Evidence for the existence of distinct mammalian cytosolic, microsomal, and two mitochondrial GrpE-like proteins, the co-chaperones of specific Hsp70 members

Abstract

We previously reported the cDNA cloning and characterization of a mammalian mitochondrial GrpE protein (~21 kDa, mt-GrpE1) and now provide evidence for the presence of distinct cytosolic (~40 kDa), microsomal (~50 kDa), and additional mitochondrial (~22 kDa, mt-GrpE2) GrpE-like members. While a cytosolic GrpE-like protein has recently been identified, the demonstration of both a microsomal and a second mitochondrial GrpE-like member represents the first in any biological system. Investigation of the microsomal and two mitochondrial GrpE-like proteins revealed that they bound specifically to Escherichia coli DnaK, and the complexes formed were not disrupted in the presence of 0.5 M salt but were readily dissociated in the presence of 5 mM ATP. The functional integrity of mt-GrpE1 and 2 was verified by their ability to specifically interact with and stimulate the ATPase activity of mammalian mitochondrial Hsp70 (mt-Hsp70). Analysis of the cDNA sequences encoding the two mammalian mitochondrial GrpE-like proteins revealed ~47% positional identity at the amino acid level, the presence of a highly conserved mitochondrial leader sequence, and putative destabilization elements within the 3'-untranslated region of the mt-GrpE2 transcript which are not present in the mt-GrpE1 transcript. A constitutive expression of both mitochondrial GrpE-like transcripts in 22 distinct mouse tissues was observed but possible different posttranscriptional regulation of the mt-GrpE1 and 2 transcripts may confer a different expression pattern of the encoded proteins.