Part:BBa_K5335003

MS2 coat protein virus-like particle loading system

The part is composed of BBa_K5335000, BBa_K5335001, BBa_K5335002, promoter J23110, J23119, rrnB T1 terminator, and ribosome binding site B0034. The whole circuit can play the role of VLP assembly, surface display of functional proteins, and the inner inclusion of functional RNA.

Expected function and circuit construction

Introduction

Our project "Bacillus Gemini" aims to improve plant immunity while killing nematodes. After a large number of previous literature review, we chose Virus-like particles (VLPs) composed of coat proteins of bacteriophage MS2 (MS2 CP) of Escherichia coli as the platform for executive functions, and displayed insecticidal proteins and plant immune activation proteins on its surface to complete the role of killing nematodes that damage plants and activating plant immunity. At the same time, we also included a dual tandem 19bp stem-loop sequence that can be specifically bound by the MS2 coat protein to verify its loading capacity as a future RNA delivery vector. We plan to make MS2 coat protein express in the engineered bacteria and form VLP, bind to the functional protein with SpyCatcher on the surface, and enclose the RNA containing a 19-bp sequence of stem rings to form a multifunctional carrier (Figure 1.).

Figure 1. VLP delivery system.

The plasmid vector was successfully constructed by homologous recombination

Our designed coat protein was constitutively expressed in the engineered bacteria. To obtain sufficient amounts of VLP, we chose to fuse the MS2 CP coding sequence to a high-copy PUC57 mini plasmid. Meanwhile, SpyCatcher-EGFP coding sequence and a 19-bp tandem stem-loop sequence were ligated downstream of the MS2 CP coding sequence (Figure 2.). Upstream of SpyCatcher, we have reserved a place to integrate Psal. Upstream of MS2 CP, we have reserved a place for access to the salicylic acid response element NahR.

Figure 2. Composite PUC57 mini plasmid vector.

We plan to verify the connection function of induced SpyCatcher and the function of loaded RNA after the successful verification of salicylic acid regulatory elements. After homologous recombination, we obtained the target plasmid vector (Figure 3.).

Figure 3. Colony PCR and sequencing confirmed the successful assembly.

Sequence and Features

Assembly Compatibility:

• 10
  COMPATIBLE WITH RFC[10]
• 12
  INCOMPATIBLE WITH RFC[12]

  Illegal NheI site found at 7
  Illegal NheI site found at 30
  Illegal NheI site found at 2317
  Illegal NheI site found at 2340
  
• 21
  COMPATIBLE WITH RFC[21]
• 23
  COMPATIBLE WITH RFC[23]
• 25
  COMPATIBLE WITH RFC[25]
• 1000
  INCOMPATIBLE WITH RFC[1000]

  Illegal BsaI site found at 392
  Illegal BsaI site found at 821
  Illegal BsaI site found at 1389
  Illegal BsaI.rc site found at 2096
  

Successful validation of protein expression

Bacterial culture and protein extraction

We preserved BL21 (DE3) strains that had previously verified the correct sequence introduction. We first explored the effect of different time periods of low temperature expression on the concentration of the same volume of purified 6x His tag protein. We cultured the same 25 mL LB medium at different temperatures. The cultivation conditions are as follows.

1). LB medium containing 60 μg·mL^-1 ampicillin was added with 100 μL of preserved bacterial solution, and cultured in a shaking bed at 37℃ at 200 rpm until OD₆₀₀ was between 0.6-0.8.

2). The culture with OD₆₀₀ meeting the requirement was removed and transferred to a shaking bed at 16℃ and 160 rpm for low temperature expression for 12 h and 16 h, respectively.

3). The low-temperature culture products were placed at 4℃ and centrifuged at 6000 rpm for 30 minutes to collect bacteria.

4). The collected bacteria were re-suspended with 8 mL of PBS buffer, broken for 10 s using an ultrasonic cell crusher, and broken for 10 s for 30 minutes. Centrifuge the crushing liquid at 12000 rpm at 4℃ for 30 minutes, separate the supernatant and precipitate, and store them at 4℃ respectively.

5). All the supernatants were purified with Ni-NTA purification kit, and the concentration of purified samples with different expression time was determined, and SDS-PAGE experiment was performed.

The expected molecular weight of SpyCather-EGFP containing 6x His tag and the complex based on SpyCatcher-SpyTag connection were 47.4 kDa and 76.5 kDa, respectively. The result is shown in Figure 4.

Figure 4. Experimental results of protein extraction

A. SDS-PAGE of total protein sample after 12 h expression. Line 1: supernatant. Line 2: precipitate. Line 3: BL21 (DE3). Line 4: Engineered bacteria introduced into the plasmid.
B. Purification results of the products expressed at 12 h. Line 1: supernatant. Line 2: BL21 (DE3) Line 3: Engineered bacteria introduced into the plasmid. Line 4 and 6: Purified sample at 12 h. Line 5: precipitate. Red boxes indicate bands that may be target proteins.
C. Purification results of the products expressed at 16 h. Line 1: supernatant. Line 2: precipitate. Line 3: BL21 (DE3). Line 4: Engineered bacteria introduced into the plasmid. Line 5 and 6: Purified sample at 16 h. Red arrows indicate bands that may be target proteins.
D. Comparison of the concentration of the same volume protein purified solution at 12 h and 16 h. Each group selected the same batch of the same volume of three bottles of bacterial culture solution, respectively purified, the same volume of purified solution to determine the protein concentration. "**" indicates p < 0.01.