ucare report
TRANSCRIPT
Production and binding of D4 protein
Bree Drda
Purpose
C. botulinium C2II domain four (D4) is a toxin binding domain protein with a general specificity towards eukaryotic cell surface glycosylation patterns which promotes cellular recognition and endocytosis of the toxin. It has not yet been characterized by crystallography or used in a biomedical application as a binding domain. Other research groups 1,2,3 have also investigated this protein and this project is a continuation of their work. The purpose of this report is to describe the methods and results of producing D4 protein and binding it to the endosomes of N2A cells. DNA electrophoresis, SDS-‐PAGE, and confocal microscopy were used.
Methods
Cell line and expression:
The DNA for D4 was amplified from the vector BDV/pGex-‐2T by PCR with a 5’ BAMHI-‐glycine extension and a 3’ ECORI extension using primers GCGGGATCCGGTCGTAAGGAAAACATCTCATCGATCAACATCATCAACG and CCGGAATTCTTAGATAATCAGTTTATCCAGTTCAATCAGAAACACGCCCGACAGAC. The vector BDV/pGEX-‐2T was obtained from synthesized DNA by Genscript with codon optimized for E. coli expression. This insert corresponded to amino acids 593-‐721 of PDB entry 2J42. The amplified fragment was ligated into the vector pGEX-‐2T by directional cloning at the restriction sites BamHI and EcoRI to make the expression vector pGEX-‐2T:D4. DH5α was transformed to propagate the vector and BL21 (DE3) was chosen as the expression host strain and transformed. The D4 sequence was confirmed by Operon sequencing services and did not contain errors. A colony was grown in 3 mL culture tubes with LB media and 100 ug/mL of ampicillin. After incubating for 18 hours at 37°C, 1 mL of the culture was added to 400 mL of LB media and grown until an OD of 0.3 – 0.5 was reached. The culture was induced with 0.5 mM IPTG once it had reached an OD of 0.5 – 0.8. It was then incubated for three hours. The cells were pelleted by centrifuging the mixture at 5,000 rpm, 4°C, for 10 minutes using 100 mL of culture/pellet. The pellets were stored at -‐20°C.
Protein extraction and purification:
A D4 cell pellet constituting 100 mL of culture volume was resuspended in 20 mL of 1 x PBS + 1% Triton pH 7.5. The sample was pressed 3 times at 1000 psig, (~16,000 psia cell pressure) using a French press. The cell debris was pelleted in a centrifuge at 10,000 rpm, 4°C, for 20 minutes and the supernatant containing the protein was decanted. This supernatant was then incubated with washed immobilized glutathione resin (Genscript) for one hour at 4°C to let the protein bind with the resin. The supernatant was washed of excess non-‐binding protein by centrifuging the resin with two volumes of 8 mL of 1 x PBS + 1% Triton. The Triton was then removed by centrifuging with three volumes of 1 x PBS. All spins involving the resin were done at 2,000 rpm, 4°C, for five minutes. The final total volume of the solution after the last wash was then reduced to 1 mL, and 10 units of thrombin were added in order to cleave the protein from the resin. The resin and the protein dissolved in the supernatant were separated using a syringe plugged with glass wool. The protein elution was concentrated to ~ten times the initial concentration using 3K spin column filters and stored at 4°C.
Protein Induction with N2A Cells:
Four well plates on a 24-‐well well plate were seeded with 100,000 N2A cells in 1 mL of EMEM + 10% FBS + 1% Pen Strep and grown at 37°C with 5% CO2. After 24 hours, three of the wells were inoculated with Bacmam 2.0 early endosome labeling kit (Molecular Probes) baculovirus at a concentration of 50 particles per mammalian cell (PPC) and left to incubate at 37°C for 24 hours. D4 protein labeled with Alexa Fluor was added to the well plates at following concentrations: 0, 1, 5, and 10 µg/mL. The cells were then fixed with 4% paraformaldehyde and stained with DAPI. The collagen coverslips were removed and mounted on glass slides. The cells were examined using confocal microscopy at 60X zoom.
Results
Protein purification gel:
Lanes 7 and 8 show the purified D4 protein after it has been cut from the resin. The amount of material present in lane 5 is considerably more than lane 9. This suggests that the protein bound to the resin in lane 5 has been cut off, which is what we see appearing in lanes 7 and 8. There are some impurities present in lane 7, which can be seen more distinctly in lane 6, which is the concentrated fraction. The purity of the protein in lanes 7 and 8 is estimated to be 90%.
Lanes 1 – Ladder 2 – Cell debris pellet 3 – Supernatant 4 – Final wash supernatant 5 – Resin, pre-thrombin 6 – Concentrated elution fraction from 10/12 purification (same cell paste lot) 7 – Elution fraction 1 8 – Elution fraction 2 9 - Resin, post-thrombin
Expected Masses kDa
D4/pGex-‐2T 40
D4 14
DNA gel of restriction digest of transformed DH5a to screen for inserted D4 DNA
This gel shows a digest of the vector plus D4 insert by BamHI/EcoRI restriction. This confirms the presence of the D4 gene in the appropriate cloning site for pGEX-‐2T. The expected masses of the fragments are 4.9 kb (pGex-‐2T) and 0.4 kb (D4). Screen for D4 inserted in pGex-‐2T: D4 in DH5α DNA gel 1 – GeneRule 1kb Plus DNA ladder 2 – colony 4 from isolate patch plate 3 – colony 5 from isolate patch plate
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Confocal microscopy, 60 X Zoom:
These figures show N2A cells that were incubated with 10 µg/mL D4 protein. Bacmam 2.0 targets specifically endosomes. The overlap of green and red areas in Figure 3 and Figure 4 shows the colocalization of Bacmam 2.0 and D4.
Figure 1: N2A cells incubated with Bacmam 2.0 (green), DAPI (blue), and Alexa Fluor 568 labeled D4 (red). a) Bacmam 2.0 b) Alexa Fluor 568 c) Bacmam 2.0 + Alexa Fluor 568 d) Bacmam 2.0 + Alexa Fluor 568 + DAPI
Figure 2: Zoom of Figure 1c
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Figure 3: Zoom of Figure 1d
From these images it appears that there is colocalization between D4 and early endosomes (Rab5a). In figure 1D there is a lack of green label, but it appears that D4 has still localized to a similar location as protein that has green labeling in the same frame. Therefore, we are assuming that this protein is most likely also colocalized to endosomes.
Discussion:
The confocal microscope images show cells with areas where Bacmam 2.0 and labeled D4 overlap (colocalization) and are represented by a yellow color. Bacmam 2.0 targets the endosomes of cells as a GFP/Rab5a fusion, bringing GFP to Rab5a locations. Rab5a is a protein that specifically localizes to early endosomes. The fact that the two are present in the same areas of cells shows that D4 is in the same subcellular location as the endosomes of cells. Therefore, it is plausible that D4 may be useful as an endosomal targeting moiety in a drug delivery application. Based on the SDS-‐PAGE results, we can see that this purification procedure produces D4 protein at an estimated 90% purity. Determining the structure of this protein with crystallography would require ~15 mg of highly pure D4 protein. This batch of D4 E. coli cells produced 0.5 mg/L. In the future, we will explore using a reactor in order to produce more D4 protein.
Appendix: D4 construct sequences
This corresponds to amino acid residues 593 to 721 with the addition of GR at the N-‐terminus for thrombin cleavage consensus.
Figure 4: N2A cells incubated with Bacmam 2.0 (green), DAPI (blue), and Alexa Fluor 568 labeled D4 (red). a) Bacmam 2.0 + Alexa Fluor 568 + DAPI b) Alexa Fluor 568 c) Bacmam 2.0 d) Bacmam 2.0 + Alexa Fluor 568
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Figure 6: Zoom of Figure 4a
1 GRKENISSIN IINDTNFGVE SMTGLSKRIK GNDGIYRAST KSFSFKSKEI 50 51 KYPEGFYRMR FVIQSYEPFT CNFKLFNNLI YSNSFDIGYY DEFFYFYCNG 100 101 SKSFFDISCD IINSINRLSG VFLIELDKLI I 131
DNA sequence confirmed by sequencing 1 GGATCCGGAT TCATGCGTAA GGAAAACATC TCATCGATCA ACATCATCAA 50 51 CGACACGAAC TTCGGCGTGG AAAGTATGAC CGGTCTGTCC AAACGTATTA 100 101 AGGGCAACGA TGGTATCTAT CGCGCGTCAA CCAAATCGTT TAGCTTCAAA 150 151 TCGAAGGAAA TTAAGTACCC GGAAGGTTTT TATCGTATGC GCTTCGTTAT 200 201 CCAGTCTTAT GAACCGTTCA CCTGTAACTT CAAGCTGTTC AACAACCTGA 250 251 TCTACTCTAA CAGTTTCGAC ATCGGCTACT ACGATGAATT TTTCTACTTC 300 301 TACTGCAACG GTTCCAAATC ATTTTTCGAC ATCAGTTGTG ATATCATCAA 350 351 CTCAATCAAC CGTCTGTCGG GCGTGTTTCT GATTGAACTG GATAAACTGA 400 401 TTATCTAAGA ATTC 414
References
1. Schleberg, C., Hochmann, H., Barth, H., Aktories, K., & Schulz, G. E. (2006) Structure and Action of the Binary C2 Toxin from Clostridium botulinum. Journal of Molecular Biology, 364, 705-‐715.
2. Eckhardt, M., Barth, H., Blöcker, D., & Aktories, K. (2000) Binding of Clostridium botulinum C2 Toxin to Asparagine-‐linked Complex and Hybrid Carbohydrates. The Journal of Biological Chemistry, 275(4), 2328-‐2334.
3. Nagahama, M., et al., Binding and Internalization of Clostridium botulinum C2 Toxin. Infection and Immunity, 2009. 77(11): p. 5139-‐5148.