Aggregate reactivation mediated by the Hsp100 chaperones

Abstract

Hsp100 family of molecular chaperones shows a unique capability to resolubilize and reactivate aggregated proteins. The Hsp100-mediated protein disaggregation is linked to the activity of other chaperones from the Hsp70 and Hsp40 families. The best-studied members of the Hsp100 family are the bacterial ClpB and Hsp104 from yeast. Hsp100 chaperones are members of a large super-family of energy-driven conformational "machines" known as AAA+ ATPases. This review describes the current mechanistic model of the chaperone-induced protein disaggregation and explains how the structural architecture of Hsp100 supports disaggregation and how the co-chaperones may participate in the Hsp100-mediated reactions.

Fig. 1

The structural model of hexameric ClpB from E. coli (side, top, and bottom views). The individual domains in each ClpB monomer are indicated: N-terminal domain (red), D1 AAA+ module (green), middle coiled-coil domain (magenta), D2 AAA+ module large sub-domain (blue), D2 AAA+ module small C-terminal sub-domain (orange). The homology model of the E. coli ClpB monomer was built from the structures of the E. coli ClpB N-terminal domain [85], E. coli D1 AAA+ module [86], and chain B from the trimeric structure of T. thermophilus ClpB [37] using MODELLER software [87, 88]. Conformation of the channel loops in D1 and D2 (visible in the top and bottom views), which were not resolved in the template structures, was refined with the automated loop-modeling module [89]. The hexamer structure was obtained by assembling six monomers into a ring and performing a 500-step energy minimization with CHARMM and Generalized Born solvent model [90, 91].

Fig. 2

The proposed mechanism of protein disaggregation mediated by Hsp100. Left: nucleotide-dependent association of monomeric Hsp100 into a cylinder-shaped hexamer. Top: ATP-stimulated binding of an aggregate (red) to the hexameric Hsp100 produces an uncommitted disaggregase complex. The formation of the uncommitted complex does not require the co-chaperones (see text). The postulated rate-limiting step (red arrow) leads to a committed disaggregase state, in which an aggregated polypeptide (blue) becomes engaged by the channel loops. The efficiency of the commitment step may be controlled by the mobility of the N-terminal domain, the conformational changes in the middle domain, and the interactions of the co-chaperones with the aggregate and/or the middle domain of Hsp100 (see text). Bottom: Non-productive dissociation of the Hsp100 hexamer from an aggregate which resists disaggregation and a productive release of an unfolded polypeptide after translocation through the Hsp100 channel.