Membrane proteins are difficult to study, resulting in limited knowledge about their structure and function, especially when compared to soluble proteins. The structure of a membrane protein is dependent on the surrounding lipid environment, making it difficult to prepare samples and limiting the choice of experimental methods that can be used to study them.
To study membrane proteins, experimental methods must be able to mimic the environment that a protein would encounter inside the body. Often, supported lipid bilayers are used to investigate lipid membranes. A variety of different lipid compositions can be made, and analysed by a wide range of techniques including neutron reflectometry. However, in most cases the application of supported lipid bilayers are focused on their interaction with soluble proteins and peptides, while the incorporation of membrane proteins in the supported lipid bilayers remains a challenging process.
In a cell, all membrane proteins of the same type have the same orientation. Recreating complex cell membranes with correctly orientated membrane proteins is challenging, and vital, if we want to learn more about the detailed structure and function of the proteins.
A team from the University of Copenhagen have proposed a new, alternative, method to create an environment to study membrane proteins, and used the SURF reflectometer at ISIS to obtain information about the structure of their uniquely formed lipid bilayers. Lead researcher Dr Alessandra Luchini explains; “The project aimed to establish a general platform for the investigation of membrane proteins, by means of surface sensitive techniques such as Neutron Reflectometry."
By using peptide-discs to mediate the formation of the supported lipid bilayer, they were able to incorporate the membrane protein molecules all with the same orientation with respect to the membrane plane and at sufficiently high concentrations to clearly see them with neutron reflectometry.
The peptide discs carry both the lipids and the membrane proteins to the support surface, and therefore can be used to self-assemble supported lipid bilayers. The peptide discs contain a phospholipid bilayer on the inside, and self-assembled peptide molecules on the outside that are both hydrophilic and hydrophobic. In previous studies, the peptide discs has been shown to be unstable, but this team are actually able to use this to their advantage. When the disk formation reverses, the lipids and membrane proteins are released onto the support surface.
In this study, the researchers used peptide discs to form a supported lipid bilayer on three different types of solid supports: silicon oxide, mica and gold. They found that their proposed mechanism works on all of these supports, as long as the surface is hydrophilic. Their method offers an alternative to those that require modifications to the support surface, or the protein itself, reducing the time and number of steps needed to prepare samples.
Two different membrane proteins were successfully loaded using the peptide disc method, enabling the group to validate the proposed method as a model system. A slightly modified version of the protein tissue factor (TF), recombinant TF (rTF), was used. Neutron reflectometry on SURF was able to confirm that the protein was in the correct orientation, and able to successfully bind to the next soluble protein in the blood coagulation cascade, the activate factor FVII (FVIIa).
“Neutron Reflectometry experiments on SURF at ISIS had a central role in developing the peptide disc mediate formation of supported lipid bilayers with membrane proteins," explains Dr Luchini; “and provided fundamental information on the structure of the investigated systems."
The full paper can be found online at: https://pubs.acs.org/doi/pdf/10.1021/acs.analchem.9b04125