Evidence for close side-chain packing in an early protein folding intermediate previously assumed to be a molten globule

• Published in: Proceedings of the National Academy of Sciences - PNAS 2014-10, Vol.111 (41), p.14746-14751
• Main Authors: Rosen, Laura E., Connell, Katelyn B., Marqusee, Susan
• Format: Article
• Language: English
• Subjects: Amino Acids - metabolism; Anilino Naphthalenesulfonates - metabolism; Biological Sciences; Chemical equilibrium; Circular Dichroism; Dimers; DNA Mutational Analysis; E coli; Enzyme Stability - drug effects; Escherichia coli; Escherichia coli - enzymology; Fluorescence; Globules; Hydrophobic and Hydrophilic Interactions - drug effects; Kinetics; Magnetic Resonance Spectroscopy; Monomers; Mutation - genetics; Nonnative species; Protein folding; Protein Folding - drug effects; Protein Structure, Secondary; Protein Structure, Tertiary; Proteins; Ribonuclease H - chemistry; Ribonuclease H - genetics; Ribonuclease H - metabolism; Spectral index; Urea - pharmacology
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1410630111

Significance Molten globules—defined as compact protein conformations with significant secondary structure but only loosely packed tertiary structure—have been hypothesized to be general folding intermediates. In this work we investigate one folding intermediate long thought to be a molten globule and find significant evidence that it likely has a well-folded region, with closely packed tertiary structure. These results suggest that the evidence for moltenness in other protein folding intermediates should be revisited and that even for fairly simple, small proteins, exclusion of water can occur before the rate-limiting step to folding. The molten globule, a conformational ensemble with significant secondary structure but only loosely packed tertiary structure, has been suggested to be a ubiquitous intermediate in protein folding. However, it is difficult to assess the tertiary packing of transiently populated species to evaluate this hypothesis. Escherichia coli RNase H is known to populate an intermediate before the rate-limiting barrier to folding that has long been thought to be a molten globule. We investigated this hypothesis by making mimics of the intermediate that are the ground-state conformation at equilibrium, using two approaches: a truncation to generate a fragment mimic of the intermediate, and selective destabilization of the native state using point mutations. Spectroscopic characterization and the response of the mimics to further mutation are consistent with studies on the transient kinetic intermediate, indicating that they model the early intermediate. Both mimics fold cooperatively and exhibit NMR spectra indicative of a closely packed conformation, in contrast to the hypothesis of molten tertiary packing. This result is important for understanding the nature of the subsequent rate-limiting barrier to folding and has implications for the assumption that many other proteins populate molten globule folding intermediates.