Mechanism of the eukaryotic chaperonin: protein folding in the chamber of secrets

Abstract

Chaperonins are key components of the cellular chaperone machinery. These large, cylindrical complexes contain a central cavity that binds to unfolded polypeptides and sequesters them from the cellular environment. Substrate folding then occurs in this central cavity in an ATP-dependent manner. The eukaryotic chaperonin TCP-1 ring complex (TRiC, also called CCT) is indispensable for cell survival because the folding of an essential subset of cytosolic proteins requires TRiC, and this function cannot be substituted by other chaperones. This specificity indicates that TRiC has evolved structural and mechanistic features that distinguish it from other chaperones. Although knowledge of this unique complex is in its infancy, we review recent advances that open the way to understanding the secrets of its folding chamber.Protein misfolding has been implicated in several human diseases that have both systemic and neurological implications, including 'conformational diseases' such as Huntington's and Parkinson's that are characterized by the accumulation of toxic protein aggregates [1]. Although small proteins with simple chain topologies can fold spontaneously, the vast majority of cellular proteins is unable to reach its native state without the assistance of elaborate cellular machinery composed of proteins known as molecular chaperones (for review, see Refs [1-4]). A complete understanding of chaperone-assisted protein folding in the cell would be an intellectual tour de force that might, eventually, lead to effective treatments for these diseases.In the cell, protein folding faces additional challenges compared with the refolding of proteins in solution [1,2,4]. For example, in vivo, the linear polypeptide chain emerges vectorially into the cytosol during synthesis on ribosomes. Because the information for the native state is encoded by the entire amino acid sequence, the nascent polypeptide chain is unable to fold stably until fully synthesized, but it exposes hydrophobic sequences into the crowded cellular milieu [4]. Details of how newly translated proteins navigate toward a final, fully functional, folded structure in vivo are not entirely understood, but it is clear that exposed hydrophobic surfaces can contribute to misfolding and aggregation. Accordingly, all cellular compartments contain many structurally and functionally distinct classes of chaperones that vary in size and complexity, ranging from those that bind only to misfolded polypeptides and prevent their aggregation to those that recognize specific classes of proteins and facilitate their folding to the native state in an energy-dependent manner [1][2][3]. In cells, these different classes of chaperones work together to form elaborate, cooperative networks that ensure the correct folding of newly translated proteins; they also ensure that potentially damaging misfolded polypeptides are cleared from the cell [5].

Functional Interaction between Phosducin-like Protein 2 and Cytosolic Chaperonin Is Essential for Cytoskeletal Protein Function and Cell Cycle Progression

“…Although CCT impacts a variety of substrate proteins, its most abundant and best characterized substrates are the cytoskeletal proteins actin and tubulin, and, accordingly, yeast ts alleles of CCT display various actin-and tubulinrelated cytoskeletal defects (Gao et al, 1992;Ursic et al, 1994;Vinh and Drubin, 1994;Siegers et al, 1999;Spiess et al, 2004). We therefore predicted that if Plp2p modulates the function of CCT, by way of direct physical interaction (Figure 1, A and B), the plp2-ts alleles might also exhibit cytoskeletal anomalies.…”

“…These barrelshaped molecular machines undergo large conformational changes to encapsulate and release bound substrate proteins during their ATP-dependent folding cycle (Spiess et al, 2004). In the eukaryotic cytosol, the chaperonin CCT ensures the correct folding and assembly of a variety of proteins.…”

“…In the eukaryotic cytosol, the chaperonin CCT ensures the correct folding and assembly of a variety of proteins. The best-characterized substrates of CCT are actins and tubulins, although several recent studies have extended the number of known CCT substrates, including several cell cycle proteins (Thulasiraman et al, 1999;Camasses et al, 2003;Siegers et al, 2003; for review, see Spiess et al, 2004). CCT cooperates with another chaperone called Prefoldin (PFD) in the folding of actins and tubulins (Geissler et al, 1998;Vainberg et al, 1998).…”

Saccharomyces cerevisiaeRot1 Is an Essential Molecular Chaperone in the Endoplasmic Reticulum

“…However, in vivo, Hsp90 seems to be engaged in the folding of specific subsets of proteins such as steroid hormone receptors and some kinases (Young et al, 2004;Caplan et al, 2007). The eukaryotic chaperonin is also thought to possess some specificity for its client proteins (Spiess et al, 2004). To understand the functions of individual chaperones in a cell, it is essential to elucidate their in vivo substrate proteins, as well as to characterize the dependency of the substrates on these chaperones.…”

MKKS Is a Centrosome-shuttling Protein Degraded by Disease-causing Mutations via CHIP-mediated Ubiquitination

“…Whereas HSP70 proteins interact with the exposed hydrophobic surfaces of unfolded and partially folded substrate proteins in concert with J-domain proteins, chaperonins sequester nonnative state proteins in cage-like structures. A group II chaperonin, chaperonin containing t-complex polypeptide 1 (CCT, also called TRiC), has been identified in the eukaryotic cytosol (Kubota, 2002;Spiess et al, 2004). CCT is a hexadecameric complex composed of eight different subunits (Kubota et al, 1994;Llorca et al, 1999) and facilitates protein folding (Tian et al, 1995;Farr et al, 1997;Meyer et al, 2003;Kubota et al, 2006).…”