The TRiCky Business of Protein Folding in Health and Disease

Abstract

Maintenance of the cellular proteome or proteostasis is an essential process that when deregulated leads to diseases like neurological disorders and cancer. Central to proteostasis are the molecular chaperones that fold proteins into functional 3-dimensional (3D) shapes and prevent protein aggregation. Chaperonins, a family of chaperones found in all lineages of organisms, are efficient machines that fold proteins within central cavities. The eukaryotic Chaperonin Containing TCP1 (CCT), also known as Tailless complex polypeptide 1 (TCP-1) Ring Complex (TRiC), is a multi-subunit molecular complex that folds the obligate substrates, actin, and tubulin. But more than folding cytoskeletal proteins, CCT differs from most chaperones in its ability to fold proteins larger than its central folding chamber and in a sequential manner that enables it to tackle proteins with complex topologies or very large proteins and complexes. Unique features of CCT include an asymmetry of charges and ATP affinities across the eight subunits that form the hetero-oligomeric complex. Variable substrate binding capacities endow CCT with a plasticity that developed as the chaperonin evolved with eukaryotes and acquired functional capacity in the densely packed intracellular environment. Given the decades of discovery on the structure and function of CCT, much remains unknown such as the scope of its interactome. New findings on the role of CCT in disease, and potential for diagnostic and therapeutic uses, heighten the need to better understand the function of this essential molecular chaperone. Clues as to how CCT causes cancer or neurological disorders lie in the early studies of the chaperonin that form a foundational knowledgebase. In this review, we span the decades of CCT discoveries to provide critical context to the continued research on the diverse capacities in health and disease of this essential protein-folding complex.

Chaperonin containing TCP-1 (CCT/TRiC) is a novel therapeutic and diagnostic target for neuroblastoma

“…Biotinylated CT20p did not bind to CCT2 150-407, while a second truncated version of recombinant CCT2 protein that retained more of the equatorial domain, (CCT2 17-526) did bind to biotinylated CT20p, Figures 8B, C . These results suggest that CT20p likely binds to a region on CCT2 (and possibly other subunits) that contains most of the equatorial domain of the subunit, potentially blocking sites for intra- and inter-ring contacts between subunits as well as portions of the nucleotide or ATP binding site, including catalytic aspartate residues ( 6 ).…”

“…Using CT20-NPs, we showed that IMR-32 neuroblastoma cells are sensitive to the peptide’s cytotoxic effects. While CT20p has not yet been tested in clinical trials, we anticipate that cytoprotective pro-tumor effects associated with HSP90 inhibitors would not transpire given that reductions in CCT levels do not induce a heat shock response ( 17 ) and cct genes have fewer heat shock elements for binding heat shock transcription factors (HSF1 and HSF2) than comparable HSPs ( 6 ).…”

“…Research from our lab and others showed that CCT2 is upregulated in multiple adult cancers (6). To investigate the relevance of CCT in pediatric cancers, we used the UCSC Xena database to compare cohorts, GTEx (normal tissues), TCGA (adult cancerous and normal tissues), and TARGET (pediatric cancerous and normal tissues) for gene expression of CCT2.…”

“…Research from our lab and others demonstrated that targeting protein folding for cancer therapy could significantly reduce tumor growth (5)(6)(7)(8). The term "chaperone addiction" was coined to describe the dependency of cancerous cells on the cellular protein folding machinery (9,10).…”

“…The CCT complex is composed of eight subunits [CCT1-8 (yeast) or CCTa-q (mammals)] and folds substrates needed for cell invasion and proliferation, such as actin, tubulin, and cell division cycle protein 20 homolog (cdc20), as well as oncoproteins like signal transducer and activator of transcription 3 (STAT3), Kirsten rat sarcoma viral oncogene homolog (KRAS), and Myelocytomatosis (MYC) (8,(13)(14)(15)(16)(17). CCT-interacting proteins are found in the ten major signaling pathways aberrantly expressed in diverse cancers (6,18). As a result, CCT subunits are often upregulated in breast, prostate, gastric, colon, lung, and other cancers (19)(20)(21)(22)(23)(24)(25)(26)(27).…”