Chaperone-assisted protein folding. Chaperones bind and release polypeptides by a cyclic pathway, shown here for E. coli chaperone proteins DnaK and DnaJ, homologs of the eukaryotic chaperones Hsp70 and Hsp40. The chaperones mainly prevent the aggregation of unfolded polypeptides. Some polypeptides released at the end of a cycle are in their native conformation; the rest are rebound by DnaJ and DnaK to repeat the cycle for further attempts at refolding. In bacteria, GrpE interacts transiently with DnaK late in the cycle (step 3), promoting dissociation of ADP and possibly DnaJ. No eukaryotic analog of GrpE is known.