Part:BBa_K2668010
Cerberus : A Molecular Binding Plateform (mSA2-CBM3a-AzF)
Sequence and Features
- 10COMPATIBLE WITH RFC[10]
- 12COMPATIBLE WITH RFC[12]
- 21COMPATIBLE WITH RFC[21]
- 23COMPATIBLE WITH RFC[23]
- 25INCOMPATIBLE WITH RFC[25]Illegal AgeI site found at 358
- 1000COMPATIBLE WITH RFC[1000]
Introduction
Cerberus part (BBa_K2668010) is composed by the CBM3a fused at the N-terminus to the biotinylated molecule-binding head (monomeric streptavidin) and at the C-terminus to the unnatural amino acid azido-L-phenylalanine (AzF). The incorporation of AzF is achieved through the recognition of the amber stop codon at the C-terminus of CBM3a by an orthogonal AzF-charged tRNA.
Coupling of molecules to the AzF head can be performed by click chemistry, either SPAAC or Cu(I)-catalyzed azide-alkyne click chemistry (CuAAC). To prove the versatility of the system, different compounds such as fluorescent proteins or paramagnetic beads have been clicked on Cerberus.
Construction
Cerberus part (BBa_K2668010) consist of a CBM3 fused with mSA2, an engineered version of Streptavidin and Rhizavidin, on his N terminus endogenous linker and containing a UAG codon on his C terminus linker for amber suppression unnatural amino acid incorporation and a TEV tag. IDT performed the DNA synthesis and delivered the part as gBlock. The construct was cloned by In-Fusion kit into the pSB1C3 plasmid and then transformed into E. coli Dh5-alpha strain. Figure 1 shows the restriction map of the resulting clones. We expected two bands at 2.4Kb and 0.7Kb Only clones 2 and 5 show the expected restriction profiles. The clones 5 was deposited in the registry of Standard Biological Parts.
Characetrisation
Modeling
Modelling this three-headed fusion protein presented a complex challenge, requiring a multi-step and multi-level strategy relying on various molecular modelling and simulation methods. We used available X-ray structure data from the Protein Data Bank, integrated 3D structure prediction tools, employed innovative robotics inspired artificial intelligence methods to explore the intrinsically disordered regions, built and parametrised the unnatural amino acid aziodphenyalanine, and finally studied its behaviour over time through molecular dynamics simulations in a large explicit solvent water box. The results of the molecular dynamics simulation comforted us in the stability of our construction as the domains kept their structure over 90ns of simulation as showed in figure 2.
Production of Cerberus
The fusion between the streptavidin linker and CBM3a platform sequences followed by the amber stop codon was fused with an His Tag and cloned into the pET28 expression vector using In-Fusion Cloning Kit. The resulting construct was co-transformed with pEVOL-AzF expression vector (coding for amino acyl tRNA synthetase (aarS)/tRNA orthogonal pair for in vivo in E. coli under L-arabinose induction) into E. coli strain BL21. Expression of the recombinant protein was induced using IPTG and L-arabinose, with addition of AzF in the medium as follow. The synthesis of aaRS/tRNA pair was first induced with L-arabinose and AzF was added at the same time and onee hour later, the production of Cerberus with IPTG. The His-tagged protein was then purified on IMAC resin charged with cobalt and used in cellulose pull down assays. Results are shown on figure 3.
Induction in the presence of AzF ( lane I+AzF) led to the expression of two major bands at 37 and 39 kDa approximately compared to control conditions, namely the non-induced (lane NI) and induced without AzF (lane I-AzF). We hypothesized that the upper and the lower band would correspond to Cerberus (expected molecular weight of monomeric Cerberus: 41 kDa), and Cerberus without the AzF and the His-tag, (i.e. Orthos) respectively. Surprisingly, both bands were still present in the elution fractions which is incoherent since Orthos lacks an His-tag and was supposed to be washed out during purification steps. To confirm our hypothesis, we analysed the same fractions by western blot using antibodies detecting the His-tag (Figure 4).
Anti-His-tag antibodies revealed a band at about 45 kDa in the sample corresponding to IPTG induction in the presence of AzF (lane I+AzF). This band is not present in control samples (lane NI and I-AzF), indicating that it corresponds to Cerberus protein (theoretical size: 41 kDa). In addition to the full length protein, we observed several extra bands which very likely correspond to proteolysis products since they are detected with the anti-His-tag antibodies. Moreover, the band at 45 kDa is clearly detected in elution samples (lanes E). The purification level of Cerberus with monomeric streptavidin in the elution samples was estimated about 62%. These data show that Cerberus was efficiently purified and can be used for pull down assays. In addition, these results show that experimental setup to produce Cerberus also leads to the production of Orthos when the amber stop codon is not recognized by the AzF-charged orthogonal tRNA. Although Orthos does not contain a His-tag at its C-terminus, the protein seems to be efficiently co-purified with Cerberus. The basis of this observation is unclear but this result may suggest that Orthos and Cerberus interact with each other, via their CBM3 or streptavidin moieties.
Validation of Cerberus
Validation using FITC (Fluorescein isothiocyanate) molecules
To test the potential of Cerberus in functionalizing cellulose, we monitored its ability to mediate an interaction between cellulose and a fluorescent compound. To generate a fluorescently labelled Cerberus protein, we performed a SPAAC reaction on 3.2 µM Cerberus protein using 31.9 µM µg of FITC-DBCO. In control experiment, 31.9 µM of FITC alone was used. These samples were then incubated with cellulose and after several washes with resuspension buffer, fluorescence levels were measured in the cellulose pellet fractions (Figure 5). We observed that the fluorescence levels in the cellulose pellet incubated with Cerberus protein clicked to FITC were about 3 times higher than in the control experiments corresponding to the cellulose pellet incubated with FITC alone. These results show that Cerberus can efficiently be conjugated with fluorescent molecules bearing a DBCO group by click chemistry and that the resulting fluorescent molecules strongly interact with cellulose. Therefore, Cerberus is both a practical and potent platform to functionalize cellulose.
Validation using paramagnetic beads
The functionality of Cerberus was further characterized using paramagnetic beads. To bind paramagnetic beads to Cerberus, we performed a click reaction using 3.2 µM of Cerberus protein and 32 µM DBCO-conjugated paramagnetic beads. In control experiment, 32 µM of paramagnetic beads were used. These samples were then incubated with cellulose, and after several washes with resuspension buffer, the magnetic capacity of cellulose using a magnet was observed and filmed (See video below. The cellulose incubated with the Cerberus protein conjugated to paramagnetic beads got quickly and was totally collected by the magnet. In contrast, in the control experiment, we only observed a slight movement of a part of the cellulose towards the magnet. This can be explained by the fact that washes do not remove all paramagnetic beads non-attached to the cellulose, or that paramagnetic beads naturally bind to cellulose but this interaction is not strong enough to resist to the washes.
Figure 5: Video of Cellulose functionnalised with magnetic beads using Cerberus Left : Negative control using Paramagnetic beads alone, Right: Cerberus-Paramagnetic beads
Conclusion and Perspectives
These results show that Cerberus has the ability to interact simultaneously with cellulose and molecules with DBCO group, indicating that this modified protein, allows to functionalize cellulose through its linker containing unnatural amino acid (AzF).
Fixation of various compounds that can be chemically functionalized can now be achieved. Next step will be to realize double fixations, for example a fluorophore on the streptavidin head and paramagnetic beads on the AzF linker. This double fixation allows to envision endless possibilities to functionalize cellulose.
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