CCT5: A Model Protein Folding Machine

The chaperonin TRiC is a 16-subunit protein complex, composed of two rings, with 8 individual subunits each (CCT1-8), that functions as a protein folding machine in eukaryotic cells. TRiC is required for the folding of approximately 10% of the human proteome, including a number of crucial cellular proteins, most notably actin and tubulin. While TRiC-mediated folding is critical for many cellular processes, its underlying mechanism is poorly understood. Mechanistic and structural elucidation of TRiC folding activity is hampered in large part by the sheer size and subunit heterogeneity of the chaperonin, and therefore most studies have focused on the functionally related but structurally less-complex chaperonins, including the thermosome and Mm-Cpn, as surrogates for TRiC architecture and function. Recently, Sergeeva et al demonstrated that human CCT4 and CCT5 subunits expressed individually in E. coli form homo-oligimeric chaperonins capable of folding native substrates. Here, we have used purified CCT5 homo-oligomer to investigate the in vitro chaperonin-dependent folding of human gD-crystallin and beta-actin. Modifications to the CCT5 primary sequence, including changes to the N and C termini, resulted in changes in folding efficiency, substrate recognition and suppression of aggregation. Structural, biochemical and computational studies on these modifications will provide insight into the mechanism of the TRiC folding machine.