Small heat shock proteins, ClpB and the DnaK system form a functional triade in reversing protein aggregation

Small heat shock proteins (sHsps) can efficiently prevent the aggregation of unfolded proteins in vitro. However, how this in vitro activity translates to function in vivo is poorly understood. We demonstrate that sHsps of Escherichia coli, IbpA and IbpB, co-operate with ClpB and the DnaK system in vitro and in vivo, forming a functional triade of chaperones. IbpA/IbpB and ClpB support independently and co-operatively the DnaK system in reversing protein aggregation. A ΔibpAB ΔclpB double mutant exhibits strongly increased protein aggregation at 42ºC compared with the single mutants. sHsp and ClpB function become essential for cell viability at 37ºC if DnaK levels are reduced. The DnaK requirement for growth is increasingly higher for ΔibpAB, ΔclpB, and the double ΔibpAB ΔclpB mutant cells, establishing the positions of sHsps and ClpB in this chaperone triade