Conserved Amino Acid Residues within the Amino-terminal Domain of ClpB are Essential for the Chaperone Activity

ClpB from Escherichia coli is a member of a protein-disaggregating multi-chaperone system that also includes DnaK, DnaJ, and GrpE. The sequence of ClpB contains two ATP-binding domains that are enclosed between the amino-terminal and carboxyl-terminal regions. The N-terminal sequence region does not contain known functional sequence motifs. Here, we performed site-directed mutagenesis of four polar residues within the N-terminal domain of ClpB (Thr7, Ser84, Asp103 and Glu109). These residues are conserved in several ClpB homologs. We found that the mutations, T7A, S84A, D103A, and E109A did not significantly affect the secondary structure and thermal stability of ClpB, nor did they inhibit the self-association of ClpB, its basal ATPase activity, or the enhanced rate of the ATP hydrolysis by ClpB in the presence of poly-l-lysine. We observed, however, that three mutations, T7A, D103A, and E109A, reduced the casein-induced activation of the ClpB ATPase. The same three mutant ClpB variants also showed low chaperone activity in the luciferase reactivation assay. We found, however, that the four ClpB mutants, as well as the wild-type, bound similar amounts of inactivated luciferase. In summary, we have identified three essential amino acid residues within the N-terminal region of ClpB that participate in the coupling between a protein-binding signal and the ATP hydrolysis, and also support the chaperone activity of ClpB.