The small heat shock protein IbpA of Escherichia coli cooperates with IbpB in stabilization of thermally aggregated proteins in a disaggregation competent state.

The small heat shock proteins are ubiquitous stress proteins proposed to increase cellular tolerance to heat shock conditions. We isolated IbpA, the Escherichia coli small heat shock protein, and tested its ability to keep thermally inactivated substrate proteins in a disaggregation competent state. We found that the presence of IbpA alone during substrate thermal inactivation only weakly influences the ability of the bi-chaperone Hsp70-Hsp100 system to disaggregate aggregated substrate. Similar minor effects were observed for IbpB alone, the other E. coli small heat shock protein. However, when both IbpA and IbpB are simultaneously present during substrate inactivation they efficiently stabilize thermally aggregated proteins in a disaggregation competent state. The properties of the aggregated protein substrates are changed in the presence of IbpA and IbpB, resulting in lower hydrophobicity and the ability of aggregates to withstand sizing chromatography conditions. IbpA and IbpB form mixed complexes, and IbpA stimulates association of IbpB with substrate.