Effects of expressing human Hsp70 and its deletion derivatives on heat killing and on RNA and protein synthesis.

We have established rat cell lines stably and constitutively expressing intact human heat shock protein (hsp) 70 and its deletion mutant derivatives. Using these stable cell lines, the functions of the various domains of human hsp70 were studied. An elevated level of human hsp70 or fragments containing the C-terminal portion of the protein, but not fragments containing the amino-terminal portion of the protein, is found to protect cells against thermal killing. Only the expression of intact human hsp70, however, can reduce the degree of heat-induced inhibition of translation and most effectively accelerate the recovery from heat-induced inhibition of transcription and translation. The peptide-binding domain in the carboxyl portion of hsp70 is of primary importance in protecting cells from thermal stress, probably through its binding to unfolded or partially folded polypeptides; this interaction, in turn, retards detrimental thermal denaturation or aggregation of key cellular proteins and protects cells from thermal stress. ATP binding and hydrolysis, on the other hand, are presumably necessary in the refolding and reassembly of thermally damaged proteins through multiple cycles of interactions between hsp70 and its targets.