Identification, Characterization, and Purification of Two Mammalian Stress Proteins Present in Mitochondria, Grp 75, a Member of the Hsp 70 Family and Hsp 58, a Homolog of the Bacterial GroEL Protein

We describe the identification, characterization, and purification of two members of the mammalian stress protein family, both of which are shown to be components of the mitochondria. The first, with an apparent mass of 58,000 daltons, is a constitutive protein whose synthesis increases in cells exposed to elevated temperatures and/or amino acid analogs and is therefore referred to as heat shock protein hsp 58 (hsp 58). The second, with an apparent mass of 75,000 daltons, is also a constitutive protein whose synthesis is increased in cells following glucose deprivation or exposure to either a calcium ionophore or 2-deoxyglucose and therefore represents a member of the so-called glucose-regulated proteins (grp 75). In cells treated with the potassium ionophore, nonactin, both hsp 58 and grp 75 were observed to accumulate in precursor form. Since nonactin has been reported to specifically inhibit the processing of cytoplasmic precursor proteins destined for the mitochondria, we investigated whether mature hsp 58 and grp 75 were components of the mitochondria. Mitochondria were isolated from rat liver and shown to contain both hsp 58 and grp 75. Indirect immunofluorescence using antibodies specific to either hsp 58 or grp 75 confirmed their presence within mitochondria. Proteolytic digestion experiments with intact mitochondria indicated that both proteins were not accessible to external proteolytic attack, suggesting that they are not exposed on the cytoplasmic face of the outer membrane. Based on a variety of biochemical and immunological criteria, grp 75 is shown to be a member of the hsp 70 family of stress proteins, while hsp 58 represents the mammalian equivalent of the bacterial groEL protein. Procedures for the purification of both hsp 58 and grp 75 are presented. The possible biochemical role of these two mitochondrial stress proteins is discussed in relation to the known biochemical function of their related stress protein counterparts.