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J. Biol. Chem., Vol. 276, Issue 43, 39586-39591, October 26, 2001
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From the Salt and heat stresses, which are often combined
in nature, induce complementing defense mechanisms. Organisms adapt to
high external salinity by accumulating small organic compounds known as
osmolytes, which equilibrate cellular osmotic pressure. Osmolytes can
also act as "chemical chaperones" by increasing the stability of
native proteins and assisting refolding of unfolded polypeptides. Adaptation to heat stress depends on the expression of heat-shock proteins, many of which are molecular chaperones, that prevent protein
aggregation, disassemble protein aggregates, and assist protein
refolding. We show here that Escherichia coli cells
preadapted to high salinity contain increased levels of glycine betaine
that prevent protein aggregation under thermal stress. After heat
shock, the aggregated proteins, which escaped protection, were
disaggregated in salt-adapted cells as efficiently as in low salt. Here
we address the effects of four common osmolytes on chaperone activity
in vitro. Systematic dose responses of glycine betaine,
glycerol, proline, and trehalose revealed a regulatory effect on the
folding activities of individual and combinations of chaperones GroEL, DnaK, and ClpB. With the exception of trehalose, low physiological concentrations of proline, glycerol, and especially glycine betaine activated the molecular chaperones, likely by assisting local folding
in chaperone-bound polypeptides and stabilizing the native end product
of the reaction. High osmolyte concentrations, especially trehalose,
strongly inhibited DnaK-dependent chaperone networks, such
as DnaK+GroEL and DnaK+ClpB, likely because high viscosity affects
dynamic interactions between chaperones and folding substrates and
stabilizes protein aggregates. Thus, during combined salt and
heat stresses, cells can specifically control protein stability and
chaperone-mediated disaggregation and refolding by modulating the
intracellular levels of different osmolytes.
Chemical Chaperones Regulate Molecular Chaperones in
Vitro and in Cells under Combined Salt and Heat Stresses*
,,, and§¶
Department of Plant Sciences, Institute of
Life Sciences, The Hebrew University of Jerusalem, 91904 Jerusalem,
Israel and § Institute of Ecology, Lausanne University, 1015 Lausanne, Switzerland
*
This work was supported in part by a grant from the United
States-Israel Binational Science Foundation and from the Israel Science
Foundation.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
Copyright © 2001 by The American Society for Biochemistry and Molecular Biology, Inc.
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