Hsp70 displaces small heat shock proteins from aggregates to initiate protein refolding

Abstract

Small heat shock proteins (sHsps) are an evolutionary conserved class of ATP-independent chaperones that protect cells against proteotoxic stress. sHsps form assemblies with aggregation-prone misfolded proteins, which facilitates subsequent substrate solubilization and refolding by ATP-dependent Hsp70 and Hsp100 chaperones. Substrate solubilization requires disruption of sHsp association with trapped misfolded proteins. Here, we unravel a specific interplay between Hsp70 and sHsps at the initial step of the solubilization process. We show that Hsp70 displaces surface-bound sHsps from sHsp-substrate assemblies. This Hsp70 activity is unique among chaperones and highly sensitive to alterations in Hsp70 concentrations. The Hsp70 activity is reflected in the organization of sHsp-substrate assemblies, including an outer dynamic sHsp shell that is removed by Hsp70 and a stable core comprised mainly of aggregated substrates. Binding of Hsp70 to the sHsp/substrate core protects the core from aggregation and directs sequestered substrates towards refolding pathway. The sHsp/Hsp70 interplay has major impact on protein homeostasis as it sensitizes substrate release towards cellular Hsp70 availability ensuring efficient refolding of damaged proteins under favourable folding conditions.

Single-molecule fluorescence-based approach reveals novel mechanistic insights into small heat shock protein chaperone function

“…During this period there was an increase in the number of Bc subunits in each Bc-CLIC1 complex. We rationalise this as the recruitment of free Bc subunits onto existing Bc-CLIC1 complexes over time, as has been suggested to occur for other sHsp-client protein interactions 24,40,44 . Varying the molar ratio between CLIC1…”

“…It is known that many sHsps, including Bc, maintain large oligomeric assemblies via dynamic subunit exchange 8,45,46 . Moreover, in both prokaryotic (IbpA and IbpB) 44 and eukaryotic sHsp systems (Hsp18.1 and Hsp16.6) 26 , sHsp-client complexes are dynamic in that sHsp subunits associate and dissociate from these complexes. Therefore, we propose that the observed accumulation of Bc onto Bc-CLIC1 complexes is regulated by the association and dissociation rates of the Bc subunits.…”

HspB1 and Hsc70 chaperones engage distinct tau species and have different inhibitory effects on amyloid formation

“…Few studies have examined the interactions between human Hsc70 and HspB1, although recent work suggests the two proteins may be physically linked through interactions with the nucleotide-exchange factor BAG3 (55). There is evidence that E. coli sHSPs work with the E. coli Hsp70 homolog, DnaK, in preventing protein aggregation (56,57). Additional work is needed to elucidate how HspB1 and Hsc70 function in the context of the full human chaperone network and how this relates to pathological protein aggregation in diseases such as tauopathies.…”

Terminal Regions Confer Plasticity to the Tetrameric Assembly of Human HspB2 and HspB3

“…Following recovery, sHsps facilitate the trafficking of non-native proteins to the energy-consuming chaperones and proteolytic machines [3], [12], [13], in some cases via the co-chaperone Bag3 [14], [15]. In higher eukaryotes, sHsps have evolved into large families, in which the role of each member in the complex proteostatic networks remains still to be defined [16], [17].…”