Role of CCT assisted functions in cellular activities

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Cytosolic chaperonin CCT plays an important role in protein folding in eukaryotic cells including mammalian cells. CCT has a highly evolved chaperoning function exerted by eight different subunits. However, details of the chaperone mechanism are still poorly understood. Here, we studied CCT-dependent cellular functions by using RNAi-mediated knockdown of CCT in mammalian cells, in addition to CCT specific recognition of substrate proteins in vitro.Using PURE system, a cell-free in vitro translation system, we found that CCT specifically recognizes proteins rich in β-sheet and prevents their aggregation. By detailed analysis, we demonstrated that CCT specifically binds hydrophobic β-strands in a manner distinct from that of the E. coli homologue GroEL, and facilitates productive folding of substrate proteins.Moreover, we found that CCT prevents aggregation and toxicity of polyglutamine-expansion proteins, which are known to aggregate in p-sheet rich structures, by RNAi-mediated knockdown and vector-based overexperssion experiments. Analysis of aggregation process suggested that CCT prevents aggregation in an early soluble stage of aggregation. Thus, CCT is a molecular chaperone that facilitates productive folding of proteins by preventing aggregation of aggregation-prone proteins including hydrophobic β-sheets and that protects cells from the toxicity of aggregation-prone proteins by modulating aggregation process.