Human Hsp40/DnaJ proteins comprise a large protein family, members of which feature the J domain (named after the bacterial DnaJ protein) (1). The J-domain spans the first 75 N-terminal amino acids and is separated from the C-terminal by a glycine/phenylalanine-rich domain (2). Members of the Hsp40/DnaJ family play diverse roles in many cellular processes, such as folding, translocation, degradation and assembly of multi-protein complexes. In particular, Hdj1, the first human Hsp40/DnaJ protein identified, plays an important role in protein translation and folding, as well as in the regulation of Hsp70 function (3). HSP40 stimulates the ATPase activity of HSP70 which in turn causes conformational changes of the unfolded proteins (4, 5). The Hsp40-Hsp70-unfolded protein complex further binds to co-chaperones Hip, Hop and HSP90 which leads to protein folding, or components of protein degradation
machinery CHIP and BAG-1 (6). Some studies have shown that the difference between HDJ1 and type 1 DNAJ proteins including HDJ2 and yeast YdjI is the result of the possession of a zinc finger domain by the latter, which helps in the function of protein folding. (7, 8).