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Membrane proteins in reverse micelles: myelin basic protein in a membrane-mimetic environment
Abstract:
The solubility, reactivity, and conformational dynamics of myelin basic protein (MBP) from bovine brain were studied in reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)-isooctane and water. Such a membrane-mimetic system resembles the aqueous spaces of native myelin sheath in terms of physicochemical properties as reflected in the high affinity of MBP for interfacial bound water. This is marked by the unusual profile of the solubility curve of the protein in reverse micelles, which shows optimal solubility at a much lower molar ratio of water to surfactant ([ H2O]/[AOT] = w0) than that reported for other water-soluble proteins. The role of counterions and/or charged polar head groups in the solubilization process is revealed by comparison of the solubility of MBP in nonionic surfactant micellar solutions. Whereas MBP is unfolded in aqueous solutions, insertion into reverse micelles generates a more folded structure, characterized by the presence of 20% alpha-helix. This conformation is unaffected by variations in the water content of the system (in the 2.0-22.4 w0 range). The reactivity of epsilon-amino groups of lysine residues with aqueous solutions of o-phthalaldehyde demonstrates that segments of the peptide chain are accessible to water. Similar results were obtained with the sequence involved in heme binding. In contrast, the sole tryptophan residue, Trp-117, is shielded from the aqueous solvent, as indicated by lack of reaction with N-bromosuccinimide. The invariance of the wavelength maximum emission in the fluorescence spectra as a function of w0 is consistent with this result.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Myelin basic protein (MBP) shows unique solubility and folding in AOT reverse micelles, mimicking myelin. MBP adopts a more folded, alpha-helical structure within these membrane-mimetic systems.
Area of Science:
- Biochemistry
- Biophysics
- Neuroscience
Background:
- Myelin basic protein (MBP) is crucial for myelin sheath structure and function.
- Understanding MBP's behavior in membrane-mimetic environments is key to elucidating its role in demyelinating diseases.
Purpose of the Study:
- To investigate the solubility, reactivity, and conformational dynamics of bovine MBP in AOT reverse micelles.
- To characterize the membrane-mimetic properties of the AOT-isooctane-water system for MBP.
Main Methods:
- Solubility studies of MBP in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles.
- Conformational analysis using techniques sensitive to protein folding (e.g., alpha-helix content).
- Reactivity assays (o-phthalaldehyde, N-bromosuccinimide) to probe solvent accessibility of protein residues.
Main Results:
- MBP exhibits optimal solubility in AOT reverse micelles at a lower water-to-surfactant ratio ([H2O]/[AOT] = w0) than other water-soluble proteins.
- MBP transitions from an unfolded state in aqueous solution to a more folded conformation (20% alpha-helix) in reverse micelles, independent of w0.
- Specific residues are accessible to solvent, while the tryptophan residue (Trp-117) is shielded.
Conclusions:
- AOT reverse micelles provide a suitable membrane-mimetic environment for studying MBP.
- MBP's conformational stability and specific residue accessibility are modulated by the reverse micelle system.
- These findings offer insights into MBP's interactions within the myelin sheath.