DnaJ domain or J-domain. DnaJ/Hsp40 (heat shock protein 40) proteins are highly conserved and play crucial roles in protein translation, folding, unfolding, translocation, and degradation. They act primarily by stimulating the ATPase activity of Hsp70s, an important chaperonine family. Hsp40 proteins are characterized by the presence of a J domain, which mediates the interaction with Hsp70. They may contain other domains as well, and the architectures provide a means of classification.