C in the buffer, whereas they were hardly detected in the case of the parent strain after the heat-shocked to 45°C. In the case of an IbpAB-overexpressing strain heated to 50°C in a growth medium, sedimentary intracellular proteins did not seem to be cosedimented with the membranes and also protein aggregation decreased, compared with the case of its parent strain heated at 50°C. These results suggest that heat-denatured intracellular proteins not only aggregate but also interact with the membranes and also that at least some of heat shock proteins including IbpAB may be involved in the suppression of protein aggregation.