EVIDENCE FOR THE EXISTENCE OF DISTINCT MAMMALIAN CYTOSOLIC, MICROSOMAL, AND 2 MITOCHONDRIAL GRPE-LIKE PROTEINS, THE CO-CHAPERONES OF SPECIFIC HSP70 MEMBERS

We previously reported the cDNA cloning and characterization of a mamm alian mitochondrial GrpE protein (similar to 21 kDa, mt-GrpE#l) and no w provide evidence for the presence of distinct cytosolic (similar to 40 kDa), microsomal (similar to 50 kDa), and additional mitochondrial (similar to 22 kDa, mt-GrpE#2) GrpE-like members. While a cytosolic Gr pE-like protein has recently been identified, the demonstration of bot h a microsomal and a second mitochondrial GrpE-like member represents the first in any biological system, Investigation of the microsomal an d two mitochondrial. GrpE-like proteins revealed that they bound speci fically to Escherichia coli DnaK, and the complexes formed were not di srupted in the presence of 0.5 M salt but were readily dissociated in the presence of 5 mM ATP. The functional integrity of mt-GrpE#l and #2 was verified by their ability to specifically interact with and stimu late the ATPase activity of mammalian mitochondrial Hsp70 (mt-Hsp70), Analysis of the cDNA sequences encoding the two mammalian mitochondria l GrpE-like proteins revealed similar to 47% positional identity at th e amino acid level, the presence of a highly conserved mitochondrial l eader sequence, and putative destabilization elements within the 3'-un translated region of the mt-GrpE#2 transcript which are not present in the mt-GrpE#1 transcript. A constitutive expression of both mitochond rial GrpE-like transcripts in 22 distinct mouse tissues was observed b ut possible different posttranscriptional regulation of the mt-GrpE#l and #2 transcripts may confer a different expression pattern of the en coded proteins.