Stabilization of protein synthesis in thermotolerant cells during heat shock. Association of heat shock protein-72 with ribosomal subunits of polysomes.

Thermotolerance is defined as the capacity of cells, following a cycle of stress and recovery, to survive a second stress which would otherwise be lethal. Whereas this is a well-documented phenomenon, the mechanisms underlying this protective event remain to be elucidated. Protection of protein synthesis appears to be one of the components in the induction of thermotolerance termed "translational thermotolerance." In the present study we show that translational thermotolerance is not the result of an increase in the concentration of cellular transcripts or the stabilization of preexisting messages, nor the preservation of the rate of amino acid uptake, synthesis of aminoacyl-tRNA, or protection from degradation of newly synthesized polypeptides. These results suggest that translational thermotolerance is the consequence of stabilization of translational initiation and/or polypeptide chain elongation during heat shock. We found that heat shock protein (hsp)-72, the major inducible form of the hsp-70 family of heat shock proteins, is associated with ribosomal subunits in polysomes of thermotolerant cells during heat shock. We hypothesize that such interaction is responsible for rescuing translational initiation and/or polypeptide chain elongation in thermotolerant cells during a subsequent stress. It is possible that hsp-72 on the ribosome is "waiting" for the nascent polypeptide to emerge from the ribosome. Such interaction may maintain the growing polypeptide in solution during stress, allowing elongation to continue, and maintaining a constant rate of translation during the stress.