Subunits of the eukaryotic cytosolic chaperonin CCT do not always behave as components of a uniform hetero-oligomeric particle

Abstract

The chaperonin CCT is an hetero-oligomeric molecular chaperone complex. Studies in yeast suggest each of its eight gene products are required for its major identified functions in producing native tubulins and actins. However, it is unclear whether these eight components always form a single particle, covering all functions, or else can also exist as heterogeneous mixtures and/or free subunits in cells. Using mouse P19 embryonal carcinoma cells, which divide rapidly, yet in retinoic acid adopt a neuronal phenotype, admixed with occasional (approximately 10%) fibroblast-like cells, together with a panel of peptide-specific antibodies raised to 7 of the 8 CCT subunits we show that; (1) adoption of a post mitotic phenotype is accompanied by reduced CCT protein expression, significantly more so for CCTbeta, CCTdelta, CCTepsilon, and CCTtheta than for CCTalpha (TCP-1), CCTgamma and CCTzeta; (2) CCTalpha is detected preferentially over other subunits in neurites of P19 neurons; (3) small amounts of CCTalpha and gamma are localised in nuclei (i.e. are not exclusively cytoplasmic), selectively so compared with other subunits; (4) numerous cytosolic foci exist in the cytoplasm which, when detected by double immunofluorescence can contain only one of the subunits probed for; (5) while a "core" chaperonin particle can be immunoprecipitated under native conditions, epitope access is modified both by nucleotides and by non-CCT co-precipitating proteins. Collectively, these findings indicate that CCT subunits are not only components of the hetero-oligomeric chaperonin particle but exist as significant populations of free subunits or smaller oligomers in cells.