MITOCHONDRIAL GRPE IS PRESENT IN A COMPLEX WITH HSP70 AND PREPROTEINSIN TRANSIT ACROSS MEMBRANES

We characterized a 24-kDa protein associated with matrix hsp70 (mt-hsp 70) of Neurospora crassa and Saccharomyces cerevisiae mitochondria. By using specific antibodies, the protein was identified as MGE, a mitoc hondrial homolog of the prokaryotic heat shock protein GrpE. MGE extra cted from mitochondria was quantitatively bound to hsp70. It was effic iently released from hsp70 by tbe addition of Mg-ATP but not by nonhyd rolyzable ATP analogs or high salt. A mutant mt-hsp70, which was impai red in release of bound precursor proteins, released MGE in an ATP-dep endent manner, indicating that precursor proteins and MGE bind to diff erent sites of hsp70. A preprotein accumulated in transit across the m itochondrial membranes was specifically coprecipitated by either antib odies directed against MGE or antibodies directed against mt-hsp70. Th e preprotein accumulated at the outer membrane was not coprecipitated by either antibody preparation. After being imported into the matrix, the preprotein could be coprecipitated only by antibodies against mt-h sp70. We propose that mt-hsp70 and MGE cooperate in membrane transloca tion of preproteins.