Identification of the key structural motifs involved in HspB8/HspB6–Bag3 interaction

Abstract

The molecular chaperone HspB8 [Hsp (heat-shock protein) B8] is member of the B-group of Hsps. These proteins bind to unfolded or misfolded proteins and protect them from aggregation. HspB8 has been reported to form a stable molecular complex with the chaperone cohort protein Bag3 (Bcl-2-associated athanogene 3). In the present study we identify the binding regions in HspB8 and Bag3 crucial for their interaction. We present evidence that HspB8 binds to Bag3 through the hydrophobic groove formed by its strands beta4 and beta8, a region previously known to be responsible for the formation and stability of higher-order oligomers of many sHsps (small Hsps). Moreover, we demonstrate that two conserved IPV (Ile-Pro-Val) motifs in Bag3 mediate its binding to HspB8 and that deletion of these motifs suppresses HspB8 chaperone activity towards mutant Htt43Q (huntingtin exon 1 fragment with 43 CAG repeats). In addition, we show that Bag3 can bind to the molecular chaperone HspB6. The interaction between HspB6 and Bag3 requires the same regions that are involved in the HspB8-Bag3 association and HspB6-Bag3 promotes clearance of aggregated Htt43Q. Our findings suggest that the co-chaperone Bag3 might prevent the accumulation of denatured proteins by regulating sHsp activity and by targeting their substrate proteins for degradation. Interestingly, a mutation in one of Bag3 IPV motifs has recently been associated with the development of severe dominant childhood muscular dystrophy, suggesting a possible important physiological role for HspB-Bag3 complexes in this disease.

NF-κB regulates protein quality control after heat stress through modulation of the BAG3–HspB8 complex

“…The involvement of BAG3-HspB8 in aggrephagy regulation is thought to take place at two levels: selectivity and induction. HspB8, as a molecular chaperone, is able to recognize aggregating proteins and to bind to BAG3 (Fuchs et al, 2009), thus presenting substrate protein to the autophagic machinery for degradation (Carra et al, 2008b). The BAG3-HspB8 complex can therefore mediate autophagy activation in close proximity to protein aggregates and the aggresome, leading to aggrephagy (Gamerdinger et al, 2011).…”

Different anti-aggregation and pro-degradative functions of the members of the mammalian sHSP family in neurological disorders

“…Alternatively, or in case HSPB members are not already expressed in the target cells, analysis on expression regulation of the various HSPB members combined with drug screens using reporter constructs may identify routes towards boosting or inducing expression of individual disease-ameliorating members. HSPB6, which only effectively suppresses shorter polyQs [1,107], also plays a role in modulating autophagy [108]. By contrast, HSPB7 does not change the rate of proteasomal degradation and does not increases the autophagic flux; rather, it appears to prevent early aggregates from nucleating into inclusions with sizes that are too large to be handled by the autophagic machinery, probably by marking these early seeds, which enables their shuttling into the autophagosomes [1,57,109,110].…”

“…HSPB9 seems to have a mild effect on TDP-43 DC. Why HSPB6 and HSPB7 showed no anti-aggregation activity towards TDP43 DC, while being very active towards mutated polyQ proteins [1,107] (table 1), is still unclear and will be investigated in the future.…”

Myofibrillar Myopathies and the Z-Disk Associated Proteins

“…BAG3 competitively binds to the ATPase domain of these chaperone proteins and alters their chaperone properties thereby targeting chaperone associated proteins for proteasomal degradation [237,239]. In fact, BAG3 has recently been shown to form a stable complex with the small Heat shock protein HspB8 and stimulate macroautophagy [236], a process that is particularly important in Huntington disease where association of BAG3 with HspB8 promotes degradation of mutant Huntingtin [236,241]. In inclusion body myositis, macroautophagy plays a role in removing -amyloid aggregates [242] and it is possible that BAG3 also plays a role in this cellular response to protein aggregates in MFM.…”