Purification and Biochemical Properties of Saccharomyces cerevisiaeMdj1p, the Mitochondrial DnaJ Homologue*

The DnaK/DnaJ/GrpE heat shock proteins ofEscherichia coliconstitute the prototype DnaK chaperone machine. Various studies have shown that these three proteins work synergistically in a diverse array of biological functions, including protein folding and disaggregation, proteolysis, and transport across biological membranes. We have overexpressed and purified the mitochondrial Saccharomyces cerevisiaeDnaJ homologue, Mdj1pΔ55, which lacks the mitochondrial presequence, and studied its biochemical properties in well defined in vitrosystems. We find that Mdj1pΔ55 interacts with DnaK as judged both by an enzyme-linked immunosorbent assay, as well as stimulation of DnaK's weak ATPase activity in the presence of GrpE. In addition, Mdj1pΔ55 not only interacts with denatured firefly luciferase on its own, but also enables DnaK to bind to it in an ATP-dependent mode. Using co-immunoprecipitation assays we can demonstrate the presence of a stable Mdj1pΔ55-luciferase-DnaK complex. However, in contrast to DnaJ, Mdj1pΔ55 does not appear to interact well with certain seemingly folded proteins, such as the ς32heat shock transcription factor or the λP DNA replication protein. Finally, Mdj1pΔ55 can substitute perfectly well for DnaJ in the refolding of denatured firefly luciferase by the DnaK chaperone machine. These studies demonstrate that Mdj1pΔ55 has conserved most of DnaJ's known biological properties, thus supporting an analogous functional role in yeast mitochondria.