ClpB is a ring-forming, ATP-dependent protein disaggregase that cooperates with the cognate Hsp70 system to recover functional protein from aggregates. How ClpB harnesses the energy of ATP binding and hydrolysis to facilitate the mechanical unfolding of previously aggregated, stress-damaged proteins remains unclear.Here, we present crystal structures of the ClpB D2 domain in the nucleotide-bound and -free states, and the fitted cryoEM structure of the D2 hexamer ring, which provide a structural understanding of the ATP power stroke that drives protein translocation through the ClpB hexamer. We demonstrate that the conformation of the substrate-translocating pore loop is coupled to the nucleotide state of the cis subunit, which is transmitted to the neighboring subunit via a conserved but structurally distinct intersubunit-signaling pathway common to diverse AAA+ machines. Furthermore, we found that an engineered, disulfide cross-linked ClpB hexamer is fully functional biochemically, suggesting that ClpB deoligomerization is not required for protein disaggregation.ATPase | chaperone | Hsp100 | protein unfoldase C lpB is an ATP-dependent protein-remodeling machine that has the remarkable ability to rescue stress-damaged proteins from a previously aggregated state. As the major protein disaggregase in cells, bacterial ClpB and its yeast (Hsp104) and plant (Hsp101) homologs are essential for thermotolerance development (1-3), and for cell survival from acute stress conditions (4).At the molecular level, ClpB is a multidomain protein composed of two tandem Walker-type ATP-binding domains (AAA+ domains), termed D1 and D2, which drive ClpB's chaperone activity. The D1 domain features the ClpB-specific M-domain, which forms a long coiled-coil (5) and is essential for protein disaggregation (6, 7). Like other type II AAA+ ATPases, ClpB forms a double-ring structure, with six copies of the D1 and D2 domains each making up a homohexamer ring (5,8). Although ClpB shares similar quaternary structure with ClpA (9), ClpC (10), and the single-ring ClpX (11) and HslU (12, 13) AAA+ ATPases, which function as the protein unfoldase components of energy-dependent proteases, ClpB does not associate with a chambered peptidase to degrade proteins. Instead, ClpB cooperates with the cognate Hsp70 system (DnaKJ/GrpE) in a species-specific manner (14, 15) to recover functional protein from aggregates (16-18).The prevailing model suggests that DnaKJ/GrpE targets the ClpB motor activity to aggregates (19,20), which is consistent with an upstream role of the DnaK system in protein disaggregation (21-23). Once targeted, ClpB disaggregates protein aggregates by extracting unfolded polypeptides (24) and threading them through the ClpB hexamer ring (21,25). In support of a direct ClpB-DnaKJ/GrpE interaction, it was reported that ClpB interacts with DnaK via the ClpB M-domain (15,26). Notably, replacing the M-domain of bacterial ClpB with that of its yeast homolog Hsp104 switched the species specificity of the bichaperone system so that ClpB now c...