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

Department of Biochemistry, University of Gdask, Kadki 24, 80-822 Gdask, Poland¹
School of Biosciences, University of Birmingham, Birmingham B15 2TT, UK²
Department of Molecular Biology, University of Gdask, Kadki 24, 80-822 Gdask, Poland³

Author for correspondence: Ewa Laskowska. Tel: +48 58 301 57 41. Fax: +48 58 301 00 72. e-mail: lasko{at}biotech.univ.gda.pl

The roles of the Escherichia coli IbpA and IbpB chaperones in protection of heat-denatured proteins against irreversible aggregation in vivo were investigated. Overproduction of IbpA and IbpB 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 °C for 4 h, decreased efficiency of the removal of protein aggregates formed during prolonged incubation at 50 °C 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 °C for 15 min. These results show that the IbpA/B proteins are required upon an extreme, long-term heat shock. Overproduction of IbpA but not IbpB caused an increase of the level of ß-lactamase precursor, which was localized in the S fraction, together with the IbpA protein, which suggests that the unfolded precursor binds to IbpA but not to IbpB. 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 IbpB co-localized with the aggregated proteins in the absence of IbpA, while in the absence of IbpB, the majority of IbpA was present in the aggregates fraction.

Keywords: protein aggregation, ß-lactamase precursor, DnaK

Abbreviations: CP, cytoplasmic and periplasmic; IM, inner membrane; OM, outer membrane; sHsps, small heat-shock proteins

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