A protein folding intermediate pulls its weight.

Proteins must acquire and maintain a specific fold to execute their biochemical function(s). In solution, unfolded proteins typically find this native structure through a biased sampling of preferred intermediate conformations. However, the initial search for these structures begins during protein synthesis, and it is unclear how much interactions between the ribosome and nascent polypeptide skew folding pathways. In this issue, Jensen and colleagues use a ribosomal force-profiling assay to show that RNase H forms a similar folding intermediate on and off the ribosome. In conjunction with measurements of the rate of RNase H unfolding on and off the ribosome, their results show that ribosomal interactions have little impact on the folding pathway of RNase H. These findings suggest that the ribosome itself does not necessarily rewire protein folding reactions.