\u03b1B-crystallin is a sensor for assembly intermediates and for the subunit topology of desmin intermediate filaments

Sarika Sharma,Jayne L Elliott,Gloria M Conover,Harald Herrmann,Roy A Quinlan,Ming Der Perng

doi:10.1007/s12192-017-0788-7

Abstract

Mutations in the small heat shock protein chaperone CRYAB (αB-crystallin/HSPB5) and the intermediate filament protein desmin, phenocopy each other causing cardiomyopathies. Whilst the binding sites for desmin on CRYAB have been determined, desmin epitopes responsible for CRYAB binding and also the parameters that determine CRYAB binding to desmin filaments are unknown. Using a combination of co-sedimentation centrifugation, viscometric assays and electron microscopy of negatively stained filaments to analyse the in vitro assembly of desmin filaments, we show that the binding of CRYAB to desmin is subject to its assembly status, to the subunit organization within filaments formed and to the integrity of the C-terminal tail domain of desmin. Our in vitro studies using a rapid assembly protocol, C-terminally truncated desmin and two disease-causing mutants (I451M and R454W) suggest that CRYAB is a sensor for the surface topology of the desmin filament. Our data also suggest that CRYAB performs an assembly chaperone role because the assembling filaments have different CRYAB-binding properties during the maturation process. We suggest that the capability of CRYAB to distinguish between filaments with different surface topologies due either to mutation (R454W) or assembly protocol is important to understanding the pathomechanism(s) of desmin-CRYAB myopathies.

Highlights

The α-crystallins are small heat shock proteins; (Carra et al 2013; Kappe et al 2003) that are part of the molecular chaperone family (Brandvold and Morimoto 2015; Kim et al 2013; Treweek et al 2014)
We report that desmin filament morphology influences CRYAB binding suggesting that it can act as a sensor detecting changes in the surface topology of IFs
It is well established that the assembly protocol, i.e. rapid versus dialysis (Herrmann et al 1999) as well as the monovalent or divalent cation concentration (Brennich et al 2014) all influence the diameter of the desmin filaments produced (Stromer et al 1987)

Summary

Introduction

The α-crystallins are small heat shock proteins (sHSPs); (Carra et al 2013; Kappe et al 2003) that are part of the molecular chaperone family (Brandvold and Morimoto 2015; Kim et al 2013; Treweek et al 2014). Protein chaperones assist the folding of nascent protein chains, bind to and prevent the aggregation of misfolded and stress-denatured proteins (Brandvold and Morimoto 2015; Strauch and Haslbeck 2016), as well as assisting oligomer assembly and the assembly of protein complexes (Ellis 2013). The α-crystallin complex (CRYAA and CRYAB) in the eye lens was first found to modulate the assembly of lenticular IFs (Nicholl and Quinlan 1994) Mutations in both CRYAB (Vicart et al 1998) and the muscle-specific IF protein desmin (Goldfarb et al 1998) were found to phenocopy each other, causing cardiomyopathy. The phenocopying by each highlights the importance of exploring in more detail the desmin-CRYAB interaction

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Conclusion