Overexpressed ribosomal proteins suppress defective chaperonins in Saccharomyces cerevisiae.

The chaperonin Cct complex of the yeast Saccharomyces cerevisiae is composed of eight different subunits encoded by eight essential genes, CCT1-CCT8. This Cct complex is responsible for the folding of a number of proteins including actin and tubulin. We have isolated and characterized 22 multicopy suppressors of the temperature-sensitive allele, cct4-1, which encodes an altered protein with a G345D replacement that diminishes ATP hydrolysis. Fourteen of the suppressors encode ribosomal proteins, four have roles in ribosome biogenesis, two have phosphatase activities, one is involved in protein synthesis and one of the suppressors corresponded to Cct4p. Some of the suppressors also acted on certain cct1, cct2, cct3 and cct6 mutations. We suggest that certain overexpressed ribosomal and other proteins can act as weak chaperones, phenotypically alleviating the partial defects of mutationally altered Cct subunits.