Sequential action of two hsp70 complexes during protein import into mitochondria.

The mitochondrial chaperone mhsp70 mediates protein transport across the inner membrane and protein folding in the matrix. These two reactions are effected by two different mhsp70 complexes. The ADP conformation of mhsp70 favors formation of a complex on the inner membrane; this 'import complex' contains mhsp70, its membrane anchor Tim44 and the nucleotide exchange factor mGrpE. The ATP conformation of mhsp70 favors formation of a complex in the matrix; this 'folding complex' contains mhsp70, the mitochondrial DnaJ homolog Mdj1 and mGrpE. A precursor protein entering the matrix interacts first with the import complex and then with the folding complex. A chaperone can thus function as part of two different complexes within the same organelle.