Monday, June 12, 2017

Dual function of the trigger factor chaperone in nascent protein folding

Christian Kaiser, Kevin Maciuba and Kaixian Liu

Multi-domain proteins often require help from molecular chaperones to fold productively, even before the ribosome has finished their synthesis. The mechanisms underlying chaperone function remain poorly understood. Using optical tweezers to study the folding of elongation factor G (EF-G), a model multi-domain protein, we find that the N-terminal G-domain in nascent EF-G polypeptides folds robustly. The following domain II, in contrast, fails to fold efficiently. Strikingly, interactions with the unfolded domain II convert the natively folded G domain to a non-native state. This non-native state readily unfolds, and the two unfolded domains subsequently form misfolded states, preventing productive folding. Both the conversion of natively folded domains and non-productive interactions among unfolded domains are efficiently prevented by the nascent chain-binding chaperone trigger factor. Thus, our single-molecule measurements of multi-domain protein folding reveal an unexpected role for the chaperone: It protects already folded domains against denaturation resulting from interactions with parts of the nascent polypeptide that are not folded yet. Previous studies had implicated trigger factor in guiding the folding of individual domains, and interactions among domains had been neglected. Avoiding these early folding defects is crucial, since they can propagate and result in misfolding of the entire protein.

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