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Here we present a method to select for novel variants of the E. coli biotin-protein ligase BirA that biotinylates a specific target peptide. The protocol describes the construction of a plasmid for the bacterial display of the target peptide, generation of a BirA library, selection and characterization of BirA variants.
Biotin is an attractive post-translational modification of proteins that provides a powerful tag for the isolation and detection of protein. Enzymatic biotinylation by the E. coli biotin-protein ligase BirA is highly specific and allows for the biotinylation of target proteins in their native environment; however, the current usage of BirA mediated biotinylation requires the presence of a synthetic acceptor peptide (AP) in the target protein. Therefore, its application is limited to proteins that have been engineered to contain the AP. The purpose of the present protocol is to use the bacterial display of a peptide derived from an unmodified target protein to select for BirA variants that biotinylates the peptide. The system is based on a single plasmid that allows for the co-expression of BirA variants along with a scaffold for the peptide display on the bacterial surface. The protocol describes a detailed procedure for the incorporation of the target peptide into the display scaffold, creation of the BirA library, selection of active BirA variants and initial characterization of the isolated BirA variants. The method provides a highly effective selection system for the isolation of novel BirA variants that can be used for the further directed evolution of biotin-protein ligases that biotinylate a native protein in complex solutions.
Biotinylation of a protein creates a powerful tag for its affinity isolation and detection. Enzymatic protein biotinylation is a highly specific post-translational modification catalyzed by biotin-protein ligases. The E. coli biotin-protein ligase BirA is extremely specific and covalently biotinylates only a restricted number of naturally occurring proteins at specific lysine residues1. The advantages of the BirA catalyzed biotinylation are currently harnessed by fusing the target protein with a small synthetic 15-amino-acid biotin acceptor peptide (AP) that is effectively biotinylated2 and allows for the highly specific and efficient in vivo and in vitro biotinylation by co-expression or addition of BirA3,4,5. Although the in vivo and in vitro BirA catalyzed biotin-protein ligation is an attractive labeling strategy, its application is limited to samples that contain AP-fused proteins. The purpose of this method is the development of new mutants of biotin-protein ligases that selectively biotinylate native unmodified proteins and, thereby expand the number of applications in which the enzymatic biotinylation strategy can be used.
Protein function can be evolved through iterative rounds of the gene mutation, selection, and amplification of gene variants with the desired function. A strong and efficient selection strategy is crucial for the directed evolution and biotin-protein ligase activity is readily selected due to the strong binding between biotin and streptavidin and its homologs6. Phage display technologies allow for the selection of phages that display biotinylated peptides7,8. Since amplification of isolated phages requires infection of a bacterial host, however, the phage selection with streptavidin creates a bottleneck in that the high-affinity binding of biotin to streptavidin is virtually irreversible under non-denaturing conditions. To ensure reversible binding of biotinylated phages, monomeric avidins with lower affinity were used which resulted in a modest ~10-fold enrichment7. We recently developed a bacterial display method for the isolation of novel BirA variants that eliminates the need for the elution from the affinity matrix and thereby removes a bottleneck from previous BirA selection systems9. Indeed, our bacterial display system allows for a >1,000,000-fold enrichment of active clones in a single selection step9, thus providing an effective selection system for the directed evolution of novel BirA variants.
Our bacterial display system consists of two components, BirA with a C-terminal 6xHis tag and a scaffold protein that allows for the surface display of a target peptide. We used the scaffold protein enhanced circularly permuted outer membrane protein X (eCPX) since the effective display of peptides can be observed at both the N- and C-termini10,11. The fusion of the target peptide sequence to the C-terminus of eCPX ensures biotinylation of bacteria expressing active BirA variants. The bacteria allow for the effective streptavidin selection as the biotinylated peptide now displays on the surface (Figure 1a).
The purpose of this method is to select for novel variants of BirA that biotinylates peptide sequences present in native proteins. The system is encoded by genes present on the plasmid pBAD-BirA-eCPX-AP, which contains an arabinose-inducible promoter controlling BirA (araBAD), and a T7 promoter controlling eCPX9 (Figure 1b). The present protocol describes the detailed procedure for 1) incorporation of a peptide derived from a target protein into the C-terminal of eCPX, 2) creation of a mutational library of BirA by error-prone PCR, 3) selection of streptavidin-binding bacteria by magnetic-activated cell sorting (MACS), 4) quantification of bacteria enrichment, and 5) initial characterization of isolated clones.
1. Insertion of Peptide Coding Sequencing Sequence in pBAD BirA-eCPX-AP
NOTE: To select for BirA variants that biotinylate a native target protein, start by identifying a 15-amino acid peptide sequence in the proteins primary sequence that contains at least one lysine (K) residue.
2. Generation of a BirA Library
NOTE: The initial BirA mutational library (Figure 1c, step 1) is created by error-prone PCR. Other methods to generate the BirA mutational library are likely to work as well.
3. Selection of Bacteria Expressing Biotinylated Peptide
NOTE: This part of the protocol covers step 2-5 of Figure 1c. It is highly recommended that the selection approach is setup using pBAD-BirA-eCPX-AP and pBAD-BirA-eCPX-AP(K10A) as positive and negative controls.
4. Quantification of Enrichment
NOTE: Quantification of the live bacteria in the "input" and "output" samples are performed after each selection round by plating of serial dilutions of the samples and subsequent counting of colony forming units (CFUs).
5. Characterization of Selected BirA Variant
NOTE: The characterization can be performed after selecting BirA variants from the first BirA library; however, the BirA variants generally have low activity towards the peptide. Therefore, an additional round of mutation and selection can also be performed before the characterization. Usually, 10 clones from the final selection round are isolated for further characterization.
Western blot of pBAD-BirA-eCPX-AP expressing bacteria produces a ~22 kDa streptavidin-reacting band consistent with the molecular weight of eCPX (Figure 2a). Unlike BirA-6xHis, biotinylated eCPX-AP was present in both uninduced and induced cultures (Figure 2a) due to a small degree of T7 promoter activity even in uninduced cultures and subsequent biotinylation of the AP by endogenous BirA. In BirA-eCPX-AP(K10A) expressing cultur...
As for all selection methods, the stringency of the washing steps is of utmost importance. Since bacteria do not need to be eluted from the beads before the amplification of the selected clones, the high affinity binding between biotin and streptavidin can be used instead of using lower affinity avidins, as previously done with the phage display system, for the selection of BirA variants7,8. This ensures that rare clones are selected and that non-biotinylated bac...
The authors have nothing to disclose.
The authors thank Mohamed Abdullahi Ahmed for the expert technician assistance. This work was supported by grants from the Lundbeck Foundation, the Novo Nordisk Foundation, the Danish Kidney Association, the Aase og Ejnar Danielsen Foundation, the A.P. Møller Foundation for the Advancement of Medical Science, and Knud and Edith Eriksen Memorial Foundation.
Name | Company | Catalog Number | Comments |
10% precast polyacrylamide gel | Bio-Rad | 4561033 | |
Ampicilin | Sigma-Aldrich | A1593 | |
ApE - A plasmid editor v2.0 | NA | NA | downloaded from http://jorgensen.biology.utah.edu/wayned/ape/ |
Arabinose | Sigma-Aldrich | A3256 | |
Biotin | Sigma-Aldrich | B4501 | |
DMSO | Sigma-Aldrich | D2650 | |
DPBS (10X), no calcium, no magnesium | ThermoFischer Scientific | 14200083 | |
DpnI restriction enzyme | New England BioLabs | R0176 | |
Dynabeads MyOne Streptavidin C1 | ThermoFischer Scientific | 65001 | |
GenElute Plasmid Miniprep Kit | Sigma-Aldrich | PLN350 | |
GeneMorph II EZClone Domain Mutagensis kit | Agilent Technologies | 200552 | |
Glucose | Sigma-Aldrich | G8270 | |
Glycerol | Sigma-Aldrich | G5516 | |
Immobilon-P PVDF Membrane | Millipore | IPVH15150 | |
IPTG | Sigma-Aldrich | I6758 | |
LS Columns | Miltenyi Biotec | 130-042-401 | |
NaCl | Sigma-Aldrich | S7653 | |
NEB 5-alpha Competent E. coli | New England BioLabs | C2987 | |
NuPAGE LDS Sample Buffer (4X) | ThermoFischer Scientific | NP0007 | |
NuPAGE Sample Reducing Agent (10X) | ThermoFischer Scientific | NP0009 | |
pBAD-BirA-eCPX-AP | Addgene | 121907 | Used a template and positive control |
pBAD-BirA-eCPX-AP(K10A) | Addgene | 121908 | negative control |
Q5 High-Fidelity DNA Polymerase | New England BioLabs | M0491 | For insertion of peptide sequence in pBAD-BirA-eCPX-AP, any high fidelity polymerase will do |
QuadroMACS Separator | Miltenyi Biotec | 130-090-976 | |
Skim Milk Powder | Sigma-Aldrich | 70166 | |
Streptavidin-HRP | Agilent Technologies | P0397 | |
T7 Express lysY/Iq Competent E. coli | New England BioLabs | C3013 | |
Tryptone | Millipore | T9410 | |
Tween-20 | Sigma-Aldrich | P9416 | |
Western Lightning Plus-ECL | PerkinElmer | NEL103001EA | |
Yeast extract | Sigma-Aldrich | Y1625 |
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