The formation of an insoluble complex in isolated nuclei incubated at physiological temperature (37 degrees C) is demonstrated. A similar complex is shown to form in the nuclei of intact cells subjected to temperatures that induce the classical heat-shock response. The formation of this complex occurs rapidly in response to hyperthermia and is induced by small increases in temperature both in vitro and in vivo. We have characterized the formation of the complex in isolated nuclei and the nuclei of intact cells. A small number of the subset of nuclear proteins involved in the complex have been identified. The significance of the loss of solubility of these proteins in the nucleus following hyperthermia is discussed.

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