Disaggregating Chaperones: An Unfolding Story

Abstract

Stress, molecular crowding and mutations may jeopardize the native folding of proteins. Misfolded and aggregated proteins not only loose their biological activity, but may also disturb protein homeostasis, damage membranes and induce apoptosis. Here, we review the role of molecular chaperones as a network of cellular defenses against the formation of cytotoxic protein aggregates. Chaperones favour the native folding of proteins either as "holdases", sequestering hydrophobic regions in misfolding polypeptides, and/or as "unfoldases", forcibly unfolding and disentangling misfolded polypeptides from aggregates. Whereas in bacteria, plants and fungi Hsp70/40 acts in concert with the Hsp100 (ClpB) unfoldase, Hsp70/40 is the only known chaperone in the cytoplasm of mammalian cells that can forcibly unfold and neutralize cytotoxic protein conformers. Owing to its particular spatial configuration, the bulky 70 kDa Hsp70 molecule, when distally bound through a very tight molecular clamp onto a 50-fold smaller hydrophobic peptide loop extruding from an aggregate, can locally exert on the misfolded segment an unfolding force of entropic origin, thus destroying the misfolded structures that stabilize aggregates. ADP/ATP exchange triggers Hsp70 dissociation from the ensuing enlarged unfolded peptide loop, which is then allowed to spontaneously refold into a closer-to-native conformation devoid of affinity for the chaperone. Driven by ATP, the cooperative action of Hsp70 and its co-chaperone Hsp40 may thus gradually convert toxic misfolded protein substrates with high affinity for the chaperone, into non-toxic, natively refolded, low-affinity products. Stress- and mutation-induced protein damages in the cell, causing degenerative diseases and aging, may thus be effectively counteracted by a powerful network of molecular chaperones and of chaperone-related proteases.

The Endocytic Fate of the Transferrin Receptor Is Regulated by c-Abl Kinase

“…Although all of the processes described above provide plausible mechanisms for how c-Abl activity might support TfR1 recycling back to the cell surface, we focused our attention on the action of the cytoplasmic chaperone Hsc70, well known to mediate a diverse set of cellular functions that include the uncoating of clathrin vesicles (44 -46), facilitating appropriate protein folding (47), and directing cellular organelles into the autophagic pathway (48,49), to name just a few. The interactions between this chaperone and the TfR1 upon inhibition of c-Abl by imatinib as assessed by co-IP (Fig.…”

Disordered form of the scaffold protein IscU is the substrate for iron-sulfur cluster assembly on cysteine desulfurase

“…However, several proteins have been identified that, like IscS, preferentially bind and stabilize a disordered target protein (20)(21)(22). Many of these are chaperone proteins that rescue misfolded proteins (23)(24)(25) or are proteins involved in protein degradation (26)(27)(28)(29)(30)(31)(32).…”

Reactivation of protein aggregates by mortalin and Tid1—the human mitochondrial Hsp70 chaperone system

“…In vitro, DnaK can perform this function when assisted by its two cochaperones, a J-domain-containing protein (either DnaJ, CbpA, or DjlA; Genevaux et al 2007) and the nucleotide exchange factor GrpE (Sharma et al 2009). Specificity of the chaperone system is conferred primarily by the DnaJ co-chaperone, which recognizes and binds to a misfolded polypeptide (Hinault et al 2010), followed by the binding of the polypeptide to a "low-affinity" DnaK, which becomes entrapped upon hydrolyzing ATP.…”

“…The strong cyto-protective, even curing effects of the Hsp70-Hsp40 chaperones systems, both in the cytoplasm and in mitochondria can be attributed to their ability to act as polypeptide unfolding enzymes that can target cytotoxic misfolded and aggregated protein conformers (Sharma et al 2009;Sharma et al 2010). In organisms and cellular compartments that also express Hsp100/ClpB homologues, the Hsp70-Hsp40 unfoldase chaperone systems benefit from a powerful synergic mechanism of forceful disaggregation that may act both upstream and downstream to the polypeptide unfoldase activity of Hsp70 chaperones (Glover and Lindquist 1998;Goloubinoff et al 1999;Weibezahn et al 2005).…”