ClpB of Escherichia coli is an ATP-dependent ringforming chaperone that mediates the resolubilization of aggregated proteins in cooperation with the DnaK chaperone system. ClpB belongs to the Hsp100/Clp subfamily of AAA؉ proteins and is composed of an N-terminal domain and two AAA-domains that are separated by a "linker" region. Here we present a detailed structurefunction analysis of ClpB, dissecting the individual roles of ClpB domains and conserved motifs in oligomerization, ATP hydrolysis, and chaperone activity. Our results show that ClpB oligomerization is strictly dependent on the presence of the C-terminal domain of the second AAA-domain, while ATP binding to the first AAAdomains stabilized the ClpB oligomer. Analysis of mutants of conserved residues in Walker A and B and sensor 2 motifs revealed that both AAA-domains contribute to the basal ATPase activity of ClpB and communicate in a complex manner. Chaperone activity strictly depends on ClpB oligomerization and the presence of a residual ATPase activity. The N-domain is dispensable for oligomerization and for the disaggregating activity in vitro and in vivo. In contrast the presence of the linker region, although not involved in oligomerization, is essential for ClpB chaperone activity.

The AAA+ protein ClpB cooperates with the DnaK chaperone system to solubilize and refold proteins from an aggregated state. The substrate-binding site of ClpB and the mechanism of ClpB-dependent protein disaggregation are largely unknown. Here we identified a substrate-binding site of ClpB that is located at the central pore of the first AAA domain. The conserved Tyr251 residue that lines the central pore contributes to substrate binding and its crucial role was confirmed by mutational analysis and direct crosslinking to substrates. Because the positioning of an aromatic residue at the central pore is conserved in many AAA+ proteins, a central substrate-binding site involving this residue may be a common feature of this protein family. The location of the identified binding site also suggests a possible translocation mechanism as an integral part of the ClpB-dependent disaggregation reaction.

The Hsp100 protein ClpB is a member of the AAA+ protein family that mediates the solubilization of aggregated proteins in cooperation with the DnaK chaperone system. Unstructured polypeptides such as casein or poly-L-lysine have been shown to stimulate the ATPase activity of ClpB and thus may both act as substrates. Here we compared the e¡ects of K K-casein and poly-L-lysine on the ATPase and chaperone activities of ClpB. K K-Casein stimulated ATP hydrolysis by both AAA domains of ClpB and inhibited the ClpB-dependent solubilization of aggregated proteins if present in excess. In contrast, poly-Llysine stimulated exclusively the ATPase activity of the second AAA domain and increased the disaggregation activity of ClpB. Thus poly-L-lysine does not act as substrate, but rather represents an e¡ector molecule, which enhances the chaperone activity of ClpB. ß