Bacteria have only one ribosome-bound chaperone and use primarily the same two chaperones to fold proteins after translation and then again after stress. But bioinformatic analyses by the authors suggested that yeast are different; whereas stresses such as heat and oxidation induced one set of cytosolic chaperones, they repressed another. The proteins from the repressed set were associated with ribosomes, and mutant lines lacking in this set were hypersensitive to translation inhibitors.
The authors propose that eukaryotes have a set of chaperones dedicated to nascent polypeptide folding during translation. This task splitting might have allowed for a better optimization of chaperone duties. “Eukaryotic cells have much larger, multidomain proteins than bacteria,” says Frydman. “Maybe this [advancement] was helped by the evolution of a chaperone machinery dedicated to ribosomes.”
Some overlap in duties might occur, as the slow growth of a mutant line lacking the major translation-linked chaperone, SSB, was partially rescued by high levels of stress-induced chaperones. Evidence suggests, however, that the replacements are more likely to be helping by cleaning up the mess of unfolded proteins rather than by contributing to cotranslational folding.