Equivalent mutations in the eight subunits of the chaperonin CCT produce dramatically different cell phenotypes.

The eukaryotic cytoplasmic chaperonin-containing TCP-1 (CCT) is a complex formed by two back-to-back stacked hetero-octameric rings that assists the folding of actins, tubulins and other proteins in an ATP-dependent manner. Here, we decided to test the significance of the hetero-oligomeric nature of CCT for its function by introducing, in each of the eight subunits in turn, an identical mutation at a position involved in ATP binding and conserved in all the subunits, in order to establish the extent of individuality of the various subunits. Our results show that these identical mutations lead to dramatically different phenotypes. For example, cells with the mutation in CCT2 have an excess of actin patches and are the only pseudo-diploid strain. By contrast, cells with the mutation in CCT7 are the only ones to accumulate juxta-nuclear protein aggregates that may reflect the absence of stress response in this strain. System-level analysis of the strains using RNA microarrays reveals connections between CCT and several cellular networks including ribosome biogenesis and TOR2 that help to explain the phenotypic variability observed

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Amit M, Weisberg SJ, Nadler-Holly M, McCormack EA, Feldmesser E, Kaganovich D, Willison KR, Horovitz A