Human CCT4 and CCT5 Chaperonin Subunits Expressed in Escherichia coli Form Biologically Active Homo-oligomers

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 Escherichia 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 8-fold double rings absent the other subunits, and these complexes carry out chaperonin reactions without other partner subunits. Background: The subunit-specific roles of the CCT subunits in the chaperonin, TRiC, have not been elucidated. Results: When expressed in E. coli , CCT4 and CCT5 form TRiC-like homo-oligomeric rings. Conclusion: TRiC does not require all eight CCT subunits to form functional rings. Significance: The unexpected formation of homo-oligomeric CCT rings provides clues into the assembly of TRiC as a complex.