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. 2018 Jun;27(6):1109-1112.
doi: 10.1002/pro.3414. Epub 2018 Apr 25.

Bicelle Size Modulates the Rate of Bacteriorhodopsin Folding

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Free PMC article

Bicelle Size Modulates the Rate of Bacteriorhodopsin Folding

Timothy C Gruenhagen et al. Protein Sci. .
Free PMC article

Abstract

The conformational equilibria of integral membrane proteins have proven extremely difficult to characterize within native lipid bilayers. To circumvent technical issues, investigations of the structure and stability of α-helical membrane proteins are often carried out in mixed micelle or bicelle solvents that mimic the membrane and facilitate measurements of reversible folding. Under these conditions, the energetics of membrane protein folding are typically proportional to the mole fraction of an anionic detergent in the micelle. However, investigations of the folding and unfolding of bacteriorhodopsin (bR) surprisingly revealed that the folding rate is also highly sensitive to the bulk molar concentration of lipids and detergents. We show here that this rate enhancement coincides with changes in bicelle size and suggest this effect arises through restriction of the conformational search space during folding. In conjunction with previous mutagenic studies, these results provide additional evidence that a topological search limits the rate of bR folding. Furthermore, this finding provides insights into the manner by which micellar and bicellar environments influence the conformational stability of polytopic membrane proteins.

Keywords: bicelles; detergents; folding kinetics; membrane protein folding; micelles.

Figures

Figure 1
Figure 1
Influence of bicelle q*‐value and size on the folding kinetics of bR. A: Previously determined values for the natural log of the observed rate constant for bR refolding (k obs) at a fixed X SDS of 0.50 are plotted against the corresponding bulk molar concentrations of lipid and detergent.13 A linear fit of the data (R 2 = 0.99) is shown for reference. B: The average bicelle diameter determined from three replicate dynamic light scattering measurements is plotted against the effective q‐value (q*) of the bicelles under each condition. Error bars reflect the standard deviation of the fitted diameter values. An unweighted linear fit of the data (R 2 = 0.99) is shown for reference. C: Previously determined values for the natural log of k obs 13 are plotted against the average diameter of the bicelles under these conditions as was determined from three replicate dynamic light scattering measurements. Error bars reflect the standard deviation of the fitted diameter values. An unweighted linear fit of the data (R 2 = 0.97) is shown for reference.

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