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spMSP1D1

Express the SpyCatcher-MSP1D1-SpyTag fusion protein to achieve self-cyclization of MSP1D1 for constructing nanodiscs, while the 6×His tag is used for subsequent protein purification.

Usage and Biology

Membrane Scaffold Protein 1D1 (MSP1D1) is a synthetic derivative of apolipoprotein A-I, which is a major component of human high-density lipoproteins. MSP1D1 is designed to self-assemble with synthetic phospholipids into discoidal nanoparticles known as nanodiscs. These nanodiscs are soluble and stable in aqueous solutions, preserving the native-like architecture of a phospholipid bilayer.They are used as a tool for studying membrane proteins and have applications in biotechnology and medicine.

However, in the production of nanodiscs, it is challenging to generate nanodiscs larger than 20 nm for the reconstitution of membrane protein complexes. In order to achieve the self-cyclization of MSP1D1 to produce circular nanodiscs (cNDs), we have fused SpyCatcher and SpyTag to the N-terminus and C-terminus of MSP1D1, respectively, to create spMSP1D1. The covalent isopeptide bond between SpyCatcher and SpyTag facilitates the self-cyclization of spMSP1D1. Compared to the methods of self-cyclization using sortase enzymes or split inteins, the introduction of the SpyCatcher-SpyTag system makes the production of spMSP1D1 more convenient and increases the yield accordingly [1].

The significance of this step in our project is to provide a theoretical basis and explore conditions for the subsequent cyclization of multimeric MSPs to construct nanodiscs. Furthermore, spMSP1D1 nanodiscs, once embedded with membrane proteins such as the Low-Density Lipoprotein Receptor (LDLR), will be used for validation in our cellular experiments.

Induction and Purification

Induction Expression

The spMSP1D1 plasmid was transformed into BL21(DE3) cells.Successfully transformed colonies were selected through colony PCR, and single colonies were picked for scaled-up cultivation. when the OD value reached 0.7, IPTG was added to a final concentration of 0.2 mM to induce expression. The cultures were shaken at 16°C, 200 rpm for 16 hours. After centrifugation, SDS-PAGE was used to verify the expression, and it was observed that spMSP1D1 was successfully expressed. (Figure 1)

Purification of spMSP1D1

We collect the bacterial culture and centrifuge to harvest the pellet.We resuspended the pellet in buffer A, sonicated to disrupt the cells, and centrifuged again. Using nickel bead affinity chromatography, we washed away impurities with a washing buffer and then eluted the protein using an elution buffer containing a gradient concentration of imidazole for purification, followed by SDS-PAGE analysis (Figure 2). It can be seen that a large amount of the target protein was purified in the 100 mM imidazole eluent and the 300 mM imidazole eluent; however, the purity was not high, with some non-specific bands present,which may be caused by dimerization or polymerization of spMSP1D1.Further purification by molecular sieve was required.

Further purify spMSP1D1 using a molecular sieve

In order to obtain higher purity proteins, molecular sieve chromatography was used to purify proteins. After SDS-PAGE analysis (Figure 3), we obtained high purity spMSP1D1 and suspected spMSP1D1 dimer proteins, which were then concentrated and measured by ultrafiltration tube for subsequent preparation of nanodiscs.

Verify spMSP1D1 dimers or oligomers by Western Blot.

We performed a Western blot using the samples eluted with 300 mM imidazole and the putative dimeric samples purified by size exclusion chromatography. By utilizing anti-His antibodies, we aim to verify the presence of spMSP1D1 dimers and oligomers in the samples. (Figure 4)

Functional Characterization

The construction and characterization of spMSP1D1 nanodiscs

We prepared nanodiscs with different lipid-to-protein ratios, and immediately performed native-PAGE characterization after the preparation was completed. Compared with lane 1, other lanes had a significant difference, which is similar to the band shape in the literature. We preliminary considered that it is successful.(figure 5)

We prepared transmission electron microscope samples using the spMSP1D1 nanodiscs and performed transmission electron microscopy imaging. The results are shown below (Figure 6). In the image, we can see that the black circular objects are the nanodiscs. Their diameter is approximately 10 nm, which is close to the theoretical value of 11nm. Surprisingly, we also observed the nanodiscs from the side.(figure 7)

The construction and characterization of spMSP1D1 nanodiscs incorporating LDLR

We have constructed nanodiscs incorporating LDLR membrane proteins using spMSP1D1 at a defined ratio with lipids, following a similar method. The ratio of LDLR, spMSP1D1, and lipids was optimized to facilitate the reconstitution of membrane protein complexes within the nanodiscs.Based on the images from negative staining electron microscopy, we believe that spMSP1D1 nanodiscs incorporating membrane proteins have been successfully created.(Figure 7)

Sequence and Features