Materials
Chemicals were purchased from Sigma-Aldrich (St. Louis, MO), ThermoFisher Scientific (Pittsburgh, PA), Agdia (Elkhart, IN), ChemImpex (Wood Dale, IL), Alfa Aesar (Ward Hill, MA), and TCI America (Portland, OR) and were used without further purification.
Instrumentation
LC-MS were obtained using an Agilent 1100 HPLC with a PLRP-S column for separation and an AB Sciex 4000 QTRAP system for detection; also, a Waters SYNAPT G2-Si Q-TOF with an M-class UPLC. TEM was conducted using a JEOL JEM-1400Plus transmission electron microscope. Bio-rad ChemiDoc MP was used for gel imaging. ELISA data were obtained using a BioTek Synergy H4 Hybrid microplate reader.
Expression of TMV
The tobacco (Nicotiana benthamiana) plants were grown for 8 weeks and infected with TMV solution with 2 weeks of incubation. The harvested infected leaves were stored at −80 °C fridge. About 100 g were blended with cold (4 °C) extraction buffer, potassium phosphate (KP) buffer (0.1 M, 1000 mL, pH 7.4) added with 2-mercaptoethanol (0.2% (v/v)). It was subsequently ground to have an effective extraction. The slurry was filtered and centrifuged at 11,000 ×g (4 °C, 20 min). The resulting supernatant was filtered again. The volume was measured and an equal amount of chloroform/1-butanol with 1-to-1 ratio was mixed (4 °C, 30 min). Another centrifugation was done at 4500 ×g for 10 min. The collected aqueous phase was mixed with NaCl (final concentration of 0.2 M), PEG 8000 (8% (w/w)), and Triton X-100 surfactant (1% (w/w)). It was stirred on ice for 30 min followed by storing for 1 h at 4 °C. It was centrifuged again at 22,000 ×g (4 °C, 15 min). The pellet was collected and resuspended in KP buffer (0.1 M, pH 7.4) and was stored at 4 °C overnight. The suspension was added carefully to 40% (w/v) sucrose gradient in KP buffer (0.01 M, pH 7.4) and centrifuged using swing bucket rotor at 96,000 ×g for 2 h. The blue band was collected with the assistance of LED light shined upward from the bottom of the centrifuge tube. The colloidal suspension from blue band was centrifuged at 360,562 ×g for 1.5 h. The pellet was resuspended in KP buffer (0.01 M, pH 7.4).
Transmission Electron Microscopy (TEM) was performed on a JEOL JEM-1400+ transmission electron microscope. Samples were prepared by incubating 5 µL of ~0.1 mg/mL TMV in water on a 300 mesh formvar-coated copper grid for 30 s. The sample was then stained with 5 μL 2% uranyl acetate for an additional 30 s. The excess liquid was wicked away with a Whatman (#1) filter paper and the grids were left to air dry. Images were taken with an accelerating voltage of 120 kV.
ESI-MS
TMV samples were prepared by denaturing 20 µL of 10 mg/mL in 40 µL of glacial acetic acid. Sample was then centrifuged at 4300 ×g for 10 s to separate the precipitated RNA. The supernatant was collected and run on an Agilent 1100 series HPLC system with a PLRP-S column for separation followed by a 4000 QTRAP mass spectrometer. The flow rate is 0.250 mL/min, and the solvent system comprises of Milli Q water, 0.1% formic acid, and pH 7.0 in a 0.1 M sodium phosphate buffer. This system was used for running both TMV and TMV-Alk samples.
Electrophoretic Mobility Assays
1% (w/v) Agarose gels were used. The sample was prepared by mixing 3 µg TMV with 5 µL Thermo Scientific 6X DNA Loading Dye. From that mixture, 4 µL was added to each well. The gel was run at 100 eV for 45 min, stained with coomassie brilliant blue, and visualized using Bio-rad ChemiDoc MP gel imager.
10% SDS-PAGE gel was used. The sample was prepared by mixing 3 µg TMV with 5 µL of SDS loading dye (β-Mercaptoethanol (5%), Bromophenol blue (0.02%), Glycerol (30%), SDS (Sodium dodecyl sulfate 10%), Tris-Cl (250 mM, pH 6.8)) and 5 µL of 0.1 M dithiothreitol. The mixture was boiled for 10 min. 4 µL of sample was added to each well and the gel was run at 100 eV for 45 min, stained with coomassie brilliant blue, and visualized using Bio-rad ChemiDoc MP gel imager.
Bottom-up Proteomics
Digestion: Trypsin digestion was performed according to manufacturer’s protocol (Thermo Scientific, Product No. 90057). Briefly, TMV samples were dialyzed into a solution of 8 M urea and 50 mM triethylammonium bicarbonate (TEAB) at pH 8.5. 500 mM DTT was added to the sample to a final concentration of 20 mM and incubated for 1 h at 60 °C. 1 M Iodoacetamide was prepared and added to the sample to a final concentration of 40 mM and incubated for 30 min at room temperature in darkness. The alkylation reaction was quenched by adding 500 mM DTT solution to a final concentration of 10 mM. The samples were then diluted to reduce the concentration of urea to 1 M by adding 50 mM TEAB, pH 8.5. Trypsin in 50 mM TEAB pH 8.5 was added to the samples to a final protease-to-protein ratio of 1:20 (w/w) and incubated for 24 h at 37 °C. Samples were then flash-frozen and stored until injection. Samples for pepsin digestion were diluted to 1 µM in TEAB pH 8.5. Samples were then mixed 1:1 (v/v) with a solution of 1.6 M GuHCl, 0.8% formic acid at pH 2.3 to prepare them for on-line pepsin digestion. Samples were then flash-frozen and stored until injection.
LC-MS: LC-MS (data presented in Figure 2) was performed using a Waters HDX manager and SYNAPT G2-Si Q-TOF. Two technical replicates of each sample were analyzed. Pepsin digest samples were digested on-line using Sus scrofa pepsin A (Waters Enzymate BEH) at 15 °C. Peptides were desalted on a C18 pre-column (ACQUITY UPLC BEH C18 VanGuard Pre-column) for 3 min at 100 μl min−1 and 1 °C. The liquid-chromatography buffer was 0.1% formic acid. Peptides were separated over a C18 column (Waters Acquity UPLC BEH) and eluted with a linear 3–40% (v/v) acetonitrile gradient for 7 min at 40 μl min−1 and 1 °C.
Mass-spectrometry data were acquired using positive-ion mode in HDMSE mode, collecting both low-energy (6 V) and high-energy (ramping 22–44 V) peptide-fragmentation data for peptide identification. All samples were acquired in resolution mode. Capillary voltage was set to 2.8 kV for the sample sprayer. Desolvation gas was set to 650 l h−1 at 175 °C. The source temperature was set to 80 °C. Cone and nebulizer gas was flowed at 90 l h−1 and 6.5 bar, respectively. The sampling cone and source offset were both set to 30 V. Data were acquired at a scan time of 0.4 s with a m/z range of 100–2,000. Mass correction was done using [Glu1]–fibrinopeptide B as a reference mass.
Data Processing: Raw data were processed by PLGS (Waters Protein Lynx Global Server 3.0.2) using a database containing Sus scrofa pepsin A and native TMV coat protein. In PLGS, the minimum fragment ion matches per peptide was 3, and methionine oxidation and N-terminal acetylation were allowed. Trypsin samples included cysteine carbamidomethylation (CAM) as a fixed modifier. The low and elevated energy thresholds were 250 and 50 counts, respectively, and the overall intensity threshold was 750 counts. Peptides were curated in DynamX 3.0 with thresholds of 0.3 products per amino acid and one consecutive product.
ELISA
96-well Microtiter Plates High Bind-Solid (ACC 00948/0005 Agdia) were coated with 100 µl of diluted (1:200) capture antibody rabbit anti-TMV IgG (CAB 57400/1000 TMV Capture antibody Agdia) in coating buffer (0.015 M Na2CO3, 0.034 M NaHCO3, NaN3 in dH2O, pH 9.6) and incubated overnight at 4 °C. The plate was washed 3× with washing buffer (0.2% (v/v) Tween-20 in PBS, pH 7.4). After washing, wells were blocked with blocking buffer (1% (w/v) BSA in washing buffer, pH 7.4) at RT for 1 h, followed by 4× washes with washing buffer. 100 μL of native TMV and mutant TMV—concentrations determined by Lowry assay (0.5–0.015 µg/ml )—were serially diluted with sample extraction buffer (0.009 M Na2SO4, 2 % (v/v) Polyvinylpyrrolidone (PVP) 40k, 0.2 % (w/v) Powdered egg (chicken) albumin, 0.003 M NaN3, 0.2% (v/v) Tween-20 in washing buffer) was then added to each well and incubated at RT for 2 h. Wells were then washed 8× with washing buffer, followed by the addition of 100 μL alkaline phosphatase-conjugated rabbit anti-TMV IgG in conjugate buffer (0.5 mg/mL BSA, 2 % (v/v) PVP 40k, and 0.003 M NaN3 in washing buffer) and incubated at RT for 2 h. Wells were then washed 8× with washing buffer, then developed by adding 100 μL one-step p-nitrophenylphosphate (PNPP) substrate for 45 min at RT. The plate was read at 405 nm, 420 nm, and 450 nm, and the absorbance values of buffer blank wells averaged and subtracted from the entire plate. Experiments were performed in triplicate.
TMV Functionalization
The diazonium salt is prepared by carefully mixing 200 μL of 0.30 M p-toluenesulfonic acid monohydrate, 75 μL of 0.68 M 3-ethynylaniline, and 25 μL 3.0 M sodium nitrite followed by incubation in ice for 1 h without light exposure. The resulting diazonium salt (50 μL) was added to a 2 mg/mL wTMV or mTMV solution in 0.1 M borate buffer at pH 8.8. This was incubated on ice for 45 min. The resulting TMV-alkyne was purified and concentrated via centrifuge filtration using an EMD Millipore Amicon Ultra Centrifugal Filter Unit (10,000 MW Cutoff) (4,303 ×g).