The co-chaperone BAG3, in complex with the heat shock protein HSPB8, plays a role in protein quality control during mechanical strain. It is part of a multichaperone complex that senses damaged cytoskeletal proteins and orchestrates their seclusion and/or degradation by selective autophagy. Here we describe a novel role for the BAG3-HSPB8 complex in mitosis, a process involving profound changes in cell tension homeostasis. BAG3 is hyperphosphorylated at mitotic entry and localizes to centrosomal regions. BAG3 regulates, in an HSPB8-dependent manner, the timely congression of chromosomes to the metaphase plate by influencing the three-dimensional positioning of the mitotic spindle. Depletion of BAG3 caused defects in cell rounding at metaphase and dramatic blebbing of the cortex associated with abnormal spindle rotations. Similar defects were observed upon silencing of the autophagic receptor p62/SQSTM1 that contributes to BAG3-mediated selective autophagy pathway. Mitotic cells depleted of BAG3, HSPB8 or p62/SQSTM1 exhibited disorganized actin-rich retraction fibres, which are proposed to guide spindle orientation. Proper spindle positioning was rescued in BAG3-depleted cells upon addition of the lectin concanavalin A, which restores cortex rigidity. Together, our findings suggest the existence of a so-far unrecognized quality control mechanism involving BAG3, HSPB8 and p62/SQSTM1 for accurate remodelling of actin-based mitotic structures that guide spindle orientation.

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.

No abstract

Small heat shock proteins (HspB proteins) form a diverse family of proteins that have evolved distinct modes of action to protect cells from proteotoxic stress. Studies conducted within the last 25 years have revealed specialized roles for some HspB, in particular for HspB1, HspB8, and its cochaperone Bag3, in the modulation of actin-based cytoskeletal dynamics under physiological and stress conditions, which might be related to their linkage to human cancer. Little is known, however, on whether and how such biological activities on signaling are connected to pathways within the quality control network. In this chapter, we examine functional relationships between HspB proteins, the cochaperone Bag3 and actin dynamics and describe the mechanisms known so far that are responsible for their modulation of actin architecture. We further discuss on how such activities might be connected to quality control network. While some pieces of the puzzle might need to be inserted differently, we hope that this review will stimulate further studies to elucidate the mechanistic behind chaperone-mediated actin remodeling by HspB proteins and their partners.