Part:BBa_K5335025

SpyCatcher-VDAL-CPPs (R9-Tag)

To enhance plant immunity, a novel fusion protein, SpyCatcher-VDAL-CPPs, was designed. This construct incorporates a SpyCatcher tag_[1] for conjugation, VDAL for immune activation_[2], and the R9 cell-penetrating peptide for intracellular delivery_[3]. Functional studies demonstrated the capacity of this fusion protein to penetrate plant cell membranes and induce an ETI response. Molecular characterization and DAB staining confirmed the successful construction and immunogenic activity of the fusion protein.
The design was verified as shown in Figure 1.

Figure 1. Experimental circuit design diagram

Usage and Biology

VDAL, an Aspf2-like protein derived from Verticillium dahliae, has been shown to activate PTI responses when expressed extracellularly _[4] and can also induce endogenous ETI immune responses when expressed intracellularly. To verify the function of the constructed circuit, a pET28a-based expression system was employed, with the T₇ promoter under the control of the lac operon. The circuit includes the engineered SpyCatcher-VDAL-CPPs protein, SpyTag-6*His protein, and SpyCatcher-CPPs protein. The SpyCatcher-SpyTag (SpyC/SpyT) system is derived from the CnaB2 domain of the Streptococcus pyogenes fibronectin-binding protein. SpyC is an immunoglobulin-like protein with a molecular weight of approximately 12 kDa, while SpyT is a short peptide consisting of 13 residues. These two components can specifically recognize each other and spontaneously form an isopeptide bond, enabling high-affinity binding. The designers aimed to utilize this covalent linkage during co-expression to verify the functionality of the SpyCatcher-SpyTag system and facilitate the purification of the target protein.
The constructed plasmid vector is illustrated in Figure 2.

Figure 2. Plasmid Vector

Experimental Verification

Transformation

After chemical transformation using calcium chloride, the constructed plasmid was introduced into E. coli BL21 (DE3). The transformed cells were plated on LB agar plates supplemented with kanamycin and incubated at 37℃ for 16 hours. Colony PCR was performed on individual colonies to verify the presence of the plasmid.
The results of the colony PCR are presented in Figure 3.

Figure 3. Agarose gel electrophoresis image of colony PCR products (target band at 2200 bp) After sequencing the selected single colonies and confirming the results, the researchers proceeded with subsequent experiments.

SDS-PAGE

SDS-PAGE detection of bacterial total protein
The single colony confirmed by sequencing was inoculated into a shake flask and cultured at 37°C for 6 hours until the OD₆₀₀ reached 0.6. IPTG was then added to a final concentration of 1 mM, and the culture was further incubated at 16°C for 18 hours. Cells were harvested by centrifugation at 8000 rpm, 4°C for 10 minutes, washed with PBS, resuspended, and centrifuged again. Finally, the cell pellet was resuspended in 1 ml of PBS to obtain a cell suspension.
The cell suspension was mixed with 5X Protein Loading Buffer and heated at 95°C for 10 minutes.E. coli BL21 (DE3) harboring the empty pET28a plasmid was processed in the same manner as a control. Samples were analyzed by 12% SDS-PAGE, and a protein band corresponding to the expected size of SpyCatcher-VDAL-CPPs + SpyTag-6*His protein (53.7 kDa) was observed.
The results are presented in Figure 4.

Figure 4. SDS-PAGE gel electrophoresis image.Compared with the control, a target band of approximately 53.7 kDa was detected.
Purification and SDS-PAGE analysis of the target protein The single colony confirmed by sequencing was inoculated into a shake flask and cultured at 37°C for 6 hours until the OD₆₀₀ reached 0.6. IPTG was then added to a final concentration of 1 mM, and the culture was further incubated at 16°C for 18 hours. Cells were harvested by centrifugation at 8000 rpm, 4°C for 10 minutes, washed with PBS, resuspended, and centrifuged again. Finally, the cell pellet was resuspended in 2 ml of Binding Washing Buffer (Sangon Biotech, Shanghai, China) containing 10 mM imidazole and 80 μl of PMSF (Sangon Biotech, Shanghai, China).
The cell suspension was then sonicated for 10 minutes with a 1-second pulse followed by a 2-second pause. The cell lysate was centrifuged at 12000 rpm at 4°C for 15 minutes. The supernatant was transferred to a new Eppendorf tube, and the pellet was resuspended in 1 ml of PBS for storage.
The supernatant was mixed with an equal volume of Binding Buffer to prepare the sample. The storage solution was slowly drained, and the Ni column was equilibrated with 5 ml of Washing Buffer. The sample was loaded onto the column in two bed volumes, and the first flow-through was reloaded.The column was washed with two bed volumes of Washing Buffer, and the flow-through was collected until the absorbance at 280 nm reached the baseline. The protein was eluted with two bed volumes of Elution Buffer, collecting 2 ml fractions each time, until the absorbance at 280 nm reached the baseline. The purified protein was obtained. (Specific experimental procedures were followed from HyPur T Ni-NTA 6FF (His-Tag) PrePacked Gravity Column Kit, Sangon Biotech, Shanghai, China)
The harvested bacterial cell pellet, the supernatant after cell lysis, the purified protein sample, and the E. coli BL21 (DE3) cell suspension containing the empty pET28a plasmid were mixed with 5X Protein Loading Buffer and heated at 95°C for 10 minutes. Samples were separated by 12% SDS-PAGE and stained with Coomassie Brilliant Blue G250 for 12 hours, followed by destaining.
The obtained results are shown in Figure 5.
Figure 5. SDS-PAGE gel electrophoresis image.The bands labeled 1, 2, 3, and 4 in the figure represent the elution fractions obtained sequentially using Elution Buffer.
==Sequence and Features== BBa_K5335025 SequenceAndFeatures