Human HSP40/DnaJ proteins comprise a large protein family, members of which feature the J domain (named after the bacterial DnaJ protein). The J-domain spans the first 75 N-terminal amino acids and is separated from the C-terminal by a glycine/phenylalanine-rich domain. There are two main types of HSP40, type 1 DNAJ proteins including HDJ2 and yeast YdjI, type II includes yeast Sis1 and human Hdj1. Whereas type I possesses a zinc finger domain which helps in the function of protein folding, type II does not. Members of the HSP40/DnaJ family play diverse roles in many cellular processes, such as folding, translocation, degradation and assembly of multi-protein complexes. HSP40 stimulates the ATPase activity of HSP70 which in turn causes conformational changes of the unfolded proteins. 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.
