The protein quality control system (ProQC system) is a system that could enhance the integrity of the protein product by preventing the translation of truncated mRNA. It involves a prefix (switch) and a suffix (trigger) on the two sides of the ORF. Our team applied it to LysPBC5 expression and improved upon BBa_K653003 to get BBa_K4204019 as a universal expression model for the Pro-QC system for future iGEM teams to use (in which the GFP could be replaced by other parts). This page compares the western blotting result with and without the ProQC system to prove its effectiveness.
Figure 1. WB result without ProQC system. (from left to right: Cas12b protein, LysPBC5 protein, LysPBC5 protein, cell lysate flow through of Cas12b protein)
Although the size of both proteins (138Kda and 38Kda, respectively) are correct*, several shorter bands also appeared on the image (meaning that they also have his tag), revealing that shorter, nonfunctional versions of the protein exist in the sample. This reflected that a bunch of truncated mRNA might be produced during transcription, which not only wastes energy and amino acids but also traps ribosomes (since truncated mRNA does not contain a stop codon).
The hollow bands in the middle of the lane of protein LysPBC5 are a consequence of the high protein concentration of purified LysPBC5. High protein concentration leads to high primary antibody levels, and thus causes a high concentration of secondary antibodies with Horseradish Peroxidase to stack in the middle of the band, which leads to fast depletion of fluorescence substrates in the center.
Considering that our product might be too expensive if we only use the traditional protein expression system which wastes a lot of resources, we find and apply the protein quality control system (ProQC system) to our project. We designed part BBa_K4204021 for the expression of LysPBC5 in our project and improved upon BBa_K653003 to get BBa_K4204019 as a universal expression model for the ProQC system for future iGEM teams to use.
We used protein expressed by BBa_K4204021 to characterize the effectiveness of the Pro-QC system. The western blotting result of LysPBC5 with the ProQC system shows significant improvement in the integrity of the protein. LysPBC5 is diluted four 4 times before being loaded. Rows 7 to 10, which correspond to eluted LysPBC5 with the ProQC system, are clearly visible in the membrane picture obtained using ECL imaging (Fig. 2), demonstrating that the LysPBC5 protein with the ProQC system is being produced appropriately and that few proteins are lost during the washing process (lanes 1-6 that contains wash through are empty or nearly empty). Western blot analysis reveals that only the correct and precise target bands are being expressed, with the mass of protein being around 40kDa (the calculated mass of LysPBC5 is 38 kDa) and no aberrant or interfering stripes in the result. Overall, the WB result with the ProQC system shows that it is useful in improving the full-length expression rate of LysPBC5.
Figure 2. The WB result of LysPBC5 protein with protein quality control system
(Lanes 1-6, 7-10, and 11-13 contain wash-through, eluted protein, and cell lysate flow-through, respectively)
[1]Yang, J., Han, Y.H., Im, J. et al. Synthetic protein quality control to enhance full-length translation in bacteria. Nat Chem Biol 17, 421–427 (2021). https://doi.org/10.1038/s41589-021-00736-3
