TRiC’s tricks inhibit huntingtin aggregation

Abstract

In Huntington’s disease, a mutated version of the huntingtin protein leads to cell death. Mutant huntingtin is known to aggregate, a process that can be inhibited by the eukaryotic chaperonin TRiC (TCP1-ring complex) in vitro and in vivo. A structural understanding of the genesis of aggregates and their modulation by cellular chaperones could facilitate the development of therapies but has been hindered by the heterogeneity of amyloid aggregates. Using cryo-electron microscopy (cryoEM) and single particle cryo-electron tomography (SPT) we characterize the growth of fibrillar aggregates of mutant huntingtin exon 1 containing an expanded polyglutamine tract with 51 residues (mhttQ51), and resolve 3-D structures of the chaperonin TRiC interacting with mhttQ51. We find that TRiC caps mhttQ51 fibril tips via the apical domains of its subunits, and also encapsulates smaller mhtt oligomers within its chamber. These two complementary mechanisms provide a structural description for TRiC’s inhibition of mhttQ51 aggregation in vitro.DOI: http://dx.doi.org/10.7554/eLife.00710.001

Structural Mechanisms of Mutant Huntingtin Aggregation Suppression by the Synthetic Chaperonin-like CCT5 Complex Explained by Cryoelectron Tomography

“…This study follows in the footsteps of previous experiments describing the structural role of TRiC chaperonin inhibition of mHTT aggregation (16). However, in this study, we present a more rigorous data classification scheme (Fig.…”

“…The construct used in this study contains 46 rather than 51 polyglutamines, putting it squarely in the range of polyglutamine repeats found in HD patients (41). In addition, the 46Q construct used here has N17 and C38 flanking regions with a C-terminal His tag, whereas the previous construct was composed of N17 and C36 flanking regions with no C-terminal tag (16,25). Both of these constructs represent the newly discovered aberrant splice variants (7,8), which have been described previously as highly toxic (42).…”

“…The equatorial domain contains the site of ATP-binding and subunit-subunit contacts, the intermediate domain acts as a hinge region, and, finally, the apical domain recognizes and binds substrates (20). By cryoelectron tomography, TRiC has been shown to interact at its apical domains with mHTTQ51-Ex1 fibrils and encapsulate oligomers through a "cap and contain" mechanism (16).…”

“…Specifically, TCP-1 ring complex (TRiC) chaperonin has been shown previously to decrease mHTT aggregation in vitro, in yeast cells, and in cell culture (16,17). TRiC (ϳ1 MDa) is a back-to-back double-ring complex composed of eight distinct subunits per ring (CCT1-8) that interacts with ϳ9 -15% of all nascent peptides, including actin and tubulin (18,19).…”

A Quantitative Proteomic Analysis of In Vitro Assembled Chromatin

“…This binding of chaperone factors suggests that chromatin assembly is associated with extensive and continuous protein folding and refolding events. Although TriC/CCT has been suggested to be mainly involved in the folding of newly translated proteins (47), there is increasing evidence that it has additional functions such as the prevention of the aggregation of poly-Q proteins (48) or the formation of macromolecular protein complexes in the nucleus (49,50). The TriC/CCT complex can also be observed on nascent and mature chromatin in vivo (13) and in interphase chromatin (12), which also supports its integral role in chromatin dynamics and metabolism.…”