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Papers In Press, published online ahead of print June 25, 2002
J. Biol. Chem, 10.1074/jbc.M203197200
Submitted on April 3, 2002
Revised on June 24, 2002
Accepted on June 24, 2002
Department of Biophysics, Bose Institute, Calcutta, W.Bengal 700 054
Corresponding Author: sidroy{at}vsnl.com
The heat-shock response in bacteria is a complex phenomenon in which sigma32 plays the central role. The DnaK/J chaperone system binds and promotes degradation of sigma32 at lower temperatures. At heat-shock temperatures, the DnaK/J mediated degradation of sigma32 is largely abolished by a mechanism, which is not yet fully understood. In this article we have shown that interaction of DnaK with sigma32 is highly temperature dependent. This interaction is completely abolished at 42 o C. To investigate the origin of such strong temperature dependence, we have monitored the structural changes that occur in the sigma32 protein upon upshift of temperature and attempted to elucidate its functional roles. Upon shift of temperature from 30 to 42 o C, the CD spectrum of sigma32 becomes significantly more positive without significant change in either tryptophan fluorescence spectra or quenchability to external quenchers. ANS binding at 42 o C is not significantly affected. The equilibrium guanidine hydrochloride denaturation of sigma32 is biphasic. The first phase shifts to even lower guanidine hydrochloride concentrations at 42 o C, while the major phase remains largely unchanged. The sigma32-core interaction remains unchanged as a function of temperature. This suggests that increased temperature destabilizes a structural element. We discuss the possible location of this temperature-sensitive structural element.
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