Impact of holdase chaperones Skp and SurA on the folding of β-barrel outer-membrane proteins

• Published in: Nature structural & molecular biology Vol. 22; no. 10; pp. 795 - 802
• Main Authors: Thoma, Johannes, Burmann, Björn M, Hiller, Sebastian, Müller, Daniel J
• Format: Journal Article
• Language: English
• Published: New York Nature Publishing Group US 01-10-2015; Nature Publishing Group
• Subjects: 101/6; 14/3; 631/337/470/1981; 631/45/470; 631/535/1262; 631/535/878/1263; 631/57/2270; Bacterial Outer Membrane Proteins - chemistry; Bacterial Outer Membrane Proteins - metabolism; Biochemistry; Biological Microscopy; Biophysics; Carrier Proteins - chemistry; Carrier Proteins - metabolism; DNA-Binding Proteins - chemistry; DNA-Binding Proteins - metabolism; Electrophoresis, Polyacrylamide Gel; Escherichia coli; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - metabolism; Life Sciences; Magnetic Resonance Spectroscopy; Membrane Biology; Membrane proteins; Microscopy, Atomic Force; Models, Molecular; Molecular chaperones; Molecular Chaperones - chemistry; Molecular Chaperones - metabolism; Peptidylprolyl Isomerase - chemistry; Peptidylprolyl Isomerase - metabolism; Physiological aspects; Protein Conformation; Protein Folding; Protein Structure; Spin Labels; Structure; Switzerland
• ISSN: 1545-9993; 1545-9985
• DOI: 10.1038/nsmb.3087

AFM and NMR are used to observe how the chaperones Skp and SurA work to fold a β-barrel outer-membrane protein, FhuA. Skp maintains FhuA in an unfolded state, and Sur A facilitates its folding.
Chaperones increase the folding yields of soluble proteins by suppressing misfolding and aggregation, but how they modulate the folding of integral membrane proteins is not well understood. Here we use single-molecule force spectroscopy and NMR spectroscopy to observe the periplasmic holdase chaperones SurA and Skp shaping the folding trajectory of the large β-barrel outer-membrane receptor FhuA from
Escherichia coli
. Either chaperone prevents FhuA from misfolding by stabilizing a dynamic, unfolded state, thus allowing the substrate to search for structural intermediates. During this search, the SurA-chaperoned FhuA polypeptide inserts β-hairpins into the membrane in a stepwise manner until the β-barrel is folded. The membrane acts as a free-energy sink for β-hairpin insertion and physically separates transient folds from chaperones. This stabilization of dynamic unfolded states and the trapping of folding intermediates funnel the FhuA polypeptide toward the native conformation.