Ribosome-tethered molecular chaperones: the first line of defense against protein misfolding?

Folding of many cellular proteins is facilitated by molecular chaperones. Analysis of both prokaryotic and lower eukaryotic model systems has revealed the presence of ribosome-associated molecular chaperones, thought to be the first line of defense against protein aggregation as translating polypeptides emerge from the ribosome. However, structurally unrelated chaperones have evolved to carry out these functions in different microbes. In the yeast Saccharomyces cerevisiae, an unusual complex of Hsp70 and J-type chaperones associates with ribosome-bound nascent chains, whereas in Escherichia coli the ribosome-associated peptidyl-prolyl-cis-trans isomerase, trigger factor, plays a predominant role.