Mitochondrial preproteins that are imported via the translocase of the mitochondrial outer membrane (Tom)70 receptor are complexed with cytosolic chaperones before targeting to the mitochondrial outer membrane. The adenine nucleotide transporter (ANT) follows this pathway, and its purified mature form is identical to the preprotein. Purified ANT was reconstituted with chaperones in reticulocyte lysate, and bound proteins were identified by mass spectrometry. In addition to 70-kDa heat-shock cognate protein (Hsc70) and 90-kDa heat-shock protein (Hsp90), a specific subset of cochaperones were found, but no mitochondria-specific targeting factors were found. Interestingly, three different Hsp40-related J-domain proteins were identified: DJA1, DJA2, and DJA4. The DJAs bound preproteins to different extents through their C-terminal regions. DJA dominant-negative mutants lacking the N-terminal J-domains impaired mitochondrial import. The mutants blocked the binding of Hsc70 to preprotein, but with varying efficiency. The DJAs also showed significant differences in activation of the Hsc70 ATPase and Hsc70-dependent protein refolding. In HeLa cells, the DJAs increased new protein folding and mitochondrial import, although to different extents. No single DJA was superior to the others in all aspects, but each had a profile of partial specialization. The Hsp90 cochaperones p23 and Aha1 also regulated Hsp90 -preprotein interactions. We suggest that multiple cochaperones with similar yet partially specialized properties cooperate in optimal chaperone-preprotein complexes.

The ATPase cycle of the chaperone Hsc70 is regulated by co-chaperones; Hsp40/DnaJ-related proteins stimulate ATP hydrolysis by Hsc70 and can bind unfolded polypeptides themselves. Conversely, various nucleotide exchange factors (NEFs) stimulate ADP-ATP exchange by Hsc70. We analyzed the purified Hsp40-related co-chaperones DJA1 (Hdj2) and DJA2 (Hdj3) and found that they had a distinct pattern of binding to a range of polypeptides. DJA2 alone could stimulate Hsc70-mediated refolding of luciferase in the absence of NEF, whereas DJA1 was much less active. The addition of the Bag1 NEF increased refolding by Hsc70 and DJA2, as did the newly characterized NEF Hsp110, but each NEF had a different optimal concentration ratio to Hsc70. Notably, the NEF HspBP1 could not increase refolding by Hsc70 and DJA2 at any concentration, and none of the NEFs improved the refolding activity with DJA1. Instead, DJA1 was inhibitory of refolding with DJA2 and Hsc70. All combinations of DJA1 or DJA2 with the three NEFs stimulated the Hsc70 ATPase rate, although Hsp110 became less effective with increasing concentrations. A chimeric DJA2 having its Hsc70-stimulatory J domain replaced with that of DJA1 was functional for polypeptide binding and ATPase stimulation of Hsc70. However, it could not support efficient Hsc70-mediated refolding and also inhibited refolding with DJA2 and Hsc70. These results suggest a more complex model of Hsc70 mechanism than has been previously thought, with notable functional divergence between Hsc70 co-chaperones.The Hsp70 family of proteins are ATP-dependent molecular chaperones that assist the folding of polypeptides. Hsp70 chaperones have a typical structure divided into ATPase and substrate-binding domains that work in an ATP-driven substrate binding cycle. The mechanism of Hsp70 proteins has been well established in studies of the Escherichia coli homolog DnaK. In the ATP-bound state, an Hsp70 chaperone has low affinity for unfolded polypeptide. After hydrolysis of ATP, Hsp70 in the ADP-bound state binds substrate with high affinity. Exchange of ADP for ATP then reverts Hsp70 to its low polypeptide affinity state. Conversion of an Hsp70 between these two nucleotide states is controlled by different co-chaperone proteins. The Hsp40/DnaJ-related co-chaperones, including E. coli DnaJ, contain J domains that stimulate ATP hydrolysis by Hsp70, and consequently substrate binding. Nucleotide exchange factors (NEFs), 2 such as GrpE in E. coli, trigger the dissociation of bound ADP from Hsp70 to allow the binding of ATP, resetting the cycle. The principles of this mechanism appear to be conserved in Hsp70 chaperones, including the major cytosolic form in humans, Hsc70 (HSPA8) (1, 2).The DnaJ-related co-chaperones are also conserved between species. Type 1 J domain co-chaperones are homologous to DnaJ throughout their sequence and have the same domain architecture. Following their N-terminal J domains, they contain a linker sequence, zinc finger and central regions, and a C-terminal homodimerization region. Unfolde...