title: GroEL/GroES promote dissociation/reassociation cycles of a heterodimeric intermediate during alpha(2)beta(2) protein assembly. Iterative annealing at the quaternary structure level.
abstract: Whereas the mechanism of GroEL/GroES-mediated protein folding has been extensively studied, the role of these chaperonins in oligomeric protein assembly remains poorly understood. In the present study, we investigated the interaction of the chaperonins with an alphabeta heterodimeric intermediate during the alpha(2)beta(2) assembly of human mitochondrial branched-chain alpha-ketoacid dehydrogenase/decarboxylase (BCKD). Incubation of the recombinant His(6)-tagged BCKD in 400 mM KSCN for 45 min at 23 degrees C caused a complete dissociation of the alpha(2)beta(2) heterotetramers into inactive alphabeta heterodimers. Dilution of the denaturant resulted in a rapid recovery of BCKD independent of the chaperonins GroEL/GroES. Prolonged incubation of BCKD in 400 mM KSCN resulted in the generation of nonproductive or "bad" heterodimers, which were unable to undergo spontaneous reactivation but capable of binding to GroEL to form a stable GroEL-alphabeta complex. Incubation of this complex with GroES and Mg-ATP led to the slow reactivation of BCKD with a second-order rate constant k = 480 M(-1) s(-1). Mixing experiments with radiolabeled and unlabeled protein substrates provided direct evidence that GroEL/GroES promote dissociation and subunit exchange between bad heterodimers. This was accompanied by the transformation of bad heterodimers to their "good" or productive counterparts. The good heterodimers were capable of spontaneous dimerization to initially form an inactive heterotetrameric species, followed by conversion to active heterotetramers. However, a large fraction of bad heterodimers were regenerated and rebound to GroEL. The cycle was perpetuated until the reconstitution of active BCKD was complete. Our data support the thesis that chaperonins GroEL/GroES mediate iterative annealing of nonproductive assembly intermediates at the quaternary structure level. This step is essential for an efficient subsequent higher order oligomerization.
authors.affiliation: Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas 75235, USA.
authors.forename: R M
authors.initials: RM
authors.lastname: Wynn
authors.forename: J L
authors.initials: JL
authors.lastname: Song
authors.forename: D T
authors.initials: DT
authors.lastname: Chuang
mesh_terms: D042942:3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide); D018835:Chaperonin 10 / Q000737:chemistry / Q000378:metabolism*; D018834:Chaperonin 60 / Q000737:chemistry / Q000378:metabolism*; D019281:Dimerization; D006801:Humans; D007658:Ketone Oxidoreductases / Q000378:metabolism; D007700:Kinetics; D009097:Multienzyme Complexes / Q000378:metabolism; D011485:Protein Binding; D020836:Protein Structure, Quaternary
chemical_list: D018835:Chaperonin 10; D018834:Chaperonin 60; D009097:Multienzyme Complexes; D007658:Ketone Oxidoreductases; D042942:3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)