HSPB8 and BAG3 cooperate to promote spatial sequestration of ubiquitinated proteins and coordinate the cellular adaptive response to proteasome insufficiency

Abstract

BCL2-associated athanogene (BAG)-3 is viewed as a platform that would physically and functionally link distinct classes of molecular chaperones of the heat shock protein (HSP) family for the stabilization and clearance of damaged proteins. In this study, we show that HSPB8, a member of the small heat shock protein subfamily, cooperates with BAG3 to coordinate the sequestration of harmful proteins and the cellular adaptive response upon proteasome inhibition. Silencing of HSPB8, like depletion of BAG3, inhibited targeting of ubiquitinated proteins to the juxtanuclear aggresome, a mammalian system of spatial quality control. However, aggresome targeting was restored in BAG3-depleted cells by a mutant BAG3 defective in HSPB8 binding, uncoupling HSPB8 function from its binding to BAG3. Depletion of HSPB8 impaired formation of ubiquitinated microaggregates in an early phase and interfered with accurate modifications of the stress sensor p62/sequestosome (SQSTM)-1. This impairment correlated with decreased coupling of BAG3 to p62/SQSTM1 in response to stress, hindering Kelch-like ECH-associated protein (KEAP)-1 sequestration and stabilization of nuclear factor E2-related factor (Nrf)-2, an important arm of the antioxidant defense. Notably, the myopathy-associated mutation of BAG3 (P209L), which lies within the HSPB8-binding motif, deregulated the association between BAG3 and p62/SQSTM1 and the KEAP1-Nrf2 signaling axis. Together, our findings support a so-far-unrecognized role for the HSPB8-BAG3 connection in mounting of an efficient stress response, which may be involved in BAG3-related human diseases.-Guilbert, S. M., Lambert, H., Rodrigue, M.-A., Fuchs, M., Landry, J., Lavoie, J. N. HSPB8 and BAG3 cooperate to promote spatial sequestration of ubiquitinated proteins and coordinate the cellular adaptive response to proteasome insufficiency.

BAG3 Pro209 mutants associated with myopathy and neuropathy relocate chaperones of the CASA-complex to aggresomes

“…3a). This is consistent with a previous independent report (Guilbert et al 2018). So, although the IPV-motifs mediate the interaction with HSPB8, we found that Pro209 mutants primarily affect the interaction with SQSTM1/p62, which, as far as we know, is not mediated by a direct interaction between the two proteins, but requires the assembly of the full CASA complex with the poly-ubiquitin chain linked to the recognized misfolded proteins.…”

Neuromuscular Diseases Due to Chaperone Mutations: A Review and Some New Results

“…Therefore, BAG3 has been considered an obligate partner for HSPB8. However, interestingly, recent research has demonstrated that some of the functions of HSPB8 are actually independent of BAG3 [407][408][409]. Hence, it is not clear to what extent the reported functions and effects of HSPB8 depend on BAG3-dependent and independent pathways.…”

Small heat shock proteins in neurodegenerative diseases

“…In cells from two dHMN patients with the K141E mutation, a similar impairment of autophagy was observed. Guilbert et al (2018) reported a role for HSPB8 in ubiquitinated microaggregate formation. Depletion of HSPB8 impaired the formation of these microaggregates and the early formation of p62 bodies.…”