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

Abstract

Heat shock of mouse L cells induces the synthesis of 2 polypeptides of MW 68,000 and 89,000. Using a fragment of a cloned gene encoding the Drosophila melanogaster MW 70,000 heat shock protein (hsp70), shows that this protein was highly conserved during eukaryotic evolution. This observation was extended 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 MW 68,000 heat shock protein (hsp68), there are mouse mRNA encoding proteins of 70,000 and 74,000 that are homologous to the Drosophila hsp70 gene. The MW 70,000 and 74,000 proteins, and their mRNA are abundant components of unstressed mouse L cells. These constitutively expressed proteins are unique polypeptides in contrast to the several isoelectric point variants of the inducible hsp68. The hsp68 or its mRNA is not detected in unstressed L cells. In addition to the mRNA corresponding to hsp68 and the MW 74,000 and 70,000 proteins, a 4th RNA is detected homologous to the Drosophila hsp70 gene but whose protein product was not identified. The hsp68 gene of mouse L cells may be a member of a multigene family and the individual family members are distinguishable by their degree of similarity but show differences in the regulation of their expression.