Unstressed early mouse embryos have been previously shown  to synthesize at very high rates 70 and 89 kD proteins belonging to the heat shock protein (HSP) family. But it was not clear whether expression of heat shock-inducible or non-inducible (cognate) genes accounted for this spontaneous synthesis. In this report we show that the 89 kD mouse HSP can be separated into two proteins by high resolution PAGE. These two components show distinct but related peptide pattern after limited proteolysis. They are synthesized from distinct mRNAs. One of these proteins-HSP 89f-is synthesized at a high rate by unstressed cells and its synthesis is rather insensitive to stress, whereas synthesis of the other protein-HSP 89s-is strongly stimulated by heat shock in fibroblasts. Both HSP 89f and HSP 89s are major proteins synthesized in unstressed mouse preimplantation embryos and embryonal carcinoma (EC) cells. After in vitro differentiation of the EC cells the spontaneous synthesis of HSP 89s decreases. Thus spontaneous expression of a mammalian inducible HSP is developmentally regulated. © 1987.
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