title: Folding of large multidomain proteins by partial encapsulation in the chaperonin TRiC/CCT.
abstract: The eukaryotic chaperonin, TRiC/CCT (TRiC, TCP-1 ring complex; CCT, chaperonin containing TCP-1), uses a built-in lid to mediate protein folding in an enclosed central cavity. Recent structural data suggest an effective size limit for the TRiC folding chamber of ∼70 kDa, but numerous chaperonin substrates are substantially larger. Using artificial fusion constructs with actin, an obligate chaperonin substrate, we show that TRiC can mediate folding of large proteins by segmental or domain-wise encapsulation. Single or multiple protein domains up to ∼70 kDa are stably enclosed by stabilizing the ATP-hydrolysis transition state of TRiC. Additional domains, connected by flexible linkers that pass through the central opening of the folding chamber, are excluded and remain accessible to externally added protease. Experiments with the physiological TRiC substrate hSnu114, a 109-kDa multidomain protein, suggest that TRiC has the ability to recognize domain boundaries in partially folded intermediates. In the case of hSnu114, this allows the selective encapsulation of the C-terminal ∼45-kDa domain and segments thereof, presumably reflecting a stepwise folding mechanism. The capacity of the eukaryotic chaperonin to overcome the size limitation of the folding chamber may have facilitated the explosive expansion of the multidomain proteome in eukaryotes.
authors.affiliation: Department of Cellular Biochemistry,  Max Planck Institute of Biochemistry, 82152 Martinsried,  Germany.
authors.forename: Florian
authors.initials: F
authors.lastname: Rüßmann
authors.forename: Markus J
authors.initials: MJ
authors.lastname: Stemp
authors.forename: Leonie
authors.initials: L
authors.lastname: Mönkemeyer
authors.forename: Stephanie A
authors.initials: SA
authors.lastname: Etchells
authors.forename: Andreas
authors.initials: A
authors.lastname: Bracher
authors.forename: F Ulrich
authors.initials: FU
authors.lastname: Hartl
mesh_terms: D000199:Actins / Q000737:chemistry* / Q000378:metabolism*; D056404:Chaperonin Containing TCP-1 / Q000737:chemistry* / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D008954:Models, Biological; D017510:Protein Folding; D055550:Protein Stability; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D017415:Ribonucleoprotein, U5 Small Nuclear / Q000737:chemistry / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D000199:Actins; D011993:Recombinant Fusion Proteins; D017415:Ribonucleoprotein, U5 Small Nuclear; D049452:Green Fluorescent Proteins; D056404:Chaperonin Containing TCP-1