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Revision as of 12:54, 17 October 2016
nbsp;Protease orthogonality
The first challenge in the construction of a new protease-based signaling cascade was the selection of appropriate proteases. The candidate proteases should recognize defined target cleavage sequences, preferably as long as possible; they should be active in mammalian cells, but not toxic to them and inducible, ideally through the reconstitution of split protein fragments. Most importantly, a large number of proteases with similar properties but with different specificities should be available to allow for modular construction of signaling pathways and logic functions and these proteases should be orthogonal to each other, meaning they should have specific cleavage sites not recognized by the other proteases in the system.
We found that the tobacco etch virus protease (TEVp) was the only protease described in the literature to match our criteria.
TEV protease is a highly specific 242 amino acids long, 27 kDa cysteine protease, that originates from the tobacco etch virus (TEV) of the Potyvirus genus.
It has a target recognition sequence of seven amino acids, ENLYFQ-S/G, where cleavage occurs after the glutamine residue and is denoted by the – symbol,
and the final residue of the recognition sequence can be either S or G, denoted by the / symbol. This substrate sequence is scarcely represented in the
proteome. TEV protease is therefore relatively non-toxic
Despite its widespread use in the biotechnology, TEVp also displays some shortcomings, the most prominent of them being self-cleavage. Substitution
of Ser-219 with Val or Pro
To overcome this lack of inducible orthogonal proteases, we looked for the characterized TEVp mutants and naturally occurring proteases closely related to TEVp that might also be used to function as split proteases.
nbsp;Results
TEVp variants
Based on the sequence alterations described by Yi et al.
Yi et al.
To test these two proteases we used a cleavable firefly luciferase (fLuc) reporter with an appropriate cleavage sequence inserted in a permissible site. We observed a significant decrease in the fLuc activity upon coexpression of the reporters with their corresponding proteases, whereas the coexpression of reporters with an orthogonal protease resulted in a much lower decrease of fLuc activity (1). These results were additionally confirmed by results from western blot where the cleaved luciferase was detected only in cells cotransfected with a reporter and its corresponding protease but not with other reporter-protease combinations (2).
HEK293T cells were transfected with the indicated fLuc:TEVs and TEVp variant constructs. Luciferase activity was measured 24h after transfection. The results are presented as normalized firefly luciferase activity (RLU).
No data has previously been reported on TEVpE and TEVpH toxicity. Therefore we performed a viability test for expression of all three TEVp variants in HEK293T cells. Even after transfection with a high amount of the plasmid for each respective protease, the cells showed high viability, with practically no difference when compared to control transfections (3).
HEK293T cells were transfected with 2000ng of the indicated protease and 500ng of the indicated reporter. Cells were lysed and analyzed by western blotting against the AU1 tag. The cleaved reporter (55 kDa) was detected only in the presence of the corresponding TEVp variant.
Cells were transfected with plasmid DNA for different TEVp variants and counted two days later. Prior to counting cells were stained with trypan blue to determine the percentage of dead cells.
TEVp homologs
Introduction of two new TEVp variants expanded our repertoire of tools, demonstrating that we can use the results of the mutational screening of protease variants, but a larger number of strictly orthogonal proteases would be required for modular design of logic gates. We therefore decided to investigate activity of de novo created split proteases from the potyviridae family.
The NIa proteases from the potyviridae group of plant viruses in general recognize a seven amino acid sequence motif as their substrate and
are classified as cysteine proteases with an active site closely related to eukaryotic serine proteases. The NIa proteases adopt a characteristic
two-domain antiparallel β-barrel fold. The active site of the protease comprises a catalytic triad: His-46, Asp-81, Cys-151 (amino acids numbered
according to the TEVp sequence) with a Gly-x-Cys-Gly motif around the active cysteine residue
We searched for sequences of different potyviruses available on UniProt, paying particular attention to any evidence of orthogonality among their target substrates. We decided to test the plum pox virus protease (PPVp), the soybean mosaic virus protease (SbMVp), and the sunflower mild mosaic virus protease (SuMMVp).
PPVp is one of the most studied potyviral proteases after the TEVp. Its substrate (PPVs) has an amino acid sequence NVVVHQ-A.
In contrast to TEVp, it has been reported that PPVp is resistant to self-cleavage at the C-terminus
SbMVp has been
recently studied by Seo et al. as a tool for protein-protein interaction studies in the soybean. The substrate (SBMVs) has been determined to be the
sequence ESVSLQ-S
Similarly, SuMMVp has been used by Fernandez-Rodriguez et al.
All selected potyviral proteases were designed as synthetic genes and tested in mammalian cells for the activity using the cyclic luciferase reporters, which results in the luciferase activity only upon cleavage. We detected an increase of luciferase activity only in the corresponding protease-reporter pairs, confirming exquisite orthogonality of the selected proteases and their activity in the human cell chassis (5).
HEK293T cells were transfected with the indicated cycLuc reporters and proteases. Luciferase activity was detected only in the presence of a protease and cycLuc reporter containing appropriate protease cleavage site.
