Purification and biochemical properties of Saccharomyces cerevisiae Mdj1p, the mitochondrial DnaJ homologue

The DnaK/DnaJ/GrpE heat shock proteins of Escherichia coli constitute 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 cerevisiae DnaJ homologue, Mdj1p Delta 55, which lacks the mitochondrial presequence, and studied its biochemical properties in well defined in vitro systems, We find that Mdj1p Delta 55 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, Mdj1p Delta 55 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 Mdj1p Delta 55-luciferase-DnaK complex. However, in contrast to DnaJ, Mdj1p Delta 55 does not appear to interact well with certain seemingly folded proteins, such as the sigma (32) heat shock transcription factor or the lambda P DNA replication protein, Finally, Mdj1p Delta 55 can substitute perfectly well for DnaJ in the refolding of denatured firefly luciferase by the DnaK chaperone machine. These studies demonstrate that Mdj1p Delta 55 has conserved most of DnacJ's known biological properties, thus supporting an analogous functional role in yeast mitochondria.