ClpB Cooperates with DnaK, DnaJ, and GrpE in Suppressing Protein Aggregation

ClpB is a heat-shock protein from Escherichia coli with an unknown function. We studied a possible molecular chaperone activity of ClpB in vitro. Firefly luciferase was denatured in urea and then diluted into the refolding buffer (in the presence of 5 mM ATP and 0.1 mg/ml bovine serum albumin). Spontaneous reactivation of luciferase was very weak (less than 0.02% of the native activity) because of extensive aggregation. Conventional chaperone systems (GroEL/GroES and DnaK/DnaJ/GrpE) or ClpB alone did not reactivate luciferase under those conditions. However, ClpB together with DnaK/DnaJ/GrpE greatly enhanced the luciferase activity regain (up to 57% of native activity) by suppressing luciferase aggregation. This coordinated function of ClpB and DnaK/DnaJ/GrpE required ATP hydrolysis, although the ClpB ATPase was not activated by native or denatured luciferase. When the chaperones were added to the luciferase refolding solutions after 5-25 min of refolding, ClpB and DnaK/DnaJ/GrpE recovered the luciferase activity from preformed aggregates. Thus, we have identified a novel multi-chaperone system from E. coli, which is analogous to the Hsp104/Ssa1/Ydj1 system from yeast. ClpB is the only known bacterial Hsp100 protein capable of cooperating with other heat-shock proteins in suppressing and reversing protein aggregation.