Team:HokkaidoU Japan/ag43
Ag43 Secretion System
Overview
Antimicrobial peptides (AMPs) have wide range of toxicity to microbes, Gram-negative/positive bacteria, archaea, fungi and viruses. However, their broad spectrum of toxicity would have harmful result for host cells. If we introduce host-toxic gene into E. coli, they would die of their toxicity. So, in order to yield host-toxic protein without killing host cells, we iGEM HokkaidoU Japan designed a safe secretion system of AMPs and using this system, we made E, coli yield thanatin, one of AMPs derived from shield bug, Podisus maculiventris.
Tandem-multimerization of thanatin
A broad-spectrum antimicrobial peptide from shield bug, thanatin, is short polypeptide composed of 21 amino acids. Thanatin contains 2 cysteine residues and disulfide bond forms between these 2 residues (Cys11 and Cys18). The disulfide bond between 2 cysteine residues is the core of active thanatin (1). This disulfide bridge stabilizes the anti-parallel β-sheets
(Fig. 1). Thanatin is very stable short polypeptide and has wide range of activity against bacteria, archaea and fungi. The detailed action mechanism of thanatin is still unknown. However, it is known that thanatin attacks the membrane of target cells and causes pore-forming.
Fig. 1 Schematic of thanatin structure. Thanatin is a 21-residue short polypeptide. It has disulfide bond between 2 cysteine residues and anti-parallel β-sheets.
C-terminal residues of thanatin are crucial to its activity rather than N-terminus. Thanatin N-terminus is comparatively tolerant towards adding or deletion of residues. However, in case of C-terminus, newly added or removed amino acid residues might be catastrophic, perhaps causing loss or remarkable decreasing of activity. Taking it into account, ligating some thanatin tandem would make it possible to yield detoxicated form of thanatin without killing host cells (Fig. 2).
Fig. 2 Image of thanatin-multimer. We designed our thanatin fragment having aspartic acid at it C-terminus. Amino side of aspartic acids are cleaved by endoproteinase AspN. Multimerized thanatin do not have toxicity, but, when it is cleaved to monomers, each monomer regains wide-spectrum toxicity.
Then, we need to recover the toxicity of thanatin after expressing detoxicated tandem-repeated thanatin. In order to realize re gaining of toxicity, we utilized endoproteinase AspN, which specifically cleaves the peptide bonds at amino side of aspartic acid residues (purchased at New England Biolabs. website). We added aspartic acid at carboxy-terminal side of thanatin. Thanatin-Asp-thanatin-Asp-thanatin… multimer is cleavable and monomerizable with AspN. Monomerized thanatin would regain pristine toxicity. We appreciate Dr. Seiichi Taguchi, a professor of Biosystems Engineering, Hokkaido University, for offering us much information about thanatin function.
Secretion of Thanatin by auto-transporter; Antigen43
An easy and rapid Collection method of expressed protein is desired. It would be laborious task to homogenize cells in order to collect inner protein or polypeptide. To collect proteins and polypeptides easily and rapidly, we utilized Antigen43 (Ag43), which is an autotransporter protein of Escherichia coli. Usually, Ag43 is surface-displayed and used for autoaggregation of E. coli (2). Autotranspoter proteins, including Ag43, are commonly composed of N-terminal signal peptide following passenger domain (α-domain), and C-terminal β-barrel translocator domain (β-domain) (Fig. 3). Translocator domain is inserted in outer membrane (OM) and forms pathway across the OM. This pathway is necessary for translocation of N-terminal passenger domain from periplasmic space to bacterial surface across the outer membrane. Therefore, passenger domain (α-domain) are displayed on the surface of bacterial cell. Signal peptide is indispensable for translocation of unfolded Ag43 into periplasmic space across the inner membrane. In addition, β-domain of Ag43 contains autochaperon domain, which mediates α-domain folding. After the secretion of passenger domain, passenger is autocatalytically cleft from β-domain, but then non-covalent bond betweenα-domain and β-domain arrows passenger remain on the surface of bacterial cells. Passenger domain is non-covalently bound to translocator domain with β-barrel, so easily released from β-domain with heat treating (60 degrees C).
Fig. 3 Structure of Antigen43. This image is referenced from (2). Ag43 contains N-terminal signal peptide, α-domain (passenger), autochaperon domain and β-barrel translocator domain.
Fig. 4 and 5 show the mechanism of Ag43 insertion into OM and secretion of thanatin on the surface of bacterial cell. First, N-terminal signal peptide is inserted into inner membrane (IM) and then, following polypeptide are translocated towards periplasmic space across IM. Next, β-domain is inserted into OM, forming β-barrel structure and finally, thanatin- multimer is displayed through β-domain on the bacterial surface. Replacing α-domain with thanatin-multimer, remaining signal peptide and β-barrel domain, would arrow the display of thanatin-multimer on the bacterial surface. So, we inserted thanatin as passenger between signal peptide and β-barrel domain. After secretion of multimerized thanatin, AspN treating cause monomerization of thanatin. Thanatin-monomer would regain antimicrobial activity.
Fig. 4 Thanatin secretion system. (a)Secretion mechanism of thantin-Ag43 fusion protein from expression of to surface-display. (b)Image of thanatin activation. On cleaving surface-displayed thanatin-multimer with AspN, thanatin-monomer will be active form.
Fig. 5 A method for thanatin-activation.
Design
We have designed sequence of thanatin with C-terminal Aspartic acid (Asp). Thanatin fragment also contains BamHI restriction enzyme site (N-terminus) and BglII restriction enzyme site (C-terminus) (Fig. 5a). Autotransporter biodevice (Fig.5b) is composed of pBAD promoter + araC (BBa_00000), N-terminal signal peptide, Ag43 translocator β-domain, and double terminator (BBa_00000). BglII restriction enzyme site is located between signal peptide and β-domain. We built a construct for thanatin detoxication and secretion by ligate Thanatin fragment (BamHI / BglII cut) with BglII cut autotransporter biodevice (Fig. 6).
Fig. 6 Schematics of fragment and plasmid that we used in this project. (a)Thanatin fragment. It contains BamHI and BglII restriction enzyme site. Aspartic acid at C-terminal side of thanatin is important for thanatin activation. (b)Autotransporter biodevice. This plasmid contains the gene of Ag43, but lacks α-domain so we can insert any fragment between signal peptide and β-domain.
Fig. 7 Image of complete version of construct.
Thanatin secretion biodevice is under control of one of inducible promoters; pBAD promoter. In the absence of L-arabinose, AraC protein act as repressor and bind to promoter region, therefore expression of downstream gene of pBAD is negatively controlled. However, on adding L-arabinose, this inducer interact with AraC and cause the conformation change of repressor. Finally, AraC is released from promoter region, leading recruiting of RNA polymerase and initiation of transcription. Thanatin-Ag43 fusion protein is inducible by adding L-arabinose. And then, AspN treating of surface displayed thanatin-multimer would realize collection of antimicrobial active Thanatin.
Experiments
thanatin multimerization
How can we create thanatin-multimer and β subunit fusion protein correctly and securely? One of some methods to create thanatin multimer is inserting thanatin fragment into thanatin-Ag43 fusion protein. Once we have got thanatin-monomer-inserted autotransporter biodevice, we can make thanatin-multimer and Ag43 fusion protein from this plasmid. Thanatin-monomer-inserted autotransporter biodevice contains BamHI/BglII scar (between signal peptide and thanatin) and BglII restriction enzyme site (between thanatin and β subunit). So, after cutting thanatin-inserted autotransporter with BglII, ligation this plasmid with thanatin fragment (BamHI/BglII cut) would make it possible to create thanatin-multimer (Fig. 7). However, in this method, there is a possibility of reverse insertion of thanatin because C-terminal BglII of thanatin might be ligated with C-terminal BglII of autotransporter.
Fig. 8 Insertion of thanatin into autotransporter biodevice. (a)Forward insertion. It is desired pattern of insertion. (b)Reverse insertion. This insertion will happens by accident. We cannot insert thanatin only in forward direction selectively.
To prevent thanatin from reverse-insertion, we utilized another method. Fig.8 shows the outline of alternative thanatin-multimerization method.
Fig. 9 Flowchart of thanatin-multimerization. (a)First, we amplified 2 types of fragment with 2 different primer sets. (b)Second, we digested 1st fragment (BamHI / SpeI cut) and 2nd fragment (BglII / SpeI cut). And finally, we ligated these 2 fragments. (c)Complete form of thanatin-multimer secretion system.
Amplify the autotransporter plasmid containing 1 thanatin by PCR with 2 primer sets below.
1st primer set: for making 1st fragment
Forward: a primer binding to pBAD region, which has an ability to regenerate BamHI restriction enzyme site.
Reverse: a primer binding to 200bp-downstream region of suffix.
2nd primer set: for making 2nd fragment
Forward: a primer binding to 100bp-upstream region of suffix.
Reverse: a primer binding to 200bp-downstream region of β subunit.
Digest the amplified 2 fragments. Cut the 1st fragment with BamHI and SpeI, and cut the 2nd fragment with SpeI and BglII.
Ligate 1st and 2nd fragment and complete the creation of thanatin-multimer.
Repeat these experimental operation to increase the number of inserted thanatin.
Result
Conclusion
