Proteins related to the mouse L-cell major heat shock protein are synthesized in the absence of heat shock gene expression.

Heat shock of mouse L cells induces the synthesis of two polypeptides of Mrs 68,000 and 89,000. Using a fragment of a cloned gene encoding the Drosophila melanogaster Mr 70,000 heat shock protein (hsp70), we have shown that this protein has been highly conserved during eukaryotic evolution. We extended this observation by probing at low stringency for the expression in mouse L cells of RNA homologous to the Drosophila hsp70 gene. In addition to the RNA encoding the inducible Mr 68,000 heat shock protein (hsp68), there are mouse mRNAs encoding proteins of Mrs 70,000 and 74,000 that are homologous to the Drosophila hsp70 gene. The Mrs 70,000 and 74,000 proteins and their mRNAs are abundant components of unstressed mouse L cells. These constituitively expressed proteins are unique polypeptides in contrast to the several isoelectric point variants of the inducible hsp68. We do not detect hsp68 or its mRNA in unstressed L cells. In addition to the mRNAs corresponding to hsp68 and the Mrs 74,000 and 70,000 proteins, we detect a fourth RNA homologous to the Drosophila hsp70 gene but whose protein product has not been identified. Our results suggest that the hsp68 gene of mouse L cells is a member of a multigene family and that the individual family members are distinguishable by their degree of similarity but show differences in the regulation of their expression.