The term nanodiscs describes a small (7-50 nm in diameter) disc-shaped structure that finds use in proteomics and biomedicine. It consists of two main components:
Schematic depiction of a nanodisc stabilizing a membrane protein.
Green: The stabilizing belt – A MSP protein in this case
Grey: Phospholipids
Orange: Stabilized membrane protein
Figure 1: Membrane proteins have hydrophobic and hydrophilic parts. Nanodiscs make them soluble in aqueous solutions.
The problem is, that membrane proteins, unlike soluble proteins, are difficult to analyze in their native environment, due to their insertion in the lipid membrane. Surfaces of membrane proteins in the hydrophobic core of the lipid bilayer are also hydrophobic, whereas surface areas in contact with the aqueous membrane environment are as hydrophilic as the surfaces of ordinary soluble proteins (2). The presence of extensive hydrophobic and hydrophilic surfaces on the same molecule is characteristic of membrane proteins. As a result, membrane proteins are not soluble in standard aqueous buffers without a solubilizing agent. For example, nanodiscs are required to solubilize them to mimic the amphipathic environment of a lipid bilayer whilst maintaining the structure of the membrane protein in a physiologically relevant state.
Figure 2: Two types of nanodiscs exist: Synthetic nanodiscs (blue stabilizer) and MSP nanodiscs (green stabilizer).
As mentioned before nanodiscs can be differentiated between their phospholipid composition and most importantly, their type of stabilizer. This stabilizer is the reason why nanodiscs in total are split into two main categories: MSP nanodiscs and Synthetic nanodiscs.
The respective names originate from the type of stabilizer that is used to keep the nanodiscs together and form them in the first place. It also decides what the lipid composition of the nanodisc is made up of. MSP nanodiscs always contain an artificial lipid composition. Meaning you have full control of it. In contrast: Synthetic nanodiscs use the native cell phospholipids to create the nanodisc. A direct comparison of the individual advantages of both nanodiscs can be found here.
able 1: Small comparison between MSP and Synthetic nanodiscs
But first, let us introduce both kinds of nanodiscs on their own, starting with the MSP nanodiscs.
MSP nanodiscs are held together by membrane scaffold proteins (MSPs). MSPs can be truncated forms of apolipoprotein (apo) A-I which wrap around a patch of a lipid bilayer to form a disc-like particle or nanodisc (5). MSPs provide a hydrophobic surface facing the hydrophobic tail of the lipids, and a hydrophilic surface on the outside. This setup makes nanodiscs highly soluble in aqueous solutions. Once assembled into nanodiscs, membrane proteins can be kept in solution in the absence of detergents (5).
Size: The size of an MSP nanodisc can range between 7 – 17 nm. It is determined by the used membrane scaffolding protein. Table 2 depicts the membrane scaffold proteins that Cube Biotech offers and which nanodisc sizes it leads to. MSP nanodiscs of the same MSP protein are uniform in size and only differ +/- 1 nm in diameter. This suits them perfectly for Cryo-EM studies.
Other advantages of MSP nanodiscs
MSP nanodiscs have a number of advantages compared to other systems for membrane protein solubilization and reconstitution, in particular for ligand binding studies, analysis of conformational dynamics, and protein interaction studies (6). Nanodiscs can be used to reconstitute membrane proteins such as GPCRs or transporters in an artificial environment resembling the native membrane.
These nanodisc-stabilized proteins can be directly purified by standard chromatographic procedures. The resulting purified membrane protein-nanodisc complex can be used in applications that require access to both the physiologically intracellular and extracellular surfaces of the protein and thus allows unrestricted access to antagonists, agonists, G proteins, and other interaction partners (7).
Figure 4: Schematic image of two ways to reconstitute proteins into nanodiscs. A: Assembled nanodiscs are added to a cell-free reaction. The nascent protein can insert spontaneously. B: Detergent and MSP are added to cells expressing the protein of interest. A complex of membrane phospholipids, proteins, and MSP forms.
This selection, but also many other phospholipids have been successfully used alone or in combination (8,25). The choice of lipids has been shown to be crucial for protein activity (8), for example in cases where lipids promote protein oligomerization (25). Cell-free expression using assembled nanodiscs is a fast and easy way to screen a variety of lipids and lipid mixtures for their effect on the protein. When proteins are solubilized directly from the membrane fraction, endogenous phospholipids are carried along and incorporated into the nanodisc complex, which may enhance protein activity.
Synthetic nanodiscs are the second big option in the field of nanodisc. They differ in certain key aspects from their MSP counterparts, but also share certain similarities.
Creation of synthetic nanodiscs
In contrast to the three creation ways of MSP nanodiscs (figure 4), synthetic nanodiscs can only be created directly from intact cells. The used synthetic polymer has a dual function during this process. First, it dissolves the cell membrane, similar to a detergent. Then it forms a nanodisc structure around membrane proteins using the native cell phospholipids. A good analogy to this process is a cookie cutter that stamps the cookies out of the dough.
Figure 5: Schematic depiction of a synthetic nanodisc that stabilizes a hypothetical membrane protein. The figure legend mentions SMA as the used stabilizing polymer, but e.g. DIBMA could fill this role as well.
Synthetic nanodiscs are variable in their size. The main factor that decides their diameter is the size of the membrane protein complex that they surround and stabilize. Therefore a definitive size cannot be given for a synthetic nanodisc. But they all range in the size range that can also be found in MSP nanodiscs (table 2). This applies to all established polymers so far. If uniformous nanodisc size is desired for a synthetic nanodiscs complex, a size-exclusion chromatography (SEC) has to be performed after the stabilized membrane protein of interest has been purified by e. g. affinity chromatography with the Rho1D4-tag.
Which polymer?
The selection of different polymers for synthetic nanodiscs is constantly growing. Each with its own benefits and downsides. DIBMA for example has protein-like absorption at a wavelength of 280 nm. Meanwhile, AASTY has quite fixed nanodiscs diameters compared to other polymers.
However, this is only a small extract from a long list of pros and cons of the different polymers. We dedicated this topic its own webpage. Have a look at it!
Figure 6: DIBMA and SMA seem to be interchangeable in the first view, but there are some differences.
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