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Overcoming hysteresis to attain reversible equilibrium folding for outer membrane phospholipase A in phospholipid bilayers.

ABSTRACT: The free energy of unfolding of a membrane protein from lipids into water (?G(o)(w,l)) describes its equilibrium thermodynamic stability. Knowing this parameter gives insight into a membrane protein's sequence-structure-energy relationships. However, there are few measures of membrane protein stability because of the technical difficulties associated with unfolded and partially folded states. Here, we describe the experimental process that allowed us to measure the ?G(o)(w,l) of the outer membrane phospholipase A into large unilamellar vesicles (LUVs) of 1,2-dilauroyl-sn-glycero-3-phosphocholine. To arrive at this reversible folding condition, we screened a large number of experimental variables: temperature, incubation time, salt concentration, pH, lipid composition and liposome morphology. The principal challenge we encountered under most conditions was hysteresis between folding and unfolding titrations. A second factor that compromised reversible folding was the observation that a fraction of the protein population tended to aggregate. We found that hysteresis could be completely eliminated on a feasible timescale by conducting experiments at acidic pH, by the slow dilution of the protein in the initial titration setup and by utilizing a low concentration of a detergent as a temporary "holdase" to solubilize the protein upon its initial dilution into folding conditions. We confirmed that the detergent did not disrupt the LUVs using fluorescence emission of lipid-sensitive dyes and light scattering. The results of our parameter search should be generally useful for efforts to measure ?G(o)(w,l) for other membrane proteins.

SUBMITTER: Moon CP

PROVIDER: S-EPMC3193555 | biostudies-literature | 2011 Oct

REPOSITORIES: biostudies-literature

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Overcoming hysteresis to attain reversible equilibrium folding for outer membrane phospholipase A in phospholipid bilayers.

Moon C Preston CP Kwon Sarah S Fleming Karen G KG

Journal of molecular biology 20110824 2

The free energy of unfolding of a membrane protein from lipids into water (ΔG(o)(w,l)) describes its equilibrium thermodynamic stability. Knowing this parameter gives insight into a membrane protein's sequence-structure-energy relationships. However, there are few measures of membrane protein stability because of the technical difficulties associated with unfolded and partially folded states. Here, we describe the experimental process that allowed us to measure the ΔG(o)(w,l) of the outer membra ...[more]

PMID: 21888919

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Project description:Understanding the forces that stabilize membrane proteins in their native states is one of the contemporary challenges of biophysics. To date, estimates of side chain partitioning free energies from water to the lipid environment show disparate values between experimental and computational measures. Resolving the disparities is particularly important for understanding the energetic contributions of polar and charged side chains to membrane protein function because of the roles these residue types play in many cellular functions. In general, computational free energy estimates of charged side chain partitioning into bilayers are much larger than experimental measurements. However, the lack of a protein-based experimental system that uses bilayers against which to vet these computational predictions has traditionally been a significant drawback. Moon & Fleming recently published a novel hydrophobicity scale that was derived experimentally by using a host-guest strategy to measure the side chain energetic perturbation due to mutation in the context of a native membrane protein inserted into a phospholipid bilayer. These values are still approximately an order of magnitude smaller than computational estimates derived from molecular dynamics calculations from several independent groups. Here we address this discrepancy by showing that the free energy differences between experiment and computation become much smaller if the appropriate comparisons are drawn, which suggests that the two fields may in fact be converging. In addition, we present an initial computational characterization of the Moon & Fleming experimental system used for the hydrophobicity scale: OmpLA in DLPC bilayers. The hydrophobicity scale used OmpLA position 210 as the guest site, and our preliminary results demonstrate that this position is buried in the center of the DLPC membrane, validating its usage in the experimental studies. We further showed that the introduction of charged Arg at position 210 is well tolerated in OmpLA and that the DLPC bilayers accommodate this perturbation by creating a water dimple that allows the Arg side chain to remain hydrated. Lipid head groups visit the dimple and can hydrogen bond with Arg, but these interactions are transient. Overall, our study demonstrates the unique advantages of this molecular system because it can be interrogated by both computational and experimental practitioners, and it sets the stage for free energy calculations in a system for which there is unambiguous experimental data. This article is part of a Special Issue entitled: Membrane protein structure and function.

Dataset Information

Overcoming hysteresis to attain reversible equilibrium folding for outer membrane phospholipase A in phospholipid bilayers.

Publications

Overcoming hysteresis to attain reversible equilibrium folding for outer membrane phospholipase A in phospholipid bilayers.

Similar Datasets

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