IbpA the small heat shock protein from Escherichia coli forms fibrils in the absence of its cochaperone IbpB.

Small heat shock proteins (sHsps) associate with aggregated proteins, changing their physical properties in such a way that chaperone mediated disaggregation becomes much more efficient. In Escherichia coli two small Hsps, IbpA and IbpB, exist. They are 48% identical at the amino acid level, yet their roles in stabilisation of protein aggregates are quite distinct. Here we analysed the biochemical properties of IbpA. We found that IbpA assembles into protofilaments which in turn form mature fibrils. Such fibrils are atypical for sHsps. Interaction of IbpA with either its cochaperone IbpB or an aggregated substrate blocks IbpA fibril formation.

Role of Escherichia coli heat shock proteins IbpA and IbpB in protection of alcohol dehydrogenase AdhE against heat inactivation in the presence of oxygen.

Escherichia coli small heat shock proteins IbpA and IbpB are molecular chaperones that bind denatured proteins and facilitate their subsequent refolding by the ATP-dependent chaperones DnaK/DnaJ/GrpE and ClpB. In vivo, the lack of IbpA and IbpB proteins results in increased protein aggregation under severe heat stress or delayed removal of aggregated proteins at recovery temperatures. In this report we followed the appearance and removal of aggregated alcohol dehydrogenase, AdhE, in E. coli submitted to heat stress in the presence of oxygen. During prolonged incubation of cells at 50 degrees C, when AdhE was progressively inactivated, we initially observed aggregation of AdhE and thereafter removal of aggregated AdhE. In contrast to previous studies, the lack of IbpA and IbpB did not influence the formation and removal of AdhE aggregates. However, in DeltaibpAB cells AdhE was inactivated and oxidized faster than in wild type strain. Our results demonstrate that IbpA and IbpB protected AdhE against thermal and oxidative inactivation, providing that the enzyme remained soluble. IbpA and IbpB were dispensable for the processing of irreversibly damaged and aggregated AdhE.

Distinct activities of Escherichia coli small heat shock proteins IbpA and IbpB promote efficient protein disaggregation.

It has been proposed that small heat shock proteins (sHsps) associate with aggregated proteins and change their physical properties in such a way that chaperone-mediated disaggregation and refolding become much more efficient. Here, we investigate the influence of two Escherichia coli sHsps, IbpA and IbpB, on the properties of aggregates formed under heat shock conditions and the susceptibility of these aggregates to chaperone-dependent reactivation. Our results show that the presence of IbpA during heat denaturation is sufficient to change the macroscopic properties of aggregates. The aggregates are substantially smaller than aggregates formed in the absence of sHsps and they are stained differently on electron micrographs. Moreover, these aggregates are indistinguishable, by electron microscopy studies and sedimentation analysis, from aggregates obtained during heat denaturation in the presence of IbpA and IbpB. However, the morphological similarity between these two types of aggregates does not correlate with similar susceptibility to Hsp100-Hsp70-dependent reactivation. The presence of IbpA alone during substrate denaturation does not increase the efficiency of the subsequent Hsp100-Hsp70-dependent reactivation. On the contrary, substantial inhibition of this process is observed. IbpB associates with aggregates at high temperature due to its interaction with IbpA and releases the IbpA-mediated inhibitory effect. Our results suggest there is an interplay between IbpA and IbpB in promoting Hsp100-Hsp70-mediated disaggregation of protein aggregates. Although each seems to play a different role in this process, they cooperate to stabilize protein aggregates in a disaggregation-competent state.