There are several methods to prove the successful transportation of CelD and SacC to the outer membrane of Escherichia coli. In this project, SDS-PAGE of entire cell culture samples, SDS-PAGE of membrane fraction samples, and measurement of enzyme activity of whole-cell-system without chemical or enzymatic purification have been considered to confirm the presence of active enzymes on the external membrane of E. coli external membrane.
1.1. CelD + estA protein fusion profiles
All the insoluble fractions of the transformed strains have a significant amount of a protein that matches the predicted weight of our chimeric construct (100kDa), in comparison to their negative controls (insoluble fraction of wild type lysates)(Figure 1). There is also no significant visual difference between each induced strain; this suggests that any strain is a good host for our construct, letting reduce the number of strains in future research. According to Clontech’s buffer kit user manual, our protein could be trapped in the pellet (insoluble phase) because of its high molecular weight (100kD > 40kD) and because it is a membrane- bound protein that can form multiprotein complexes and as we did not use Clontech’s TALON CellThru for direct purification from crude cell lysates (unclarified cell lysates), which is the solution proposed by the user manual in order to further solubilize proteins. Unclarified cell lysates were not further processed.
Figure 1. Protein profile of cell lysates from culture experiments of E. coli Bl21 SI, C43, XL1 Blue, Rosetta Gami and BW27783. (a) BL21 TNI (1)IF (2)SF. C43 TI (3)IF (4)SF - TNI (5)IF (6) SF - WT (7)IF (8)SF. (9)PMWM. Xl1 blue, TI (10)IF (11)SF - TNI (12)IF (13) SF - WT (14)IF (15)SF. (b) Rosetta Gami TI (1)IF (2)SF - TNI (3)IF (4) SF - WT (5)IF (6)SF. BW27783 TI (7)IF (8)SF - TNI (9)IF (10) SF - WT (11)IF (12)SF.(13) PMWM. BL21SI, TI (14)IF (15)SF. TI – transformed and induced. TNI – transformed and no induced, WT – wild type (C-). SF – soluble fraction. IF – insoluble fraction. PMWM – protein molecular weight marker
Future research should include identification of protein membrane display by periplasm extraction, Blue native electrophoresis for isolation of membrane protein complexes in enzymatically active form and SEM (Scanning Electron Microscope).
1.2.1. Whole-Cell CelD+estA Activity
In the whole-cell activity assay (Figure 2) The glucose concentration in celD + estA strain was of 332.04 µM and in the Negative Control (C-) was of 275.85 µM that is a difference in the glucose concentration of 57 µM. The result of the t- test was the rejection of the null hypothesis, suggesting that the difference between them is significant.
Figure 2. Whole-Cell Cellulase Activity. The IUPAC Filter Paper Assay of celD+ estA and the Negative Control (C-). The glucose concentration in celD + estA strain was of 332.04 µM and in the Negative Control (C-) was of 275.85 µM.
1.2.2. Cell-Lysate CelD+estA Activity
In the cell-lysate cellulase activity assay (Figure 3) The glucose concentration in the soluble fraction of celD-estA was of 358 µM and in the Negative Control (C-) was of 323 µM.In the insoluble fraction, the glucose concentration of the celD-estA was 374 µM and in the Negative Control (C-) it was 264 µM. The difference in soluble and insoluble fractions with its negative control was 35 µM while the difference in the insoluble fraction was 110 µM. The result of the t-test was the rejection of the null hyphothesis, suggesting that the difference between them is also significant.
Figure 3. Cellulase Activity of Cell lysates.The IUPAC Filter Paper Assay was assessed for the soluble and insoluble fraction of the celD+estA strain and the Negative Control (C-). The glucose concentration in the soluble fraction of celD-estA was 358 µM and in the Negative Control (C-) it was 323 µM.In the insoluble fraction, the glucose contentration of the celD-estA was 374 µM and in the Negative Control (C-) it was 264 µM.
2.1. OmpA+sacC Construction
The final genetic construction for ompA + sacC was accomplished without the translation terminator sequence (BBa_K633015).
Approximately 3 kb of the linearized plasmid containing ompA + sacC was detected in all lanes and 1.25 kb of restriction fragment was visualized in the lane 6. (Figure 4)
Figure 4. 0.7% Agarose Gel.
(Lane 1: Negative control (non-digested plasmid), 2: Linealized plasmid of ompA+sacC with EcoRI, 3: Linealized with XbaI, 4: 1kb DNA Ladder, 5: Linealized with SacI, 6: Digested with NheI, 7: Linealized with SpeI, 8: Linealized with PstI)
2.2. OmpA+sacC Expression
A visible protein band of the expected molecular wight (62.8 kDa) of the fusion protein (ompA+sacC) could not be confirmed by SDS-PAGE (Figure 5). However, as Lee et al. (2004) have proven, the fusion protein could hardly be detected by Coomassie blue staining as its expression was below the detection level of the method used. Our result may be due to the same reason.
Figure 5. Tris-Glycine SDS-Polyacrylamide Gel of ompA + sacC. Gel A visualizes the 1D protein profiles of the soluble and insoluble fractions of XL1 Blue Wild Type (lane 2, 3), induced XL1 Blue + ompA + sacC plasmid (lane 4, 5) and non-induced XL1 Blue + ompA + sacC (lane 6, 7), the soluble and insoluble fractions of Rosetta Gami Wild Type (lane 8, 9), non-induced Rosetta Gami + ompA + sacC plasmid (lane 10, 11) and induced Rosetta Gami + ompA + sacC (lane 12, 13), and the soluble and insoluble fractions of induced BW27783 + ompA + sacC (lane 14, 15). Gel B shows the 1D protein profiles of the soluble and insoluble fractions of induced BW27783 + ompA + sacC (lane 1, 2), the soluble and insoluble fractions of BL21 SI wild type (lane 4, 5), non-induced BL21 SI + ompA + sacC (lane 6, 7), and induced BL21 SI + ompA + sacC (lane 8, 9), the soluble and insoluble fractions of C43 wild type (lane 10, 11), non-induced C43 + ompA + sacC (lane 12, 13), and induced C43 + ompA + sacC (lane 14, 15).
2.3. SacC is active in whole cell assays
Whole cells of the E. coli strain (BL21SI) +sacC+ompA produced a fructose concentration of 350.71±60.97 µM which is 149.36 µM higher than the negative control cells (Figure 6). A T-test with 2 tails and alpha value of 0.05 was carried out, and the null hypothesis of "the population means are the same" was rejected, indicating that there is a difference between the fructose concentration in the control strain and those of the sample strains. And although further investigation is required, the evidence we have is a strong indicator that the enzyme is active in the outer membrane of E. coli.
Figure 6. SacC Activity Assay Graph. Each bar indicates the fructose concentration generated by the sacC activity of non-transformed BL21 SI (left) and those of the BL21 SI + ompA + sacC plasmid.
Further research will be focused on SDS-PAGE with a more efficient staining/blotting technique, expression of sacC fused to estA protein fragments, and more sacC enzymatic assays.
• Lee SH, Choi JI, Park SJ, Lee SY & Park BC (2004) Display of Bacterial Lipase on the Escherichia coli Cell Surface by Using FadL as an Anchoring Motif and Use of the Enzyme in Enantioselective Biocatalysis. Applied and Environmental Microbiology. Vol.70(9):5074–5080.
• Schägger, H., & Gebhard, v. J. (1991). Blue native electrophoresis for isolation of membrane protein complexes in enzymatically active form. Analytical Biochemistry , 199 (2), 223-231.