Part:BBa_K2382005

Thioredoxin-FGD fusion protein

Sequence and Features

Assembly Compatibility:

• 10
  COMPATIBLE WITH RFC[10]
• 12
  INCOMPATIBLE WITH RFC[12]

  Illegal NotI site found at 1068
  
• 21
  INCOMPATIBLE WITH RFC[21]

  Illegal BglII site found at 1117
  Illegal BamHI site found at 405
  Illegal XhoI site found at 411
  Illegal XhoI site found at 1383
  
• 23
  COMPATIBLE WITH RFC[23]
• 25
  INCOMPATIBLE WITH RFC[25]

  Illegal NgoMIV site found at 777
  Illegal NgoMIV site found at 975
  Illegal AgeI site found at 741
  
• 1000
  COMPATIBLE WITH RFC[1000]

Usage and Biology

By ligating these two different parts as a fusion protein, it is supposed to raise the solubility of our protein, F420-dependent glucose-6- phosphate dehydrogenase (FGD). We also call this part the "Synthetic FGD" in order to make a distinction with the original FGD from Australia.

This is a fusion protein made out of Thioredoxin and FGD. Those two proteins have different functions. Thioredoxin can help the protein folding correctly, and have the fusion protein produced in soluble form that are biologically active. FGD could help the electrons transferring from G6P(Glucose 6-phosphate) to the F420(It is a flavin derivative which plays a role like a coenzyme that could help the redox reactions in Methanogens such as Actinobacteria, especially in bacterial lineages.) Finally, the F420 would be reduced to F420H2, attaching two hydrogens and electrons on it. This fusion protein will help us transferring electrons and reduce aflatoxin via MSMEG5998.

Characterization of the Thioredoxin-FGD fusion protein

Expression results

IPTG induction

Two of the composite parts (BBa_K2382005 and BBa_K2382006) were synthesized by Allbio Life Co., Ltd and put into the standard backbone pSB1C3. First, we transformed two of our own composition parts into E. coli BL21 (DE3) strain to express our proteins. Then IPTG was used to induce the expression system, since the plasmid in our project had T7 promoter. We sonicated E. coli and did 9500 rpm and 13000 rpm centrifugation to remove the cell pellet and obtain the supernatant. To confirm the suitable concentration of cell supernatant, we do western blot. The results are demonstrated in figure 1C. After centrifuging for two times, we could find a high percentage of proteins in the pellet (the 9500 P group) and small amount of protein exist in the supernatant ( the 13000T group ).

Fig 1C:Cell lysates were analyzed by SDS-PAGE and coomassie brilliant blue staining. 9500 T meant the initial sample obtained after sonication; 9500 P and 13000 T meant the pellet and the supernatant gotten after 9500 rpm for 20 min; 13000 P and 13000 Su meant the pellet and the supernatant obtained after 13000 rpm for 20 min.(C) Samples contained Australian and synthetic FGD.

Protein purification, and dialysis

After extracting the cell lysates, we used nickel-resin column to purify our target proteins from the cell lysates because all of our proteins were tagged with 6 histidines at their C-terminal ends. After protein purification, protein dialysis with diaysis buffer containing 150 mM NaCl, 20 mM Tris-HCl (pH=7.5), and 20% glycerol to remove imidazole in our purified proteins, we did SDS-PAGE gel electrophoresis to ensure our target proteins were successfully purified (Fig. 2A ). The molecular weights of these proteins are listed in the Table 1. The standard BSA proteins were used to quantify the concentration of target proteins.

Table 1: Two expressed recombinant proteins and their molecular weights are listed.

Proteins	Molecular weight
Synthetic MSMEG5998	32.4 kDa
Synthetic FGD	51.5 kDa

Figure 2B: Concentration of proteins was quantified by SDS-PAGE and standard BSA samples with 0.16, 0.31, 0.63, 1.25, 2.5, 5, 10 mg/ml. (B) Two recombinant proteins expressed by the plasmids from DNA synthesis.

Protein solubility analysis

To know whether the solubility of our two enzymes (MSMEG_5998 and FGD BBa_K2382002) increased after fusing enzymes with thioredoxin, we dissolved all cell lysates which containing pellet and supernatant and did western blot to detect the content of our target proteins. All proteins were detected by anti-6x His Tag antibody because all of them contained a 6-histidines tail when bacteria expressed them. In Fig. 3, we could find there was good expression of both Australian and synthetic MSMEG5998 in the “13000 Su” group when compared with the “13000 P” group. This result meant that most proteins were dissolved in the supernatant while few proteins deposited in the cell pellet after 13000-rpm centrifugation. However, we could not observe good solubility in both Australian and synthetic FGD because there were little or no difference between the “13000 Su” group and the “13000 P” group.

Figure 3: Cell lysates in the process of two times centrifuge were analyzed by western blot. The abbreviations of five groups were the same as Fig. 1.

Enzyme Function Results

Enzyme Activity Assay

The conditions of reaction to degrade aflatoxin by MSMEG5998 were modified from Taylor’s study[6]. All concentrations of reactants are listed in Table 2 and 32 μM aflatoxin was used. We first mixed all reactants in eppendorfs and then put them at 22°C.

Figure 7A: MSMEG5998 could significantly degrade aflatoxin at time manner in vitro. (A) Direct 365 nm absorbance were detected after mixing Australian/synthetic MSMEG5998 and Australian/synthetic FGD and other reactants at 0th and 8th hour in the environment of pH=7.5 and 22℃.

In Fig. 7A, we compared two proteins, MSMEG5998 and F420-dependent glucose-6-phosphate dehydrogenase (FGD) expressed from Taylor’s vectors (from Australia) and from our synthetic vectors. We found that both the Australian and synthetic MSMEG5998 have great activity and degraded aflatoxin B1 by more than 60%. The effect of the synthetic one may be better than the Australian one but there were no statistic significance.

Table 2: The substance concentration of the aflatoxin-degradation reaction. For convenience sake, we called G6P, F420, FGD, and tris buffer as the reactants.

Name	Concentration
Aflatoxin B1	32 or 10 μM
MSMEG5998	0.1 μM

Reactants

Glucose-6-phosphate (G6P)	2.5 mM
F420	5 μM
F420-dependent glucose-6-phosphate dehydrogenase (FGD)	0.225 μM
Tris-HCl (pH=7.5)	25 mM

However, only Australian FGD has activity to reduce F420 into F420H2 and help the reaction. This finding corresponds with our dry lab results. Therefore, we used Australian and synthetic MSMEG5998 and Australian FGD to do the same experiment again to figure out whether the degradation percentage was dependent of time and whether the main reason of degradation was MSMEG5998. </p>

Figure 7B: MSMEG5998 could significantly degrade aflatoxin at time manner in vitro. (B) The same way as (A) but Australian/synthetic MSMEG5998 and Australian FGD were used and the reaction were detected at 0th, 2nd, 4th, 6th, and 8th hour. a, p < 0.001 compared to the 0th hour of the synthetic MSMEG5998(+) group; b, p < 0.001 compared to the 0th hour of the Australian MSMEG5998(+) group; c, p < 0.001 compared to the same time of the Australian MSMEG5998(+) group.

Figure 7C: MSMEG5998 could significantly degrade aflatoxin at time manner in vitro. (C) The same way as (B) but the degradation percentage were detected by ELISA. Because the initial concentration of aflatoxin (10000 ng/ml) was too high to be detected by the ELISA, we didn’t demonstrate the initial data.

The results were detected by direct 365 nm absorbance (Fig. 7B) and by ELISA (Fig. 7C). We found out that the degradation percentage was time-dependent. The synthetic MSMEG5998 had better activity than Australian MSMEG5998. The former was able to degrade 83% aflatoxin after 8 h while the latter could only degrade 52% aflatoxin.

References

Carsten Berndt, Christopher Horst Lillig, Arne Holmgren, Thioredoxins and glutaredoxins as facilitators of protein folding, In Biochimica et Biophysica Acta (BBA) - Molecular Cell Research, Volume 1783, Issue 4, 2008, Pages 641-650, ISSN 0167-4889, https://doi.org/10.1016/j.bbamcr.2008.02.003. (http://www.sciencedirect.com/science/article/pii/S0167488908000700)