Abstract
The heat shock transcription factor (HSF) mediates the induction of heat shock gene expression. The activation of HSF involves heat shock-induced trimerization, binding to its cognate DNA sites, and the acquisition of transcriptional competence. In this study, the oligomeric properties of Drosophila HSF were analyzed by equilibrium analytical ultracentrifugation and gel filtration chromatography. Previous findings showed that trimerization of purified Drosophila HSF was directly sensitive to heat and oxidation (1). Here we report that low pH, in the physiological range, also directly induces HSF trimerization and DNA binding in vitro. Furthermore, the induction of HSF trimerization by low pH is synergistic with the actions of heat and oxidation. Since heat or chemical stress leads to a moderate decrease of intracellular pH, we suggest that intracellular acidification may contribute to activating the heat shock response in vivo.
Highlights
All heat shock transcription factor (HSF) contain two highly conserved regions: a DNAbinding domain at the amino terminus and an adjacent trimerization domain with clusters of hydrophobic heptad repeats (HR-A/B) separated by a short spacer [9]
We demonstrate that low pH in the physiological range of pH 7.5 to 6.5 can directly and reversibly induce trimerization and DNA binding of purified Drosophila HSF in vitro
This in vitro sensitivity of Drosophila HSF to pH is generally consistent with earlier reports showing that the DNA binding of HSF can be induced by low pH in crude human and Drosophila cell extracts [42, 43] and with a recent study showing pH effects on bacterially expressed mouse HSF1 protein [38]
Summary
(Received for publication, August 31, 1998, and in revised form, October 28, 1998). Min Zhong, Soon-Jong Kim‡, and Carl Wu§ From the Laboratory of Molecular Cell Biology, NCI, National Institutes of Health, Bethesda, Maryland 20892-4255. The activation of HSF involves complex signaling pathways, including the loss of feedback repression imposed by the constitutive HSP70 proteins [25,26,27,28,29,30], serine phosphorylation of HSF [31,32,33,34,35,36,37], and direct effects of temperature (38 – 43) To test these mechanisms, we established an in vitro assay for reversible trimerization and DNA binding of Drosophila HSF purified from nonshocked or heat-shocked Sf9 cells [1]. Such a change of pH may play an additional regulatory role in heat shock response in vivo
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