ClpB and Hsp104 (ClpB/Hsp104) are essential proteins of the heat-shock response and belong to the class 1 family of Clp/Hsp100 AAA(+) ATPases. Members of this family form large ring structures and contain two AAA(+) modules, which consist of a RecA-like nucleotide-binding domain (NBD) and an alpha-helical domain. Furthermore, ClpB/Hsp104 has a longer middle region, the ClpB/ Hsp104-linker, which is essential for chaperone activity. Unlike other Clp/Hsp100 proteins, however, ClpB/Hspl04 neither associates with a cellular protease nor directs the degradation of its substrate proteins. Rather, ClpB/Hsp104 is a bona fide molecular chaperone.. which has the remarkable ability to rescue proteins from an aggregated state. The full recovery of these proteins requires the assistance of the cognate DnaK/Hsp70 chaperone system. The mechanism of this "bi-chaperone" network, however, remains elusive. Here we review the current understanding of the structure-function relationship of the ClpB/Hsp104 molecular chaperone and its role in protein disaggregation. (C) 2003 Elsevier Inc. All rights reserved.