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Immunoglobulin heavy chain binding protein (BiP) is present in the lumen of the mammalian endoplasmic reticulum, where it associates transiently with a variety of newly synthesized secretory and membrane proteins or permanently with mutant proteins that are incorrectly folded. We describe a unique member of the Saccharomyces cerevisiae 70-kDa heat shock protein gene family (HSP70) that encodes a protein homologous to mammalian BiP. The DNA sequence contains a 2046-nucleotide open reading frame devoid of introns, and examination of the predicted amino acid sequence reveals features not found in most other yeast HSP70 proteins but which are present in BiP. Most notable are a 42-residue sequence at the N terminus that exhibits characteristics of a cleavable signal sequence and a C-terminal sequence, -His-Asp-Glu-Leu, that is involved in determining endoplasmic reticulum localization in yeast. The 5' flanking region of this gene contains two overlapping sequences between nucleotides -146 and -169 that closely resemble consensus heat shock elements. The yeast BiP gene is strongly heat shock-inducible, whereas the BiP genes in various other species are either weakly or non-heat-inducible. We demonstrate that a functional BiP gene is essential for vegetative growth. An evolutionary comparison of amino acid sequences of 34 HSP70 proteins from 17 species suggests that BiP genes share a common ancestor, which diverged from other HSP70 genes near the time when eukaryotes first appeared.

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.