"Prompt" heat shock proteins: translationally regulated synthesis of new proteins associated with the nuclear matrix-intermediate filaments as an early response to heat shock.

The response of mammalian cells, such as HeLa cells, to prolong exposure to increased temperature (termed heat shock) has been well characterized. In these studies new mRNA is synthesized for several proteins whose translation is best seen after a return to 37 degrees C. We show here another response to increased temperature of a distinctively different character. A set of at least 50 newly detectable proteins, exclusively associated with the nuclear matrix-intermediate filaments (NM-IF) fraction, is synthesized immediately upon exposure to high temperature. These are of very low abundance or nonexistent in the unstressed cell and none appear to correspond to the "classic" heat shock proteins produced after new transcription. Prior treatment with actinomycin D has little effect on these "prompt" proteins, and they appear to be made from preexisting mRNAs that are activated at the increased temperature. The protein synthesis in the soluble, cytoskeletal, and chromatin fractions is strongly reduced by the increased temperature, while the labeling of the prompt proteins associated with NM-IF complex rapidly rises severalfold above that in control cells. Additionally these results suggest that the four cell fractions are not arbitrary cell divisions; rather they represent physiologically significant compartments in the cell.