The Escherichia coli small heat-shock proteins IbpA and IbpB prevent the aggregation of endogenous proteins denatured in vivo during extreme heat shock

The roles of the Escherichia coli lbpA and lbpB chaperones in protection of heat-denatured proteins against irreversible aggregation in vivo were investigated. Overproduction of lbpA and lbpB resulted in stabilization of the denatured and reversibly aggregated proteins (the S fraction), which could be isolated from E. coli cells by sucrose gradient centrifugation. This finding is in agreement with the present model of the small heat-shock proteins' function, based mainly on in vitro studies. Deletion of the ibpAB operon resulted in almost twofold increase in protein aggregation and in inactivation of an enzyme (fructose-1,6-biphosphate aldolase) in cells incubated at 50 degreesC for 4 h, decreased efficiency of the removal of protein aggregates formed during prolonged incubation at 50 degreesC and affected cell viability at this temperature. IbpA/B proteins were not needed for removal of protein aggregates or for the enzyme protection/renaturation in cells heat shocked at 50 degreesC for 15 min. These results show that the IbpA/B proteins are required upon an extreme, long-term heat shock. Overproduction of lbpA but not lbpB caused an increase of the level of beta-lactamase precursor, which was localized in the S fraction, together with the lbpA protein, which suggests that the unfolded precursor binds to lbpA but not to lbpB. Although in the wild-type cells both E. coli small heat-shock proteins are known to localize in the S fraction, only 2% of total lbpB co-localized with the aggregated proteins in the absence of lbpA, while in the absence of lbpB, the majority of lbpA was present in the aggregates fraction.