ACTIVATION OF THE DNA-BINDING ABILITY OF HUMAN HEAT-SHOCK TRANSCRIPTION FACTOR-1 MAY INVOLVE THE TRANSITION FROM AN INTRAMOLECULAR TO AN INTERMOLECULAR TRIPLE-STRANDED COILED-COIL STRUCTURE

Heat stress regulation of human heat shock genes is mediated by human heat shock transcription factor hHSF1, which contains three 4-3 hydrop hobic repeats (LZ1 to LZ3). In unstressed human cells (37 degrees C), hHSF1 appears to be in an inactive, monomeric state that may be mainta ined through intramolecular interactions stabilized by transient inter action with hsp70. Heat stress (39 to 42 degrees C) disrupts these int eractions, and hHSF1 homotrimerizes and acquires heat shock element DN A-binding cability. hHSF1 expressed in Xenopus oocytes also assumes a monomeric, non-DNA-binding state and is converted to a trimeric, DNA-b inding form upon exposure of the oocytes to heat shock (35 to 37 degre es C in this organism). Because endogenous HSF DNA-binding activity is low and anti-hHSF1 antibody does not recognize Xenopus HSF, we employ ed this system for mapping regions in hHSF1 that are required for the maintenance of the monomeric state. The results of mutagenesis analyse s strongly suggest that the inactive hHSF1 monomer is stabilized by hy drophobic interactions involving all three leucine zippers which may f orm a triple-stranded coiled coil. Trimerization may enable the DNA-bi nding function of hHSF1 by facilitating cooperative binding of monomer ic DNA-binding domains to the heat shock element motif. This view is s upported by observations that several different LexA DNA-binding domai n-hHSF1 chimeras bind to a LexA-binding site in a heat-regulated fashi on, that single amino acid replacements disrupting the integrity of hy drophobic repeats render these chimeras constitutively trimeric and DN A binding, and that LexA itself binds stably to DNA only as a dimer bu t not as a monomer in our assays.