ISOLATION AND CHARACTERIZATION OF A CDNA-ENCODING RAT MITOCHONDRIAL GRPE, A STRESS-INDUCIBLE NUCLEOTIDE-EXCHANGE FACTOR OF UBIQUITOUS APPEARANCE IN MAMMALIAN ORGANS

In contrast to the E. coli chaperones DnaK, GroEL and GroES, cDNAs enc oding mitochondrial homologues of DnaJ and GrpE from higher eukaryotes have yet to be reported. Based on peptide sequences, we have isolated a cDNA encoding a 217 residue nuclear encoded precursor of rat mitoch ondrial GrpE (mt-GrpE) including a typical mitochondrial presequence o f 27 residues. Western blotting revealed that the 21 kDa GrpE homologu e is present exclusively in the mitochondrial fraction where it compri ses only similar to 0.03% of the total soluble protein, while Northern blotting showed that the mt-GrpE transcript is present in most if not all organs. By contrast to other mitochondrial chaperones, the levels of mt-GrpE and its transcript in cultured cells are only marginally i ncreased in response to the proline analog L-azetidine 2-carboxylic ac id but not by heat shock. Furthermore, members of the GrpE family exhi bit a much lower degree of sequence identity than do the well studied members of the Hsp70, Hsp60 and Hsp10 families.