Secondary and Tertiary Structure Formation of the β-Barrel Membrane Protein OmpA is Synchronized and Depends on Membrane Thickness

The mechanism of membrane insertion and folding of a β-barrel membrane protein has been studied using the outer membrane protein A (OmpA) as an example. OmpA forms an eight-stranded β-barrel that functions as a structural protein and perhaps as an ion channel in the outer membrane of Escherichia coli. OmpA folds spontaneously from a urea-denatured state into lipid bilayers of small unilamellar vesicles. We have used fluorescence spectroscopy, circular dichroism spectroscopy, and gel electrophoresis to investigate basic mechanistic principles of structure formation in OmpA. Folding kinetics followed a second-order rate law and is strongly depended on the hydrophobic thickness of the lipid bilayer. When OmpA was refolded into model membranes of dilaurylphosphatidylcholine, fluorescence kinetics were characterized by a rate constant that was about fivefold higher than the rate constants of formation of secondary and tertiary structure, which were determined by circular dichroism spectroscopy and gel electrophoresis, respectively. The formation of β-sheet secondary structure and closure of the β-barrel of OmpA were correlated with the same rate constant and coupled to the insertion of the protein into the lipid bilayer. OmpA, and presumably other β-barrel membrane proteins therefore do not follow a mechanism according to the two-stage model that has been proposed for the folding of α-helical bundle membrane proteins. These different folding mechanisms are likely a consequence of the very different intramolecular hydrogen bonding and hydrophobicity patterns in these two classes of membrane proteins.