Redefining the Chaperone Mechanism of sHsps: Not Just Holdase Chaperones

Ecroyd, Heath

doi:10.1007/978-3-319-16077-1_7

Cited by 8 publications

(9 citation statements)

References 97 publications

(109 reference statements)

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“…This certainly does not exclude the possibility that dodecamers also can dissociate into subunits that interact with substrates. The possibility that flexible NTR arms may be available for substrate interaction on the outer surface of intact oligomers, and not only on a smaller population of transient dimers present during dynamic subunit exchange, was also recently suggested in a revised model of sHsp activity ( 49 , 50 ). It remains to be shown under what conditions the substrate proteins interact directly with the dodecamers and under what conditions they interact with dissociated subunits.…”

Section: Discussionmentioning

confidence: 98%

Structural model of dodecameric heat-shock protein Hsp21: Flexible N-terminal arms interact with client proteins while C-terminal tails maintain the dodecamer and chaperone activity

Rutsdottir¹,

Härmark

Weide³

et al. 2017

Journal of Biological Chemistry

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Small heat-shock proteins (sHsps) prevent aggregation of thermosensitive client proteins in a first line of defense against cellular stress. The mechanisms by which they perform this function have been hard to define due to limited structural information; currently, there is only one high-resolution structure of a plant sHsp published, that of the cytosolic Hsp16.9. We took interest in Hsp21, a chloroplast-localized sHsp crucial for plant stress resistance, which has even longer N-terminal arms than Hsp16.9, with a functionally important and conserved methionine-rich motif. To provide a framework for investigating structure-function relationships of Hsp21 and understanding these sequence variations, we developed a structural model of Hsp21 based on homology modeling, cryo-EM, cross-linking mass spectrometry, NMR, and small-angle X-ray scattering. Our data suggest a dodecameric arrangement of two trimer-of-dimer discs stabilized by the C-terminal tails, possibly through tail-to-tail interactions between the discs, mediated through extended IXVXI motifs. Our model further suggests that six N-terminal arms are located on the outside of the dodecamer, accessible for interaction with client proteins, and distinct from previous undefined or inwardly facing arms. To test the importance of the IXVXI motif, we created the point mutant V181A, which, as expected, disrupts the Hsp21 dodecamer and decreases chaperone activity. Finally, our data emphasize that sHsp chaperone efficiency depends on oligomerization and that client interactions can occur both with and without oligomer dissociation. These results provide a generalizable workflow to explore sHsps, expand our understanding of sHsp structural motifs, and provide a testable Hsp21 structure model to inform future investigations.

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Section: Discussionmentioning

confidence: 98%

Structural model of dodecameric heat-shock protein Hsp21: Flexible N-terminal arms interact with client proteins while C-terminal tails maintain the dodecamer and chaperone activity

Rutsdottir¹,

Härmark

Weide³

et al. 2017

Journal of Biological Chemistry

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“…sHsps utilize flexible hydrophobic surfaces to interact with exposed hydrophobic surfaces of client proteins, which are made available during misfolding or denaturation (Sharma et al, 1997; van Montfort et al, 2001; Haslbeck et al, 2005; Jaya et al, 2009). Both high affinity and dynamic low affinity interactions of sHsps with client proteins have been described (Bruinsma et al, 2011; McDonald et al, 2012; Mattoo and Goloubinoff, 2014; Ecroyd, 2015). Generally, high affinity interactions are associated with amorphous misfolding clients while sHsp effects on amyloid fibril formation are often due to weak or transient client interactions (Kulig and Ecroyd, 2012; Treweek et al, 2015).…”

Section: Structural Plasticity Of Shspsmentioning

confidence: 99%

Small Heat Shock Proteins, Big Impact on Protein Aggregation in Neurodegenerative Disease

et al. 2019

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Misfolding, aggregation, and aberrant accumulation of proteins are central components in the progression of neurodegenerative disease. Cellular molecular chaperone systems modulate proteostasis, and, therefore, are primed to influence aberrant protein-induced neurotoxicity and disease progression. Molecular chaperones have a wide range of functions from facilitating proper nascent folding and refolding to degradation or sequestration of misfolded substrates. In disease states, molecular chaperones can display protective or aberrant effects, including the promotion and stabilization of toxic protein aggregates. This seems to be dependent on the aggregating protein and discrete chaperone interaction. Small heat shock proteins (sHsps) are a class of molecular chaperones that typically associate early with misfolded proteins. These interactions hold proteins in a reversible state that helps facilitate refolding or degradation by other chaperones and co-factors. These sHsp interactions require dynamic oligomerization state changes in response to diverse cellular triggers and, unlike later steps in the chaperone cascade of events, are ATP-independent. Here, we review evidence for modulation of neurodegenerative disease-relevant protein aggregation by sHsps. This includes data supporting direct physical interactions and potential roles of sHsps in the stewardship of pathological protein aggregates in brain. A greater understanding of the mechanisms of sHsp chaperone activity may help in the development of novel therapeutic strategies to modulate the aggregation of pathological, amyloidogenic proteins. sHsps-targeting strategies including modulators of expression or post-translational modification of endogenous sHsps, small molecules targeted to sHsp domains, and delivery of engineered molecular chaperones, are also discussed.

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“…The necessity to sequester a monomeric histone may relate to the folding kinetics of H3-H4 relative to nuclear import. NASP may therefore be regarded as having a 'holdase' function, as has been suggested for the ATP-independent small heat shock protein family of molecular chaperones (Jakob et al, 1993;Mymrikov et al, 2017;Ecroyd, 2015;Taipale et al, 2014;Mogk and Bukau, 2017;Reinle et al, 2021). In addition, the presence of NASP in downstream complexes, mediated by interactions outside of the TPR-H3 peptide interface (Bowman et al, 2017), further supports NASP having a role in guiding the transition from monomeric import to a heterodimer capable of DNA deposition.…”

Section: Discussionmentioning

confidence: 76%

A specific role for Importin-5 and NASP in the import and nuclear hand-off of monomeric H3

Pardal

Bowman

2021

Preprint

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Core histones package chromosomal DNA and regulate genomic transactions, with their import and deposition involving a dedicated repertoire of molecular chaperones. Histones H3 and H4 have been predominantly characterised as obligate heterodimers, however, recent findings have alluded to the existence of a significant pool of monomeric histone H3 in the nucleoplasm. Using a combination of in vitro and in vivo experiments, here we show that monomeric H3 and H4 use an Importin 5 (Imp5) dependent pathway for their nuclear import, distinct from Importin 4 (Imp4) previously described for H3-H4 dimers. Using mutants that disrupt the histone fold, we show monomeric H3 loses its interaction with Imp4, but retains interactions with Imp5 and the chaperone NASP. H4 monomeric mutants similarly bind Imp5 and not Imp4, however, they lose interaction with NASP, retaining their interaction with the HAT1-RBBP7 complex instead. In vitro experiments revealed that Imp5 and NASP are mutually exclusive in their binding, suggesting a facilitated hand-off mechanism. Furthermore, new H3 accumulates rapidly in a NASP-bound complex after nuclear translocation. NASP can assemble into three distinct co-chaperoning complexes, including a novel complex containing NASP, H3 and the putative ubiquitin ligase UBR7, a NASP-H3-H4-RBBP7 subcomplex and the previously characterised NASP-H3-H4-ASF1-HAT1-RBBP7 multi-chaperoning complex. Here we propose an alternative import pathway and folding mechanism for monomeric H3 and H4 that involves Imp5, rather than Imp4, and hands off to nuclear chaperones NASP, RBBP7 and HAT1.

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Redefining the Chaperone Mechanism of sHsps: Not Just Holdase Chaperones

Cited by 8 publications

References 97 publications

Structural model of dodecameric heat-shock protein Hsp21: Flexible N-terminal arms interact with client proteins while C-terminal tails maintain the dodecamer and chaperone activity

Structural model of dodecameric heat-shock protein Hsp21: Flexible N-terminal arms interact with client proteins while C-terminal tails maintain the dodecamer and chaperone activity

Small Heat Shock Proteins, Big Impact on Protein Aggregation in Neurodegenerative Disease

A specific role for Importin-5 and NASP in the import and nuclear hand-off of monomeric H3

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