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rig, 1,3.3. Structure of a heat shock (transcription) factor from tomato. H2N- N-terminus; DNA-binding binding domain on the HSE; HR 1,2,3 regions for oligomerisation via hydrophobic interactions; NLS nuclear localisation sequence; Trp central tryptophan of the activator element. (After Nover and Hohfeld 1996)

the oligomerised HSF (trimer) is active, i.e. binds to DNA and HSPs;

• a cluster of alkaline amino acids C-terminal to HR 1, 2: the signal peptide for import into the nucleus (NLS: nucleus localisation signal);

• peptide motifs with a central tryptophan (Trp) as activator element which can interact with the transcription apparatus and thereby activate it. In larger HSFs the activator region contains a further hydrophobic heptade.

A Heat Shock Factor Must First Be Activated

Monomeric HSFs are inactive, i.e. they cannot bind to a heat shock element. The pool of HSFs is probably kept small in the normal state of the cell because of binding to the free HSPs (HSP70 and probably also HSP90). After dissociation from the HSP70/90 (and perhaps due to heat), the HSF changes its form so that it is able to form trimers. The trimer formed in the cytoplasm must now be imported into the nucleus to bind to the heat shock element of the promoter. The activator region becomes active because of binding to the HSE, and transcription can start.

What is the "actual signal" in the reaction chain leading to the activation of HSFs? The prerequisite for the activation of the HSFs is the dissociation of the HSF-HSP70/90 complex. Such dissociation would, of course, be favoured by a decrease in the free pools of HSP70/90. But how can the free pool of these HSPs decrease as a result of heat shock? These proteins bind as a consequence of the heat shock to other, denatured, proteins. It may be readily assumed that heat-denatured housekeeping proteins bind the free HSP70.

HSF inactive

Heat shock

HSF inactive

Heat shock

Denatured protein resulting from heat shock: HSP 70 and HSP 90 are limiting and are released from the complex.

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