Part:BBa_K2686002
Encapsulin protein with HexaHistidine insert
This part encodes a modified Thermotoga maritima Encapsulin protein BBa_K2686001 and has an additional HexaHistidine (GGGGGGHHHHHHGGGGG) insert between amino acids 43 and 44, forming a loop on the interior surface of the encapsulin monomer providing higher heat resistance and stability, and better hydrodynamic properties (Moon et al., 2014).
Usage and Biology
The part can be used to deliver cargo, both on the outer surface of the nanoparticle by fusing a peptide in between the 139/140 Amino Acids as well as the protein's C terminus. Cargo proteins can also be loaded inside the nano-cage using a tag binding to Encapsulin's interior surface (Cassidy-Amstutz et al., 2016). The protein is modified with an additional amino acid sequence (GGGGGGHHHHHHGGGGG) between positions 43/44 granting it better stability and high heat resistance (Moon et al., 2014).
Sequence and Features
- 10COMPATIBLE WITH RFC[10]
- 12COMPATIBLE WITH RFC[12]
- 21INCOMPATIBLE WITH RFC[21]Illegal BglII site found at 77
Illegal BglII site found at 492 - 23COMPATIBLE WITH RFC[23]
- 25COMPATIBLE WITH RFC[25]
- 1000INCOMPATIBLE WITH RFC[1000]Illegal SapI.rc site found at 426
Illegal SapI.rc site found at 457
Characterization
A variety of different characterization techniques were used to assess the properties of the encapsulin protein cage.
Expression
A cell free expression system was used to synthesize the encapsulin proteins in vitro. The TX-TL cell free system is a robust way to express proteins (Sun et al., 2013), and we used the protocol developed by the 2017 EPFL iGEM team Aptasense.
Assembly
The self assembly of the encapsulin 60-mer was first examined using SDS PAGE, where a high band is expected to form due to the high molecular weight and size of the 1.9MDa complex.
DLS Measurements
Intensity
Counts
Volume
Conclusion
Since the intensity weighted distribution shows how well particles with different sizes are identified from a fit to the autocorrelation function of the scattering, any protein aggregates even when present in small numbers can greatly vary the result. As for the count and volume, they are converted from the intensity using the protein's refraction index and absorbance. Since these are not exactly known we had to use the presets for proteins and water, which introduces errors (Malvern user manual). By analyzing these three measurements we can determine that the actual size of our protein is likely to be in between these extremes (larger than 18.166nm and smaller than 32.674nm). There are different hypotheses, first of all that the DLS measurement by intensity is biased and that the counts and volume based sizes represent the true size, or alternately that the DLS intensity measurement is accurate and that the 180-mer of encapsulin is formed. Overall, these results suggest that the construct forms multimers.
In vivo Characterisation by Team UCL 2019
Scale-up Procedure
Day 1
Different batches of BL21(DE3) competent cells were transformed with pSB1C3 plasmids containing BBa_K2686002 sequence coding for the T. maritima T=1 encapsulin monomer. Transformed cells were grown in LB agar plates containing chloramphenicol and glucose. Plates were incubated at 37°C overnight.
Day 2
Transformed colonies containing pSB1C3 + BBa_K2686002 were used to prepare overnight starter cultures containing a total of 5 mL LB broth and chloramphenicol (5 μL). Cultures were incubated at 37 °C overnight.
Day 3
A 50 mL scale-up culture was prepared from a single starter culture containing cells carrying pSB1C3 + BBa_K2686002. The culture was incubated at 37°C until it reached an OD of 0.6. Once they reached OD 0.6, the cultures were induced by addition of 400 μΜ IPTG. The cultures were left to grow again overnight at 37 °C.
Day 4
The culture was collected and transferred into a 50 mL Falcon tube. It was spun for 10 minutes at 5000 rpm in order to pellet the cells. Then the supernatant was discarded, and the pellet frozen at -80 °C.
Expression Analysis
According to our expression tests, this part was not suitable for successful production and purification of HexaHistidine-containing T. maritima encapsulin monomers in E. coli BL21(DE3). In vivo encapsulin production was hindered by the aggregation of the protein monomers at different production temperatures (i.e. 37°C and 18°C). This was evidenced by the SDS-PAGE gels which we run to test the presence of T. maritima encapsulin monomers in the insoluble, soluble and heat-purified fractions obtained from our cell lysates. As shown in Figure 1, the HexaHistidine-containing T. maritima encapsulin monomers (~ 32 kDa), which were produced in vivo by E. coli BL21(DE3), were concentrated in the insoluble fraction of BBa_K2686002-expressing bacteria even when the temperature of post-induction incubation was decreased from 37°C (Figure 1A) to 18°C (Figures 1B) to favour protein expression.
Self-Assembly
Due to the lacking or low-level solubility of T. maritima encapsulin monomers produced by in vivo expression of this part, these could not assemble to form the loadable 60-mer protein shells in in vivo systems. This was evidenced by dynamic light scattering (DLS). Under normal (i.e. soluble) conditions, T. maritima T=1 encapsulin monomers self-assemble into an encapsulin cage with a diameter of approximately 20-24 nm. Nevertheless, after performing DLS in the heat-purified soluble fractions obtained from BBa_K2686002-expressing bacteria, we observed that, at both temperatures of post-induction incubation (i.e. 37°C and 18°C), no signal was obtained for molecules ranging that size in the samples used. Instead, as displayed in Figure 2, DLS studies only detected the presence of monomers (diameter ~ 1 nm) and aggregates (diameter > 100 nm) of BBa_K2686002-encoded T. maritima encapsulin monomers at both production temperatures. Thus, DLS confirmed that, as it was observed from the SDS-PAGE gels, BBa_K2686002-encoded encapsulin monomers were mostly insoluble in in vivo protein expression systems and unable to self-assemble into T. maritima encapsulins.
Conclusion
We hypothesised that there may be some problems in T. maritima encapsulin in vivo expression, resulting in low quantity of protein when applying the heat purification method for which BBa_K2686002 had originally been designed. Firstly, a few other bacterial proteins may be stable at the temperature at which the heat-purification was performed and, subsequently, co-purify with the encapsulins that were being targeted. In fact, we observed that this was the case in cell-based protein expression systems, as, in the SDS-PAGE gel, we detected bands indicating the presence of proteins with molecular weights different to 32 kDa (Figure 1). This revealed that not only the encapsulin monomers had been heat purified and, therefore, there were additional proteins present in the purified analysed sample. This unspecific protein co-purification would not only reduce the purity of our target protein in the purified soluble sample but could even contribute to lowering the solubility of the previously existing parts coding for encapsulin monomers (Figures 1 and 2).
To overcome the challenges that arose with the in vivo solubility and self-assembly capacity of the monomers encoded by this part, the UCL 2019 iGEM team re-evaluated the purification by expressing BBa_K2686002 with T7 promoter (BBa_J64997) and a strong RBS (BBa_K2306014). After showing that said features allowed protein expression in vivo, Team UCL 2019 designed BBa_K3111103. which encodes T. maritima encapsulin monomers with an inserted StrepII tag that allowed for in vivo expression and purification using affinity chromatography.
Improvement by iGEM Kyoto 2019
References
Moon, H., Lee, J., Min, J. and Kang, S. (2014). Developing Genetically Engineered Encapsulin Protein Cage Nanoparticles as a Targeted Delivery Nanoplatform. Biomacromolecules, 15(10), pp.3794-3801.
Putri, R., Allende-Ballestero, C., Luque, D., Klem, R., Rousou, K., Liu, A., Traulsen, C., Rurup, W., Koay, M., Castón, J. and Cornelissen, J. (2017). Structural Characterization of Native and Modified Encapsulins as Nanoplatforms for in Vitro Catalysis and Cellular Uptake. ACS Nano, 11(12), pp.12796-12804.
Shimizu, Y., Inoue, A., Tomari, Y., Suzuki, T., Yokogawa, T., Nishikawa, K. and Ueda, T. (2001). Cell-free translation reconstituted with purified components. Nature Biotechnology, 19(8), pp.751-755.
Sun, Z., Hayes, C., Shin, J., Caschera, F., Murray, R. and Noireaux, V. (2013). Protocols for Implementing an Escherichia coli Based TX-TL Cell-Free Expression System for Synthetic Biology. Journal of Visualized Experiments, (79).
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