Human CCT4 and CCT5 chaperonin subunits expressed in E. coli form biologically active homo-oligomers.
ABSTRACT Chaperonins are a family of chaperones that encapsulate their substrates and assist their folding in an ATP-dependent manner. The ubiquitous eukaryotic chaperonin, TCP-1 Ring Complex (TRiC), is a hetero-oligomeric complex composed of two rings each formed from eight different CCT (Chaperonin Containing TCP-1) subunits. Each CCT subunit may have distinct substrate recognition and ATP-hydrolysis properties. We have expressed each human CCT subunit individually in E. coli to investigate whether they form chaperonin-like double ring complexes. CCT4 and CCT5, but not the other six CCT subunits, formed high molecular weight complexes within the E.coli cells that sedimented about 20S in sucrose gradients. When CCT4 and CCT5 were purified, they were both organized as two back-to-back rings of eight subunits each, as seen by negative stain and cryo-electron microscopy. This morphology is consistent with that of the hetero-oligomeric double-ring TRiC purified from bovine testes and HeLa cells. Both CCT4 and CCT5 homo-oligomers hydrolyzed ATP at a rate similar to human TRiC, and were active as assayed by luciferase refolding and human γD-crystallin aggregation suppression and refolding. Thus both CCT4 and CCT5 homo-oligomers have the property of forming eight-fold double rings absent the other subunits, and these complexes carry out chaperonin reactions without other partner subunits.
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ABSTRACT: The features in partially folded intermediates that allow the group II chaperonins to distinguish partially folded from native states remain unclear. The Archaeal group II chaperonin from Methanococcus Mauripaludis (Mm-Cpn) assists the in vitro refolding of the well-characterized β-sheet lens protein human γD-Crystallin (HγD-Crys). The domain interface and buried cores of this Greek key conformation include side chains, which might be exposed in partially folded intermediates. We sought to assess whether particular features buried in the native state, but absent from the native protein surface, might serve as recognition signals. The features tested were a) paired aromatic side chains; b) side chains in the interface between the duplicated domains of HγD-Crys, and c) side chains in the buried core which result in congenital cataract when substituted. We tested the Mm-Cpn suppression of aggregation of these HγD-Crys mutants upon dilution out of denaturant. Mm-Cpn was capable of suppressing the off-pathway aggregation of the three classes of mutants indicating that the buried residues were not recognition signals. In fact, Mm-Cpn recognized the HγD-Crys mutants better than WT and refolded most mutant HγD-Crys to levels twice that of WT HγD-Crys. This presumably represents the increased population or longer lifetimes of the partially folded intermediates of the mutant proteins. The results suggest that Mm-Cpn does not recognize the features of HγD-Crys tested – paired aromatics, exposed domain interface, or destabilized core – but rather recognizes other features of the partially folded β-sheet conformation that are absent or inaccessible in the native state of HγD-Crys.Protein Science 03/2014; 23(6). · 2.86 Impact Factor